Course 05 · Lesson 2 of 6 · Nutrients, labels, and sources
Understanding labels and formulations
A fertilizer's three big numbers describe the product, not the feed you mix from it. Learn to read a label line by line: its phosphate and potash figures, nitrogen forms, chelated and missing nutrients, release statement, and the line that sets your can's strength.
The question
What do the numbers and directions on a fertilizer label tell you?
By the end of this lesson, you will be able to:
- Interpret a label's analysis, including its nitrogen forms and its phosphate and potash figures, and convert those figures to phosphorus and potassium.
- Distinguish a product's formulation, what is in it and how concentrated it is, from its application rate, how much goes into each liter of water, and its interval, how often you feed.
- Find the release characteristics and the line of directions that apply to your tree and container.
About 18 minutes to read and explore
May on the west balcony. Like every case in this course, it is an instructional example, set in one temperate Northern Hemisphere city at about 40° N (USDA zone 7a). On the slatted shelf, the gift juniper and the nursery juniper are pushing out soft spring tips, and on its bench by the back wall, the trident maple, repotted in early March, is in leaf again. Their owner has never fed any of the three, and this month they mean to start.
On a Saturday in mid-May, the owner stands in a garden center's fertilizerA product that supplies mineral nutrients, such as nitrogen, phosphorus, and potassium, for roots to absorb. It supplies no energy or carbohydrates that the tree can use, so it cannot make up for too little light or too few healthy leaves, and too much can harm roots.Open the glossary aisle with a 250 g (about 9 oz) tub of water-soluble powder in one hand and a 500 mL (about 17 fl oz) bottle of liquid concentrate in the other. The powder's numbers, 14-4-14, are bigger than the liquid's 8-4-6, and the owner is sure that makes the powder the stronger feed. On the shelf below, a 1 L (about a quart) ready-to-use bottle reads 0.03-0.01-0.02, which looks like almost nothing at all. Like every product in this course, all three are fictional, and each carries a fictional label made for this course.
Whatever they buy has to suit the balcony's routine. There is no outdoor tap, so every can is mixed at the kitchen sink and carried out in the 2 L (about half a gallon) can with its fine watering roseThe cap pierced with many small holes on the end of a watering can's spout, which breaks the flow into a gentle shower. A fine rose wets a pot's whole surface evenly without washing the substrate away or cutting channels through it.Open the glossary (Course 03, Lesson 3), and the trees drink the kitchen tap's moderately hard water. Turning the bottles over, the owner finds the backs crowded with percentages, chemical names, “chelated (EDTA)”, a release statement, and three lines of directions for different uses.
Course 01 promised that the three big numbers are not quite what they seem (Course 01, Lesson 10). Which of these products makes the stronger feed, and which line on its label applies to a bonsai on a balcony? This lesson reads a fertilizer label line by line: what the three numbers measure, the forms its nutrients come in, how and when it releases them, and the directions that turn a product into a feed for your tree.
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Alt text: Five fertilizer containers stand in a row on a pale wooden garden-center shelf, each with a blank label panel: a teal bottle under a cream measuring cap, a low white tub with a blue lid and a small white scoop on top, a clear one-liter bottle of pale liquid with an orange cap, a clear jar of small round cream and amber granules under a violet lid, and a kraft paper bag with a window showing round brown cakes.
Natural editorial photograph of five fertilizer containers standing in a row on a shelf in a garden center's fertilizer aisle, on a bright overcast morning, in soft, even daylight from high windows. Eye-level viewpoint, straight on, the row filling the middle of the frame, with a little of the shelves above and below softly out of focus. Calm, warm light and a muted natural palette, consistent with the site's homepage photographs; no HDR look and no lens flare. It is an illustrative scene of generic, invented products, not a record of real ones. Horizontal 3:2.
The shelf: a plain, pale wooden shelf about 30 cm (12 in) deep, clean and uncluttered, against softly blurred shelving in muted greens and browns. The five containers stand left to right with about a hand's width between them, each turned to face the camera, each with a plain, blank label panel in a muted shade of its own color and no writing of any kind.
Left to right: a 500 mL (about 17 fl oz) bottle of liquid concentrate in deep teal plastic under a cream screw cap shaped like a small measuring cup; a low, wide white plastic tub about 12 cm (5 in) across with a cobalt-blue lid, a small white level scoop resting on the lid; a clear 1 L (about a quart) plastic bottle with a small molded handle and an orange flip cap, full of pale, faintly golden liquid; a clear plastic jar about 10 cm (4 in) tall under a violet lid, filled with small, smooth, round granules 2 to 4 mm across in cream and pale amber; and a kraft paper stand-up bag about 20 cm (8 in) tall with a clear window showing flat, round, brown cakes about 2.5 cm (1 in) across.
Accuracy: the containers are plain and generic, of the kinds sold for houseplants and gardens, with no embossed or molded lettering; every label panel is a flat, blank area of color; the granules are smooth, round, and glossy like coated fertilizer, never crystals or pebbles; the cakes are dense, matte, and brown like pressed seed meal; the teal bottle is opaque, and the ready-to-use liquid is nearly clear, only faintly tinted.
Do not include: text, labels with writing, letters, numbers, logos, brand names, brand marks, barcodes, price tags, shelf signs, people, hands, plants, spilled fertilizer, or any packaging that resembles a real brand.
A label, line by line
Here is the 8-4-6's whole label: each part as it is printed on the left, with one row for what it leaves out, and what each tells you on the right. Every label in this course follows the same order, and many real ones come close: the name and the three numbers, the guaranteed analysisThe part of a fertilizer label that lists each nutrient the product contains, as a percentage of its weight, often with the forms its nitrogen takes and which metals are chelated. Its first three figures are the N-P-K numbers. It describes what is in the product; how strong a feed it makes also depends on the rate on the line for your use.Open the glossary of each mineral nutrientA chemical element, such as nitrogen, potassium, or iron, that a tree needs and takes up through its roots from the water around them. Mineral nutrients help build and run cells, but they supply no energy and only a small share of the tree's mass.Open the glossary it contains, what those nutrients are derived from, how they are released, the directions, and the cautions.
| On the label | What it tells you |
|---|---|
| All-purpose liquid fertilizer 8-4-6 (concentrate, 500 mL bottle, cap marked 5 mL and 10 mL) | The name, the three numbers, and the form. A concentrateA fertilizer sold strong, to be diluted with water at the label's rate before use, such as a liquid measured in milliliters into a watering can. A ready-to-use product is already diluted, which is why its numbers look tiny: poured straight, it can make a feed about as strong as a concentrate's can.Open the glossary is sold strong and diluted before use. “All-purpose” means made for many kinds of plants, not for bonsai in particular. |
| Guaranteed analysis | The amounts the maker guarantees, each nutrient as a percentage of the product's weight. It describes the bottle, not your can. |
| Total nitrogen (N) 8.0% | The first of the three numbers: 8 g of nitrogen in every 100 g of concentrate. Nitrogen is always given as the element. |
| 2.4% ammoniacal nitrogen | Part of the 8 percent, as ammoniumA positively charged form of nitrogen that roots take up directly, listed on labels as ammoniacal nitrogen. Substrate particles can hold a little of it by cation exchange. Roots taking it up and soil life turning it into nitrate both tend to make the root zone more acidic over time, which is why feeds for acid-loving plants rely on it.Open the glossary: usable at once, and held a little by the mix between waterings. |
| 3.4% nitrate nitrogen | Part of the 8 percent, as nitrateA negatively charged form of nitrogen that roots take up directly, listed on labels as nitrate nitrogen. Substrate particles do not hold it, so it moves freely with water, reaching roots easily and washing out of a pot readily. Feeds rich in nitrate tend to raise the root zone's pH a little over time.Open the glossary: usable at once, and the form that washes out of a pot most readily. |
| 2.2% urea nitrogen | Part of the 8 percent, as ureaA form of nitrogen common in fertilizers, uncharged when it dissolves, that soil life turns into ammonium within days in a moist, warm root zone and more slowly in a cold one. On a label, urea nitrogen behaves much like ammonium a little later, including its tendency to make the root zone more acidic.Open the glossary, which soil life turns into ammonium within days in a warm, moist pot. The three forms add up to the total. |
| Available phosphate (P₂O₅) 4.0% | The second number, given as phosphateOn many fertilizer labels, the form in which phosphorus is declared: phosphorus pentoxide, written P₂O₅, a way of counting rather than a compound in the bottle. The actual phosphorus is about 0.44 times the phosphate figure, so 4 percent phosphate is about 1.7 percent phosphorus. In roots and substrates, phosphate is also the ion roots take phosphorus up as; the context makes clear which is meant.Open the glossary. The product holds about 1.7 percent phosphorus, the element itself (4 × 0.436). “Available” is the label's term for phosphate that dissolves in the standard tests, not a promise about what roots can reach. |
| Soluble potash (K₂O) 6.0% | The third number, given as potashOn many fertilizer labels, the form in which potassium is declared: potassium oxide, written K₂O, a way of counting rather than a compound in the bottle. The actual potassium is about 0.83 times the potash figure, so 6 percent potash is about 5 percent potassium.Open the glossary. The product holds about 5 percent potassium (6 × 0.83). |
| Phosphate and potash are given as P₂O₅ and K₂O, as on many labels. | A reminder that the second and third numbers are counted as oxides, not elements. Labels in some countries print phosphorus and potassium directly, so check which yours uses. |
| Magnesium (Mg) 0.5% | A secondary nutrient, the atom at the center of every chlorophyll molecule. Moderately hard tap water brings some as well. |
| Sulfur (S) 0.7% | Another secondary nutrient, part of proteins. Here it arrives with the magnesium, as magnesium sulfate. |
| Boron (B) 0.02% · Copper (Cu) 0.005%, chelated (EDTA) | micronutrientA mineral nutrient that plants need only in tiny amounts, such as iron, manganese, zinc, copper, or boron. Tiny amounts are still essential: a shortage of iron, for example, can yellow new leaves between their veins.Open the glossary, needed in tiny amounts and just as essential. chelateA molecule that wraps around a metal nutrient, such as iron, and keeps it dissolved where it would otherwise turn into solids that roots cannot use. Chelates differ in how far into alkaline conditions they keep iron available, so labels name the type: common EDTA chelates suit acidic to near-neutral root zones and become less reliable as the pH rises past neutral, while EDDHA-type chelates are made for alkaline ones.Open the glossary means the copper is held in a molecule, here EDTA, that keeps it dissolved. |
| Iron (Fe) 0.10%, chelated (EDTA) | Iron, needed to make chlorophyll, held by EDTA. EDTA suits acidic to near-neutral root zones and loses effectiveness as the pHA scale, usually 0 to 14, that measures how acidic or alkaline a solution or substrate is: 7 is neutral, lower numbers are acidic, and higher numbers are alkaline. It affects which mineral nutrients stay dissolved and available to roots.Open the glossary rises past neutral, so this iron is less reliable in an alkaline pot. |
| Manganese (Mn) 0.02%, chelated (EDTA) · Molybdenum (Mo) 0.0005% · Zinc (Zn) 0.01%, chelated (EDTA) | Three more micronutrients, two of them chelated the same way. |
| (no line for calcium) | The bottle supplies no calcium. That is fine on moderately hard tap water, which brings some; a tree watered only with rainwater or reverse-osmosis water needs a feed that lists calcium and magnesium. |
| Derived from: potassium nitrate, ammonium nitrate, ammonium phosphate, urea, magnesium sulfate, boric acid, copper EDTA, iron EDTA, manganese EDTA, sodium molybdate, zinc EDTA. | The compounds the nutrients come from. They confirm the nitrogen forms and show which metals are chelated. |
| Release: Water-soluble: available to roots as soon as it is applied. | Everything is dissolved when it goes on, so it is available at once and washes through over the next waterings: it follows your timing. |
| Directions. Shake well. Measure with a measuring syringe or the cap. | Measure, never pour by eye. A syringe reads single milliliters, and the cap's marks read 5 mL and 10 mL. |
| Outdoor containers, including bonsai kept outdoors: 4 mL per 1 L of water, every 14 days while plants are actively growing. | The line for a bonsai that lives outdoors: a application rateHow much of a product to use for a given amount of water or pot, as a label's directions state for each use, such as 4 mL per 1 L of water for outdoor containers. Together with the interval, how often it is applied, it sets how much a tree receives, and either one can exceed the label on its own.Open the glossary (4 mL in each liter of water), an interval (every 14 days), and a condition (only while it is actively growing). The balcony's 2 L can takes 8 mL. |
| Houseplants and indoor bonsai: 2 mL per 1 L of water, every 14 days in spring and summer; every 28 days in autumn and winter if still growing. | The line for a tree kept indoors: half the outdoor rate, and a longer interval in the darker months, when an indoor tree grows slowly. |
| Garden beds: 8 mL per 1 L of water, every 14 days. | For plants in the ground, at twice the outdoor rate. Never a line for a pot. |
| Water dry potting mix with plain water first, then apply to the moist mix until a little runs from the drainage holes. | On a feeding day, a pot that checks Drying or drier gets plain water first, then the feed once it drains to Moist, enough that a little runs from the holes. |
| Cautions. Do not use more than directed. Keep out of reach of children and pets. Store closed, away from frost. | The directions are a ceiling as well as a guide: more than directed adds salts, not growth. Keep the bottle closed, out of reach, and frost-free. |
Two parts of a label do most of the work. The analysis says what is in the bottle, and the directions line for your use says how much of it goes into your can, and how often; everything else refines one of the two. Neither says whether a tree needs feeding today. A label describes supply, the first step on the way from a bottle to a new shoot, not whether the roots can take it up or the leaves have the light to use it (Course 01, Lesson 10). The sections below take the parts in order.
The three numbers
The three numbers on the front, such as 8-4-6, are the label's headline. They are often called the N-P-K numbersThe three numbers on the front of a fertilizer, such as 8-4-6: the percentages by weight of nitrogen, phosphate, and potash, in that order. They describe how concentrated the product is, not how strong a feed it makes, which also depends on how much goes into each liter of water. Some countries print phosphorus and potassium as the elements instead.Open the glossary, after the chemical symbols for nitrogen, phosphorus, and potassium. Each is a percentage by weight, always in the same order, so a kilogram of the 8-4-6 concentrate holds 80 g of nitrogen. They say how concentrated the product is, not how strong a feed it makes; that comes later in this lesson.
Course 01 called the three numbers nitrogen, phosphorus, and potassium (Course 01, Lesson 10), and that is what they are about. On this label and many others, though, the second and third are declared in an older form: phosphorus as phosphate, written P₂O₅, and potassium as potash, written K₂O. Neither compound is actually in the bottle. They are a way of counting, and part of the weight they count is oxygen, so the second and third numbers overstate the phosphorus and potassium themselves.
To find the actual phosphorus, multiply the phosphate figure by about 0.44 (0.436 more exactly); to find the actual potassium, multiply the potash figure by about 0.83. The 8-4-6 therefore holds 8 percent nitrogen, about 1.7 percent phosphorus (4 × 0.436 = 1.74), and about 5 percent potassium (6 × 0.83 = 4.98). Nitrogen needs no converting, because labels give it as the element. Penn State Extension's guide to how much phosphorus and potassium are really in a fertilizer works with the same factors.
Not every country prints oxides. Australian labels, for example, give phosphorus and potassium as elements, and European rules allow the element in place of the oxide or beside it, so one product can carry different numbers in different places. The label's own words tell you which you are reading: “phosphate (P₂O₅)” and “potash (K₂O)” are oxides, while “phosphorus (P)” and “potassium (K)” are the elements. Read the label used where you live, and put two products in the same terms before you compare them.
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Nitrogen comes in forms
Under its total, the 8-4-6 splits its 8 percent nitrogen by form: 2.4 percent ammoniacal nitrogen, 3.4 percent nitrate nitrogen, and 2.2 percent urea nitrogen. The forms matter because roots take nitrogen up mostly as one of two ionAn atom or small group of atoms carrying an electric charge, positive or negative, such as potassium, calcium, or nitrate. Roots absorb mineral nutrients as ions dissolved in the water around them.Open the glossary, nitrate or ammonium (Course 01, Lesson 10), and each behaves differently in a pot:
- Nitrate nitrogen is nitrate, usable as soon as it reaches the roots. It carries a negative charge, so the mix does not hold it: it moves with the water, reaching roots easily and washing out of the holes just as easily (Course 04, Lesson 5).
- Ammoniacal nitrogen is ammonium, also usable at once. It carries a positive charge, so particles of akadamaA Japanese granular clay-like growing-medium material. Grades differ in particle size and hardness; its water retention and gradual breakdown must be evaluated for the container, climate, and intended maintenance interval.Open the glossary and organic matter can hold some of it between waterings, by cation exchangeThe ability of some soil and substrate particles to hold positively charged nutrient ions, such as potassium, calcium, and magnesium, on their surfaces and release them into the surrounding water in exchange for other ions. It helps a substrate store nutrients between fertilizer applications.Open the glossary.
- Urea nitrogen is urea, which carries no charge. Soil life turns most of it into ammonium within days in a warm, moist pot, more slowly in a cold one, so it works much like ammonium a little later.
- Water-insoluble nitrogen, which organic feeds such as seed-meal cakes list, is still locked in plant or animal material. Soil life frees it over weeks, by mineralizationThe release of mineral nutrients as dissolved ions, such as ammonium and phosphate, when bacteria and fungi break down organic matter such as the meal in a fertilizer cake. It runs fastest when the root zone is warm, moist, and airy and nearly stops when it is cold or dry, which is why organic feeds work slowly in a cold spring.Open the glossary, faster when the pot is warm and moist (Lesson 3).
The forms also move the root zone's pH, slowly; Course 04 names fertilization among the things that shift it (Course 04, Lesson 5). Roots taking up ammonium give off acid, and soil life turning ammonium into nitrate releases more, so feeds rich in ammonium and urea tend to make the root zone more acidic over time. Roots taking up nitrate give off alkaline ions instead, so feeds rich in nitrate tend to raise the pH a little. A feed made for acid-loving plants leans on ammonium and urea for exactly this reason, and its label says so. Whatever the form, every feed adds soluble saltsDissolved minerals, from fertilizer and from tap water, that can accumulate in a container's substrate as water evaporates. A high concentration makes it harder for roots to take up water and can damage them. Buildup can leave a white crust on the pot or surface, though hard tap water alone can leave a similar crust, so a crust by itself does not show that roots are harmed.Open the glossary to the root zone (Course 01, Lesson 10).
The rest of the analysis
Below the three numbers, the analysis lists everything else the product guarantees. Magnesium and sulfur come first. With calcium, they are the three macronutrientA mineral nutrient that plants need in relatively large amounts: nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur. Nitrogen, phosphorus, and potassium are the three behind the N-P-K numbers on fertilizer labels.Open the glossary that labels often call secondary nutrients (Course 01, Lesson 10). The name is a fertilizer-trade grouping, not a sign that a tree needs less of them, and in a mineral bonsai mix they still have to come from the water or the feed. Then come six micronutrients, from boron to zinc, one of them, molybdenum, at less than a thousandth of a percent, each essential all the same (Lesson 1).
Four of them are marked “chelated (EDTA)”. A chelate is a molecule that wraps around a metal nutrient and keeps it dissolved where it would otherwise react with the water and the mix and turn into solids that roots cannot use. Iron is where this matters most. In an alkaline root zone, iron forms solids that roots cannot absorb, though it is still there (Course 01, Lesson 10): a question of nutrient availabilityWhether a mineral nutrient in the root zone is dissolved, in a form roots can take up, and close enough to reach them, which depends on the conditions around the roots, such as moisture, pH, and the substrate itself. A nutrient can be present in the substrate yet unavailable to the tree, and an available nutrient still has to be absorbed by living roots.Open the glossary, not of supply. Chelates differ in how far into alkaline conditions they keep iron available, so labels name the type. Common EDTA and DTPA chelates suit acidic to near-neutral root zones and lose effectiveness as the pH rises past neutral, while EDDHA-type chelates are made for alkaline root zones. Colorado State University Extension's Iron chlorosis of woody plants describes that drop in EDTA and DTPA and names the chelates made for a high pH.
That is why the chelate type matters for an azalea on hard tap water, whose alkalinityThe dissolved carbonates and bicarbonates in water that neutralize acid. It is not the same as pH: pH says how acidic or alkaline water is, and alkalinity how much acid it can neutralize. Hard water is usually high in it, softening does not remove it, and with repeated watering it slowly raises a substrate's pH, which matters most for acid-loving trees such as azaleas.Open the glossary slowly raises the root zone's pH (Course 03, Lesson 5). The 8-4-6's EDTA iron is less reliable there, while the acid-loving feed's iron is chelated as EDDHA, which its label says “stays available in alkaline conditions”. Read the chelate type once the pH is known, and expect a chelate to treat the symptom while the water goes on raising the pH (Lesson 5).
The derived-from list names the compounds the nutrients come from. On the 8-4-6 it confirms the nitrogen forms (potassium nitrate and ammonium nitrate for the nitrate, ammonium nitrate and ammonium phosphate for the ammonium, and urea), shows magnesium sulfate supplying both magnesium and sulfur, and names each chelated metal as an EDTA. A metal listed as a plain sulfate is not chelated, so it has no such protection in an alkaline root zone.
What a label leaves out can matter as much as what it lists. The 8-4-6 has no calcium line: it supplies none. On the example city's moderately hard tap water that does not matter, because water hardnessThe amount of dissolved calcium and magnesium in water. Hard water leaves limescale in kettles when it is heated, and a white crust on pots as it evaporates; the crust itself does no harm. Hard water usually carries alkalinity too, which can slowly raise a substrate's pH.Open the glossary brings calcium and some magnesium with every watering (Course 03, Lesson 5). Rainwater and reverse-osmosis water bring almost none, so a tree watered only with them depends on its feed for both (Course 03, Lesson 5). That is the 14-4-14 powder's purpose. Its front reads “With calcium and magnesium, for rainwater, reverse-osmosis water, and other soft water”, and its analysis lists calcium at 5.0 percent and magnesium at 2.0 percent. Its low middle number is deliberate, because calcium and a lot of phosphate cannot stay dissolved together. Many all-purpose feeds carry little or no calcium, so for a tree on soft water, look for both lines.
Practice check · not scored
Go deeper · optional, not assessedWhy labels print oxides
The habit dates from nineteenth-century chemistry, when the mineral content of rocks, ashes, and fertilizers was reported as oxides, and fertilizer rules in many countries kept it for phosphorus and potassium. Neither oxide is in the product: the phosphorus is there in phosphate salts, and the potassium in potassium salts such as potassium nitrate. The factors come from the weights of the atoms. Phosphorus pentoxide (P₂O₅) is two phosphorus atoms and five oxygen atoms, and the phosphorus makes up 0.436 of its weight. Potassium oxide (K₂O) is two potassium atoms and one oxygen atom, and the potassium makes up 0.830 of it. European fertilizer law gives the same two factors and lets a label print the element instead of the oxide, or beside it (Regulation (EU) 2019/1009, Annex III).
A chelate works by its grip. EDTA and its relatives are larger molecules that close around a single metal ion and hold it at several points at once, like a claw, which is what the Greek word behind “chelate” means. Held this way, iron stays dissolved instead of joining with oxygen and phosphate in solids that roots cannot use. As the pH rises past neutral, EDTA's grip on iron weakens and more of the iron settles out of solution, while EDDHA's grip holds in alkaline water. A chelate protects only the iron it carries, though. It does nothing to the pH, so whatever made the root zone alkaline, usually the water, is still at work.
How and when it releases
Every label in this course carries a release statement; on a real label, look for words such as “water-soluble” or “slow-release”, or a duration with a temperature. The statement tells you when a feed's nutrients become available to roots and how long they keep coming, which decides how you time it. The course's products show the three kinds:
| Kind, and what its label says | What it means for timing |
|---|---|
| water-soluble fertilizerFertilizer that dissolves completely in water, sold as a liquid concentrate, a powder, or an already diluted ready-to-use liquid, and applied in a watering. Its nutrients are available to roots at once, and the thorough waterings that follow carry some of them out of a free-draining pot, so it is applied regularly, on the label's interval, to a moist root zone.Open the glossary (the 8-4-6, the 14-4-14 powder, the ready-to-use bottle, and the acid-loving feed): “Water-soluble: available at once.” The 8-4-6 words it “available to roots as soon as it is applied.” | Available at once, and starts washing through with the next thorough waterings, so it follows your timing and stops when you stop. |
| controlled-release fertilizerGranules of soluble fertilizer inside a coating that lets water in and nutrients out slowly, for the period its label states at a stated root-zone temperature. Warmth speeds the release and cold slows it, and the granules keep releasing whether or not the tree is growing, so they are counted as the tree's feed until they are spent.Open the glossary (the granules): “Each granule's coating lets nutrients out slowly for about 4 months at an average root-zone temperature of 21 °C (70 °F). Release is faster when warmer and slower when cooler.” | Feeds for months from one application, faster in a warm pot and slower in a cold one, and keeps releasing whether or not the tree is growing. |
| organic fertilizerFertilizer made from plant or animal materials, such as seed meals, fish, or bone. Soil organisms must break down most of it before roots can take up its nutrients, so it releases faster when the root zone is warm and moist and slowly when it is cold or dry. The word describes the source, not gentleness, and its legal meaning varies by country.Open the glossary (the seed-meal cakes): “Nutrients are released as soil organisms break the cakes down: faster when warm and moist, slowly when cool or dry.” | A small part is available at once; the rest follows the weather, because soil life sets the pace: steady when warm and moist, little when cold or dry. Its directions give a replacement interval, not a duration. |
Two details are easy to miss. A controlled-release duration is a rating at a stated temperature, not a promise: in a pot that averages warmer than 21 °C, the granules run out sooner, and in a cooler one they last longer. Granules like these are often already in the mix of nursery stockOrdinary plants grown by a nursery for gardens and landscapes and sold at garden centers, usually in black plastic nursery pots. It is affordable and usually healthy, which makes it a common first bonsai, but it is grown to look full rather than for bonsai structure, so its trunk is often thin and its root flare buried.Open the glossary (Course 02, Lesson 8), which is why the granules' own directions warn about other slow-release fertilizer already in the pot; Lesson 3 says what to do then. And an organic label's analysis says in numbers what its release statement says in words: of the cakes' 5 percent nitrogen, only 0.5 percent is water-soluble, and the other 4.5 percent waits for soil life to free it.
None of the three is better in itself. Which suits a tree depends on the weather, the pot, and how often someone is there to feed it, and Lesson 3 follows all three through a season.
Directions: rate, interval, and use
The directions turn a product into a feed. Most labels give several lines, one for each use, and only one of them is yours. Each line has the same parts: who it is for, a rate, an interval, and its conditions.
Find the line for your use
The 8-4-6 gives three lines, and a bonsai's line is set by where it lives, not by the word “bonsai”. A tree kept outdoors in a pot belongs on the first, “Outdoor containers, including bonsai kept outdoors”, whatever its size. A tree kept indoors belongs on the second, “Houseplants and indoor bonsai”, at half the outdoor rate: in a room's dimmer light a tree makes less sugar, grows more slowly, and uses less (Course 01, Lesson 6). A tree that spends its summers outdoors and its winters indoors changes lines when it moves. The third line, “Garden beds”, is for plants in the ground, at twice the outdoor rate; in a pot, its can would be far stronger than the label means for containers.
If a product has no line for containers, choose one that does, and where it gives separate lines for outdoor and indoor plants, use the one for where your tree lives. Never borrow a line written for beds or lawns.
Rate, units, and measuring
After the use comes the rate: how much product for how much water, or for how much pot. Read the units before the number, because labels measure in different ways:
- Milliliters of liquid per liter of water: the 8-4-6's 4 mL per 1 L.
- Grams, or level scoops, of powder per liter of water: the 14-4-14's 1 level scoop (1 g) per 1 L.
- Grams per liter of pot volume, for granules scattered on the surface: the granules' 3 g per 1 L of pot volume.
- Cakes per width of pot: the seed-meal cakes' 1 cake for every 10 cm (4 in) of pot width.
- No dilution at all: the ready-to-use bottle, poured straight.
For a feed mixed in water, the amount for your can is the rate times the water in the can. On the balcony, the 8-4-6's 4 mL per 1 L in the 2 L can comes to 8 mL. Lesson 6 works through other cans and units.
Measure with the tool the label names. For a liquid, that is a measuring syringe, which reads single milliliters, or the bottle's cap with its marks at 5 mL and 10 mL; the balcony's 8 mL falls between the cap's marks, so the owner uses a syringe. For a powder, it is the product's own level scoop, scraped flat. A spoon from the cutlery drawer is not a measure: spoons differ in size, and nobody fills one the same way twice.
Image placeholder · Illustration · 1600×1067
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Alt text: An illustration of four objects on a pale counter, left to right: an open teal bottle with a clear measuring syringe standing in its neck, the syringe's barrel partly filled with amber-gold liquid; the bottle's cream cap, upturned, with two fill rings inside; a small white scoop holding a level measure of pale powder; and, set a little apart on the right, an ordinary steel kitchen teaspoon.
A refined scientific illustration: ink linework with soft watercolor washes on warm off-white paper (#f8f7f4), muted natural palette, light from the upper left. Four objects on a plain, pale counter, seen from slightly above at a three-quarter angle and arranged left to right with space between them (no borders). It is an illustration of typical tools, not a record of particular products. Horizontal 3:2.
Left: an open bottle of liquid fertilizer concentrate in deep teal plastic, about 18 cm (7 in) tall, its cap removed and its blank label panel facing the viewer. A clear plastic 20 mL (0.7 fl oz) measuring syringe without a needle stands in its neck at a slight lean, its tip below the liquid, its barrel about half full of translucent amber-gold liquid and its plunger part drawn back. The barrel's graduations are plain tick marks.
Center left: the bottle's cream plastic measuring cap, upturned on the counter and slightly translucent, with two raised fill rings molded inside it, one at about a third of its height and one at about two thirds, and no markings beside them.
Center right: a small white plastic level scoop, about the size of a thimble, holding pale cream granular powder scraped exactly flat across its rim, with a few loose grains on the counter beside it.
Right, set a little farther apart than the others: an ordinary stainless-steel kitchen teaspoon from a cutlery drawer, lying empty on the counter, its bowl a different size and shape from the scoop.
Accuracy: the syringe is a needle-free oral or measuring syringe, never a medical injection syringe; the concentrate is amber-gold and translucent, never blue or green; the scoop's powder is level, not heaped; the cap's fill rings are plain raised lines; the teaspoon is plain cutlery, not a measuring spoon.
Do not include: text, labels with writing, letters, numbers, logos, brand names, brand marks, people, hands, needles, spills, drips on the counter, measuring jugs with printed scales, watering cans, or plants.
Interval and conditions
After the rate comes the interval, how often, with its conditions. The 8-4-6's outdoor line says “every 14 days while plants are actively growing”, and the condition is part of the line: a tree resting in winter, or not yet growing again after root work, is outside it (Lesson 4). The houseplant line adds a second interval for the darker months, every 28 days in autumn and winter if the tree is still growing.
One more condition covers every line: “Water dry potting mix with plain water first, then apply to the moist mix until a little runs from the drainage holes.” Feed poured onto a dry root ball stays concentrated around roots that are already short of water (Course 03, Lesson 4). So on a feeding day, check the pot first, and if it checks Drying or drier, give it a thorough wateringWater applied evenly across the whole surface, slowly enough to soak in, until it runs from the drainage holes, and then confirmed by a check that the root ball wetted through. It usually takes two or three passes a minute or two apart. Water at the holes alone shows only that water reached the bottom, not that the roots got it.Open the glossary, let it drain to Moist, and then feed.
The cautions finish the directions. Every label in this course carries two, “Do not use more than directed” and “Keep out of reach of children and pets”, and some add their own: the 8-4-6's “Store closed, away from frost”, the 14-4-14 powder's “Mix it on its own: do not combine it with other fertilizer concentrates in the same can”, and the granules' “Do not apply again within 4 months”. Each is a rule the feeding planA written plan for feeding one tree, short enough to fit on a card: the product and its label line, the amount for your can, the interval, when to start, taper, and stop in growth stages, when to water first or hold, who feeds, where it is logged, and when to review it. It turns a label's directions into decisions already made, so that anyone who helps can follow them.Open the glossary has to keep.
Strength and frequency
A line's rate and its interval are two separate limits. Strength is the dilutionMixing a measured amount of a fertilizer concentrate into a measured volume of water, at the label's rate for your use or a stated fraction of it, such as half strength. The amount is the rate times the water: if a label's rate is 4 mL per 1 L, a 2 L can takes 8 mL. Getting it right says how strong the can is, not whether the tree should be fed today.Open the glossary, how much product goes into each liter; frequency is the interval, how often a can goes on. Together they set how much a tree receives over a month. The 8-4-6 at its outdoor line every 14 days puts two cans a month on a tree; the same can every 7 days puts on four, twice the label's supply, though every can was mixed exactly right. A label can be exceeded by frequency alone, as surely as by strength.
That is the question behind a neighbor's potted boxwood that Course 03 met, fed “at full strength every week” since spring, with a white crust on its rim and browning leaf edges (Course 03, Lesson 5). “Full strength” describes only the dilution. To know whether a tree has been fed beyond its label, you need the interval too, which a dated feeding recordA dated line for each feeding: the tree or trees fed, the product and the label line used, the amount in the water, the strength, the state of the mix before feeding, the weather, and who fed. Kept in one log that everyone who feeds can see, beside the watering log, and read before each feed, it prevents doubled or missed feedings and shows at each review whether the plan works.Open the glossary shows at a glance.
Feeding often is not a fault in itself. Some labels direct weekly feeding at their own dilution, and following one is not overfeeding. Bonsai growers also sometimes feed weaker and more often than a line, such as half strength every 7 days in place of full strength every 14, which delivers a similar total. That is a different timing for the same supply, not a gentler dose for a weak or newly arrived tree; Lesson 4 says what it changes and what it does not.
Practice check · not scored
Concentration is not dose
Back at the garden center, the owner's question was which product makes the stronger feed. The three numbers alone cannot answer it.
Predict first
The 8-4-6's, about twice as much. Each liter of its can holds 4 mL of a concentrate that is 8 percent nitrogen: counting a milliliter as about a gram, which is close enough to compare cans, that is about 0.32 g of nitrogen. Each liter of the powder's can holds 1 g that is 14 percent nitrogen, about 0.14 g. The powder is the more concentrated product, and its line simply puts far less of it in each liter. Which can is stronger?, below, lets you mix both.
Whatever the products, the method is the same: multiply the nitrogen percentage by the amount the line puts in each liter. At their outdoor lines, a liter of the 8-4-6's can carries about 0.32 g of nitrogen and a liter of the powder's about 0.14 g, so the product with the bigger numbers makes the can with about half the nitrogen.
Two different things hide in the word “strong”. A product's formulation, what it contains and how concentrated it is, is printed in its analysis. Its application rate, how much of it goes into each liter of water, is printed in its directions, beside the interval that says how often. The strength of the can you carry out depends on both: the concentration times the amount in each liter. Two products, each at its own label rate, can make cans of quite different strength, and both can be right, because each maker writes its lines for the strength it intends.
A ready-to-use bottle is already diluted
The ready-to-use bottle's tiny numbers, 0.03-0.01-0.02, are the clearest case. Its directions begin “Ready to use: do not dilute”: the maker has already mixed it, so what is in the bottle is the feed. Poured straight, each liter of it, which weighs about 1,000 g, carries 0.03 percent nitrogen, about 0.3 g: close to the 0.32 g in a liter of the 8-4-6's outdoor can, and about twice the 8-4-6's houseplant line. The numbers look like almost nothing because they describe a feed, not a concentrate.
What a ready-to-use product saves is measuring; what it costs is volume. Its one line is for houseplants, indoor bonsai, and small pots, in place of a watering every 14 days in spring and summer. On the balcony, every 2 L can would take two whole bottles, where the 8-4-6's 500 mL bottle makes about 60 cans at 8 mL each.
Image placeholder · Illustration · 1600×1067
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Alt text: An illustration in two halves of two identical metal watering cans with their front walls cut away to show the liquid inside. On the left, a short amber-gold stream falls from a measuring syringe into a full can and fades into the water. On the right, a clear bottle pours pale liquid straight into the other can, and an empty bottle stands beside it. Both cans hold the same scattering of small magenta diamonds.
A refined scientific illustration: ink linework with soft watercolor washes on warm off-white paper (#f8f7f4), muted natural palette, light from the upper left. Two drawings side by side at the same scale, separated only by space (no borders), on a plain, pale counter, seen straight from the front at the height of the cans. It is a simplified illustration of a hypothetical setup, not a record of particular products. Horizontal 3:2.
Both drawings: an identical round metal watering can of about 2 L (about half a gallon), dull galvanized gray, with a long spout ending in a round fine rose, its front wall cut away in a clean vertical slice to show it filled almost to the top with clear liquid. The liquid has only the faintest warm gold cast. Scattered evenly through the liquid of each can are about twenty small magenta diamond shapes, about the same number and spacing in both cans, so that the two read as about equally strong at a glance.
Left drawing: just above the can's open top, a clear 20 mL measuring syringe without a needle, drawn on its own as in a technical plate, points down and releases a short, thin stream of translucent amber-gold concentrate. Where the stream meets the water it thins into a faint gold swirl that fades within a finger's depth. An opened bottle of concentrate in deep teal plastic, with a blank label panel, stands beside the can.
Right drawing: a clear 1 L plastic bottle with a small molded handle, its orange cap off, drawn on its own and tipped over the can's open top, pours a steady stream of pale, faintly golden liquid straight in. An identical bottle, empty, stands beside the can.
Accuracy: the two cans are the same can, filled to the same level; the diamonds are flat, clean, magenta diamond shapes, a diagram's marks rather than objects, and they appear only inside the liquid; the concentrate is amber-gold, never blue or green; the ready-to-use liquid is nearly clear.
Do not include: text, labels with writing, letters, numbers, arrows, logos, brand names, brand marks, people, hands, plants, splashes, bubbles, or any diamonds outside the liquid.
Two cans, mixed side by side
Which can is stronger? brings the garden-center question home to the west balcony's kitchen counter, beside the sink where every can is mixed before it is carried out. Two 2 L watering cans stand side by side, with a 20 mL measuring syringe, the 8-4-6's measuring cap, and the powders' scoops. Can A · Stays as it is holds the 8-4-6 at its outdoor line, 8 mL in the can, with that line quoted from its label. Can B · Your changes starts with the 14-4-14 powder at its outdoor line, 2 level scoops (2 g). Change B with Product (8-4-6 liquid, 14-4-14 powder, Ready-to-use, or Acid-loving feed), then choose its line under Measure by by picking it on that product's label. A chip on each can shows what goes in, such as “8 mL” or “2 bottles, straight”.
Under Your call, say which can will carry more nitrogen, A, B, or About the same, or press Skip. Then press Mix both cans: the run measures each feed, pours it in, tops each can up to its 2 L line, stirs, and compares, as magenta diamonds for nitrogen spread through each can and then rise, one at a time and at the same pace in both cans, into a column above it, with A's height outlined on B's column. Under the cans, In the can reads each can's strength against its own label, and for B, Against A compares it with A. B's label shows the product's name and three numbers above its directions; on a phone, the four products sit as buttons, two by two, above it. Try these:
- Run the first comparison as it comes, with B on the 14-4-14 powder at its outdoor line. Use Pause, Back, and Next to stop at Measure, Pour in, Top up, Stir, and Compare, or press Skip to the end.
- Choose Ready-to-use for B and mix again: two 1 L bottles fill B straight, with no water added.
- Choose the 8-4-6 liquid for B, and pick its Houseplants and indoor bonsai line, then its Garden beds line, under Measure by, mixing each. Read In the can each time.
- Choose the Acid-loving feed and mix it against A; its diamonds are drawn hollow, the look for ammonium, because it has no nitrate. Press Replay to watch any run again, and find all five challenges to finish.
Simulation
Which can is stronger?
Can A· Stays as it is
The 8-4-6 liquid, measured by its outdoor line:
Outdoor containers, including bonsai kept outdoors: 4 mL per 1 L of water, every 14 days while plants are actively growing.
In the 2 L can: 8 mL
Can B· Your changes1 of 2 differ from can A
Measure byPick a line on B's label.
Powder
Water-soluble fertilizer for soft water
14-4-14
Directions
Outdoor containers and bonsai kept outdoors:
1 level scoop (1 g) per 1 L of water, every 14 days in active growth; stir until dissolved.
Houseplants and indoor bonsai:
1 level scoop per 2 L of water, every 14 days.
In the 2 L can: 2 scoops (2 g)
Calls: none yet
On the counter Both 2 L cans are half full of tap water. A gets 8 mL of the 8-4-6; B gets 2 scoops (2 g) of the 14-4-14.
Can A8 mL of the 8-4-6 at its outdoor line
- In the can
- Plain water
- Against A
- The reference
Can B2 scoops (2 g) of the 14-4-14 at its outdoor line
- In the can
- Plain water
- Against A
- Not compared yet
| Can A | Can B | |
|---|---|---|
| In the can | Plain water | Plain water |
| Against A | The reference | Not compared yet |
- Nitrogen in the water, in any form: each diamond is the same amount
Result
Which can carries more nitrogen?
Press Mix both cans. When both cans are mixed and compared, the result says which carries more nitrogen, and why.
Challenges
Find all five to finish.
0 of 5 found
Find bigger numbers that make a weaker can, not found yet.
Compare a product with bigger numbers, each mixed at its line for outdoor pots.
Match A with a bottle you pour straight, not found yet.
Look for a label whose directions say not to dilute.
Find another 8-4-6 line that makes about half of A, not found yet.
Give B the 8-4-6 too, then try its other lines.
Find a line far too strong for a bonsai pot, not found yet.
Some lines on a label are written for plants that are not in pots.
Compare the feed made for acid-loving trees, not found yet.
It is one of B's products. Mix it against A.
Text description
A kitchen counter beside the sink, where the west balcony's owner mixes every can before carrying it out, in May. Morning light comes in at a window over the sink, and the balcony door stands ajar at the end of the counter. Two red 2 L watering cans stand side by side, each with a cutaway window that shows its water. Can A, on the left, stays as it is; can B, on the right, is the one you change. A 20 mL measuring syringe, the products, their scoops, and the 8-4-6's measuring cap stand by. Every label here is a fictional label made for this course.
Can A holds the 8-4-6 liquid, measured by the line on its label for outdoor containers, including bonsai kept outdoors: 4 mL per 1 L of water, every 14 days while plants are actively growing. In the 2 L can: 8 mL.
For can B, Product offers four products, each shown with its whole label (its name, its three numbers, and its directions), and Measure by picks one of that product's own directions lines on the label. Each line's amount for the 2 L can appears in a chip on the can. The products and their lines:
- The 8-4-6 liquid, 8-4-6: 8 percent nitrogen. Shake well. Measure with a measuring syringe or the cap. Outdoor containers, including bonsai kept outdoors: 4 mL per 1 L of water, every 14 days while plants are actively growing. In the 2 L can: 8 mL. Houseplants and indoor bonsai: 2 mL per 1 L of water, every 14 days in spring and summer; every 28 days in autumn and winter if still growing. In the 2 L can: 4 mL. Garden beds: 8 mL per 1 L of water, every 14 days. In the 2 L can: 16 mL.
- The 14-4-14 powder, 14-4-14: 14 percent nitrogen. Outdoor containers and bonsai kept outdoors: 1 level scoop (1 g) per 1 L of water, every 14 days in active growth; stir until dissolved. In the 2 L can: 2 scoops (2 g). Houseplants and indoor bonsai: 1 level scoop per 2 L of water, every 14 days. In the 2 L can: 1 scoop (1 g).
- The ready-to-use bottle, 0.03-0.01-0.02: 0.03 percent nitrogen. Ready to use: do not dilute. Houseplants, indoor bonsai, and small pots: use in place of a watering every 14 days in spring and summer; pour onto moist potting mix until a little runs from the holes. In the 2 L can: 2 bottles, straight.
- The acid-loving feed, 10-4-6: 10 percent nitrogen. Azaleas, camellias, and other acid-loving plants in outdoor containers and bonsai kept outdoors: 1 level scoop (1 g) per 1 L of water, every 14 days in active growth. In the 2 L can: 2 scoops (2 g).
Mix both cans plays five stops on one clock for both cans. Measure: the syringe draws amber concentrate from its bottle, a scoop is dipped heaped and struck level, or the first ready-to-use bottle tips over the empty can. Pour in: the feed goes into each can where it lands, low in the water. Top up: a jug of tap water fills each can to its 2 L line; ready-to-use bottles fill a can on their own, with no water added. Stir: the feed spreads through all 2 L. Compare: one diamond after another, at the same pace in both cans, each can's nitrogen rises out of the water into a column above it, so the two heights can be compared, with a dashed outline of A's height drawn on B's column; a faint trace of each diamond stays in the water.
Nitrogen is drawn as small magenta diamonds, each the same amount in every can, so a can with twice the nitrogen holds twice the diamonds. The acid-loving feed's diamonds are drawn hollow with a small plus, the look for ammonium, because its nitrogen is ammonium and urea that soil life turns into ammonium, with no nitrate. Water carrying feed keeps its color, with only a faint gold cast that grows with strength. The strength of a can comes from both the label's numbers and its line: the nitrogen percentage times the amount the line puts in each liter.
Under each can, In the can gives its strength against its own label's line for outdoor pots, or against its only line, in words: Plain water, Weaker than the label, At the label rate, Stronger than the label, Far stronger than the label. For can B, Against A compares its nitrogen with A's: About the same as A, Less than A, About half of A, More than A, About twice A. At Far stronger than the label, a note adds: Fed like this regularly, salts build up and can harm roots (Lesson 5).
Before each run you can call which can will carry more nitrogen, A, B, or About the same, or skip. Once a can B has been mixed, or when B is mixed exactly like A, no call is offered. Five challenges are found from your runs: Find bigger numbers that make a weaker can; Match A with a bottle you pour straight; Find another 8-4-6 line that makes about half of A; Find a line far too strong for a bonsai pot; Compare the feed made for acid-loving trees. A finish card appears when all five are found, and Start over clears everything.
The 14-4-14 made the weaker can. Its two scoops read At the label rate under In the can, yet Against A read About half of A: the powder's bigger numbers applied to 2 g of product in the can, while the 8-4-6's smaller ones applied to 8 mL. The ready-to-use bottles came out About the same as A, their tiny numbers spread through a whole 2 L of feed that needed no water at all.
The lines mattered as much as the products. The 8-4-6's houseplant line made about half of A, and In the can read Weaker than the label for an outdoor pot: the line is written for indoor trees, and for a tree outdoors it is weaker than the label's starting point. Its garden beds line made about twice A and read Far stronger than the label, with the note that a can like that, fed regularly, builds up salts that can harm roots. The acid-loving feed made a can with less than half of A's nitrogen, and strength was never the reason to choose it: its nitrogen, all ammonium and urea, tends to make the root zone more acidic, and its iron is chelated as EDDHA to stay available in alkaline conditions (Lesson 5). In every run, the bigger numbers on the front never made the stronger can by themselves; the concentration times the amount per liter that each line put in did.
Before the first can
The bonsai decision
Read the label for your use before the first can
Starting point
The west balcony owner buys the 8-4-6 liquid. It has a line for bonsai kept outdoors, and the kitchen tap's moderately hard water already brings the calcium it leaves out, so the 14-4-14's calcium, meant for soft water, is not needed; the ready-to-use bottle would take two bottles for every can. At the kitchen counter, before the first can, they read the label once more and write down:
- The analysis, and how it is printed: 8-4-6, with phosphate and potash given as P₂O₅ and K₂O, so about 1.7 percent phosphorus and 5 percent potassium; its nitrogen as ammonium, nitrate, and urea.
- What is missing, and how it releases: no calcium, which the tap water supplies; water-soluble, so it is available at once and washes through over the next waterings (Lesson 3).
- The line for the use: outdoor containers, including bonsai kept outdoors, 4 mL per 1 L of water every 14 days while the trees are actively growing, so 8 mL in the 2 L can, measured with the syringe.
- The conditions and cautions: water first if the pot checks Drying or drier, and feed once it drains to Moist; never more than directed; the bottle closed, out of reach, and away from frost.
The two junipers start on that line this month. The trident, repotted in March, starts only once its new growth shows that its roots are working again (Lesson 4).
This assumes
- The product is labeled for container plants, and each tree is healthy and growing in a pot that drains.
- You can measure milliliters or grams: with a measuring syringe, the marked cap, or the product's own level scoop.
- The water is the example city's moderately hard, unsoftened tap water.
- The months belong to the course's Northern Hemisphere example climate. In the Southern Hemisphere, swap north and south, and shift the months by half a year.
Change course if
- The label has no line for containers or for indoor bonsai: choose a product that has one, and where it gives both, use the line for where the tree lives. Never borrow a line written for garden beds or lawns.
- The tree is watered only with rainwater or reverse-osmosis water: choose a feed whose label lists calcium and magnesium, such as the 14-4-14 powder (Course 03, Lesson 5).
- An acid-loving tree, such as an azalea, is on hard water: once its pH is known, choose a feed whose iron is chelated to stay available in alkaline conditions, such as the acid-loving feed's EDDHA iron (Lesson 5).
- Granules or cakes are already in the pot: find out what they are, count them as the tree's feed, and add no liquid feed until the granules are spent or the cakes come off (Lesson 3).
- The tree is declining or wilting: this is not a feeding problem: investigate before feeding (Lesson 5).
Back to the question
What do the numbers and directions on a fertilizer label tell you?
What the product holds and how to turn it into a feed, though the numbers alone never tell you how strong your can will be. The three numbers are percentages by weight of nitrogen, phosphate, and potash; on many labels the last two are counted as oxides, so the 8-4-6 holds about 1.7 percent phosphorus and 5 percent potassium. The rest of the analysis names the nitrogen forms, the other nutrients, and which metals are chelated, and what it leaves out, such as calcium, matters too. The release statement says when nutrients become available to roots. The directions give a line for each use, with a rate, an interval, and conditions. The line you choose sets the amount in each liter, and your can's strength is the concentration times that amount. No line says whether your tree needs feeding today.
Key takeaways
- The three numbers are percentages by weight of nitrogen, phosphate, and potash. Where a label gives phosphate and potash as oxides, multiply by about 0.44 and 0.83 to find the phosphorus and potassium.
- Nitrate is usable at once and washes out easily, ammonium is usable and held a little by the mix, and urea becomes ammonium within days. Feeds rich in ammonium and urea tend to make the root zone more acidic over time.
- Read what a label leaves out as well as what it lists. A tree on rain or reverse-osmosis water needs a feed that lists calcium and magnesium. For an azalea on hard water, find the pH first: if it is alkaline, choose iron chelated to stay available in those conditions.
- Strength is the dilution a line gives and frequency is its interval, and either one can exceed the label on its own: full strength every week on a 14-day line doubles the supply.
- A product's numbers describe its concentration, not your can. The can's strength is that concentration times the amount per liter that the line for your use puts in, so compare cans, not bottles.
Optional · not required for completion
Read one real label line by line
Find a fertilizer you have, or a product's label online, and read it the way the table in “A label, line by line” does. Write down its analysis and whether phosphate and potash are given as oxides, then convert them to phosphorus and potassium. List its nitrogen forms, find its line for containers (or for indoor plants, if your tree lives indoors) with its rate, units, and interval, and note anything it leaves out, such as calcium. Would it suit a tree watered only with rainwater?
No fertilizer? Work through the table in “A label, line by line” with its right-hand column covered, and say what each line tells you before you uncover it.