Liquid, solid, organic, and mineral sources

Early April in the small backyard. 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), where the last spring frost usually comes in late April. On a cold, gray morning, one of the household's two adults sets seed-meal cakes on the new juniper, the Japanese garden juniper (Juniperus procumbens ‘Nana’) in a 4 L (1 gallon) black nursery pot of open, bark-based mix at the patio's lawn edge. It came home last May, settled over the summer, and grew well, and the nursery has confirmed that the granules it was sold with are spent: the conditions Course 02 set for a first feed (Course 02, Lesson 8).

Many bonsai growers use cakes like these, and one set lasts weeks: their label says one cake for every 10 cm (4 in) of pot width, replaced every 4 to 6 weeks. Three weeks later the cakes look just as they did the morning they went on, pale, firm, and dry on top, though the mix beneath them is moist, and the juniper looks as it did in March. The nights have stayed near freezing and the days cool. One of the adults wonders whether the cakes are doing anything at all, and whether a few more would help.

On the west balcony in late July, the owner is planning the two weeks away that every August brings. The trident maple and both junipers have been on the 8-4-6 liquid since late spring, each can mixed at the kitchen sink and carried out in the 2 L (about half a gallon) can. While the owner is away, the friend will come every second day, daily in a heat wave, with the tested drip kit watering every pot in between, and no helper has ever been asked to feed anything (Course 03, Lesson 6). Could a scoop of controlled-release granules, scattered before the trip, or a few seed-meal cakes feed the trees while the friend only waters? Or should feeding simply wait?

Every product in this course is fictional, with a label made for the course (Lesson 2), but each behaves like the real kind it stands for. Both questions are about timing: when a source releases what it carries, and what sets the pace, whether the owner's can, the weather, or the life in the pot. This lesson compares how liquid feeds, controlled-release granules, and organic cakes release their nutrients, what speeds, slows, or washes each one out, and how to choose the source whose timing fits your tree, your weather, and your routine.

Every source ends as dissolved ions

Roots take up mineral nutrients as ions dissolved in the water around them: potassium and calcium as positive ions, nitrogen mostly as nitrate or ammonium, and phosphorus as phosphate (Course 01, Lesson 10). Every fertilizer you give the roots, whatever it is made of and however it is sold, does its work only once it has become those ions in the root zone's water. The sources differ in how they get there, and how fast.

A mineral fertilizer is made of salts, mined or manufactured, whose nutrients are ions the moment they dissolve: stirred into a can, the 8-4-6's potassium nitrate is simply potassium and nitrate (Lesson 2). Urea, also on its list, is the exception: it dissolves without becoming ions, and soil life turns most of it into ammonium within days in a warm, moist pot, more slowly in a cold one. “Mineral” here means salts like these, not the mineral particles of a bonsai mix, such as pumice and lava, which supply almost nothing of their own (Lesson 1).

An organic fertilizer is made from plant or animal material. The seed-meal cakes are rapeseed meal, fish meal, and bone meal pressed together, and most of their nitrogen and phosphorus is bound up in proteins, bone, and other material that roots cannot take up as it is. Bacteria and fungi break the meal down for its carbon and release the nitrogen and phosphorus they do not need as dissolved ions: ammonium, which other microbes then turn into nitrate, and phosphate. That release by soil organisms is mineralization, one part of decomposition, and only after it can roots use most of what a cake carries. The soil life keeps the carbon; the tree gets only the ions left over.

So the nitrate a root takes up from a cake is the same ion it would take up from a bottle. What the source changes is when the ions appear, how many appear at once, and what sets the pace. “Liquid or solid” and “organic or mineral” are separate questions, and the one that matters most is a third: whether a feed is already dissolved when it goes on, held back by a coating, or waiting for soil life. The course's six products show how the answers combine:

• The 8-4-6 liquid and the ready-to-use bottle: liquid and mineral, dissolved before they go on.
• The 14-4-14 powder and the acid-loving feed: solid and mineral, but you dissolve them in the can, so they work exactly as a liquid does.
• The granules: solid and mineral, inside a coating that sets their pace.
• The seed-meal cakes: solid and organic, released by soil life.

Liquid and water-soluble feeds

A water-soluble fertilizer dissolves completely in water and goes on as part of a watering: a liquid concentrate such as the 8-4-6, a powder such as the 14-4-14 or the acid-loving feed, or the ready-to-use bottle, already diluted. Its nutrients are available to roots as soon as the feed wets the root zone.

That makes a soluble feed the most precise source you can use. You measure each can at the label's rate for your use, so you know what went in and when, and you can change the next can or stop: within a few waterings, most of what the last feed brought has been taken up or carried out of the pot.

The same speed has a cost. A free-draining bonsai mix holds water in its small pores and lets the rest run out of the holes, and the water that runs out takes dissolved nutrients with it (Course 03, Lesson 3). Nitrate, a negative ion, is barely held by the mix and leaves most readily; ammonium, potassium, calcium, and magnesium, all positive, are held a little by cation exchange on akadama and bark and leave more slowly. So a feeding's supply is highest the day it goes on and falls with each thorough watering after it, and a soluble feed has to be repeated on the label's interval while the tree grows: every 14 days for the 8-4-6 on an outdoor bonsai.

Spraying a dilute feed onto the leaves, foliar feeding, puts in far less than it seems to. Leaves take in only small amounts, mainly of a few micronutrients such as iron, and iron stays in the leaves the spray touched, so leaves that open later get none of it (Lesson 1). Roots take up nearly all of a tree's mineral nutrients (Course 01, Lesson 10), so a spray is at most a short-term measure, never a substitute for feeding the roots, as Washington State University's review of the evidence, The myth of foliar feeding, also finds.

Feed the moist mix

A soluble feed goes onto a root zone that is already moist: the 8-4-6's label says to water dry potting mix with plain water first, then apply the feed to the moist mix until a little runs from the drainage holes. Poured onto a dry root ball, much of a feed runs down the easy paths and out of the holes, the channeling of a dried mix (Course 03, Lesson 3), and what soaks in sits as a strong solution in the little water left, against roots already short of it (Course 01, Lesson 10).

So a feeding day starts with the usual check below the surface (Course 03, Lesson 2). A pot that checks Moist can take the feed as it is; one that checks Drying or drier gets a thorough watering first, and the feed once it has drained, in slow passes until a little runs from the holes. Lesson 4 fits feeding days into hot and wet weeks.

Controlled-release granules

A controlled-release fertilizer is soluble fertilizer in a coat. Each of the granules, the course's 14-8-11, is a small bead of mineral salts inside a thin resin coating. Course 02 met their kind as the “small round fertilizer pellets” left in nursery pots, “slow-release fertilizer from the grower” (Course 02, Lesson 8). This course calls them controlled-release granules, or the granules: many feeds release slowly, organic cakes among them, but only a coating controls the pace this way.

They go on once, at the label's rate: for the course's granules, 3 g per 1 L of pot volume, scattered on the surface and lightly covered or mixed into the top layer, once in spring. On a moist mix, water seeps in through the coating and dissolves the salts inside; the granule swells a little, and the dissolved nutrients seep back out into the water around it, a little each day, for months. Moisture lets the water in; temperature sets the pace.

So a granule's label states how long it lasts at a stated temperature: for the course's granules, “about 4 months at an average root-zone temperature of 21 °C (70 °F)”, faster when warmer and slower when cooler. Ohio State University Extension's guide, Controlled-release fertilizers, names coating thickness and temperature as the two factors that matter most, notes that most makers rate their granules at 21 °C and some at 28 °C (82 °F), and warns that a root zone warmer than the rating releases faster than stated. Read the temperature on a label as well as the months.

An outdoor pot in the example climate spends a cool spring well below 21 °C and much of July and August above it. Granules set out at bud break therefore release little through a cold April, speed up as the weather warms, and run fastest in the heat; in a hot year they can be spent by late summer, and in a mild one they are still releasing into autumn. Granules that seem to do nothing in a cold April are waiting for warmth, and more of them would only add to the surge when it comes. A dark pot in sun runs hottest of all: more than 45 °C (113 °F) has been measured just inside the sunny side of black nursery pots on summer afternoons (Course 02, Lesson 5), so granules in such a pot run through their months early.

They keep releasing whether the tree grows or not

A granule answers to warmth and water, not to the tree. In a heat wave, when a tree's growth stalls and it can use little, its granules release fastest; in a mild autumn, after its shoots have stopped extending, they keep going. Once they are in, they cannot be stopped quickly: granules mixed into the top layer cannot be picked out, and those on the surface can be scraped off only where you can see them. That is why the label says to apply once in spring and not again within 4 months, and to reduce the rate if other slow-release fertilizer is already in the pot. That line gives no amount, so this course keeps it simpler: while other slow-release fertilizer is still releasing, add no granules at all.

In return, granules give months of feeding with no can to mix, and because the coating holds most of the fertilizer back, a wet week washes out only what has already been released. For a tree nobody can reach every two weeks, that can be worth the loss of control, as long as the granules are counted as its only feed for their months.

Granules already in a nursery pot

Nurseries often feed this way, so a tree from a nursery may come with granules already in its mix. Ask which product it is and when it went on, note both in its arrival record (Course 02, Lesson 8), and count the months from that day, not from the day you bought the tree, allowing for nursery sun on a black pot.

The backyard trident shows how that works out (Lesson 1). It came home in leaf two springs ago with the nursery's granules in its mix, rated, the nursery says, for about four months at 21 °C, and they were spent before that first summer was out. Their empty coatings stayed in the mix for a year or more, the “old fertilizer pellets” Course 03 found beside its moss (Course 03, Lesson 2). From above, an empty coating can look much like a full one; pressed between finger and thumb, a full granule is firm, and a spent one is soft or hollow and often squashes flat.

Organic cakes, pellets, and meals

The seed-meal cakes' label spells out an organic feed's timing. Of their 5 percent nitrogen, its guaranteed analysis lists 0.5 percent as water-soluble and 4.5 percent as water-insoluble: about a tenth dissolves in the first waterings, and the rest stays locked in the meal until soil life releases it. Their potash is soluble too, so it reaches the roots with the first waterings, and their phosphorus, much of it in the bone meal, comes out slowly as the meal breaks down. The release line is plain: nutrients are released as soil organisms break the cakes down, faster when warm and moist, slowly when cool or dry.

Other organic feeds come as loose meals, as pellets, and as liquids made from fish or seaweed. They differ in strength and speed, but every one that soil life must break down follows the same rules.

Soil life sets the pace

The releasing is done by bacteria and fungi on a cake and in the mix beneath it, and each watering carries what they have released down to the roots. Like the roots beside them, they work fastest when it is warm, moist, and airy, and slow almost to a stop when it is cold or dry (Course 04, Lesson 5). Colorado State University Extension's notes, Organic fertilizers, put the threshold for that activity at soil warmer than about 10 °C (50 °F), with enough moisture, and warn that cold or dry soil delays the release. So the same cake behaves differently through a season:

• In a cold spring, it releases little for weeks, whatever the calendar says.
• In warm, moist weeks, it softens, darkens, grows its white mold, and releases quickly, fastest when the weather is warm and wet at once.
• In a hot, dry, or windy spell, a cake on the surface dries between waterings, and its release stops and starts with each one.
• As the nights cool in autumn, it slows again.

That is what the household saw: the new juniper's cakes sat pale and firm through the cold weeks of April, then darkened and softened under a fuzz of white mold in the warm, moist days of early May, as the juniper's new tips began to extend. The household replaces them every 6 weeks.

Living with cakes on the surface

Most of what a cake asks of you is on its label:

• Where, and how many. One cake for every 10 cm (4 in) of pot width, set on the surface away from the trunk, and never buried. Buried among the roots, a cake breaks down where they need air.
• White mold. A harmless white mold may grow on the cakes. It is part of the breakdown, not a disease of the tree.
• Flies, birds, and pets. Fish meal smells, and birds and pets may carry cakes off. A cake basket, a small cage pinned over each cake, keeps it in place.
• Replacing. Every 4 to 6 weeks from spring to late summer, lifting off what is left of the old cakes first.

More cakes than the label's number crowd the surface and cover the mix, so a watering pools between them and runs off at the rim instead of soaking in evenly. As they soften they mat into a damp layer that hides the surface from your checks, and once the weather warms they release more than the tree can use.

Organic is not automatically gentle

The word “organic” on a fertilizer describes its source, plant or animal material, not its strength or its safety. Its legal meaning varies by country, and on its own the word does not mean gentle, slow in every condition, balanced, or unable to harm roots. Once soil life has broken a cake down, its nutrients are the same dissolved salts a bottle supplies, and salts beyond what a tree can use do the same harm whatever they came from: they make water harder for its roots to take up and can damage root tips and leaf edges, which is fertilizer burn (Course 01, Lesson 10).

Organic products also differ as much as mineral ones: seed meals and bone meal release slowly, others release much of their nitrogen quickly, and the same Colorado State notes warn that one of the quick ones, blood meal, can burn plants with the ammonia it releases if over-applied. Because soil life sets the pace, an organic feed is also the least predictable source in a pot, slow for weeks in the cold and then quick when warmth and moisture arrive together, which is when a crowded surface or a second feed on top becomes more than the tree can use. Stacking an organic feed on another feed is still stacking. As Course 04 said of substrate components, choose by what a material does, not by a slogan about “organic” or “inorganic” (Course 04, Lesson 3).

What speeds, slows, or washes out a feed

Every source answers to the same few conditions, each in its own way: temperature, the water that runs through the pot, the mix, and its pH. Warmth speeds soil life and coatings alike, and a soluble feed is dissolved at any temperature, though cold roots take up little of it (Course 01, Lesson 10). So cold slows cakes and granules in early spring, when a tree's first growth runs largely on stored reserves and it needs least, while heat speeds granules most in the weeks a stressed tree's growth may stall, when their supply becomes more than it can use and soluble salts start to build.

Water, rain, and the waterings between feeds

Moisture is what lets each source work: a dry cake stops releasing, and a dry granule takes in no water. But water also carries dissolved nutrients away. Each thorough watering runs a little of the root zone's dissolved feed out of the holes, nitrate most readily, and the more water runs through, the more goes:

• Daily watering in a heat wave runs more water through the pot than watering every few days, so a liquid feed's supply falls faster between feeds.
• Heavy or steady rain on a pot open to the sky can wet it right through and rinse dissolved salts out on the way (Course 03, Lesson 5), including some of the last feed. Granules keep releasing through it, and warm, wet weather speeds the cakes.
• Small sips that never drain do the opposite: nothing runs through, and what each feed leaves behind builds up as salts (Course 03, Lesson 4).

A wet spell washes a liquid feed out faster, but it is not a reason for an extra one. A few days of lower supply cost a healthy, growing tree little, and the next feed on the label's interval replaces what went; feeding early or doubling the next can adds more than the tree can use, often to a root zone the rain has left wet. A deliberate rinse, leaching, answers evidence of salts, such as a crust with browning leaf edges after feeding, not a wet week (Course 03, Lesson 5).

The mix and its pH

The mix decides how much of a feed stays between waterings. Akadama and organic matter such as bark hold some of the positively charged nutrients and trade them back into the water over the following days; pumice and lava hold very little, none of them holds much nitrate, and bark gives back only a little of its own as it breaks down (Lesson 1). Their cation exchange capacity is room to hold some, not a store (Course 04, Lesson 5), so even in a mix rich in akadama or bark, a soluble feed's supply falls within days.

The pH decides which nutrients stay usable. In an alkaline root zone, iron, manganese, and zinc form solids that roots cannot take up, though they are still in the pot (Course 01, Lesson 10), and the alkalinity of the example city's moderately hard tap water slowly raises a mix's pH (Course 03, Lesson 5). Whether a feed's iron helps there depends on the form its label names (Lesson 2), and Lesson 5 follows the household's azalea through what to do.

Two pots through one season

When does the feed reach the roots? runs two pots through one season in the courtyard from Course 04: two matching young trident maples side by side on the courtyard bench, in matching training pots of the documented mix of akadama, pumice, and lava (Course 04, Lesson 6), with morning sun and afternoon shade, watered by the checks from the brass tap from bud break in early April to mid October. Pot A · Stays as it is gets the 8-4-6 at its outdoor line, 8 mL in the 2 L can every 14 days once the spring flush is growing, held through a heat wave and given when it breaks, at half strength once shoots stop extending, and none once the leaves color: the plan Lesson 4 writes for the courtyard maple. Under Pot B's feed, choose Seed-meal cakes, replaced every 6 weeks, Granules, once at bud break, The same liquid as A, or Nothing at all, and under The year, which both pots share, A mild year, A cold spring, A hot summer, or A wet summer.

Before each run you can answer Your call, “Which pot will be short of feed more often while the tree is growing?”, with A, B, or About the same, or press Skip. Then press Run the season, and use Pause, Back, Next, Replay, or Skip to the end. For each pot, Supply in the root zone compares what is in the pot with what the tree can use that week, The tree's demand names that need, This week says what is happening, and Short in growing weeks counts the weeks the pot was Short while demand was Rising or High, the only weeks that count. The season, week by week, the strip under the scene, fills in as the run goes; once a run has started, each week in it is a button that shows that week, and on a phone, seven month buttons do the same. Find all five challenges to finish. Try these:

1. Start with the settings it opens on: Seed-meal cakes, replaced every 6 weeks in A cold spring, the same feed and weather as the new juniper's April. Make your call or press Skip, press Run the season, and watch both pots through April and May.
2. Keep A cold spring, switch Pot B's feed to Granules, once at bud break, and press Run the season again. Then change The year to A hot summer and watch the heat wave in July, and then to A mild year and watch September and October.
3. Change The year to A wet summer and watch Pot A's row in the strip through the rainy weeks, then compare its Short weeks with a run in A mild year.
4. Run Nothing at all once, in any year.

In A cold spring, Pot B's cakes barely broke down for the first weeks, as the new juniper's had, and it mattered less than it looked. At bud break the tree's demand was Low, because the first flush ran largely on stored reserves, so the cakes' small supply read Enough. They read Short in four weeks that count, all as the flush extended before the mix warmed, then Enough or Plenty once it was warm and moist, and the older leaves, faintly paler for two weeks, were green again by late May. Pot A's liquid followed its owner's timing: Plenty in a week with a feeding day, Enough in the week between, and, in the weeks that count, Short only in a week before a feed, after daily watering in the heat or hard rain had carried the last one out. A wet summer gave the liquid its most short weeks, seven of them, and still the answer was the next feed on the interval, not an extra one; a single short week slowed the tree's growth a little and never showed in its color.

The granules followed the temperature, not the tree: Short in three weeks of the cold spring, Plenty in hot weeks, and in the heat wave, while the tree's leaves hung limp and its growth stalled, “More than the tree can use: salts building”, with a pale salt crust at the rim. In A hot summer they were spent and read Short by late August; in A mild year they were still releasing in September and October, after the tree's growth had slowed. Nothing at all read Short from mid April on, because the mix and the tap water bring almost no nitrogen, phosphorus, or potassium, and after three weeks of it the older leaves began to pale. Every source ran short somewhere, for its own reason, and only the granules piled up more than the tree could use, in the weeks it could use least. The choice is less about which source feeds best than about whose timing you can live with: yours, the weather's, or the soil life's.

Choosing a source for your tree and routine

No source suits every tree. Each fits a different routine, a different water, and a different tree, and a plan for one tree usually needs only one. Side by side:

How often you can feed

• You check the tree most days. A soluble feed is the easiest to time, adjust, and stop, and each feed can follow the check. This is the course's starting point for a tree you see every day or two, like the bright window's ficus or the courtyard's trees.
• You are away about a week at a time. A 14-day interval leaves room for it: if a feed falls due while you are away, give it the day you get back, a few days late, and keep the interval from there. A feed that falls a few days late is not made up with a double one, and granules added to cover a week away would stack on the liquid for months.
• You are away for weeks at a time, or cannot feed on an interval. For a healthy tree, the simplest answer is Not now: hold the feed while you are away, then feed as planned on your return, at the plan's rate, with no catch-up dose: a healthy tree can skip a few feeds. Where absences of several weeks come more than once in a season, granules or cakes set out beforehand can carry the feeding instead, counted in the feeding plan as the tree's only feed for as long as they release.

Worked example: the west balcony in August

The owner weighs the three choices against the balcony in August:

• Granules scattered in late July would go against their own label, which says to apply them once in spring, and would release fastest in the hottest weeks of the year, in pots the drip kit keeps moist. They would go on releasing for months after the owner came home, so the 8-4-6 would have to stop for the granules' months, not two weeks, and with it the easy taper as growth slows in September.
• Cakes on the three pots would dry in August heat between the friend's visits, since a drip emitter wets the mix at one spot rather than the cakes, so their release would come and go; four floors up, they would draw birds and flies, and the friend would be left to judge mold and missing cakes.
• Holding the feed asks nothing of the friend. The three trees are growing, the nursery juniper again after its dry spell earlier in the month (Lesson 4), and a feed late or skipped costs them little.

So the plan is Not now: hold the feed for the two weeks. The last feed goes on as planned before the trip, and feeding resumes on the owner's return at the plan's rate, with no catch-up dose; with a 14-day interval, a two-week trip leaves at most one feed late or skipped. Lesson 6 writes that into the friend's card.

Indoors, on soft water, and for acid-loving trees

• Indoors. A soluble feed on its houseplant and indoor bonsai line is the easiest to fit to a tree inside: the bright window's ficus gets the 8-4-6 at that line every 14 days in spring and summer and every 28 days in autumn and winter while it keeps growing under its grow light (Lesson 4). Cakes would bring mold and the smell of fish meal into the living room.
• On rain or reverse-osmosis water. A tree watered only with soft water depends on its feed for calcium and magnesium (Course 03, Lesson 5), so it needs a soluble feed whose label lists both. The household's ficus, watered from the rain barrel since last October apart from midwinter jugs filled at the hose tap, went onto the 14-4-14 powder at its outdoor line in late May, when it moved out to the lawn; the 8-4-6 and the granules list magnesium but no calcium, and the cakes calcium but no magnesium (Lesson 2).
• An acid-loving tree on hard water. Once a reading shows its root zone has turned alkaline, the acid-loving feed fits: all of its nitrogen is ammonium and urea, which tend to acidify the root zone over time, and its iron is chelated as EDDHA, a form made for alkaline root zones, where the 8-4-6's EDTA iron is less reliable. The lasting fix is the water itself, and Lesson 5 follows the household's azalea through that change.

One plan for everything in the pot

Every source in a pot feeds the same root zone, so they add up. Before you add a feed, count what is already there: granules from the nursery, cakes from last month, a can someone else gave the tree. Then keep one source per tree: add nothing new until the old source is spent or lifted off, which for granules means waiting out their months, longer after a cool season. Write each change in the tree's feeding record with the date (Lesson 6), so whoever feeds next knows what is in the pot.

Choose the source whose timing fits