What nutrients do

June 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). On the bench in the western third of the yard, with morning sun and afternoon shade from the neighbor's maple, stands the household's trident maple (Acer buergerianum), now in its third summer with them. They bought it in leaf from an outdoor nursery two springs ago, and it still grows in the pot it came in: a black plastic nursery pot about 18 cm (7 in) across, filled with the grower's bark-based mix, which also held controlled-release fertilizer granules.

This June it looks different. The older leaves at the base of this year's shoots have paled to an even yellow-green, and the shoots are shorter than last year's, with fewer leaves. Yet its roots are firm, with white tips at the drainage holes; its root zone dries back from Moist toward Drying between thorough waterings, with nothing standing under the pot (Course 03, Lesson 1); it gets the same morning sun as last year; and no pests show. No one has ever fed it. The nursery says its granules were rated to last about four months at 21 °C (70 °F): they were spent in its first summer.

The household's two adults share the trees, one doing the morning round before work and the other the evening round. One of them wants to give the trident some “plant food” today. The other wonders why it grew well all last year without any, and what a feed could give it that sun and water do not.

What does fertilizer actually give a tree, and why does a tree in a pot run short when a garden tree does not? This lesson explains what mineral nutrients do in a tree and what fertilizer can and cannot supply, how the place a change shows first narrows down which nutrient could be short, why a shortage can slow growth before any leaf changes color, and why a tree in a small pot of bonsai mix runs low unless it is fed while it grows.

Fertilizer supplies minerals, not food

A tree builds itself mostly from carbon, hydrogen, and oxygen. Its leaves take carbon dioxide from the air and water from the roots and, with the energy of light, join them into sugar by photosynthesis. That sugar is both its building material and its fuel. From it the tree makes its other carbohydrates, such as cellulose for the walls of new wood and starch for storage, as well as the lignin that stiffens wood, and every living cell spends some of it for energy (Course 01, Lesson 6). Everything else arrives through the roots as mineral nutrients, elements dissolved in the water around them. Together they make up a few percent of a tree's dry weight at most (Course 01, Lesson 6).

Small as that share is, nothing works without it. Nitrogen is part of every protein, including the enzymes that run each step of growth. Nitrogen and magnesium are part of every molecule of chlorophyll, the pigment that captures light, and iron is needed to build it (Course 01, Lesson 6). Calcium holds the walls of new cells together, and potassium keeps the water balance that holds leaves firm. Course 01 compared a tree to a woodshop in which light is the power, carbon dioxide the lumber, and mineral nutrients the screws and tools. A shop short of screws works slowly however much power and lumber it has, and a truckload of screws builds nothing without the power and the lumber.

A fertilizer supplies those elements and none of the sugar a tree builds with and runs on: no carbohydrate and no energy. So it cannot make a tree grow in poor light, heal a wound, or hurry its recovery after root work, which is why fertilizing a tree because it looks unwell is a habit worth breaking (Course 01, Lesson 10). It is also why “plant food,” printed on many real packages, misleads: a tree makes its own food, and what the package holds are the screws. The products this course uses, starting in Lesson 2, are fictional, with labels made for the course, and each is named for what it is: a fertilizer.

So the household's trident is not short of food: in the morning sun, its leaves still make sugar, though its paler leaves make less of it. What it may be short of is one or more of the elements it builds with, and the rest of this lesson is about how to narrow that down and why a tree in a pot runs short at all.

What the main nutrients do

Course 01, Lesson 10 listed the mineral nutrients a tree needs: six macronutrients, needed in relatively large amounts, and eight micronutrients, needed in tiny amounts but just as essential. Here are the six macronutrients and iron, with what each does and what you tend to see when it runs short. The table is a map, not a list to memorize: the next section draws one pattern out of its last column, and Lesson 5 shows why that column cannot diagnose a tree on its own.

Iron's shortage usually has a different cause from the others. In a pot given a complete fertilizer, iron is rarely missing; more often it is present but locked away, because in an alkaline root zone it turns into solids that roots cannot take up. That is a question of nutrient availability, not supply, and more of the usual feed does not answer it (Course 01, Lesson 10). Lesson 5 shows what does.

The other seven micronutrients are needed in such small amounts that a fed pot rarely runs short of them, but each is essential. Manganese and chlorine take part in the step of photosynthesis that splits water. Zinc and copper are parts of many enzymes. Boron works in cell walls and growing tips. Molybdenum lets a tree turn the nitrate it takes up into protein, and nickel lets it break down urea, a form of nitrogen common in fertilizers. Many labels list some of these, and Lesson 2 reads those lines.

Too much of one nutrient can hold back another

More is not better. Positively charged nutrients compete with one another as roots take them up, so roots in a mix loaded with potassium take up less magnesium and calcium. A complete feed used at the rate on its label keeps them in proportion; heaping on one nutrient does not. Nor does extra phosphorus grow more roots or flowers (Lesson 2).

Excess nitrogen shows in the growth itself. Given more nitrogen than it needs, a tree grows long, soft shoots with long gaps between large leaves: tissue that pests and diseases attack more readily, and the opposite of the short shoots and small leaves a bonsai in refinement needs. Lesson 4 matches feeding to the growth you want. Beyond what a tree can absorb and use, any extra fertilizer only adds soluble salts to the root zone, which make water harder for roots to draw in and, at high strength, damage root tips (Course 01, Lesson 10).

Where a shortage shows first

A tree does not spread a shortage evenly. Some nutrients can be taken back out of a leaf after they have arrived and sent through the phloem, the living transport tissue of the inner bark, to wherever the tree is growing. Others, once built into a leaf, stay there. That difference is nutrient mobility, and it decides where a shortage shows first:

• Nutrients the tree can move: nitrogen, phosphorus, potassium, and magnesium. When the roots cannot supply enough, the tree takes them from its older leaves to keep its new growth going, so the older leaves show the shortage first.
• Nutrients that stay put: calcium and iron. New leaves and growing tips depend on what arrives from the roots now, so the newest growth shows the shortage first, while the older leaves stay green.

These are tendencies, and kinds of tree vary. Sources disagree about sulfur and several of the micronutrients, so this course does not sort them. The two lists are the ones Course 01, Lesson 10 gave as optional depth, and the University of Florida's extension notes on nutrient movement state the same rule: shortages of mobile nutrients appear first in older leaves, and shortages of immobile ones in new growth (Movement of plant nutrients).

What the change looks like

Where a change shows is half the pattern; how it shows is the other half. Three looks recur:

• Pale all over: the whole leaf turns a lighter green to yellow-green, veins included. A nitrogen shortage starts this way in the older leaves and, as it continues, pales the whole tree.
• Yellow between green veins: the blade yellows while the veins stay green, a kind of chlorosis called interveinal chlorosis. On the newest leaves of a broadleaf tree it is the well-known look of iron shortage; on older leaves it points more toward magnesium.
• Brown edges: the margins yellow, then brown and dry. On older leaves it can come from a potassium shortage, but far more often it comes from salts, drought, or scorch (Lesson 5).

Conifers break the pattern for iron. Their needles have no network of veins to stay green, so on junipers, pines, and other conifers an iron shortage shows as an overall yellowing of the needles, as Colorado State University Extension's notes on iron chlorosis describe (Iron chlorosis of woody plants). And a juniper's soft new tips are naturally paler than its older foliage: that is how a healthy juniper grows, not a shortage (Course 01, Lesson 14).

Together, where a change shows and how it looks narrow the list. The household's trident fits the first pattern: its older leaves paled first, evenly and veins included, while its newest are still green, the look of a nutrient it can move, most likely nitrogen. The narrowing stops there. A nutrient deficiency, too little of a nutrient reaching the growing tissues, looks the same whether the pot holds too little or the roots cannot take up what is there, and roots short of air, low light, salts, pests, disease, and the season can all produce the same looks. So treat a pattern as a clue that tells you what to check, not as a diagnosis. Lesson 5 works through the checks, and Course 07, Tree Health and Diagnosis, goes further.

Yellowing that is not a shortage

Some yellowing is a tree working as it should:

• Autumn color. Before a deciduous tree drops its leaves, it breaks down their chlorophyll and proteins and moves much of their nitrogen and phosphorus into its twigs, bark, and roots for next spring. As the green goes, the yellow and orange pigments that were in the leaves all along show through (Course 01, Lesson 13). The whole crown colors together as the days shorten. That is the season, and this course's verdict for it is Nothing to fix: normal for the season.
• Older foliage shed by conifers. Junipers and other evergreens drop some of their oldest, innermost foliage each year, mostly in autumn: it yellows and browns inside the foliage pads while the outer foliage stays fresh (Course 02, Lesson 6). Many junipers also turn bronze or purplish in winter cold, a color that fades in spring (Course 01, Lesson 13).
• Low light and roots short of air. A tree in too little light, or with its roots in a pot that stays wet, can pale or yellow with plenty of feed in its pot. Neither is a shortage that more feed can fix, and this lesson returns to both below.

A shortage can slow growth before it shows

Color is not the first thing a shortage changes. When a nutrient runs a little short, a tree first grows less: it makes fewer and smaller new leaves and shorter shoots, spreading what it has so that the leaves it keeps stay green. Only as the shortage deepens do the leaves themselves run short and change color. A shortage mild enough to slow growth without any change in color is a hidden shortage. Crop scientists call it hidden hunger, and the North Carolina agriculture department's guide to plant tissue reports describes it as a crop lacking a nutrient without any visible symptoms (Understand your plant tissue report). It is why green leaves alone do not prove that a tree has all the minerals it needs (Course 01, Lesson 6).

You cannot see a hidden shortage, but you can infer one, in four steps:

1. Judge the growth against what this kind of tree usually does now. Look at how far the shoots extend, the number and size of the new leaves, and the buds, against what a healthy tree of its kind does at this stage of the season and against this tree's own past growth: its vigor (Course 01, Lesson 14). Last year's growth is the easiest yardstick. A band of bud scale scars, fine rings around the stem, marks where each year's shoot began, so on a tree that grows one main flush a year you can measure this year's shoots against last year's on the same branch (Course 01, Lesson 5).
2. Read the feeding and substrate history. A tree with nothing coming in, such as an unfed tree in a mostly mineral mix or in an old nursery mix whose granules are spent, has been drawing down a small store, and each thorough watering has carried some of it away. The section on small pots below explains why.
3. Rule out the other limits: the roots, the water, the light, and pests. A tree short of any of them grows less too, and feeding fixes none of them.
4. Then feed at the label rate, and judge the new growth over the following weeks. If the shoots extend more strongly and the new leaves come out full-size and green, supply was the limit. If nothing changes, look past the feed (Lesson 5).

Laboratories can measure the nutrients in a sample of leaves or of the mix, and growers of valuable trees sometimes use them, but for one bonsai such tests are rarely practical, and their results still need the tree's history to read (Course 07).

The household's trident is past this stage. Its growth was probably already slowing late last summer, after a spring flush that ran on stored reserves and before any leaf paled; this year the shortage has deepened enough to shorten its shoots and pale its older leaves.

Two young tridents, one change

Plant scientists learned where each shortage shows by growing plants that are alike in every way but one. In an omission trial, one nutrient is left out of one plant's supply, so whatever changes can be traced to it. Where does a shortage show first? runs one. Two young trident maples of the same age, grown from cuttings, stand in matching training pots of pumice and lava, which bring almost no nutrients of their own, on a plain trial bench in a sunny yard in June, both in active growth. Both are watered by the checks with rainwater, so every nutrient they get comes from the solution they are given. Tree A, drawn in ink and marked Stays as it is, gets a complete nutrient solution of the kind experiments use, not one of this course's products. Tree B, drawn in red and marked Changed, gets the same care with one change, which you choose: a nutrient left out, a quarter as much of every nutrient, or a change to its light or its pot. Below each tree, a large panel shows This season's shoot from its base, where the oldest leaves are, to its tip, where the newest are, with a round inset of its roots at its foot. In B's panel, a dashed line marks A's tip, and the key under the panels names the small diamonds that stand for the nutrients. On a phone, the two shoots stand side by side under a smaller picture of the trees, with the readouts in a table beneath them. Try these:

1. Under B's one change, start with No nitrogen. Under Your call, answer Where will B show it first? with Older leaves first, Newest leaves first, All over the tree, or Only in its growth, or press Skip. Your call locks when the run starts, and the run stamps it Called it or Surprise. You call each change only the first time you grow it.
2. Press Grow six weeks. Use Pause, Back, Next, the week buttons, or Skip to the end to stop on any week, and Replay to watch it again. For each tree, read Older leaves, Newest leaves, New growth, Roots, and What is happening. Watch the small diamonds on each shoot, named in the key: which leaves do they come from, and where do they go?
3. Read the result card, then run the other changes in turn: No iron, A quarter of A's feed, Complete feed, deep shade, and Complete feed, pot standing in water. While a new run plays, the previous result stays in the card as Last run.
4. Find all four challenges to finish, from Find a shortage that shows on the newest leaves first to Find two different causes that both yellow the older leaves.

With No nitrogen, B fell behind before any leaf changed: at week 2 its New growth read Slow while every leaf was still Green. From week 3 its oldest leaves turned Pale all over, veins included, as the nitrogen in them moved up the stem to the tip, and the paleness climbed the shoot pair by pair. By week 5 New growth read Little new growth, and by week 6 the whole shoot was pale, its newest leaves small. No iron ran the other way. The older leaves stayed Green, and from week 2 each new pair came out Yellow between green veins, paler than the pair before, the newest nearly cream by week 6, while the shoot kept growing. The little iron arriving from the roots went to the tip, and little left the older leaves.

A quarter of A's feed, a quarter as much of every nutrient, changed no color at all. Every leaf stayed Green and only a little smaller, but from week 3 growth was Slow, and by week 6 B's shoot ended about two leaf pairs behind A's: a hidden shortage, visible only against A. The last two changes kept the complete feed. In Complete feed, deep shade, every leaf came out lighter green and thinner, on long, stretched shoots leaning toward the light, and by week 5 the lowest leaves yellowed and dropped. Little moved along the stem, because the feed sat in the pot with too little sugar to build with. In Complete feed, pot standing in water, the oldest leaves yellowed and dropped from week 2, the crown drooped in the afternoons, and by week 4 its New growth read Stalled. It looked like the nitrogen shortage, and inside the tree it was one, but the Roots row read Browned tips, and the roots inset showed them surrounded by plenty of feed they could not take up.

So the place a change showed first narrowed down the cause without ever naming it. A missing nutrient paled the older leaves first, roots short of air yellowed them first, and shade paled a tree with a full feed in its pot. Read the pattern, then check the roots, the water, and the light before you decide it is the feed.

Why a tree in a small pot runs low

A garden tree's roots have a great volume of soil to explore (Course 04, Lesson 1). That soil is rich in organic matter that soil life keeps breaking down, the roots keep growing into fresh ground, and even the tree's fallen leaves rot where they land and return what they held. A bonsai's roots share a few liters of mix, most of it the same mix the tree was potted in. Its supply has few sources, and one steady loss:

• The mix itself brings little. Pumice and lava rock release almost nothing. Akadama, a granular clay-like material from Japan, holds some positively charged nutrients, such as potassium, calcium, and magnesium, on its surfaces by cation exchange, but that is a holding capacity, not a store: it keeps some of what the water and the feed bring between waterings, and it makes none of its own (Course 04, Lesson 5).
• Organic components give a little, and can take a little. Bark and other organic matter release some nutrients as soil life breaks them down, a process called mineralization that runs slowly and unevenly (Course 04, Lesson 5). Fresh bark can do the opposite for a while, because the microbes breaking it down take up nitrogen from the water around them.
• A nursery pot may still hold fertilizer. Many nurseries mix controlled-release granules into their pots, and those keep releasing for months, so find out what is in a new tree's pot before you add any feed: ask the seller, read the product's label if you can get it, and note what you learn in the tree's arrival record (Course 02, Lesson 8). Once spent, their empty coatings can stay in the mix for a year or more looking much as before, so granules you can see do not prove that anything is still being released. Lesson 3 explains how they work and how long they last.
• Every thorough watering carries some out. The water that runs from the holes takes dissolved nutrients with it (Course 03, Lesson 3), nitrate most readily, because the mix's exchange sites hold positively charged nutrients and nitrate is negatively charged (Course 01, Lesson 10). Steady rain on an open pot does the same.
• The water brings calcium and magnesium, or does not. The example city's moderately hard tap water supplies some of both at every watering, the same minerals that leave its white crust, and little or no nitrogen, phosphorus, or potassium. Rainwater and reverse-osmosis water supply almost none, so a tree watered only with them depends on its feed for calcium and magnesium (Course 03, Lesson 5).

So a pot of bonsai mix holds a small, leaky supply. While a tree is growing, the nutrients its new leaves, shoots, and roots are built from must keep arriving, and in a pot they arrive mostly as feed. A tree in active growth in a bonsai mix therefore needs regular, moderate feeding: steady enough to keep up with what growth uses and watering carries away, never so heavy that salts build up. A resting tree, bare in winter or held back by cold, needs none.

The household's trident shows the budget run down: two summers without feed, while thorough waterings carried nitrate out of the holes. Its roots, water, and light check out and no pests show, so a shortage, most likely of nitrogen, is the best explanation of what it shows. The adult who wanted to feed it is right about what it needs, if not about what to call it: mineral nutrients, at the rate on a fertilizer's label, with the next weeks' growth to show whether supply was the limit. Lesson 2 reads the label, and Lesson 4 builds the plan.

Fertilizer cannot stand in for light, roots, or water

Feeding helps only where supply is what holds a tree back. Between a bottle of fertilizer and a new shoot, nutrients must be supplied to the pot, dissolve and reach the roots, be taken up by living roots, and be built into growth with sugar made in light, and fertilizer widens only the first of those steps (Course 01, Lesson 10). Uptake needs living fine roots, air in the pores, moisture, suitable warmth, and sugar from the leaves. Two common problems pale or yellow a tree while plenty of feed sits in its pot, and Tree B showed both in the trial above:

• Too little light. In a dim spot, a tree grows pale, thin, stretched shoots with long gaps between small leaves, and its innermost and lowest leaves may yellow and drop: etiolation (Course 02, Lesson 4). A juniper in shade thins and pales the same way, its foliage sparse and open. Its leaves make too little of the sugar that growth is built from, so extra fertilizer mostly stays in the pot as salts. Give it more light first.
• Roots short of air. In a pot that stays saturated or stands in water, the roots run short of oxygen, their tips die back, and the tree takes up little of what surrounds them (Course 03, Lesson 4). The older leaves yellow and drop, just as in a shortage of a nutrient the tree can move, because the leaves really are short of nitrogen while the pot holds plenty. More feed adds salts to roots that are already struggling. Empty the standing water and let it drain, and hold the feed until new growth shows the roots are working again.

Roots that have dried out, been cut back, or sit in cold mix also take up little, whatever the pot holds. When poor growth or color has a possible cause like these that you have not yet checked, the call is Investigate before feeding: check the roots, the water, and the light first. It never means feeding by default, and Lesson 5 works through the checks in order.

Feed while it grows, and look before you change the feed