Chapter 4. What You Should Know About Nutrients

Nitrogen, phosphorus and potash are the "big three" of the many nutrient elements needed in the soil by plants for proper growth. These three are listed by numbers in that order on every package or bag of fertilizer (thus a 5-10-5 fertilizer product contains 5 per cent nitrogen, 10 per cent phosphorus and 5 per cent potash or potassium).

These "plant foods" do not occur as pure elements but as com­pounds with other chemicals. These compounds may be simple or quite complex but they share one quality—they can be attacked by bacteria, fungi and other organisms and broken down into quite simple products which plants can absorb. A fundamental quality of these simple products is that they must be soluble in water.

The Way Plants Feed

If we are to understand how plants feed, an old misconception must be discarded at once. Plants cannot "eat" the way animals do. Plants have no alimentary canal, no means of using undigested organic compounds such as protein, bone, straw and other "foods" applied to soil to supply them with nutrients. Even after such mate­rials have gone through thorough decomposition in a compost pile, they may need to be broken down or rotted still more before their complex protein can be reduced to "available food" for plants.

Whether we call this process "decay," "digestion" or "organic breakdown," it involves exposing plant and animal wastes and by­products to soil organisms. These use part of the foods for their own life processes, but leave behind less complicated materials as end products. These simpler materials are water soluble and can be taken up by roots of plants.

Another widely believed misconception is that plants reach out for fertilizer in the soil, drawn by some force which tugs at the roots. Vivid proof that this is not so was shown to me in the studio-green­house of John Nash Ott who photographs plant growth in lapsed time. Plants were growing in a soil-filled box with a glass front which allowed a clear view of the roots. Fertilizer was placed here and there in the soil. But the roots grew in various directions, apparently un­affected by the fertilizer. In fact, it could be seen that some roots had passed within a fraction of an inch of one concentration of fertilizer.

Perhaps the most complicated substance plants can use directly is ammonia, a relatively simple molecule of nitrogen and hydrogen. Rhododendrons and other acid-soil plants can use ammonia directly, but many others require it to be broken down still further into nitrate nitrogen.

Contrast this simple chemical with the protein molecule, so com­plex that it offers all-but-insurmountable obstacles to chemists at­tempting to synthesize it. Contrary to what many gardeners think, direct absorption of such a complex molecule is beyond the capacity of any plant root. (One apparent contradiction of this statement is the capture and absorption of insects by certain insectivorous plants, such as the Venus Fly Trap and the Sundews. Insects are lured into special organs where they are caught and "digested" before being used by the plant as food. However, the digestive organ of these plant oddities is a specialized leaf in which protein is fermented into ammonia and nitrogen compounds. These can be absorbed by the leaf and used just as these compounds are used when absorbed by other plants through roots.)

To repeat (and it's worth repeating), the simple nitrogen com­pounds which garden plants always need, the phosphorus they use at certain stages of growth, the potash so vital to woody plants, the sulfur so often ignored in discussions of plant nutrition, and several other chemicals vital to plant growth must all be in soluble form. Otherwise they are as inaccessible to plants as if they did not exist. This does not mean that the best material is the most soluble. Often we need controlled solubility—especially with nitrogen—to give a longer feeding period.

MIGHTY NITROGEN

Of all food elements needed by plants, none is more important than nitrogen. It is popular to call the "nitrogen cycle" (a process by which nitrogen is used and reused, over and over again) the most important single biological process in the world. While the nitrogen cycle is vital to the continued existence of every living organism, it is, of course, only one of several such basic processes, none of which could be halted without destroying all life.

Nitrogen is so important to plant nutrition that its concentration in a given soil tends to be the # 1 factor which controls growth. What we call a worn-out soil is often the result of farming or gardening practices which have exhausted native reserves of this vital element and made no provision for replacement. Nitrogen usually determines whether a soil is rich or poor, whether yields will be high or low.

One reason why nitrogen is so important is that it is essential to all tissues involved in growth and reproduction. Research has proved that the rate of growth in plants is more dependent upon this element than on any other single material.

Where Does It Come From?

We may talk glibly about organic versus inorganic nitrogen, but regardless of whether it occurs as part of animal or plant protein or as any other nitrogen compound, every atom of nitrogen came orig­inally from the atmosphere. Once captured from the skies (whether precipitated by lightning or trapped by a nitrogen-fixing organism), nitrogen must be built into plant protein in order to be available to living organisms. Animals are wholly dependent upon plants to supply them with nitrogen; animals cannot use the simple nitrogen compounds which plants extract from the air.

The atmosphere does not give up this element lightly. Although above every acre of soil there floats a reserve supply of about 150,000 tons of free nitrogen, this is almost totally inaccessible to plants. Minute amounts are brought down as fixed oxides of nitrogen by powerful lightning flashes. Certain soil-inhabiting bacteria, which are primitive plants (though lacking in the chlorophyll of their higher relatives), are able to fix nitrogen from the air. Other bacteria (those that form nodules on roots of legumes like clovers, peas, and beans, to name a few) are also able to convert this element into a form which they use for their own growth, supplying what is left to their host plants.

Except for a miniscule amount of nitrogen fixed electrically by man, these limited sources (limited in comparison with the vast unused store floating above) must satisfy the craving of every living thing for this vital ingredient of existence. Once captured, it might not be held for long, since each time nitrogen is converted from one form to another, it struggles to escape.

The Nitrogen Cycle

An intricate pattern is traced by nitrogen as it is captured, used and released by plants and animals. This is commonly called the nitrogen cycle. At each stage some nitrogen returns to the atmos­phere directly because not all of it can be used. Thus, while we call this a nitrogen cycle, a complete recycling through all stages without some return to the atmosphere is never achieved.

Atmospheric nitrogen can enter soil in one of two ways. The first is by direct precipitation from air when the nitrogen is fixed as oxides by electrical discharges during thunder storms. The second is by fixation by specialized soil bacteria or by other forms of bacteria that live on the roots of legumes.

When plant roots absorb nitrogen which was previously fixed by one of these two processes, it is converted into protein by the plant. Any unused portion may return to the atmosphere or be blotted up by avid soil bacteria and fungi which are not specialized and thus cannot fix their own supply. Animals feed on plants, but are con­tinually returning matter (and finally their bodies) to the soil to be reused by other plants. No living thing can escape dependence upon the nitrogen cycle.

Manna from the Sky

An old French saying my mother taught me, as I protested against bad weather in spring that kept me from outdoor play, went something like this, "April snow is as good as sheep manure." This holds more than a grain of truth, since spring rains and snows do bring down nitrogen in oxide form from the atmosphere. According to figures collected at various stations throughout the world, the amount brought down may vary from 2 to 8 pounds per acre.

Since fertilizing was a big problem for Old World farmers, they learned to leave their fields rough-plowed (with large clods) in fall, to allow winter rain and snow to enter and penetrate the soil quickly, so this manna from the sky would not be lost.

In home gardens today, rough plowing just for this purpose would not be worth while, since less than an ounce of usable nitrogen per thousand square feet would thus be captured. Most of us throw away more than that much nitrogen in the dust that clings to the empty bag of low-cost, easy-to-use fertilizer.

Nor would a present-day farmer find it profitable to rough plow for this purpose alone. The capture of 2 to 8 pounds of nitrogen per acre would not make much of a dent in the 150 pounds or more per acre he would have to replace after harvesting a 100-bushel corn crop. This does not mean that rough-plowing in fall is obsolete, but today we continue this operation because of other benefits which justify it.

Restoring Nitrogen

Before modern chemistry came to the rescue, farmers and garden­ers had two ways to replace nitrogen consumed by crops. One was to use manures and other animal wastes in amounts as large as could be afforded. I recall, as a boy, walking many blocks to find livery stables and grocery delivery barns where manure was being thrown away and could be had for the hauling. Our own mare, a prodigious "oat burner," could not produce this precious stuff fast enough to maintain our one-acre vegetable garden and home orchard. In the race for this largess I had to compete with half a dozen neighborhood boys. Only our next-door neighbor, the local banker, who kept both a team and a milch cow, was exempt from this competition.

Manure is still valued in many places in the world, as witness the Pennsylvania Dutch farmers and French peasants who accumulate it as a miser amasses gold. In America, the automobile, the growth of city and surrounding suburban areas and other factors have con­spired to make barnyard manure almost unknown to millions of gardeners.

The only other nitrogen-replacement method available to farmers up to the nineteenth century was to grow cover crops of legumes such as clovers, alfalfas, peas and beans to capture atmospheric nitrogen through the bacteria growing on the roots. This practice was once part of the farmer's bible, but is slowly falling into disuse. Modern farmers find it much more profitable to apply one dose of low-cost liquid ammonia—the work of a few minutes—than to de­vote every second or third year to growing cover crops that bring no cash return. (In mentioning this modern trend, I am by no means giving it unqualified endorsement. I cannot help but feel that in our rush for cash income we are exhausting basic fertility in soils, using up elements which seem less critical than nitrogen, yet, when gone, will cause decreases in yields just as surely as will a nitrogen de­ficiency.)

Fortunately, today's gardener does not have to raid manure piles or grow cover crops in order to maintain soil fertility. For the price of a couple of movie tickets, the home grower can replace all the nutrient elements he removed in a year's harvest. The average garden plot is so small and fertilizer cost is such a minor factor that any elaborate organic system of conserving nitrogen would be point­less (except, of course, that a program of conserving and augmenting organic supplies in soils is essential for many other reasons).

Nitrogen Reserves in Soil

The "furrow slice" (the depth to which a horse-drawn plow could "bite") was set years ago at seven inches. In richer, heavier soils, nitrogen tends to accumulate in this upper layer. On rich Midwestern prairie loams, a furrow slice may contain as much as 7,500 pounds of nitrogen per acre. The next lower seven inches may hold only half as much, while the nitrogen content seven inches lower is down to 25 per cent of that of the furrow slice.

In lighter soils, this accumulation pattern is reversed. The more sand and gravel a soil contains, the deeper into the soil the nitrogen tends to move. This is important to know when handling such a soil. It suggests the importance of double digging and trenching (two soil-improvement operations described in Chapter Twelve) to bring the richer layer to the surface. Too, it suggests the value of growing deep-rooted plants which can penetrate to the layer where nutrients have accumulated.

Organic Nitrogen Compounds

During the warm-weather times of the year, when soil bacteria and fungi are working at top speed, ammonia and nitrate nitrogen are present in the soil in considerable amounts. Nevertheless, most of the nitrogen in soils exists as organic compounds. Although a plant's root system cannot "eat" them, these compounds still are valuable sources of nutrition (as discussed in Chapter Five). Or­ganic compounds are mentioned briefly here because they often give a false picture of fertility when the soil is tested, particularly when the analysis is turned over to the beginner without explanation.

In a fertile, humus-rich soil, availability of nitrogen will vary from season to season or even from day to day. In early spring, none may seem to be present because all the free nitrogen has been taken up by soil organisms. With practically no bacterial or fungal action going on in the cold soil, nitrogen is not released. Weekly tests as the soil warms up will show a gradually increasing nitrogen supply, with a high point late in June (in the region north of the Ohio River). This levels off soon thereafter and gradually subsides until July, when a sharp deficiency of nitrogen may be registered (a partially false reading). If summer rains are abundant (preventing drought-death of soil organisms) this leveling-off in July may not take place. When cooler weather comes in fall, nitrogen will again accumulate as plants use less and less of it. Yet the surplus will again gradually be blotted up by bacteria, fungi, actinomyces and protozoa, until a nitrogen "deficiency" is again registered.

Farther south, where soil organisms can attack organic matter over a much longer period, it is soon used up, so that its end product —humus—has little chance to accumulate. In tropical countries, where decay is continuous, humus formation is a negligible factor in soil fertility.

PHOSPHORUS: AN ELUSIVE ELEMENT

Although our knowledge of how plants actually use phosphorus is still elemental, we are much better off than we were just before World War II. Production of radioactive forms of phosphorus in atomic piles has made possible a study of its movement through plant tissues, unlocking many secrets of a decade or more ago.

Why is phosphorus a difficult element to maintain in soil in a form that plants can use? It is extremely quick to react with—and be locked up by—other chemicals. In fact, experts estimate that less than 1 per cent of the total phosphorus reserves of a given soil are ever used.

We once thought that phosphorus was used by plants only as they approached maturity and was not essential to young growth. Old garden books are full of recommendations for "hardening soft growth" with phosphorus (and potash). You may still see references such as "apply phosphorus to tomatoes toward maturity to 'firm up the fruit.'" Many special dahlia and potato fertilizers were low in nitrogen but high in phosphorus because phosphorus was thought to bring about earlier maturity. Actually, young growth in particular needs phosphorus. It is so essential to such growth that if it is not present in sufficient quantities for all parts of the plant, it will be withdrawn from older leaves and translocated to more active grow­ing tips and young foliage. For this reason, a phosphorus deficiency is among the first things that plant nutritionists suspect when a plant's lower foliage is poor but younger leaves seem normal.

Phosphorus is a major ingredient in the nuclei of cells, and is present as well in cytoplasm surrounding each nucleus. We know that phosphorus has something to do with transfer of inheritance factors from one generation to the next. Exactly how it works, we do not know.

Out of Balance

For all its importance, phosphorus is removed by plants in amaz­ingly small amounts compared with amounts applied to soil in order to supply it. A crop of corn may remove less than 25 pounds of phosphorus per acre. Yet to supply that amount, between 200 and 300 pounds of superphosphate may have to be applied. Hay may remove only 2 to 3 pounds of actual phosphorus from an application of 50 to 100 pounds of superphosphate.

Because of its tendency to lock up, phosphorus accumulates when high-phosphate fertilizers are used regularly. I have seen analyses of lawn soils from the Chicago area, where such fertilizers had been applied for several years without letup, in which the phosphorus content was so high that the soil itself could have been used as a low-grade source of that element!

An old-time phosphate fertilizer is bone meal. It is still used by many, but is a poor value because of its low solubility. It is often said that one application of bone meal will last in soil for 15 years— which is presented as an argument in its favor. (The pros and cons of bone meal are discussed under fertilizers in Chapter Five.) I know of an instance of bone meal remaining virtually unchanged in the soil for half a century.

Not All Is Lost

Even though they may be locked up by chemical action, all forms of phosphorus are not lost forever. True, certain combinations with iron or aluminum are so highly resistant to change that they can never be dissolved by any chemical that would be safe to use on soils in which plants are growing. Other phosphorus compounds, however, such as fluorapatite and hydroxyapatite, do become avail­able by weathering, by bacterial action and by exposure to soil acids and alkaline solutions. For example, raising the pH of an acid soil from 5.5 to a reading of 6.4 increased the availability of phos­phorus to ten times the original level. In another case, reducing the pH of a higher alkaline soil (from 8.3 to 6.9) resulted in a 500 per cent increase in phosphorus availability.

Phosphorus Does Not Move

One difficulty experienced in supplying phosphorus to plants is its lack of mobility in soil—a result of its low solubility. Phosphorus may become slightly soluble and move somewhat in soil water, but even when this happens, it will hardly have time to move far before it is fixed in less soluble form. For this reason, if plants are to obtain enough phosphorus, their roots must grow out to meet it. Phos­phorus is the mountain, roots are Mahomet.

Because phosphorus moves so little once it is in contact with soil, placement is highly important. The usual practice of scattering superphosphate on top of the soil is of little value, at least to the current growth. Because it is so stable, the phosphorus will still be in place, unused, when soil is prepared the following year.

Adding superphosphate to topsoil just before spading or tilling does have the virtue of getting some of it down into the ground. However, any of the material that remains above the area in which roots grow is of no use to them.

For maximum use, the best placement for phosphorus fertilizers is in the soil, worked down deeply before planting. It should be as close to the root zone as possible. For shallow-rooted plants such as petunias, lettuce or alyssum, this means within 3 to 4 inches of the surface, but for deep-rooted woody plants, such as trees and shrubs, it might mean working in superphosphate to a depth of 3 to 4 feet.

In The Lawn

Soils for lawns present a special problem. Mistakes in feeding vegetables and annuals can be corrected a year later, but it is not easy to roll up an established sod to incorporate superphosphate. Since it does not move downward, phosphorus in liberal amounts should be used in original lawn soil preparation, with hopes that this will become slowly available through the years.

On lawns, maintaining proper pH will be of tremendous help. Fortunately, superphosphate is not harmful to roots and can be used liberally to build up a reserve for the future.

Organic Matter to Conserve Phosphorus

One of the roles of organic matter is conserving phosphorus. Organic matter keeps up soil moisture. A characteristic of phos­phorus is that when it exists as small crystals, it is much more soluble than when larger crystals form. In moist soil, smaller crys­tals are formed. These go into solution more readily.

Too, as organic matter decays it produces certain organic acids such as tartaric, isocitric, and so on. These combine rapidly with any free iron and aluminum to form metal-organic ions which do not combine readily with phosphorus. By using up free iron and alu­minum, these acids prevent formation of less-soluble compounds of phosphorus.

In addition, organic matter itself contains considerable phos­phorus, the amount depending upon the origin of the organic com­pound. As cells of decomposition bacteria and fungi die, they release their phosphorus for use by higher plants.

The amount of organic phosphorus available varies from time to time. If liberal amounts of nitrogen are present, soil organisms may increase so rapidly that instead of releasing nutrients they will use up all surplus food and cause a temporary shortage. Fortunately, the life span of these organisms is short, so that plant roots will not have to wait long before the food elements are again available.

Phosphorus in Early Growth

Because phosphorus is needed by young plant growth, it should be applied early in the season. At least half the total annual phos­phorus consumption by annuals and perennials will be absorbed before these plants have made one-fifth their annual growth. In case of grasses, early uptake may be even higher: perhaps 80 to 90 per cent of their annual consumption will be taken up during the first few weeks of growth.

This need for phosphorus early in the growth cycle poses a problem. Phosphorus should be supplied just before it is needed, but not too far ahead of need. For grasses and perennials, this means in early spring. Bedding plants, however, and tender vegeta­bles such as tomatoes, eggplant and peppers usually are not set outdoors until early June. Superphosphate worked under in April would already be combining in less soluble compounds by June, and so would not be of maximum value to these late-set plants.

Here the so-called transplanting solutions serve a useful purpose. These are chemicals to be dissolved in water and applied to seed­lings as they are transplanted. These solutions are low in nitrogen but high in phosphorus. They are completely soluble and are taken up by seedlings and transplants before phosphorus  fixing can take place.

Growth Pattern of Woody Plants

Trees and shrubs do not seem to benefit from spring applications of phosphorus (and other nutrients) to the same extent as other types of plants. This is no doubt due to their different growth pat­tern. That tremendous canopy of leaves produced so quickly in spring by a mature elm could not possibly be manufactured from foods absorbed from cold, wet spring soil, in which most nutrients would be locked up and unavailable. Instead, this growth comes from food stored in tissues a year before. Maple sap is a case in point. Its sweetness as it is tapped for maple syrup in late winter comes from natural sugars stored the previous summer.

Growth of trees begins with elongation of terminal buds and leaf­ing out of dormant foliage buds long before bacteria can begin their work in the cold soil around the tree roots. It is not uncommon for maples, for example, to finish their flowering and produce their first leaves before the last traces of snow have disappeared. Twig elonga­tion and production of new leaves continue without interruption until about August first north of the Ohio River, and for about two weeks longer south of that line.

Quickly Available in Solution

About August first, most trees will shed a few leaves as though anticipating autumn. Most of the foliage, however, continues to function in food manufacture, but twigs stop growing in length. Instead, they begin to swell in girth. This indicates a storage of starches and sugars in the wood, a process which continues until frost kills all foliage. Almost the last act of the growing season is a withdrawal of all food from the leaves and a halt to chlorophyll formation, thus bringing on the pageant of fall foliage color.

If soluble phosphorus is applied in summer, just before twigs start to increase in diameter, it will be stored along with elaborated starches and sugars, ready for next spring's burst of growth. If, however, it is applied in spring, it will not affect growth a great deal and, by August, it will be locked up and of little use to the tree.

POTASH: THE THIRD "ESSENTIAL"

Potash is classed with nitrogen and phosphorus as one of the three essential (major) fertilizer elements. This seems surprising in light of the relatively small amounts of potash removed from the soil by some crops. A 25-bushel per acre oat crop, for example, removes only 5 pounds of potash. Yet this element is highly important to several basic functions in plants.

It helps check the tendency of nitrogen to produce soft, rapid growth. It is essential to formation of starch and sugar and to trans­port of these materials inside plants. Plants fed liberally with potash suffer less in drought. Potash stiffens cereal straws, increases oil con­tent of oil-bearing seeds and has an important role in plant protein formation. Plants which store starches and sugars in tubers or corms, such as dahlias and potatoes, quickly show injury or decline when they are suffering potash deficiency. Later, the keeping qualities of the tuber, bulb or corm will be seriously affected if potash is too low for normal growth.

Other types of plants do not readily show signs of potash short­ages. If soil is only slightly deficient in potash, plants tend to remain smaller in all parts, yet they flower, fruit and reach full maturity. It is only when potash-deficient plants are compared directly with those fed liberally with potash that the difference can be seen.

Some crops do use rather large amounts of potash. A single acre of celery may use up as much as much as 200 pounds. On the other hand, grain may remove very little if the straw is plowed under. An excellent way to maintain potash reserves is to return all plant residues to the soil.

In the home garden a well-managed compost pile can produce organic matter that will help to sustain potash in the soil, partic­ularly if extra table wastes are added to the refuse gathered from the garden. A lawn on which clippings are allowed to remain will need to be fed only half as much potash as needed by an always cleanly raked lawn.

Clay Soils Rich in Potash

Clay soils may not always show a response following addition of potash fertilizers. Clay particles hang onto this element tenaciously, yet release it readily to plant roots. When fertilized regularly, clay soils tend to accumulate potash, since rates recommended for most crops are usually made with sandy soils in mind.

In home gardens, regular use of a good mixed fertilizer plus addi­tions of compost should insure all the potash needed. Sandy soils, particularly if strongly acid, are another matter. Sandy soils, mucks and peats have little or no reserves of potash on which plants can draw, and little capacity to hold what is applied. For this reason, fertilizer applications in such soils should be split, so that about one-fourth of the potash goes on in early spring, half in mid-summer and the final one fourth as crops are nearing maturity. While potash is vital for early growth, not much is used at this stage. Toward ma­turity the demand is much greater. Thus, if all of it is applied in spring, a shortage may develop by fall on sandy soils.

Heavy Applications Sometimes Needed on Clay

Following World War II, a rush to suburban living absorbed mil­lions of acres of farmland around American cities. Many farm own­ers, realizing what was going to happen, stopped regular soil mainte­nance and let crops use up fertility that had been built up through the years.

In such soils, potash (even on heavy clays) was depleted to a point where deficiency symptoms developed. I have seen a number of spec­ulative housing developments where the minimum of black soil that was applied came from just such impoverished former farm fields. Heavy applications of potash were needed to bring the soil up to good tilth again. As a result, even where black clay soil is deep, I recommend that you apply extra potash if you are developing a garden in a new housing development. Once clay soil has been brought up to a high potash level, ordinary applications plus com­post should keep it will supplied.

Sodium vs. Potash

Chemically, potash and sodium are enough alike so that many plants will absorb sodium if potash is in poor supply. This presents a problem in certain western soils where sodium is high. Large amounts of potash are needed in such soils to override the sodium and thereby keep plants from absorbing this useless and sometimes harmful chemical.

There are, however, a few plants which seem to need small amounts of sodium for normal growth. Among these are beets, cab­bage, celery and turnips. If these do not do well, the use of nitrate of soda as a source of nitrogen will sometimes increase yield and crop quality.

Other crops, particularly asparagus and tomatoes, seem to be able to use a certain amount of sodium if potash is low, yet seem to be neither hurt nor helped by the substitution—up to a certain point. If sodium is too high, they will suffer.

In general, with these exceptions, sodium can be considered non-essential as a trace element and definitely harmful if present in quan­tity. In fact, many crabgrass-killing chemicals contain a form of sodium.

THE "MINOR" ELEMENTS

The word "minor" as applied to elements such as iron, boron, magnesium, calcium, sulfur, zinc and so on, does not refer to their importance but to the amounts present in soil for use by plants.

Calcium and Magnesium

Although used by plants for different functions, calcium and mag­nesium should be discussed together. They often occur in the same "limestone" used for "sweetening" acid soils. Thus when a liming has a favorable effect on plant growth it is hard to tell whether the improvement is due to changes in pH, or to the effect of calcium on plant cells, or to the vital effect of magnesium on chlorophyll formation.

In other cases, calcium may override magnesium and cause a deficiency. If present in excess, magnesium may also create a prob­lem by starving the plant for calcium.

In areas where limestone is high in calcium and lacking in mag­nesium, it may pay to use finishing lime from a building-material yard to supply magnesium. A light dusting of finishing lime on the soil every second or third year should be enough, unless fertilizers high in sulfates have been used. In this case, an annual application of finishing lime may be needed to replace magnesium washed out as Epsom salts (magnesium sulfate). Epsom salts are highly soluble and readily washed out of soil.

Calcium is a vital plant nutrient, particularly during early growth. It is needed to form cell walls and to serve as a building block in protein. As might be guessed from its role in neutralizing soil acids, it also helps tame acids formed during growth which might otherwise harm plant tissues.

Magnesium, an essential element in chlorophyll formation, has been exhausted from many older cultivated soils. Overtiming with calcium, without also adding magnesium, may hinder chlorophyll formation in plants. To check whether magnesium is needed, mix a tablespoonful of Epsom salts to a quart of water and spray it on some foliage. If the leaves turn a darker green, a shortage of mag­nesium in the soil is indicated. A dusting of finishing lime between the plants (avoid hitting foliage and stems) will be of benefit.

Neglected Sulfur

Sulfur is seldom mentioned in discussions of plant nutrition and is never listed as an essential ingredient on fertilizer bags. It may sur­prise you to learn that plants utilize sulfur as much as or more than they do phosphorus. For example, while a crop of cabbage may use up only 25 pounds of phosphorus per acre, it will extract 40 to 50 pounds of sulfur.

During the latter part of the nineteenth century, scientists be­lieved that sulfur was not an essential plant nutrient. During that period, all analyses for essential elements were made by burning plant tissues and analyzing the ash. Since sulfur is volatile at ashing temperatures, it went out the flue and did not appear in the residue.

I was fortunate in being able to study under the great Cyril G. Hopkins during the last years of his life. It was he who pointed out that sulfur probably is washed down out of the atmosphere by rain and snow in much the same way as nitrogen oxides are precipitated. He predicted that no sulfur shortages would occur in areas where fumes from factory chimneys were belching this element into the air in quantities sufficient for normal growth.

In such areas, accumulations amount to about 50 pounds per acre a year. In areas at a distance from industrial centers, annual accumu­lation is less than 5 pounds, not enough for normal crop needs.

Fortunately, many fertilizer materials used today are sul fates and supply sulfur along with the element in combination with it. Require­ments of some crops for sulfur are quite high, particularly those in the mustard family, the brassicas—alyssum, stock, candytuft, nas­turtium, and hesperis among flowers, and cabbage, broccoli, cauli­flower, radish, turnip, and kale among vegetables. Onions, too, need sulfur to develop their tear-jerking odor.

Sulfur is important because it is a basic element in protein manu­facture by the plant. If it is in short supply, older leaves are robbed to supply younger, more active leaves and growing tips. If the plant continues to "starve" for sulfur, protein synthesis stops while amino-acids, cystine and other nitrogen-bearing compounds accumulate in plant tissues; these unused building blocks of protein cannot be set in final place for lack of sulfur.

Perhaps the second most important role of sulfur is in synthesis in the plant of the so-called plant hormones or growth regulators.

Home gardens seldom lack sulfur. In addition to sulfur washed down from the atmosphere, quantities of this element are provided by humus and other organic matter in the soil, or by the fertilizers that contain some sulfates, such as sulfate of ammonia and super­phosphate. Thus, while sulfur is fully as essential as nitrogen, phos­phorus and potash, it can be generally taken for granted.

Iron

Although the majority of garden soils are well supplied with iron, one of the most common plant troubles is a chlorosis or yellowing of foliage, a symptom of iron deficiency. This is due either to a lack of iron (in rare cases) or to a locking up of this vital element in a soil of too high pH. Iron chlorosis often appears with dramatic sudden­ness following an overdose of lime. Sulfur, as noted, plays a vital role in acidifying overly sweet soil.

As might be judged from its effect on green foliage color, iron plays an important role in chlorophyll formation. Since lack of chlo­rophyll prevents plants from manufacturing starch needed for energy and growth, iron-starved plants become unthrifty. Lime is therefore "verboten" where broadleaved evergreens such as rhodo­dendrons and other acid-soil, iron-dependent plants are growing. Also, there is a still-undefined but apparently unfavorable relation­ship between iron and such elements as copper, manganese and zinc in the soil.

Zinc

Although plants contain and need as little as one part per million of zinc, this element is essential to growth of many plants and possi­bly to all. Its role in plant nutrition was discovered comparatively recently. Zinc deficiencies were first observed in Florida and Cali­fornia. Its most important role seems to be in seed formation. Peas and beans grown in zinc-deficient soils may form small, seedless pods. If some zinc was present in the soil earlier but was exhausted before flower pollination, plants make some growth and rob older foliage of zinc to mature the seeds formed when pods begin to set. Often enough zinc is "borrowed" so that a near-normal crop will be set on stunted plants.

Zinc also plays an important role in cell formation. When zinc is lacking, cells do not divide but continue to enlarge in size. Appar­ently, without zinc the nucleus is incapable of dividing to form new cells.

Like sulfur, zinc enters into synthesis of such vital products of plant metabolism as protein and plant growth regulators.

An unusual characteristic of zinc should be noted: It seems to be scarcest where organic matter is most abundant. Most other metallic minor elements such as iron and boron are more readily available in the presence of organic matter, but apparently this does not hold true for zinc.

Boron

Nearly 400 years ago, borax was shipped from Central Asia to Europe for use as a fertilizer—one of the first chemicals to be used in feeding plants. Despite this early use of a boron-bearing material, it was not until 1915 that boron's essential role in plant nutrition was fully established. Because boron is used in such minute amounts, modern chemical methods were needed to make the necessary an­alyses to detect its role. It is unique in that the lack of as little as one or two parts per million in soil may produce deficiency symp­toms; and, conversely, if boron is present in concentrations of only 10 to 15 parts per million, it may be toxic. A ton of cut alfalfa will contain less than two ounces of boron drawn from the soil, but those two ounces are vital to alfalfa growth. If boron is not present, ter­minal buds of the plants die, forcing side shoots to develop. In turn, tips of these shoots die, producing a plant full of short stubs with dead ends.

Boron has other uses in plant nutrition, many of them critical. It enters into cell division, affects flowering and fruiting, stimulates pollen grains into germinating, affects translocation of water in plant tissues and enters into many metabolic processes. Like several other elements, boron is linked with calcium in its effects on plants. Symp­toms of boron and calcium deficiencies are much alike. When cal­cium uptake is low, plants need less boron. When calcium use is high, boron deficiencies develop more rapidly. The two chemicals should be in a certain ratio to work well together—eighty parts calcium to one part of boron as maximum and 600 parts calcium to one of boron as a minimum. There is also evidence of a relationship between boron and potash, but the exact nature of this has not been clarified.

The addition of organic matter to soil releases boron that has been locked up in an insoluble form and makes it available to plants. Soil moisture also affects boron availability. As long as the soil is moist, boron remains soluble but in dry soil it reverts to an insoluble form.

Boron is credited with affecting fifteen different functions in plant growth. Certainly it is a mighty midget of an element.

Copper

Copper is both a poison and a nutrient. One of the earlier chemical weed killers was copper sulfate. The famed Bordeaux mixture, per­haps the first chemical fungicide, is a copper material that is used to destroy a fungus, which is, of course, a form of plant life.

The fact that copper is a nutrient was not proved until 1927. About that time, lack of copper was proved to be the cause of a slow decline in vigor of citrus trees. Its lack seems to affect many functions of plant growth, yet its role has not as yet been well denned. Two places where it may be in short supply are in mucks and in sandy soils of Florida. One way to test whether copper is needed is to spray plants with a weak solution of Bordeaux mixture. If copper is lacking, a marked improvement in foliage color and vigor will be evident in a week. For soil treatment, an application of 10 pounds of copper sulfate per acre is recommended.

Manganese

Many obscure and puzzling diseases have been traced to man­ganese deficiency without too many clues as to how this affects plants. Recently, chlorophyll research with radioactive isotopes has shown why the lack of manganese causes so much trouble. Along with iron, it is vital in chlorophyll formation and, if it is missing, production of starches and sugars is severely checked.

In most cases, manganese deficiency alters the color of foliage in some way. When leaf veins remain a dark green but areas between veins turn yellow or brown and finally break up, you can suspect a lack of manganese. It is seldom toxic when present in excess but in tobacco fields in Kentucky and Connecticut signs of poisoning have been noted. Again, leaf color is affected, with severe chlorosis and yellowing of the foliage.

Overtiming is often a cause of a shortage of manganese since this element readily locks up (becomes insoluble) in alkaline soils. For­merly the addition of sulfur was recommended when manganese de­ficiencies were suspected. This worked only if manganese was pres­ent originally and had locked up because of too high a pH. Thus when this element seems to be needed, lowering the pH to about 6.0 by the addition of sulfur is recommended. The manganese itself is supplied by applying 10 pounds of manganese sulfate per acre, or four ounces to 1,000 square feet of garden area. If the soil is already acid, 3 to 5 pounds of manganese sulfate per acre should be enough.

Molybdenum

Although needed in fantastically small amounts (an ounce will supply enough to fertilize an acre for several years!), molybdenum is being recognized more and more as a vital micro-nutrient. Its most important function is to help certain free-living bacteria to fix nitro­gen directly from the atmosphere, without the need for growing a crop of legumes. True, this molybdenum action is hardly a major source of nitrogen, but moly (as it is called for short) is needed for other purposes anyhow. If a soil contains none of it, clovers, toma­toes, certain fruit trees and a number of other plants will not grow. The amount needed is one part in 100,000,000 parts of soil, yet this barely-detectable bit of molybdenum is critical.

Unlike many other metallic elements, moly is released by liming. Normally, soils which are limed regularly are not deficient in molybdenum.

OTHER ELEMENTS

At various times in recent years, other elements have been studied in relation to plant nutrition but the need for them is not well estab­lished. Cobalt, for example, is taken up by plants but plants grow well without it. It is, however, essential to animal growth and lack of it in forage plants causes serious deficiency diseases in cattle. These ailments go by such names as salt-sickness, bush-sickness, pining, pine, vinkish and dasing (the last four are English dialect names) and marasmus (in Australia).

Certain plants need some chlorine to grow but this element is so often present in fertilizers, table wastes or organic matter that it is never deficient in garden soils. Iodine and fluorine have not been proved essential, although they are absorbed by most plants.

TOXIC EFFECTS

Arsenic is an element which is likely to be toxic in soils. Where an apple orchard has been sprayed for years with lead arsenate, arsenic can build up substantially. This seems to do very little harm to apple trees, but if the area is later subdivided, home owners may have a hard time growing grass or other plants on the contaminated soil. Arsenic is taken up by plants in place of phosphorus but does not substitute for it nutritionally.

Another place where arsenic can be harmful is on old golf courses. Lead arsenate is used for grub-proofing turf and also to control knot-weed, crabgrass and annual bluegrass (Poa annua). If used year after year, arsenic may build up and override the intake of phos­phorus by plants, thus starving them for that element. Growth will be retarded, plants will be stunted and they will mature late. Very sensitive plants, such as the stone fruits, may show injury in the form of shot-holes in the leaves or as marginal scorch.

The remedy for arsenic-sick soils is not easy or cheap: it involves application of as much as 500 to 1,000 pounds per acre of super­phosphate. Or the same amount of iron sulfate can be applied. How­ever, superphosphate is not only cheap but safer; iron sulfate used at this high rate will badly injure or kill grasses in lawns; in fact it was once recommended as a weed killer.

Selenium, molybdenum, fluorine and lead also are taken up by plants. These elements are poisonous and thus can harm humans (or animals) who eat contaminated plants. About the only time these elements need concern the home gardener is when they get into soil in the vegetable plot. The most dangerous of these elements is sele­nium, used by many African violet growers as a soil insecticide. It works beautifully for this purpose. However, selenium is a danger­ous and lethal chemical. Never discard a plant grown in selenized soil (or the old soil itself) onto the compost pile. There is too much chance that some of the treated soil will eventually wind up in the vegetable garden, where it could be responsible for sickness and death.

Chapter Digest

Nutrients are commonly called "plant foods" but, like human food, they must be broken down or "digested" into simpler forms before plants can use them. Soil bacteria and fungi perform this essential job. Abo, plant roots can only take up nutrients in solution, so everything must be soluble in water. Nitrogen, phosphorus and potash—the "big three"—plus a num­ber of "minor" elements play definite though often inter-related roles in plant growth. A properly fertilized soil is one in which these elements are always available in amounts adequate to assure maximum flower and "fruit" as well as vegetative (stem and leaf) development.

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