CHAPTER I
PLANT NUTRIENTS
There is some confusion in the use of the terms "nutrient," "plant food," etc., as applied to the nutrition and growth of plants. Strictly speaking, these terms ought probably to be limited in their application to the organized compounds within the plant which it uses as sources of energy and of metabolizable material for the development of new cells and organs during its growth. Botanists quite commonly use the terms in this way. But students of the problems involved in the relation of soil elements to the growth of plants, including such practical questions as are involved in the maintenance of soil productivity and the use of commercial fertilizers for the growing of economic plants, or crops, are accustomed to use the terms "plant foods," or "mineral nutrients," to designate the chemical elements and simple gaseous compounds which are supplied to the plant as the raw material from which its food and tissue-building materials are synthetized. Common usage limits these terms to the soil elements; but there is no logical reason for segregating the raw materials derived from the soil from those derived from the atmosphere.
The essential difference between these raw materials for plant syntheses and the organic compounds which are produced within the plants and used by them, and by animals, as food, is that the former are inorganic and can furnish only materials but no energy to the organism; while the latter are organic and supply both materials and potential energy. It would probably be the best practice to confine the use of the word "food" to materials of the latter type, and several attempts have been made to limit its use in this way and to apply some such term as "intake" to the simple raw materials which are taken into the organism and utilized by it in its synthetic processes. But the custom of using the words "food," or "nutrient," to represent anything that is taken into the organism and in any way utilized by it for its nourishment has been followed so long and the newer terms are themselves so subject to criticism that they have not yet generally supplanted the loosely used word "food."
If such use is permitted, however, it is necessary to recognize that only the green parts of green plants can use this inorganic "food," and that the colorless plants must have organic food.
To avoid this confusion, the suggestion has recently been made that all of the intake of plants and animals shall be considered as food, but that those forms which supply both materials and potential energy to the organism shall be designated as _synergic foods_, while those which contain no potential energy shall be known as _anergic foods_. On this basis, practically all of the food of animals, excepting the mineral salts and water, and all of the organic compounds which are synthetized by plants and later used by them for further metabolic changes, are synergic foods; while practically all of the intake of green plants is anergic food.
It is with the latter type of food materials that this chapter is to deal; while the following and all subsequent chapters deal with the organic compounds which are synthetized by plants and contain potential energy and are, therefore, capable of use as synergic food by either the plants themselves or by animals. It will be understood, therefore, that in this chapter the word "food" is used to mean the anergic food materials which are taken into and used by green plants as the raw materials for the synthesis of organic compounds, with the aid of solar energy, or that of previously produced synergic foods. In all later chapters, the term "food" will be used to mean the organic compounds which serve as the synergic food for the green parts of green plants and as the sole supply of nutrient material for the colorless parts of green plants and for parasitic or saprophytic forms (see page 16).
PLANT FOOD ELEMENTS
The raw materials from which the food and tissue-building compounds of plants are synthetized include carbon dioxide, oxygen, water, nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, and iron. The two gases first mentioned are derived directly from the air, through the respiratory organs of the plant. Water is taken into the plant chiefly from the soil, through its fibrous roots. All the other elements in the list are taken from the soil, nitrogen being derived from decaying organic matter (the original source of the nitrogen is, however, the atmosphere, from which the initial supply of nitrogen is obtained by direct assimilation by certain bacteria and perhaps other low forms of plant life), and the remaining ones from the mineral compounds of the soil.
Carbon dioxide and oxygen, being derived from the air, are always available to the leaves and stems of growing plants in unlimited supply; but the supply available to a seed when germinating in the soil, or to the roots of a growing farm crop, may sometimes become inadequate, especially in soils of a very compact texture, or "water-logged" soils. In such cases, the deficiency of these gaseous food elements may become a limiting factor in plant growth.
Water is often a limiting factor in plant growth. Experiments which have been repeated many times and under widely varying conditions show that when water is supplied to a plant in varying amounts, by increasing the percentage of water in the soil in which the plant is growing by regular increments up to the saturation point, the growth of the plant, or yield of the crop, increases up to a certain point and then falls off because the excess of water reduces the supply of air which is available to the plant roots. Hence, abundance of water is, in general, a most essential factor in plant growth.
Under normal conditions of air and moisture supply, however, the plant food elements which may be considered to be the limiting factors in the nutrition and growth of plants are the chemical elements mentioned in the list above.
AVAILABLE AND UNAVAILABLE FORMS
The plant food materials which are taken from the soil by a growing plant must enter it by osmosis through the semi-permeable membranes which constitute the epidermis of the root-hairs, and circulate through the plant either carried in solution in the sap or by osmosis from cell to cell. Hence, they must be in water-soluble form before they can be utilized by plants. Obviously, therefore, only those compounds of these elements in the soil which are soluble in the soil water are _available_ as plant food. The greater proportion of the soil elements are present there in the form of compounds which are so slightly soluble in water as to be _unavailable_ to plants. The processes by which these practically insoluble compounds become gradually changed into soluble forms are chiefly the "weathering" action of air and water (particularly if the latter contains carbonic acid) and the action of the organic acids resulting from decaying animal or vegetable matter or secreted by living plants.
THE VALUE OF THE SOIL ELEMENTS AS PLANT FOOD
Analyses of the tissues of plants show that they contain all of the elements that are to be found in the soil on which they grew. Any of these elements which are present in the soil in soluble form are carried into the plants with the soil water in which they are dissolved, whether they are needed by the plant for its nutrition or not. But in the case of those elements which are not taken out of the sap to be used by the plant cells in their activities, the total amount taken from the soil is much less than is that of the elements which are used in the synthetic processes of the plant. Hence, much larger proportions of some elements than of others are taken from the soil by plants. The proportions of the different elements which are used by plants as raw materials for the manufacture of the products needed for their growth varies with the different species; but a certain amount of each of the so-called "essential elements" (see below) is necessary to every plant, because each such element has a definite rôle which it performs in the plant's growth. A plant cannot grow to maturity unless a sufficient supply of each essential element comes to it from the soil.
From the standpoint of their relative value as raw materials for plant food, the elements which are present in the soil may be divided into three classes; namely, the _non-essential_, the _essential and abundant_, and the _critical_ elements.
The first class includes silicon, aluminium, sodium, manganese, and certain other rarer elements which sometimes are found in soils of some special type, or unusual origin. These elements seem to have no rôle to play in the nutrition of plants; although silicon is always present in plant ash and sodium salts are found in small quantities in all parts of practically all plants. Nearly all species of plants can be grown to full maturity in the entire absence of these elements from their culture medium. Occasional exceptions to this statement in the case of special types of plants are known, and are of interest in special studies of plant adaptations, but need not be considered here.
The second group includes iron, calcium, magnesium, and, generally, sulfur. All of these elements are essential for plant growth, but are usually present in the soil in ample quantities to insure a sufficient supply in available form for all plant needs. Recent investigations have shown, however, that there are many soils in which sulfur is present in such limited quantities that many agricultural crops, when grown on these soils, respond favorably to the application of sulfur-containing fertilizers. In such cases, sulfur is a "critical" element.
The "critical" elements are those which are essential to the growth of all plants and which are present in most soils in relatively small proportions and any one may, therefore, be the limiting factor in plant growth so far as plant food is concerned. These are nitrogen, phosphorus, potassium, and (possibly) sulfur.
RÔLE OF PLANT FOOD ELEMENTS IN PLANT GROWTH
The use which a plant makes of the elements which come to it from the soil has been studied with great persistency and care by many plant physiologists and chemists. Many of the reactions which take place in a plant cell are extremely complicated, and the relation of the different chemical elements to these is not easily ascertained. It is probable that the same element may play a somewhat different rôle in different species of plants, in different organs of the same plant, or at different stages of the plant's development. But the usual and most important offices of each element are now fairly well understood, and are briefly summarized in the following paragraphs. It should be understood that a thorough and detailed discussion of these matters, such as would be included in an advanced study of plant nutrition, would reveal other functions than those which are presented here and would require a more careful and more exact method of statement than is suitable here. However, the general principles of the utilization of soil elements by plants for their nutrition and growth may be fairly well understood from the following statements.
=Nitrogen= is a constituent of all proteins (see Chapter XIII). Proteins are apparently the active chemical components of protoplasm. Since it is in the protoplasm of the green portions, usually foliage, of plants that the photosynthesis of carbohydrates and the synthesis of most, or all, of the other tissue-building materials and reserve food substances of the plant takes place, the importance of nitrogen as a plant food can hardly be over-emphasized. Nitrogen starvation produces marked changes in the growth of a plant. Leaves are stunted in growth and a marked yellowing of the entire foliage takes place; in fact, the whole plant takes on a stunted or starved appearance. Abundance of nitrogen, on the other hand, produces a rank growth of foliage of a deep rich color and a luxuriant development of tissue, and retards the ripening process. In the early stages of growth, the nitrogen is present most largely in the leaves; but when the seeds develop, rapid translocation of protein material into the seeds takes place, until finally a large proportion of the total supply is deposited in them.
Nitrates are the normal form of nitrogen in the soil which is available to plants. During germination and early growth, the young seedling uses amino-acids, etc., derived from the proteins stored in the seed, as its source of nitrogen; and experiments have shown that similar forms of soluble organic nitrogen compounds can be successfully fed to the seedling as an external food supply. Soluble ammonium salts can be utilized as sources of nitrogen by most plants during later periods of growth, particularly by the legumes. But for most, if not all, of the common farm crops whose possibilities in these respects have been studied, it has been found that a unit of nitrogen taken up as a nitrate is very much more effective in promoting growth, etc., than is the same unit of nitrogen in the form of ammonium salts.
While the proteins are finally stored up largely in the seeds, or other storage organs, they are actively at work during the growing period in the cells of the foliage parts of the plant. Hence, the popular statement that "nitrogen makes foliage" is a fairly accurate expression of its rôle. Inordinate production of straw in cereal crops and of leaves in root crops often results from liberal supplies of available nitrogen in the soil early in the growing season. If the crops develop to normal maturity, this excessive foliage growth has no harmful results, as the surplus material which has been elaborated is properly translocated into the desired storage organs; but, unfortunately, the retarding effect of the surplus nitrogen supply upon the date of maturing of the crop is often associated with premature ripening of the plants from other causes, with the consequence that too large a proportion of the valuable food material is left in the refuse foliage material of the crop. Crops which are grown solely for their leaves, such as hay crops, lettuce, cabbage, etc., profit greatly by abundant supplies of available nitrogen; although when foliage growth is stimulated in this way the tissue is likely to be thin-walled and soft rather than firm and solid.
=Phosphorus= is likewise an extremely important element in plant nutrition. But phosphorus starvation produces no such striking visible effects upon the growth of the plant as does lack of nitrogen. Abundance of available phosphorus early in the plant's life greatly stimulates root growth, and later on it undoubtedly hastens the ripening process; hence, this element seems to act as the exact antithesis of nitrogen.
The rôle of phosphorus, or of phosphates, in the physiological processes of the cell seems to be difficult to discover. The element itself is a constituent of some protein complexes and of the lecithin-like bodies (see page 141) which are supposed by some investigators to play an important part in determining the rate of chemical changes which take place in the cell and the movement of materials into and out of it. It is an essential constituent of the nucleus, and a meager supply of phosphorus retards, or inhibits, mitotic cell-division. Photosynthesis of sugars and the condensing of these into starch or cellulose takes place in plants in the absence of available phosphorus; but the change of these insoluble carbohydrates back again into soluble and available sugar foods does not.
Phosphorus is taken from the soil by plants in the form of phosphates. Much study has been given to the problem of the proper supply of available soil phosphates for economic crop production. Any discussion of soil fertility and fertilization which did not devote large attention to the conditions under which phosphates become available as plant food would be wholly inadequate; but such a discussion would be out of place here.
The final result of an ample supply of phosphates in hastening the ripening process and stimulating seed production, as contrasted with that of an over-supply of nitrogen, has led to the popular statement that "phosphates make seeds." This statement, while not strictly accurate, is a fairly good summary of the combined results of the rôle of phosphorus in the plant economy. Large amounts of phosphorus are stored in the seeds. The two facts that large amounts of these compounds are thus available to the young seedling and that relatively large proportions of phosphates are taken from the soil by the plant during its early stages of growth are undoubtedly connected with the need for rapid cell-division at these periods in the plant's life.
=Potassium.=--The popular expression that "potash makes sugars and starch" is a surprisingly accurate description of the rôle of this element in plant metabolism. Either the photosynthesis of starch, or the changes necessary to its translocation (it is not yet certain which) is so dependent upon the presence of potassium in the cell sap that the whole process stops at once if an insufficient supply is present. The production and storage of sugar, or starch, in such root crops as beets, potatoes, etc., diminishes in direct proportion with a decreasing supply of potassium as plant food. The grains of the cereal crops become shrunken as a result of potassium starvation; and are plump and well filled with starch in the endosperm when sufficient potassium is available for the crop's needs.
The general tone and vigor of growth of the plant is largely dependent upon an ample potassium supply; potash-hungry plants, like those which have been weakened by any other unfavorable conditions, have been found to be more susceptible to injury by disease, than those which are well nourished with this food element. But potassium-starvation does not produce any pathological condition of the cell contents; its absence simply prevents the possibility of the development of the necessary carbohydrates for vigorous growth.
There is no known difference in the availability, or effectiveness, of potassium from the different forms of compounds containing it which may be present in the soil. Apparently, the only essential is that the compound shall be soluble so that it can be absorbed into the plant through the root-hairs. Of course, the acid radical to which the basic potassium ion is attached may, in itself, have some beneficial or deleterious influence which gives to the compound as a whole some important effect in one case, which might not follow in the case of another type of compound; but the relative efficiency as plant food of a given unit of potassium seems to be the same regardless of the nature of the compound in which it is present.
=Calcium= is an essential plant food element but its physiological use has not yet been definitely established. It seems to stimulate root-development, and certainly gives vigor and tone to the whole plant. It is commonly believed that calcium is in some way connected with the development of cell-wall material. It has been reported that the stems of grasses and cereal plants become stiffer in the presence of ample calcium, but this may be due to greater turgidity rather than to strengthened cell-walls. Calcium remains in the leaves or stem as the plant ripens, but it is not clear that this has anything to do with the stiffness or weakness of the stem, or straw, of the plant. Experiments with algæ have shown that in the absence of calcium salts mitotic cell division takes place, showing that the nucleus functions properly, but the formation of the new transverse cell-wall is retarded. This is the only direct evidence that has been reported that calcium has any connection with cell-wall formation.
Certain species of plants, notably many legumes, require such large amounts of calcium salts for their growth as to give to them the popular appellation of "lime-loving plants." Other plants, known as "calciphiles," while not actually showing abnormally large percentages of calcium in their ash, flourish best on soils rich in lime. On the other hand, certain other species, known as "calcifuges," will not grow on soils which are even moderately rich in lime; in what respect these differ in their vital processes from others which demand large amounts of calcium, or those which flourish on soils rich in lime, has not been determined, however.
The beneficial effect of alkaline calcium compounds in the soil, in correcting injurious acidity, in improving the texture of clay soils, and in promoting the proper conditions for bacterial growth, is well known; but this has no direct connection with the rôle of calcium as plant food. Furthermore, calcium salts in the soil have a powerful influence in overcoming the harmful, or toxic, effects of excessive amounts of soluble salts of magnesium, sodium, or potassium, in the so-called "alkali soils" (i.e., those which contain excessive amounts of water-soluble salts). The probable explanation for this fact is pointed out in a later paragraph of this chapter (see page 14); but this property of calcium probably has no connection with its physiological uses as plant food.
=Magnesium=, like phosphorus, is finally stored up mostly in the seeds, not remaining in the leaves and stems, as do calcium and potassium. This fact, together with other evidence obtained from experiments in growing plants in culture solutions containing varying amounts of this element, has led certain investigators to the conclusion that the rôle of magnesium is to aid in the transport of phosphorus, particularly from older to more rapidly growing parts of the plant. More recent investigations have shown, however, that magnesium has other rôles which are probably more specific and more important than this one. It is now known that magnesium is a definite constituent of the chlorophyll molecule serving, as will be shown (see