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.