PLANT FOODS

Southland Times, Issue 22734, 9 November 1935, Page 12

PLANT FOODS

QUANTITIES AVAILABLE ( IN THE SOIL

EFFECTS ON CROP PRODUCTION The quantities of each plant food naturally available in the soil cannot yet be determined. Analysis of soils and crops certainly give some guide to manurial requirements, but so far actual experiment is the only method by which more light can be obtained on this difficult subject. The following brief extracts from “The Application of Fertilizers to the Orchard and Garden,” by R. A. Boyle, M.Sc., A.A.C.1., are very interesting and helpful. The author obviously knows his subject very thoroughly indeed. In the first place, he says, a plant depends upon two media for its existence —the air and the soil. So long as it remains impossible to forecast precisely every change that is likely to occur in the weather from day to day in a growing period, we shall never be able to specify just what alterations to the soil condition or content will need to be made to effect the maximum possible production. Nevertheless, such is the goal at which we aim, and extraordinary advances in our knowledge have been made within less than a century. The greater portion of the structure of a plant is built up from water, and the carbon, existing in combination with oxygen as a gas, in the atmosphere. The amounts of other elements, e.g., nitrogen, phosphorous and potassium, etc., in the plant are comparatively small, though of extreme importance. All of the following elements appear to be more or less essential to normal plant life—carbon, hydrogen, oxygen, nitrogen, phosphorous, potassium, calcium, magnesium, iron and sulphur. The availability of others, such as sodium, boron, manganese and silicon, although not apparently essential to growth, may, if sufficiently low, have a detrimental effect upon satisfactory production. Balance Upset.

As crop production proceeds, the elements essential to plant life—plant foods—are drawn from the soil in proportions depending upon their availability in the soil, the crop and the conditions of growth. Gradually the elements in greatest demand or least supply become more scarce and the balance of plant food is upset. The mere act of replenishing one element may cause another automatically to become deficient. For example, there may be a shortage of nitrogen in a certain soil, but as a result of the increased growth due to added nitrogen, there may be insufficient available phosphate to maintain this extra growth and so a phosphate deficiency results. Application of nitrogen and phosphate may in turn expose a relative scarcity of potash when only with a mixed fertilizer having its nitrogen, phosphate, and potash contents suitably balanced to suit all conditions may efficient production be attained.

Plants absorb nitrogen, as well as all the other essential elements, only through their roots. Before they can thus feed, it is considered that the. necessary elements should be in water' solution. . . . The interstices or free spaces in between all particles of soil are occupied by air and water . . . This more or less permanent water content of a soil is known as the soil solution. It is a solution because it contains, dissolved within it, certain portions of most of the chemical elements which went to form the minerals from which the soil degenerated. It would also contain the dissolved portions of any fertilizers which had been added. It is considered that it is from this solution that plants derive their supply of nutrients. This soil solution is found to be for ever changing in concentration. It is apparently extremely sensitive to many factors. Elements may go out of solution as well as in to solution. The art of successful soil treatment would appear to be that of creating and maintaining a soil solution suitable to each variety of plant. . . A very informative chapter is headed “The chief functions of the most important plant foods,” and gives as follows some of the main symptoms noted in connection with deficiencies of nitrogen, phosphate and potash:— Nitrogen—Stunted browth, yellowishgreen leaves, early defoliation, bare shoots, poor fruit setting, and a reduced root system. Phosphate—Phosphate has a most important influence upon the root system of a plant, promoting root growth. Stunting of the root shoots and also the leaves occurs in phosphate-starved fruit trees. Phosphate also has an important bearing upon the reproductive processes of a plant. It tends to hasten the ripening processes and its presence in sufficient quantity is necessary for satisfactory blossoming. The addition of this plant food appears to be favourable to bacterial activity and also seems to assist the plant to derive other nutritents from the soil, e.g., potash. Potash—Plants grown with a shortage of potash exhibit the following characteristics:—Leaves a poor, dull colour and tend to die early at the tips and along the edges; stems are weak and plants do not stand up well; pot-ash-starved plants are the first to suffer in a bad season or to succumb to disease; fruit trees show leaf scorch and defoliation and restriction of root growth; stunted growth and small grain; over nitrogen effects may more easily be produced. Similarly definite symptoms arising from the deficiencies of other less-used elements are being observed, e.g., calcium, magnesium, sulphur, iron, etc.