Cyril G. Hopkins
CRITICAL PERIODS IN PLANT LIFE
IN this connection we may well consider another cause of differences in crop yields quite out of proportion to the difference in soil treatment. There may be, and often are, critical periods in the life of plants, when some small measure of assistance may change prospective failure into marked success. Thus, it is not infrequently a question of life or death with the clover plant when the nurse crop is removed; and, while most of the plants may die at that time on untreated land, a good stand of clover may be saved where a very light application had been made of manure, fertilizer, or soil stimulant. The nutrient value of the application may not be sufficient for half a ton of clover, but the difference in yield may amount to one or two tons; and from the larger crop larger residues are left on and in the soil, resulting in a larger crop of grain the following year. Thus, enormous credit may be given, which is not at all deserved, on the basis of total plant food concerned.
One of the most critical periods in the life of the corn plant is at the time the ears are forming, and an ample supply of moisture appears to be especially necessary at that time. If a severe summer drouth is coincident with this critical period, the yield of grain is likely to be small; and any soil treatment which has the effect either of hastening or retarding the development of the plant, and thus of bringing the earing time either before or after the drouth, may very markedly affect the crop yield.
The farmer is usually most anxious for conditions under which his wheat will "fill" well, and since this is influenced very appreciably by the temperature during this critical period, it follows that very marked effects upon both yield and quality may sometimes result from any soil treatment that causes the wheat to "fill" either a few days earlier or a few days later than on the untreated land. On the other hand, the treatment, whether applied as a crop stimulant or in a system of permanent soil improvement, may sometimes be the means of bringing this critical period at the time when the weather conditions are most unfavorable, while the untreated land may mature a larger crop at the more favorable time.
An instance has been reported of a field treated with half a ton per acre of raw phosphate having produced a crop of 45 bushels of oats free of rust, whereas only 20 bushels of badly rusted oats were produced from similar seed on adjoining untreated land. Two influences may help to produce this difference: the added phosphorus tends to balance the food ration and thus to strengthen the oats against the fungous disease (and against lodging, as well), and also to hasten the maturity by which the crop escapes the rust which might attack the plants maturing later and perhaps under weather conditions more favorable for the development of the disease. As was stated by the author to the farmer who reported this experience, the marked difference in yield is not to be credited even largely to the phosphorus because of the plant food for its own sake, but rather to a combination of influences to which the added phosphorus proved to be the key.
While such examples may serve temporarily as good advertisements for the treatment applied, they are just as misleading for wide application as are the occasional reports of damage to crops produced by applying manure. All of this serves to emphasize the importance of having some fundamental knowledge upon which to base definite systems of permanent agriculture. For this purpose we must rely primarily upon the absolute facts furnished by chemistry and mathematics and be guided only by the results of carefully conducted and long-continued experiments. Single examples can be found in support of almost any practice or theory that can be advanced; but a mere experience, though it be repeated, invariably with the same result, for fourscore times, furnishes no proof whatever that the octogenarian will live to celebrate another birthday.
A small amount of readily available plant food, such as 50 pounds of sodium nitrate per acre as a top dressing for wheat on poor land in a cold spring, may produce a sufficient increase in yield to more than pay the cost of the nitrate. Likewise, 100 pounds of "ammoniated bone and potash," carrying perhaps 2 pounds of nitrogen, 4 pounds of phosphorus, and 2 of potassium may be dropped in or near the hill of corn with a "fertilizer attachment" to the planter, and, under adverse conditions of soil and season, the crop increase may show some profit. It should be clearly understood, however, that all such systems of fertilizing are of themselves only an aid to soil depletion, because the "good start" thus given to the crop enables it to draw upon the soil itself for larger supplies of one or more elements of plant food than would be furnished by the untreated soil and the fertilizer applied.
Quite independent of any such practices, the landowner should make ample provision for maintaining the fertility of the soil, on normal soil, by large use of phosphorus and farm manure or legume crops and crop residues, sufficient limestone being applied when necessary to prevent or correct soil acidity. Where this is done, however, the use of "starters" is usually unnecessary and unprofitable. Indeed, the dropping of a small quantity of fertilizer in the hill of corn (or near it) is sometimes a source of damage, not so much because it may injure the seed or young plant, but because it does not encourage the normal development of the root system in proportion to the early growth of the plant, and as a consequence the crop may suffer from drouth later in the season much more than the unfertilized corn.