CHAPTER IV

SOIL MOISTURE AND PLANT

FEEDING

SOIL moisture in abundance, continuously

maintained, is an indispensable factor in the growth of every crop. It constitutes from 70 to 90 per cent of the actively growing part of every plant. Through the action of light in the green of leaf, water entering from the soil and carbon dioxide entering from the air react to produce the largest part of all the dry substances of plants. No plant food in the soil is available to the crop until it is dissolved in the soil moisture, and through its solvent action and its movement through the plant, water conveys all plant food to the tissues and distributes it to the growing parts. It is not strange, therefore, that the right amount of water in the soil is so important a factor in the production of large yields.

But, important and large as is the part

water plays in crop production, it is powerless to produce results if other essential factors are absent or are not sufficiently abundant. When phosphorus, potassium, nitrogen, or any other essential plant food element is deficient in the soil, an abundance of soil moisture will make the largest crop possible under such conditions, but not as large as it could have been had other plant foods been abundant. On the other hand it matters not how plentiful other plant food substances may be in the soil, the crop must be limited by the amount of available water. When all plant food substances are continuously abundant, the size of the crop is limited by the number of plants and their ability to feed and grow.

In our 10-year measurements of the amount of water used by crops it was found that in average round numbers about 450 pounds of water must be taken from the soil and most of it passed through the crop for each pound of dry substance produced above ground. With good soil conditions and an abundance of plant food it was possible to

secure yields from ordinary crops of six tons of dry substance per acre. Six tons of dry substance means 24 tons of sugar beets and tops; it means 7 tons of hay containing 15 per cent moisture; 92 bushels of wheat and straw, and 756 bushels of potatoes per acre. These are large yields, but nevertheless possible and practicable under intensive cultural methods where there is maintained an abundance of plant food and soil moisture. But the right amount of moisture means about 4 inches of effective rainfall for each ton of dry substance produced, or 24 inches during the growing season. We get less than this, and it is not distributed so as to be most effective.

In China and Japan, where they must raise large crops every year or starve, they have been compelled to irrigate, although they have a larger summer rainfall than we. More than half of the cultivated lands of Japan are irrigated every year, but they also fertilize highly, applying, on the average, from three to five tons of manure in some form per acre every year.

We do not sufficiently appreciate in our practice that to double the crop on the ground we must at the same time double the crop feeding. This means that the soil must turn over to the crop double the amount of nitrogen, phosphorus and potassium, and that double the amount of soil moisture must be taken up by and passed through the crop. Abundant harvests occur only where there is abundant moisture and abundant food at the time when the crop does its most rapid feeding and most rapid growing. If either soil moisture or accessible plant food, or both, are deficient at the critical period a reduced harvest is inevitable. The Chinese and Japanese, working always small areas, watch these critical periods with the greatest care, and whatever may be deficient they make a desperate effort to supply at just the right time.

Then, where large yields are expected, it is very important to understand that less water is required for a crop, in proportion to the yield, when the soil is abundantly rich. We found that when in abundantly rich soils

corn, oats and potatoes used an average of 355 pounds of water for each pound of dry product; on the same soils, after they had become impoverished by repeated cropping without fertilization, the same crops used an average of 625 pounds of water for each pound of dry produce.

It is also true that a continuous abundance of soil moisture will permit a soil poor in other plant food to produce the largest possible yields for such soils, and from this it follows that, where there is an especially rich soil, an abundance of soil moisture is extremely desirable, and that where the soil is poor it is doubly important that the soil moisture should be kept at the best amount.

Where all plant food substances are in great abundance in the soil the yields are almost exactly in proportion to the amount of moisture which can be supplied to the soil without making it too wet. Thus we found, in our trials, that corn gave a yield of 11,000 pounds of dry substance; oats 8,000 pounds, and potatoes 7,000 pounds per acre when it was given but 2 inches of water more than