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REASONS FOR CULTIVATING THE SOIL. 



By Milton Whitney . 

 Chief of the Division of Agricultural Soils, TJ. S. Department of 



Agriculture. 

 From Year Book for 1895. 



HOW WATER ENTERS THE SOIL. 



"Water is the most abundant substance found in living crops. Not 

 only does it form by far the largest proportion of all fresh vegetable 

 substance, but, on account of less through evaporation from the leaves 

 of growing plants and the necessity of replacing this loss, thirty or 

 forty times more water is needed during the growing period of a crop 

 than is contained in the crop when harvested. Plants require a large 

 amount of water for their life and growth, and it is necessary that the 

 supply should be abundant at all times. If the evaporation from the 

 plant grately exceeds the amount taken in through the roots, the leaves 

 wilt and the plant suffers. 



Therefore one of the most important functions of the soil in its 

 relation to crop production is the maintenance of a proper supply of 

 water. Rain falls, on an average, in the humid portion of the United 

 States for two or three days in succession, and is then followed by 

 an interval of eight or ten days of fair weather. As plants are fixed 

 in their relative positions in the earth, the soil, in order to supply them 

 with water during the fair-weather period, has to offer such a resistance 

 to the percolation of the rain than an adequate supply shall be held back. 

 On account of this resistance, due to the friction which the rain encoun- 

 ters in the minute spaces between the soil grains through which it has 

 to pass, the movement is very slow and only part of the water sinks 

 below the reach of plants before the next rainfall occurs. 



The resistance which soils, owing to their difference in texture, offer 

 to the percolation of the rain varies greatly. Light, sandy soils main- 

 tain comparatively little moisture, because the spaces between the grains 

 are comparatively large and there is relatively but little resistance to the 

 flow of water, so that the rainfall moves down quite rapidly until there 

 is only 5 or 10 per cent of moisture present in the soil. Strong clay 

 soils, on the other hand, have very minute spaces for the water to move 

 through, and consequently offer a very great resistence to the percolation 

 of the rain. These soils maintain, as a rule, from 15 to 20 per cent of 

 their weight of water. 



Different plants grow best with different amounts of water. For 

 instance, the pasture grasses thrive on a soil which is too moist for 

 Indian corn, or even for the largest and surest yield of wheat. Some 

 classes of tobacco thrive well on soils whieh are very retentive of mois- 

 ture, while other classes can only be grown with success on drier soils. 

 We are not concerned in this article with the amount of moisture which 

 different soils maintain or with the amount of moisture required by 

 different kinds of plants. We must recognize, however, that it is not 

 possible nor desirable to maintain the same amount of water in all soils, 

 for if this were done there would not be the opportunity for diversity 

 in agriculture which we have under existing conditions. 



While water is maintained for a time in the soil, as already explained, 

 it is liable to be lost to the growing crop by evaporation from the sur- 



