208 NATURE AND PROPERTIES OF SOILS 



of rainfall and its distribution, but also by evaporation, tlie 

 character of tlie soil, and the presence of a crop. As the rain- 

 fall increases, percolation increases, being much greater in 

 New York, for example, than in Utah. Evaporation has a 

 marked influence, reducing drainage losses to a considerable 

 degree. The drainage through sandy soils is generally larger 

 than through clayey soils under strictly humid conditions and 

 where run-ofP is a factor. When evaporation is high, sandy 

 soils have been known to percolate very much less than those 

 of a heavier nature.^ Field crops, in that they utilize a large 

 amount of moisture, have always been found to reduce per- 

 colation losses. 



The loss of moisture by percolation is the least objectionable 

 feature of the phenomenon, since it is often necessary, espe- 

 cially during the spring and summer, to rid the soil very 

 quickly of superfluous water. The loss of nutrient salts is 

 more vital, since the materials so carried away might be used 

 by plants. The loss of nitrogen, calcium, and potassium from 

 a bare clay loam at Cornell University ^ over a period of ten 

 years averaged, respectively, 69, 398, and 72 pounds an acre 

 annually. This is equivalent to an acre loss of 419 pounds 

 of sodium nitrate, 995 pounds of calcium carbonate and 137 

 pounds of potassium chloride every year, which is a larger 

 amount of nutrient material than is removed by an average 

 crop. 



Control of percolation is exerted, not so much to save water, 

 as to conserve nutrients. As water enters a soil it moves 

 downward and is continually changing into the capillary state. 

 If the absorptive capacity of the soil is high, little of the rain- 

 fall may appear as drainage. The presence of organic matter 

 and the influence of good tillage will do much toward check- 

 ing drainage losses. Once the absorptive capacity of the soil 



=^ Fraps, G-. S., Losses of Moisture and Flant Food hy Percolation: Tex. 

 Agr. Exp. Sta., Bnl. 171, 1914. 

 * Unpublished data. Cornell Agr. Exp. Sta., Ithaca^ K Y. 



