SOILS AND FERTILIZERS 383 



The color of a soil may indicate the presence of a component 

 in itself of actual benefit to the plant. Light-colored clayey soils 

 generally contain goodly amounts of potash, and real black soils 

 contain organic material in abundance. Color may also indicate 

 the manner of formation of a soil, or the parent rock strata from 

 which it was formed. 



As a physical property, color can be of direct agricultural value 

 only through the effect that different colors have in radiating and 

 absorbing heat and possibly in regulating light intensity, for many 

 chemical and bacterial reactions are influenced by that factor. The 

 high quality of the wines made from grapes grown on red soils on 

 the hillsides of northern Europe may be due to the greater amount 

 of heat the grapes have had during the growing season. 



The dark colored soils absorb the heat of the sun more readily 

 than the light colored soils. Red soils, their color being near the 

 heat end of the spectrum, absorb heat readily. In the early part 

 of the growing season, before the cultivated crops shade the ground, 

 or in the case of crops that do not cover the ground, this variance 

 of heat absorption by different colored soils is a factor of consider- 

 able economic importance. Melons have been ripened in a very 

 inclement climate by covering the soil to the depth of one inch 

 with charcoal. 



Washing of Soils. There are large areas of land in the differ- 

 ent sections of the country that are being ruined by the washing and 

 gullying of the soils. The amount of washing depends upon the 

 amount of surface run off. The amount of run off and the effect 

 it has, will be modified to a large extent by the character of the 

 soil and subsoil, <by the length and steepness of the slope, by the 

 tillage practiced, and by the vegetation produced. Nothing will 

 completely ruin land more quickly than washing, especially gul- 

 lying. 



The washing produced by the run off is of two kinds, sheet- 

 washing or general surface washing, and gullying. In the former 

 the water is spread over a uniformly sloping surface where there 

 is little or no tendency to collect into streams. If the soil and 

 covering are uniform in character, the wearing of the water will 

 be the same for all points. Ordinarily, however, it is not so simple 

 as this. More frequently the gradient is not uniform, the general 

 slope will contain many smaller slopes, the water will collect into 

 streams in the draws ana this accumulation of water will give greater 

 volume with less resistance, and consequently the water will attain 

 greater velocity. The washing, or work, that a stream does varies 

 with the velocity of the water and this depends upon four things : the 

 slope, the resistance to movement, the volume of water, and the 

 amount of sediment carried in suspension. 



Swift streams have much greater power to wash than slow ones. 

 In general, the transporting power of running water varies as the 

 sixth power of its velocity, or doubling the velocity increases the 

 carrying power 64 times and trebling the velocity increases the 

 carrying power 729 times. Hence it is that gullies form so rapidly 



