The Physical Character of Soils 25 



Loess, or wind-borne soils, are made up of very fine 

 particles held together by mineral matters, so that in cul- 

 tivation they approach the nature of the sandy soils. 

 Differences in the physical nature of soils explain the 

 difference in the degree to which air and the roots of plants 

 penetrate them. In a soil of a mellow texture the roots of 

 many plants strike down several feet. The tap root of 

 the alfalfa plant in a mellow subsoil will go straight 

 down, while on a soil the surface of which is sandy 

 but underlain by a very compact clay of great fineness 

 of particles we have found the roots of alfalfa striking 

 this clay and then running horizontally, so that the 

 plants came under the blighting influence of a summer 

 drought. 



But a soil too loose in texture becomes a leachy soil. 

 This is the case with deep sands, through which the water, 

 laden it may be with plant food, soon gets beyond the 

 reach of plants, and constant appHcations of fertiUzers are 

 needed to keep up its productiveness. The barrenness of 

 sand hills is largely due to this leaching away of plant 

 food. Hence, in the ameHoration of such soils, appUca- 

 tions of clay and hme have been found useful in binding 

 the particles and making them less open and leachy. The 

 rotation of crops, and the addition of organic matter in 

 decayed vegetations, also improve the leachy sands, for 

 on account of their extreme fineness the particles of vege- 

 table decay have a greater power to retain moisture than 

 matter in any other form. This black humus or organic 

 decay is of the first importance in agriculture, not only as 

 a carrier of plant food that has contributed to the structure 

 of other plants, but as a mellower of a heavy soil, as a 



