332 SOILS: PROPERTIES AND MANAGEMENT 



mum quantity of any constituent required to produce 

 a good crop. Hilgard has shown that the addition of 

 four or five volumes of quartz sand to one volume of a 

 heavy, but highly productive, black clay soil greatly 

 increased the productiveness, while diluting the potash 

 content of the mixture to 0.12 per cent and the phosphoric 

 acid to 0.03 per cent. It is evident that in this soil the 

 plant-food materials were in a condition to be easily 

 taken up by the plant when the physical condition of the 

 soil was suitable. 



If these small quantities of food elements had been 

 distributed in the sand particles as well as in the original 

 clay, the result would doubtless have been different. 

 Suppose, for example, that fifty per cent of the potash 

 and phosphoric acid had been in the sand particles and 

 the remainder in the clay; in that case the former, in a 

 soil exposing much the less surface to dissolving liquids, 

 would be proportionately less soluble, and as the minimum 

 quantity is approached, as shown by the more dilute soil's 

 yielding less than the other, the effect would doubtless 

 have been to decrease the production. In some soils, 

 particularly those of arid regions, the larger particles 

 may carry much of the mineral nutrients, in which case 

 it is quite evident that a higher percentage of fertility 

 is required than in soils carrying the plant-food material 

 largely in the small particles. 



234. Nature of the subsoil. The nature and com- 

 position of the subsoil is naturally a factor in determining 

 soil productiveness, and must be considered as well as 

 the top soil. An impervious subsoil, or a very loose 

 sandy one, will confine the productive zone largely to 

 the topsoil and hence require a greater proportionate 

 amount of fertility in that part of the soil. 



