INFLUENCE OF TEXTURE ON SOLUBILITY 271 



of the fertilizing materials in these constituents will 

 affect the minimum amounts required. Hilgard has 

 shown that the addition of four or five volumes of quartz 

 sand to one of a heavy but highly productive black clay 

 soil greatly increased the productiveness, while diluting 

 the potash content of the mixture to .12 per cent and 

 the phosphoric acid to .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 amounts 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 50 per cent of the potash 

 and phosphoric acid had been in the sand particles and 

 the remainder in the clay, the former which expose 

 much the less surface to dissolving liquids would be 

 proportionately less soluble, and as the minimum quan- 

 tity is approached, as shown by the more dilute soil 

 yielding less than the other, the effect would doubtless 

 have been to decrease the production. (See page 86.) 

 In some soils, particularly those of the arid region, 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. 



132. Nature of subsoil. The nature and compo- 

 sition of the subsoil is naturally a factor in determining 

 soil productiveness, and must be considered as well 

 as the soil. An impervious subsoil, or a very loose sandy 

 one, will confine the productive zone largely to the top 



