282 SOILS 



amount of plant food it contains, yet mere richness 

 is a very valuable asset of a farm soil, and no man 

 can afford to disregard it in the modern emphasis 

 on good texture and other desirable attributes. 



The actual richness of a soil in plant food de- 

 pends largely upon its origin and its fineness. A 

 leachy, sandy soil, for example, is not likely to con- 

 tain more tnan a third as much plant food as an 

 alluvial clay; a limestone soil is usually richer than 

 a slate soil, and so on. 



The Soil a Storehouse of Plant Food. The point 

 that needs to be emphasised most, however, is not 

 that farm soils vary greatly in native richness, but 

 that practically all farm soils, including those that 

 we consider poor, contain a vast amount of plant 

 food. 



The analyses of representative soils in the 

 Appendix show that all of them contain almost 

 unbelievable quantities of the plant foods that we 

 buy and apply so grudgingly. An average farm 

 soil usually contains about 4,000 Ibs. of nitro- 

 gen, 6,000 Ibs. of phosphoric acid, and 20,000 

 Ibs. of potash per acre in the upper eight 

 inches of soil. "Worn-out" soils, which scarcely 

 produce enough to pay for cropping them, often 

 contain nearly as much plant food as this while 

 some rich soils have over 6,000 Ibs. of nitrogen, 

 10,000 Ibs. of phosphoric acid, and 50,000 Ibs. of 

 potash per acre in the first eight inches. Besides all 

 this large amount of plant food in the surface soil, 

 the soil below the first eight inches usually con- 

 tains nearly as much, and a part of this can be used 

 by the roots of most farm crops. 



These figures are astounding to those who have 

 believed that a soil gradually ceases to be produc- 

 tive because the plant food in it becomes exhausted. 



