74 THE ESSENTIALS OF AGRICULTURE 



crop is all that will be produced. If there is sufficient phosphorus 

 available for only half of this crop, only half the crop can be 

 secured. The same rule will hold for the other elements. In 

 other words, plants must have a balanced food supply, just as 

 animals must have a balanced ration if they are to make a satis- 

 factory growth. The table shows that crops like clover, alfalfa, 

 and cowpeas require large quantities of nitrogen, though part of 

 their nitrogen may through the medium of bacteria be taken 

 directly from the air which is in the soil. 



79. The amount of plant food in soils. The total amount of 

 plant-food elements is large in some soils and comparatively 

 small in others. The amount of phosphorus is usually less than 

 that of either nitrogen or potassium, but phosphorus is used by 

 crops in smaller quantities than is either of the other plant foods. 

 Soils containing considerable clay contain much potassium, while 

 sandy soils contain little. Very fertile soils contain in the top 

 foot the area from which the bulk of plant food is taken 

 enough nitrogen to produce about 80 corn crops of 75 bushels 

 each, enough phosphorus to produce about 200 such crops, 

 and enough potassium to produce 1000 such crops. This is 

 assuming that both corn and stover are removed from the land 

 and that nothing in the way of fertilizers is returned. 



It cannot be assumed, however, that all of the plant food in 

 a soil can ever be removed by crops, because the yields would 

 become so low as to be unprofitable long before the plant food 

 was entirely exhausted. On the other hand, it must be borne in 

 mind that plant roots penetrate deeper than one foot and that 

 the undersoil also contains large quantities of plant food, espe- 

 cially of phosphorus and potassium. In soils whose topography 

 is such that the surface is gradually being washed away, new soil 

 layers are slowly brought within the root zone. Often, especially 

 in clay soils, this is sufficient to maintain the potassium supply, 

 because of the large amount of potassium contained in the 

 subsoil, but it will not keep up the phosphorus supply. It will 

 be necessary to add phosphorus to our soils if they are to be 

 kept productive. 



