THE COW PEA. 21 



needs from the air. If, however, the supply of either 

 potash or phosphoric acid in the soil are insufficient, then 

 only a corresponding amount of nitrogen will be absorbed 

 from the atmosphere. 



The practical application of this is then, that "the 

 more phosphoric acid and potash there are supplied to the 

 growing crop, the larger will be the amount of nitrogen 

 drawn from the air." This condition is well described as 

 making the crop " nitrogen hungry," or, to state the proposi- 

 tion in other words, if liberal quantities of phosphoric acid 

 and potash are supplied and nitrogen not given, the plant 

 absorbs sufficient nitrogen from the air to balance the sup- 

 ply of phosphoric acid and potash already used, and not a 

 particle more. Nitrogen is more expensive than any other 

 element in a complete fertilizer, hence it is economy to 

 assist the cow pea crop to secure, free of cost, the largest 

 possible quantity of nitrogen from the air. The larger the 

 amount of phosphoric acid and potash supplied to the crop, 

 to the limit of its healthy feeding, the greater is the assimi- 

 lation of the free nitrogen of the air; and so, financially, the 

 larger the investment in phosphoric acid and potash, within 

 the above limits, the greater the repayment or dividend in 

 the form of the more expensive nitrogen. The limit of 

 plant growth is determined largely by the mechanical con- 

 ditions of the soil and a suitable supply of moisture to make 

 the plant food available. Plants grown on soils in good 

 condition can use a larger amount of fertilizer than those 

 on soils too hard and dry for the full development of roots. 



