PART IV. 



MANURES. 



CHAPTER LXXXVI. 



GENERAL SUMMARY. 



TV MANURING is governed by what is called the Law of 

 Minimum. Nitrogen alone will produce no crop, if 

 Phosphoric acid, Potash, Lime &c. are absent. Similarly 

 Phosphoric acid and Potash alone without Nitrogen &c. will 

 produce no crop. If by adding 8 Ibs. of N, 10 Ibs. of Potash 

 and 5 Ibs. of Phosphoric acid, one gets 14 maunds of oats, by 

 adding 8 Ibs. of N. and only 5 Ibs. of Potash and 2\ Ibs. of 

 Phosphoric acid, one would obtain only 7 maunds. The Law 

 of Minimum holds good even for moisture, light and heat : a 

 minimum proportion of moisture, light and heat are needed 

 for the growth of every crop. This minimum requirement of 

 food and other conditions of growth, is different for different 

 crops; but crops have not been so minutely studied yet to 

 enable one to give a tabular statement of minimum require- 

 ments for each variety of every crop. Chemical analysis may 

 prove certain amounts of plant food present, even in an avail- 

 able form, which are theoretically sufficient for obtaining the 

 maximum yield. But even in such cases, manuring has given 

 good result. Chemical analysis also does not take into con- 

 sideration the potentiality of a soil for accumulating nitrogen, 

 due to the growth of microbes under proper conditions of 

 moisture, porosity, heat and presence of lime. As much as 

 100 Ibs. of nitrogen per acre may be accumulated from the air, 



