166 CROP GROWING AND CROP FEEDING 



nitrates of the bases that may exist in the soil such as lime, magnesium or 

 potassium. This last form has no power to attack ammonia, but must have 

 a nitrite for its food, and the previous work of breaking down the organic 

 matter and the release of ammonia must be done by other plants. Both of 

 these organisms are present in enormous numbers in any cultivated soil, but 

 most largely, of course, in those having a large amount of nitrifiable organic 

 matter. 



The work of both organisms goes on together. Nitrogen added to the 

 soil in the form of ammonia must go through the oxidation process conducted 

 by these minute plants, and be changed into a nitrate before our crops can 

 use it. This shows the importance of having nitrogen present in a fertilizer 

 in two forms. Nitrate of soda, for instance, is already in the form of a 

 nitrate, and plants can use it at once if they are in active growth ; if not used 

 at once it soon washes from the soil. Then the supply of nitrogen must be 

 kept up through the nitrification of organic ammonia in the form of cotton 

 seed meal, fish scrap, castor pomace, tankage, etc., generally used in the man- 

 ufacture of a complete fertilizer. 



At Eothamsted, England, the nitrous organisms were found abounding 

 in the surface soil and down to three feet from the surface in a clay soil, but 

 the organism which makes nitrates from the nitrites is found in the surface 

 soil only, although it may do its work in a sandy soil at a greater depth. 



NITRATES ARE EASILY DRAINED FROM THE SOIL. 



It was found at Rothamsted, that the drainage water from a field annu- 

 ally cultivated in wheat without manure, carried off 3.2 parts of nitrogen to 

 every million parts of water, on an average, during the year. If this much 

 is continually being carried off from a field that has been cultivated without 

 manure for fifty years, how much greater must be the loss of nitrates from 

 land regularly fertilized and left bare, as the cotton lands of the South have 

 been for many generations. This loss from bare soil is one of the chief 

 reasons for a proper rotation. In soils occupied by leguminous plants the 

 subsoils become rich in nitrates drained downwards. 



CROPS WHICH PREVENT LOSS OF NITROGEN. 



"Cereal crops, whose growing period is confined to spring and early sum- 

 mer, are very poor conservers of soil-nitrogen." Continuous, clean culture 

 will soon impoverish a good soil. Permanent pasture has been shown to be 

 the best conserver of nitrogen in the soil. But all land cannot be kept thus 



