82 PLANT PRODUCTS 



in the soil, and is still going down deeper and deeper. The 

 fixation of nitrogen in soil is usually dependent upon the 

 presence of leguminous crops. At Cockle Park the leguminous 

 crop concerned is undoubtedly wild white clover, but 

 in different parts of the world other leguminous crops would 

 play the same part. Nitrogen that has been fixed in the 

 soil, or obtained in the soil by any other means, is converted 

 by other soil bacteria into nitrites and nitrates. It is the 

 latter that form the nitrogen food of the plant. Much can 

 be done in practice to improve the rate at which nitrifica- 

 tion proceeds. In calcareous soils the nitrification proceeds 

 at a much greater rate than in soils deficient in lime. Clays 

 can be made to nitrify much faster if they are opened up 

 so that they admit air. The chief requirements for the 

 oxidation of nitiogenous matter in the soil are air, warmth, 

 moisture, and lime. Tillage and bulky manures will supply 

 more air to the soil, and control the water supply as well, 

 lyime may need to be added directly to the soil. When 

 the soil is closely packed, saturated with water, and air 

 excluded, denitrification may occur (p. 51). The fixation 

 of nitrogen in the soil by soil bacteria is facilitated by 

 a good supply of suitable organic matter, such as the straw 

 in farmyard manure, phosphatic manure, a good supply 

 of potash, and satisfactory conditions for the growth of the 

 bacteria. 



Soils and Fertilizers. —The relationship between the soil 

 and the fertilizer used is an important point that must be con- 

 sidered. To some extent this has already been discussed in 

 Part I. Generally speaking, lime is a necessity for the sound 

 working of any of the fertilizers, with the exception of basic slag 

 and calcium cyanamide, which both contain a certain amount 

 of lime. Soils that are very deficient in one of the ingredients 

 will respond specially to that particular ingredient at first, 

 but it not infrequently occurs that as soon as one has 

 satisfied the main need of the soil, a second order of necessity 

 makes its appearance. There are many soils whose chief 

 demand is phosphate, and very little good can be done to 

 such soils until phosphates have been supplied. Afterwards 



