Manurial Needs and Resources. 149 



Nitrogen. 



may be applied in the form of complex organic compounds, such 

 as are present in animal and vegetable matter, as already described ; 

 in the form of ammonium compounds, or, lastly, in the form of 

 nitrates. Organic nitrogenous substances are contained in kraal 

 manure, guano, various oil cakes, human excreta, animal matter of 

 all kinds, blood, offal from butchers, bones, and similar refuse. 

 Before such substances can be utilised by plants, it is necessary, as 

 already stated, that they undergo the process of nitrification, and 

 therefore that the soil contain some carbonate of lime. Ammonium 

 salts, e.g., sulphate of ammonia, have also to be nitrified before they 

 can be absorbed into the plant. Moreover, the acid of the ammonium 

 compound has to be combined with some base from the soil, so that 

 with these manures, the presence of much carbonate of lime in the soil 

 is essential. 



Nitrates, on the other hand, are directly assimilated, and have 

 not to undergo any previous change. They, however, are not retained 

 by the soil, as are ammonium salts and most other manures, and 

 therefore should not be applied until the plant is well rooted and 

 can absorb them. 



Phosphoric Acid. 



This may be applied in three forms. 



1. As tricalcium phosphate, e.g., in bones. 



2. As acid calcium phosphate, e.g., in superphosphate. 



3. As tetra-calcium phosphate, e.g., in basic slag. 



The first has the disadvantage of being insoluble in water, 

 therefore difficult to distribute through the soil. It is slow in its 

 action, as it is only absorbed by the plant after it has come into 

 solution in the soil water by action of carbonic and other acids 

 produced by chemical changes in the soil or plant. The second form 

 of phosphatic manure has the advantage of being easily soluble in 

 water, and therefore quickly distributes itself throughout the soil. 

 It is, however, soon converted by the calcium carbonate into the first 

 form. Nevertheless it is readily available to the plant, because of its 

 fine state of sub- division and good distribution throughout the soil. 

 This form of phosphatic manure can only be used successfully on 

 soils containing a fair amount of carbonate of lime. The third form 

 of phosphatic manure, though insoluble in water, is much more readily 

 soluble in saline solution than the first, and, if applied in a sufficiently 

 fine state of sub-division, is rapidly assirnilated by plants. Basic 

 slag contains free lime, in addition to its tetra-calcium phosphate, and 

 is especially suited to soils poor in lime. 



It gives excellent results on soils rich in organic matter, probably 

 because the free lime in it promotes nitrification, while the vegetable 

 acids resulting from the decay of the organic matter aid in the 

 solution of the phosphoric acid. 



