PHOSPHORUS DETERMINATIONS 359 



Nitrogen and Organic Matter. Nitrogen is an index to the 

 supply of organic matter. When the percentage of nitrogen is 

 high, the organic matter is abundant; when very low, the need of 

 organic matter becomes evident. Many highly productive silt 

 loams, for example, contain 0.25 per cent and more of nitrogen, 

 while many others, particularly the lighter-colored types, contain 

 about 0.15 per cent. Frequently the nitrogen content of certain 

 long-cropped silt loams is as low as 0.09 per cent. Such a low 

 percentage, together with field observations, generally indicates 

 the urgent need of more nitrogen and organic matter. Thus, 

 special manuring may be recommended, the plowing under of 

 some legume as a green manuring crop, and the growing of more 

 and better clover. Whenever possible, it is instructive to compare 

 the nitrogen content and organic matter of any long-cropped soil 

 with a virgin sample of the same soil collected along the fence. 

 Such comparisons usually support the reasons commonly given 

 why crop yields are much less now than in former years. 



Phosphorus determinations are not only interesting but of 

 much value, since this element is regarded by many as the key to 

 the maintenance of soil fertility. Highly productive silt loams 

 commonly analyze 0.09 to 0.12 per cent, or more, of phosphorus. 

 Some long-cropped silt loams contain no more than 0.03 per cent. 

 Practically all low determinations of phosphorus indicate the need 

 of phosphate fertilizers. When examining into the phosphate 

 needs of any particular soil, it is also important to take into con- 

 sideration soil acidity, the fertility factors, the cropping history 

 of the fields, and the character of the crops grown. 



Some silt loams, for example, are comparatively well supplied 

 with phosphorus, as high as 0.08 per cent, but because they are 

 deficient in active organic matter, or because of poor aeration, 

 they respond to soluble phosphate fertilizers (Fig. 126). Again, 

 most acid, black silt loams are in need of phosphates even though 

 they may contain fairly good supplies of phosphorus (see index) 

 (Fig. 137). 



It is important to know that certain soils are well supplied 

 with phosphorus, because this fact will decide whether it is neces- 

 sary to rely wholly upon commercial fertilizers for the source of 

 phosphorus, or to adopt such measures as to render available the 

 phosphorus already present in the soil. During the improvement 

 of such soils by the addition of organic matter and by liming, 

 it is usually necessary to supply available phosphorus in the form 



