CH. Ill] Determination of Organic Matter 41 



sound, for some of the mineral matter undergoes changes 

 on heating accompanied by alterations in weight. Nor 

 does the method discriminate between the undecom- 

 posed and the decomposed material which as we have 

 seen behave very differently in the soil. In some labora- 

 tories {e.g., at Rothamsted) the larger fragments of un- 

 decomposed material are removed by sifting and blow- 

 ing, and estimated separately. But more usually the 

 whole of the material is estimated together. 



Another method of discovering how much organic 

 matter is present in the soil is to determine the per- 

 centage of nitrogen (p. 228). This is important because 

 it gives an indication of the nitrogen reserves in the soil, 

 but again it tells us nothing about the state in which 

 these exist and whether they are useful or not. Table I 

 gives typical examples and shows that there is no very 

 clear connection between the productiveness of the soil 

 and the percentage of nitrogen or the loss of organic matter. 



Table I. Percentage of nitrogen and organic matter 

 m typical soils and subsoils 



