10 



SOIL FERTILITY, 



coming within the different limits of the standard previously 

 referred to, are as follows : — 



These figures show the amount of phosphoric acid and 

 potash in the soils examined which by weathering agencies 

 may become available to crops. 



The organic matter of the soil is a very important 

 constituent owing to the very great influence it has upon 

 the chemical, physical, and biological conlition of the 

 soil. 



In connection with organic matter, in soil analysis 

 determinations of the " loss-on-ignition ' (volatile and 

 organic matter), humus, and nitrogen, are generally made. 

 The loss obtained by igniting a soil, includes chemically 

 combined water and sometimes carbon dioxide from carbon- 

 ates, and therefore does not represent organic matter only. 

 If the humus (the organic matter most readily available 

 for crop use) be determined, no information concerning the 

 amount of total organic matter is obtained. The estimation 

 of the total nitrogen in the soil includes that from nitrates 

 and the nitrogen of all the organic matter, but from this 

 determination the amount of nitrogen which is available, 

 or which will soon become available, cannot be estimated. 

 Some investigators report both the percentage of total 

 soil nitrogen and of nitrogen in the humus. The following 

 extract from Hilgard* is given as bearing upon this matter — 



" It thus appears that on the average the humus of the arid 

 soils contains about three and a half times as much nitrogen 

 as that of the humid ; that in the extreme cases, the differ- 



*Soils. E. W. Hilgard. 



