COMMERCIAL FERTILIZERS 



83 



is thought by many to enable the farmer or the soil expert to judge as to 

 the character of the fertilizer needed. This, however, is not the case, 

 and such chemical analyses are as a rule of very little help in this respect. 

 The chief difficulty with this method lies in the fact that such analyses 

 do not determine the availability of the plant food present. Another 

 method which is fairly satisfactory is to make pot tests with the soil in 

 question and for the crops to be grown. Such tests may frequently be 

 completed in a shorter period of time than can field tests. They are not, 

 however, so satisfactory as field tests because the crops are not grown 

 under field conditions. 



Effect of Fertilizers on the Growth of Sweet Clover. 



Soil from virgin cut-over land in Pennsylvania. 

 Ca — Lime. N — -Nitrogen. P — -Phosphorus. K — Potash. 



Effect Modified by Soil and Crop. — The fertilizer to be used is deter- 

 mined both by the needs of the soil and the crop grown. A commercial 

 fertilizer is beneficial chiefly because of the plant-food elements it supplies. 

 Its best action is accomplished when the soil is in good physical condition 

 and when there is a good supply of moisture and organic matter. The 

 effect of a fertilizer under one set of soil conditions may be reversed when 

 the conditions are materially changed. Under favorable conditions, 

 for example, nitrification in the soil might proceed with sufficient activity 

 to supply a certain crop with all the nitrogen needed for normal growth. 

 The following season being cold and accompanied by an excess of moisture 

 might result in slow nitrification, and this might materially diminish the 

 growth of the crop. In one case nitrogen in a readily available form 



