Plants as Affected by Insufficient Food. 157 



started early in the spring, when the soil is too cool 

 for active nitrification (255). The surface soil is apt 

 to be poor in nitrates in spring, because they are often 

 washed down by the autumn and winter rains. 



Ammonium sulfate is changed to nitrates in the soil 

 before it is used by plants, and hence is less prompt in 

 its action than sodium nitrate. It is more tenaciously 

 held by the soil than scdium nitrate and is therefore 

 less likely to be lost by washing. 



262. Phosphorus is used by plants in the form of 

 soluble phosphoric acid, which exists in the soil in 

 combination with lime, iron and alumina, as phosphates 

 of these substances. It may be purchased in the form 

 of mineral phosphate of lime, ground bone, wood ashes, 

 odorless phosphates, etc. The first two are insoluble 

 in water unless treated with a strong acid, when they 

 are known as acid phosphate or superphosphate. 

 Phosphoric acid is not readily washed out of the soil, 

 even in its soluble form. 



263. Potassium is used by plants in the form of pot- 

 ash, i. e., potassium combined with oxygen. Potash ex- 

 ists in the soil mainly in combination with chlorin 

 (chlorid or muriate of potash), with sulfurie acid (sul- 

 fate of potash), or with nitric acid (nitrate of potash). 

 All these forms of potash are freely soluble in water and 

 are immediately available as plant food. Nitrate of 

 potash (saltpeter) is a most valuable fertilizing mate- 

 rial, since it contains both potash and nitrogen, but 

 unfortunately its price is too high to permit of its use 

 for this purpose. The muriate, either pure or in crude 

 form (kainit), and sulfate may, on the other hand, be 



