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 sodium 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 com- 

 bination 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 sulfuric 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 



