UNFAVORABLE FOOD SUPPLY 149 



materials. The former is available as plant-food when 

 dissolved in the soil water. It is best applied imme- 

 diately before the planting of a crop or in small quan- 

 tities at intervals during growth, since it is in danger of 

 being washed out of the soil in drainage water. Sodium 

 nitrate is especially useful for garden crops started early 

 in the spring, when the soil is too cool for active nitrifi- 

 cation (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, and the like. 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 (sulfate 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 material, since 



