Plants as Affected by Insufficient Food. 151 



ammonium sulfate (sulfate of ammonia), and in organic 

 materials. The former is available as plant food when 

 it is dissolved in the soil water. It is best applied 

 immediately before the planting of a crop or in small 

 quantities at intervals during growth, since it is in dan- 

 ger of being washed out of the soil in drainage water. 

 Sodium nitrate is especially useful for garden crops 

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

 nitrification (256). 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. 



263. Phosphorus is used by plants in the form of solu- 

 ble phosphoric acid, which exists in the soil in combina- 

 tion 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 un- 

 less 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. 



264. Potassium is used by plants in the form of potash, 

 i. e. , potassium combined with oxygen. Potash exists in 

 the soil mainly in combination with chlorin (chlorid or 

 muriate of potash), with sulfur ic acid (sulfate of potash), 

 or with nitric acid (nitrate of potash). All these forms 

 of potash are freely soluble in water and are immediately 



