Normal Soil and Its Requirements 15 



plants can readily assimilate them. These too must 

 first be acted upon by certain soil bacteria. 



1. Changes of Phosphates. Phosphates as 

 they commonly occur in nature are but little soluble 

 in water. This is why they cannot be used in their 

 first form, although they are required by most plants. 

 Soils deficient in this element may be improved by 

 such fertilizers as superphosphate of lime, ground 

 bone, phosphate rock, or Thomas slag. In the pro- 

 cess of decomposition that organic matter must un- 

 dergo as it becomes available for plant food, large 

 quantities of carbon dioxide are liberated which 

 unite with the water in the soil to form carbonic acid. 

 This acid attacks the insoluble phosphates, trans- 

 forms them into superphosphates, the only form 

 soluble in water, and renders them available to 

 plant life. 



2. Changes in Potassium, Sulphur, and Iron. 

 Like phosphorus, potassium, sulphur, and iron are 

 made available for plants through the indirect action 

 of soil bacteria. The carbon dioxide and other 

 organic acids produced during the fermentation of 

 organic matter, attack the potash feldspar which 

 occurs in the soil. The product is potassium car- 

 bonate which is soluble in water and hence readily 

 taken up by plants. The nitric acid which is formed 

 during nitrification may also combine with the raw 

 potash in the soil forming potassium nitrate which is 

 a form available for plants. 



As a result of the activity of soil bacteria, hydrogen 

 sulphide is evolved from the decomposition of pro- 



