14 Diseases of Greenhouse Crops 



trosomonas and Nitrosocoscus. The nitrates are then 

 oxidized into nitric acid and nitrates, through the 

 work of the bacterium, Nitrobacter. The nitrates 

 are the only forms of nitrogen which plants can use. 



C. Action of Soil Flora on Mineral Sub- 

 stances. Inert mineral substances, like the organic 

 matter in the soil, must first be acted upon by cer- 

 tain soil bacteria to be converted to a form which 

 plants can readily assimilate. 



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 im- 

 proved by such fertilizers as superphosphate of lime, 

 ground bone, phosphate rock or Thomas slag. In 

 the process of decomposition of organic matter a 

 large quantity of carbon dioxide is liberated, which 

 unites with the water in the soil to form carbonic 

 acid. This acid attacks the insoluble phosphates, 

 transforms them into superphosphates — the only 

 form soluble in water, — and renders them available 

 to plant life. 



2. Changes in Potassium^ Sulphur^ and Iron. 

 The carbon dioxide and other organic acids pro- 

 duced during the fermentation of organic mat- 

 ter, attack the potash feldspar which occurs in 

 the soil. The product is potassium carbonate 

 which is soluble in water and hence readily taken 

 up by plants. The nitric acid which is formed dur- 

 ing nitrification may also combine with the raw pot- 



