42 THE PHYSIOLOGY OF PLANTS 



by the addition to the soil of a manvire containing phosphates, 

 the more insoluble ammoniacal magnesium phosphate is 

 formed. In stables the giving off of ammonia is often miti- 

 gated by strewing gypsum, the sulphuric acid of which com- 

 bines with the ammonia to form the ammonium sulphate. 

 The same effect may be more speedily and more completely 

 attained by the addition of kainit ; and a similar action to 

 that of the magnesium sulphate of the kainit would be pro- 

 duced by sulphate of iron (green vitriol). 



In light soils, therefore, in which there is little humus, clay, 

 or other absorptive substances, manuring with kainit would 

 enrich the soil by causing the ammonia, which becomes 

 volatilised in the decomposing manure, to be fixed and re- 

 tained in the soil. The common salt which is also present 

 acts as a solvent and distributing agent for other salts. 



(&.) Fhosphoric Acid. 



This very important nutritive substance, which counter- 

 balances the excessive addition of nitrates, is immediately 

 absorbed in the soil, if in solution, by the heavy metals and 

 the earths, as these substances will at once combine to form 

 insoluble phosphates. The acid is then generally found com- 

 bined in the form of calcium and magnesium phosphates, and 

 in smaller quantities as iron and aluminium phosphates. The 

 richer, therefore, a soil is in the first place in carbonates of 

 lime and magnesium, the more we shall be able to saturate 

 it with phosphoric acid. And the same effect will be pro- 

 duced by those silicates (sesquisilicates) which yield on decom- 

 posing hydrated aluminium oxide and iron oxide. If lime 

 is present in the soil in the form of gypsum, it can take up 

 phosphoric acid, gradually giving off sulphuric acid as it does 

 so. The phosphoric acid is only available to the roots 

 directly in the form of its soluble combinations with alkalies 

 (potassium and sodium). 



(c.) Nitrogen. 



This substance, as important as potassium to plant life, 

 represents 79 per cent, of the air we breathe, the remaining 



