SOME CONCEPTIONS OF THE SOIL 11 



fixation processes, especially those of a biological nature, is 

 a feature of practical soil improvement. 



Phosphorus has its origin in the mineral apatite (Ca 5 - 

 (P0 4 ) 8 (C1,F)) and exists in the soil not only in this form 

 but as tri-calcium phosphate (Ca 3 (P0 4 ) 2 ), iron and alum- 

 inum phosphates (FeP0 4 and A1POJ and in certain other 

 inorganic complexes. It also exists in organic combinations 

 of a constantly varying nature. It probably is utilized by the 

 plant as a simple phosphate such as the mono- or di-calcium 

 salt (CaH 4 (P0 4 ) 2 and Ca 2 H 2 (P0 4 ) 2 ). 



Potassium, as already stated, occurs in the soil in orthoclase 

 and microcline (KAlSi 3 8 ), in mica, especially muscovite 

 (H 2 KAl 3 Si 3 12 ), and in other aluminum silicates, both hy- 

 drated and non-hydrated. These complex forms supply potash 

 to the soil solution and thus to the plant at a more or less 

 rapid rate in the bicarbonate, carbonate, chloride, nitrate, and 

 sulfate forms. 



Calcium, while necessary in the soil as a nutrient, also 

 functions as an amendment in that it seems to preserve a 

 proper soil reaction. It is possible that this relationship is as 

 much nutritive as strictly chemical. Calcium exists in the soil 

 in many minerals, of which calcite, plagioclase feldspar, horn- 

 blende and augite are perhaps the most important. It is 

 carried as an absorbed compound by kaolinite and similar 

 materials. Calcium becomes available in the soil as the ni- 

 trate, bicarbonate, chloride, phosphate, and sulfate. 



Sulfur is found in the soil in rather small amounts and 

 generally forms a part of the organic matter. Inorganically 

 it usually occurs as a sulfate combined with the common 

 bases. In this form it is available to plants. The original 

 source 1 of most of the soil sulfur has been pyrite (FeS 2 ), the 



1 Considerable sulfur is brought to the soil in atmospheric precipita- 

 tion. From 5 to 150 pounds an acre a year have been reported. Wilson, 

 B. D. Sulfur Supplied to the Soil in Bain Wacer, Jour. Amer. Soc. 

 Agron., Vol. 13, No. 5, pp. 226-229. 1921. 



