194 THE SOIL: INORGANIC MATTER 



for sodium chloride, and 0.5 per cent, for sodium sulphate. 

 On clay soils this tolerance is less. Crops vary in their 

 ability to withstand the alkali, alfalfa, sugar beets, and 

 radishes being better able to withstand it than the grains 

 and celery. 



Methods of reclaiming alkali soils are for the most part a 

 matter of rotation, culture, the growing of resistant crops, 

 prevention of evaporation, flooding, and underdrainage. 

 Black alkali can be remedied by the application of gypsum 

 or land plaster. This reacts with the sodium carbonate to 

 form calcium carbonate and sodium sulphate, the former 

 harmless and the latter relatively so. 



145. Summary. — With the exception of nitrogen, the 

 plant food elements are derived from mineral particles in the 

 soil, and besides the decomposition products of organic 

 matter, the gases in the soil have a solvent effect on these 

 mineral particles. The composition of soil gases is some- 

 what different from that of the air, in that oxygen varies 

 from 10 to 20 per cent, and carbon dioxide from 11 to 1 

 per cent. As oxygen is used up, carbon dioxide is evolved. 

 Methane and hydrogen sulphide are also present, due to 

 anaerobic decomposition of organic matter, but not to any 

 great extent. Carbon dioxide and oxygen are the most 

 important gases which serve as solvents for the minerals. 

 Soil solvents, besides carbonic acid and oxygen in solution, 

 are organic acids, inorganic acids, and soluble salts. 



Phosphoric acid is made soluble by the action of water 

 and carbon dioxide on apatite, or on tricalcium phosphate 

 derived from it. Potash is made soluble by the action 

 of water and carbon dioxide on orthoclase feldspar, pro- 

 ducing at the same time kaolinite which is a hydrated 

 aluminium silicate. The latter forms the basis of clay. 

 Water and carbon dioxide also render soluble some potash 

 from mica. Calcium is derived from limestone or dolomite, 

 being dissolved in the form of the bicarbonate by the action 

 of water and carbon dioxide. Limonite is a source of iron. 

 Quartz is an important constituent of all soils, being 

 the basis of sand, although, of course, it does not contain 

 an essential element. 



