The Soil 55 



an effective means of rendering the surface soil more pervious 

 to water. 



The wettest soil does not always supply the largest amount of 

 water to a crop. A peaty soil holds most water, but it is held 

 so firmly by the colloid matter as to be unavailable to plants. 

 A stiff clay fails in a drought as the water in this class of soils 

 is also firmly held and moves with difficulty. Soils composed of 

 silt or extremely fine sand are those which yield water most 

 effectually to a growing crop. 



Chemical changes occurring in soils. The chemical changes 

 going on in soil are numerous and complex. The mineral matter 

 is subjected to the same influences as led to its breaking down in 

 the formation of soil from the original rock. These changes are, 

 however, hastened because of the great quantity of carbon di- 

 oxide produced by the decay of organic matter. Fragments of 

 feldspar are decomposed with formation of silicic acid, potassium 

 carbonate and kaolin or clay. The clay remains behind, but the 

 silicic acid and potassium carbonate may in part be dissolved and 

 either carried away in the drainage, or may be absorbed by the 

 roots of plants or by some of the absorptive constituents of soils. 

 Calcium carbonate or limestone is dissolved by water containing 

 carbon dioxide, which is true of all soil waters, and is in part 

 carried away in the drain or absorbed by certain soil constituents. 



Calcium phosphate, as it exists in minerals, is nearly insoluble 

 in water, but through the action of the soil water containing 

 carbon dioxide in solution, it is changed to more soluble forms 

 and therefore becomes available to plants. In contact with cer- 

 tain forms of iron and aluminum in the soil the soluble calcium 

 phosphates may be changed to iron and aluminum phosphates 

 and held back in the soil in finely divided condition, and though 

 then quite insoluble in water, may still be dissolved by the acid 

 juices of the plant's roots. 



Absorption of soluble plant food by soils. If the plant food 

 made soluble by the chemical chang< s occurring in soils were not 



