SOIL-PLANT RELATIONSHIPS AND PLANT NUTRITION 375 



Calcium usually far outweighs the other bases, and calcium and magnesium 

 together may account for 90-95 per cent of the total exchangeable bases. In 

 a soil with a total exchange capacity of 15 milliequivalents per 100 g., with 

 calcium accounting for 60 per cent of the exchangeable bases, there would 

 be 3600 pounds exchangeable calcium per acre to the depth of 6 inches. If 

 potassium accounted for 3 per cent of the exchangeable bases, there would be 

 approximately 350 pounds of exchangeable potassium in the same volume of 

 soil. These amounts are far more substantial in relation to the amounts taken 

 up by an acre planting than are the quantities found in the soil solution at 

 any one time. However, a total analysis of the soil would show much larger 

 amounts of such elements as magnesium and potassium, and although the 

 nonexchangeable form is not directly available to plants, these nutrients are 

 slowly released to replenish the supply of the exchangeable form as this is 

 depleted by continual removal of vegetation. This is particularly important 

 in the case of potassium, which often forms the major inorganic component 

 of plant tissue and yet which is not ordinarily a major exchangeable base. 

 The chemistry of soil potassium indeed presents some unusual features, which 

 have been much studied, because even drying and subsequent wetting of 

 clays has been found to change the partition of potassium between exchange- 

 able and non-exchangeable forms. 



Much more could be said about the properties of the clay and organic 

 colloids of soils, but it is sufficient here to indicate that the salient and out- 

 standing difference between soils and nutrient solutions as environments for 

 the growth of plants lies in the form of presentation of cations, the major 

 ones, such as potassium and calcium, and minor ones, such as iron, manganese, 

 zinc, and copper. 



However, not all the major nutrient ions are cations. There is yet to be 

 accounted for such ions as nitrate, phosphate, sulfate, and borate. Nitrogen 

 availability in soils ordinarily depends on a sequence of microbiological trans- 

 formations. The soil nitrogen reserves are wholly organic, and the rate of 

 release therefrom is controlled by the activity of an array of organisms, mostly 

 unspecialized. The primary end product is the ammonium ion, which is 

 retained by clay and organic colloids, as are the other bases, and which can 

 be exchanged with root surfaces and utilized directly by some plants. 



However, microbial conversion to nitrate ordinarily takes place; it is 

 usually not possible to find more than a trace of ammonia in soils. The rate- 

 limiting step is the liberation of ammonium from the organic reserve, not 

 the oxidation of ammonium to nitrate. The nitrate ion, accompanied by a 

 cation from the exchangeable bases, is found in the soil solution. It does not 

 appear to be retained in any way by the clay colloids. Soil clays do exhibit 

 anion-exchange reactions, somewhat similar to cation-exchange reactions, 

 but at a lower level quantitatively, and probably of significance only with 

 such anions as phosphate and molybdate. The sulfate and chloride ions, like 



