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The Cornell Reading-Courses 



stands alone. Available nitrogen is derived from the decay of the organic 

 matter in the soil. The original supply of the element is the atmosphere, 

 of which it forms about eighty per cent. But this free nitrogen is not 

 available to the higher plants, and the operation of certain microscopic 

 plants in the soil, called bacteria, is essential to its collection and elabora- 

 tion into available forms. 



The plant-foods derived from the rock particles are termed mineral 

 elements and are used in different quantities by different plants. They 

 are arranged above in the order of the quantities used by plants. In 

 the soil particles they occur in various chemical combinations, and the 

 amounts present vary greatly in different soils. In an acre of soil a foot 



Fig. 8. — A clay soil in bad tilth (physical condition). Too lumpy and coarse 



deep, there is usually many hundred times as much of these plant-food 

 elements as is used by a single crop. Similar amounts occur in the subsoil. 

 The elements are taken up by plants in solution in the soil water. In 

 the soil particles they are very slightly soluble, and their availability is 

 largely controlled by the fineness of the soil, its permeability, the amount 

 of water present, the temperature and ventilation, and the amount of 

 humus and lime in the soil. These constitute a further reason for the 

 proper adjustment of the soil factory. The" use of commercial fertilizer 

 in a soil in poor condition would have small effect to make it fertile. This 

 is the reason for presenting the diagram on the first page, in which com- 

 mercial fertilizers are placed as the last instead of the first means usually 

 to be employed in order to increase crop yields. 



It is not usually possible to determine by chemical analysis the kind 

 of fertilizer needed by a particular soil , since such analysis gives no infor- 



