CHAPTER IX 

 MANURES 



THE food-constituents in soil may, from one point of view, be 

 roughly divided into two classes, according to the condition in 

 which they are present, namely, those which are soluble in water 

 or in very weak acid, and which are, therefore, immediately 

 available for absorption by the roots of plants ; and those which 

 are insoluble, becoming available for food only gradually, after 

 being converted into soluble compounds by the slow processes of 

 chemical change occurring under the action of water, air, carbon 

 dioxide, etc. We have no means of ascertaining with certainty 

 the amount of readily available food-supplies in a soil, though 

 approximate values may be obtained in some cases by determining 

 the quantities which are dissolved by a weak solution of citric 

 acid. Only a portion of this available food is ever removed by 

 any crop : the roots do not penetrate every space where the food 

 exists, and even if they did, their power of absorbing it would be 

 arrested or counterbalanced by other forces long before they had 

 entirely depleted the soil of the soluble salts in it. Indeed, an 

 excess of available food over and above the actual requirements 

 of the plant is essential to the well-being of that plant ; and plants 

 differ materially as to the magnitude of the excess which is neces- 

 sary for their welfare : two different plants may remove about 

 the same amount of food-material from the soil, but the one will 

 not flourish unless there is present a larger amount of such material 

 than that which would produce an excellent crop of the other. 

 It is not merely a question of the amount of food present, but of 

 the power of the plant to reach it and assimilate it. 



The simpler question of the amount of readily available food 

 in the soil is complicated by the gradual change of the materials 

 from the insoluble into the soluble form, and the rate of this 

 change, besides depending on physical circumstances, must 

 depend to a considerable extent on the rate at which the soluble 

 constituents are removed, and, hence, on the nature of the 

 crops grown in the soil. Judging by the results with wheat in 



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