418 SOILS: PBOPJSBTim AND MANAGEMENT 



which the plants are grown.^ This is the way in which 

 it is generally believed that soluble fertilizer salts benefit 

 plant growth. Insoluble plant-food materials have a 

 similar, but less active, result because they do not increase 

 the concentration of the soil solution to as great an extent. 



339. Quantities of plant-food material removed by 

 crops. — The utilization of mineral substances by crops 

 is a source of loss of fertility to agricultural soils. In a 

 state of nature, the loss in this way is comparatively small, 

 as the native vegetation falls on the ground, and in the 

 process of decomposition the ash is almost entirely returned 

 to the soil. Under natural conditions, soil usually in- 

 creases in fertility; for, while there is some loss through 

 drainage and other sources, this is more than counter- 

 balanced by the action of the natural agencies of disin- 

 tegration and decomposition, and the fixation of atmos- 

 pheric nitrogen affords a constant, though small, supply 

 of that important soil ingredient. 



Wlien land is put under cultivation, a very different 

 condition is presented. Crops are removed from the 

 land, and only partially returned to it in manure or straw. 

 This withdraws annually a certain small proportion of 

 the total quantitv of mineral substances, but, what is of 

 more immediate importance, it withdraws all of this in a 

 readily available form. 



The following table, computed by V/arington, ^ shows 



1 Hall, A. D., Brenchley, W. E., and Underwood, L. M. 

 The Soil Solution and the Mineral Constituents of the Soil. 

 Philosoph. Trans. Royal Soc. London, Series B, Vol. 204, pp. 

 179-200. 1913. Also, Lyon, T. L., and Bizzeli, J. A. The 

 Plant as an Indicator of the Relative Density of Soil Solutions. 

 Proc. Am. See. Agron., Vol. 4, pp. 35-49. 1912. 



2 Warington, R. Chemistry of the Farm, pp. 64-65. Lon- 

 don. 1894. 



