164 THE CHEMICAL ANALYSIS OF SOILS [chap. 



removed by the crop; even the minimum of o-oi per 

 cent., soluble in citric acid, which has been suggested, 

 as marking the limit of fertility, means about 250 lbs. 

 per acre in the surface layer 9 inches deep : and few 

 crops will take away as much as 50 lbs. per acre of phos- 

 phoric acid or 150 lbs. per acre of potash. No crop 

 searches the soil so thoroughly for food as does the 

 solvent acid : if we assume that the roots themselves by 

 their excretion of carbon dioxide effect some of the 

 solution, it is obvious that they come in contact with but 

 a small proportion of the soil particles ; nor can the soil 

 water, limited in amount and moving slowly, attack the 

 soil with the vigour displayed by an acid which is 

 continuously shaken with a comparatively small propor- 

 tion of soil. Even in the case of material so essentially 

 * available " as a manure soluble in water, the whole of 

 the manure applied is never recovered in the crop ; e.g., 

 in the experiments with wheat at Rothamsted, only 73 

 per cent, and with mangolds 78 per cent, of the nitrogen 

 supplied as nitrate of soda has been recovered in the 

 crop, though there was an abundant supply of the other 

 manurial constituents. In the same way, on the plot 

 with an excess of nitrogen there was recovered only 

 36 per cent, of the phosphoric acid supplied as super- 

 phosphate, and 50 per cent, of potash supplied as sulphate 

 of potash. 



In other words, the "available" plant food in the 

 soil represents not that which the succeeding crop will 

 remove, but that which it can draw upon : how much 

 it will acquire will depend on a variety of factors, such 

 as the nature of the plant, the texture of the soil, 

 the supply of water, and other necessaries of nutrition. 



No method akin to solution in dilute citric acid 

 has yet been devised for determining what proportion 

 of the nitrogen reserves in the soil is likely to be avail- 



