I.] AMOUNT OF PLANT FOOD IN SOIL 21 



eventually reach the plant. In addition, we have 

 various salts going into solution in the acids used 

 for the analytical process, and these include pre- 

 cisely the substances that have already been indi- 

 cated as constituents of the ash of plants — amongst 

 metals, calcium, magnesium, potassium, sodium, with 

 iron and aluminium in quite disproportionate amounts; 

 sulphuric and phosphoric acids, chlorine and silica 

 supply the non-metals. Read as percentages some 

 of these amounts seem small enough, but they repre- 

 sent enormous quantities of material in the soil, as 

 will be realised when they are correlated with the 

 fact that the layer of soil at Rothamsted, nine inches 

 deep, which is taken for analysis, weighs, over the area 

 of one acre, rather more than two and a half million 

 pounds. Translating, then, the percentages into pounds 

 per acre, o-i per cent, of nitrogen becomes 2500 lb., 

 o-ii of phosphoric acid becomes 2750 lb., and the 

 potash rises to 6750 lb. ; also, these quantities are in 

 the surface soil only, without considering the lower 

 layers into which the plant roots penetrate freely. 

 A comparison of the materials in the soil with those 

 taken away by ordinary crops at once leads to results 

 which seem paradoxical ; the stock of plant food in 

 the soil is so much greater than any requirements 

 of the crop that further additions of the same stuff 

 in the shape of fertilisers would seem to be needless. 

 The accompanying table (III.) shows the amounts 

 of various materials per acre which are on the average 

 drawn from the soil by various crops at Rothamsted. 



Roughly speaking, an average soil contains enough 

 plant food for a hundred full crops, yet without fresh 

 additions of plant food as manures the production will 

 shrink in a very few years to one-third or one-fourth of 

 the average full crop. Once, however, the yield has 



