FOOD OF PLANTS IN SANDY SOILS AND LOAMS. 141 



though the latter soil should contain a much larger 

 amount of organic substances. 



The ' absorption numbers ' of two different arable 

 soils afford no criterion for determining the quality of 

 the soil or the amount of nutritive substances which it 

 contains ; they merely tell us that, in the one soil, the 

 elements of the food of plants will spread beyond cer- 

 tain places, further than in the other ; that the one soil 

 opposes greater obstacles to their diffusion than the 

 other. The farmer, in learning the strength of these 

 obstacles, finds out by experience whether they exert a 

 beneficial or adverse influence upon the cultivation of 

 his fields, and ascertains the means of removing the 

 injurious or strengthening the beneficial influences. 



On comparing a fruitful sandy soil with an equally 

 fruitful loam or marl, as regards the nutritive sub- 

 stances contained in them, we are surprised to find that 

 the sand with one-half or even one-fourth, of the total 

 substances contained in the loam, will furnish an 

 equally rich harvest. To understand this circumstance 

 properly, we must remember that the nutrition of a 

 plant depends less upon the quantity, than upon the 

 form of the nutriment in the soil ; just in the .same 

 way as, for example, half an ounce of animal charcoal 

 presents as large an acting surface as a pound of wood 

 charcoal. If the smaller quantity of nutritive sub- 

 stances in the sandy soil presents as large a surface for 

 absorption as the larger quantity of those substances in 

 the loam, the plants must thrive as well upon the 

 former as upon the latter. 



If a cubic decimetre of a fruitful loam is mixed 

 with 9 cubic decimetres of silicious sand, so that every 

 particle of sand is surrounded with particles of loam, 

 as many root-fibres and particles of loam will come 

 into contact in the mixed as in an equal volume of the 

 unmixed soil ; and if all the particles of loam can yield 

 the same nutriment, plants will receive from the mixed 

 just as much as from the unmixed soil, though, on the 

 whole, the latter is ten times richer. 



All fruitful sandy soils consist of a mixture of sand 



