FOOD OF PLANTS IN SANDY SOILS AND LOAMS. 139 



learning the strength of tliese obstacles, finds out by 

 experience whether they exert a beneficial or adverse in- 

 fluence 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 substances 

 contained in them, w^e 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 nutri- 

 ment 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 w^ood charcoal. If the smaller 

 quantity of nutritive substances 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 mth 9 

 cubic decimetres of silicious sand, so that every particle 

 of sand is sm-rounded with particles of loam, as many 

 root-fibres and particles of loam wiU come mto contact in 

 the mixed as in an equal volume of the unmixed soil ; 

 and if all the particles of loam can yield the same nutri- 

 ment, 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 fruitfrd sandy soils consist of a mixture of sand with 

 more or less clay or loam ; and as sihcious sand has a very 

 limited power of absorbing potash and the other mineral 

 constituents of plants, the ingredients of the sup[)lied 

 manure, which have become soluble, spread sooner and 



