96 SOILS AND FERTILIZERS 



quantity of available plant nutrients, and the greater the 

 productiveness of the soil is likely to be. Hence, it is 

 desirable to conserve the supplies of these substances 

 and to augment them, if possible, by their judicious ap- 

 plication in the form of farm manures and other fertilizing 

 materials, and especially to maintain the store of organic 

 matter. 



112. Water-soluble matter in soil. — Although soil is 

 very slightly soluble in water, an extract of soil made with 

 water contains all of the substances required by plants. 

 The solution obtained by extracting a soil with water is 

 probably not identical in composition or concentration with 

 the solution presented to the root-hairs of plants for their 

 nourishment, because the plant by the excretion of carbon 

 dioxide, and possibly in other ways, aids in dissolving plant 

 nutrients. It is probably true, however, that the solution 

 obtained by water is the nearest approximation that we have 

 to the solution presented to roots and is, for that reason, 

 deserving of attention. 



113. Relation of water-soluble matter to productiveness. 

 — It might be expected that there would be a direct relation 

 between the productive capacity of a soil and the quantities 

 of plant nutrients in its water extract, and that this relation 

 would hold between different soils. This would imply that, 

 as between two or more soils ; the plant-food materials dis- 

 solved by water would, in general, be proportional to the 

 quantities of the readily available constituents in the soil. 

 It has been demonstrated that such relations do obtain 

 between certain soils, but it has not been proven that this 

 is invariably the case. Indeed it is probable that soils which 

 differ little in their productivity would not, in every instance, 

 show such a direct proportional relationship. Experiments 

 with four good and four poor soils showed the following 

 averages for their crop yields and water extracts. 



