SOILS AND FERTILIZERS 315 



potash than of the chlorine. If some silver nitrate is passed slowly 

 through finely divided charcoal, the salt is decomposed by this selec* 

 tive absorption, some free nitric acid comes through, and small span- 

 gles of silver can be seen in the charcoal. 



On account of this absorbing power it is altogether unlikely that 

 fertilizers applied to soils in ordinary amounts will noticeably increase 

 the concentration of free water in the soil. The material for these 

 minerals may come from great distances or from great depths, as a 

 result of the slow but profound movement of ground waters. They 

 form a very important part of the soil fertility. 



Every one knows how important it is to have a plentiful supply 

 of moisture for the growing crop, but few realize the value of organic 

 matter in maintaining the supply in time of drought. The organic 

 matter acts as a sponge. Its effects upon moisture content are evi- 

 dent in two ways, (1) in the increased storage capacity and absorb- 

 ent power for water, and (2) in preventing evaporation by its re- 

 tentive power and by allowing a more perfect surface mulch than 

 could otherwise be formed. Sand permits of rapid percolation with 

 comparatively small amounts of water retained. If different amounts 

 of organic matter are added to the sand, the quantity of water re- 

 tained increases with the increase of organic matter. 



Besides this holding back of nutrient soluble salts, the absorp- 

 tive power of a soil has an important part in determining its physical 

 condition. There is a relation between the quantity of soluble salts 

 absorbed by the soil and the size and compactness of the soil-grain 

 aggregates; and upon the size and arrangement of these soils aggre- 

 gates, in turn, the optimum water content of the soil depends. The 

 openness of a soil determines its ventilation, and thus likewise its 

 opportunity for securing proper oxidation and for the forming of 

 humus. The optimum water content as determined by plant growth 

 coincides in general with the percentage of moisture at which the 

 soil has its maximum volume, and at this maximum the soil also 

 shows its greatest openness or minimum resistance to root penetration. 

 Likewise, through its absorptive power the soil removes from soil 

 solution and soil atmosphere the waste products thrown off by grow- 

 ing roots, which products are detrimental or inhibitory to continued 

 growth. 



The absorption of nutrient salts by the soil not only maintains 

 a proper soil solution, but also exercises a marked influence upon the 

 size of the soil aggregates. This soil structure in turn governs the 

 water-holding power, the porosity, and consequently the aeration of 

 the soil ; and these factors are of the utmost importance to the bac- 

 terial life in the soil, as well as to the changes in the organic and 

 mineral constituents. (Bu. Soils B. 32, 51, 52, 55.) 



Texture of Soils. The texture of the soil is determined by the 

 proportion of the different sized mineral particles it contains. The 

 texture determines to a large extent the relative power of soils to 

 hold water. Under equal conditions of depth and exposure coarse- 

 grained soils hold less moisture and more air than fine-grained soils, 

 so that taken as a whole they contain less solution, and consequently 



