456 ANNUAL REPORT OF THE Off. Doc. 



take up their food iu a soluble condition, hence changes must take 

 place iu these substances before they can serve as plant food. The 

 plant roots themselves assist to some extent in causing this change, 

 thus rendering the constituents soluble, still, the great factor in 

 this change is the action of those natural forces, water, sun and air. 

 Water has a solvent effect upon the constituents of soils, and unless 

 it can freely penetrate and reach every portion of the soil its power 

 is very much reduced. That is, if the soils are so compact and dense 

 as to ])revent the free penetration and movement of water in 

 the soil, the plants are not able to readily obtain their food, and 

 thus make maximum growth and development, because enough is not 

 distributed throughout those soil layers to which the plant roots are 

 limited. On the other hand, soils that are too open and porous, 

 tbat are made up of rather coarse particles of mineral substances, 

 as, for example, sandy soils, made up chiefly of quartz, the water 

 penetrates and moves too freely, and the dissolved constituents 

 that are present are too readily carried away from those parts of 

 the soil in which the plants obtain their food, thus again not pro- 

 viding the best condition for its appropriation. 



Furthermore, the reservoirs of water under these soils, or in the 

 subsoils, and which are also extremely essential, are rapidly de- 

 pleted, as the water escapes very freely from the soil in the form 

 of vapor, and this prevents the plants from obtaining so large a 

 proportion of the water that is stored in the earth, as if the condi- 

 tions of soils were such as to prevent the free escape from the sur- 

 face. Between these two extremes of soils, first, those with very 

 finely divided particles, making the soil too compact and thus im- 

 pervious to water; and second, those that are made up of coarse 

 particles', we may have a mixture of the two, which is probably well 

 represented by what is understood as a loam, which possesses in 

 part the properties of each in such a degree as to make the move- 

 ment of the water more nearly perfect in respect to its supply to 

 plants. That is, we obtain the solvent effect of the water, as well 

 as the free movement of it, which contributes both to the solution of 

 constituents and their movement throughout the soil where needed. 

 It, therefore, frequently happens that from soils reasonably rich 

 or even very rich in chemical constituents, results are obtained 

 that are far below what might be expected from the composition of 

 the soil, because their physical character is such as to prevent the 

 movement of water, and consequently the necessary changes which 

 make it possible for the plant to obtain its food. These considera- 

 tions are also true in a degree in reference to the action of the 

 other natural agencies, air and warmth. Only in the case of those 

 soils which possess good proportions of the various substances mak- 

 ing up soils, do we receive the full benefit of Nature's aids, 



