SOILS AND FERTILIZERS 381 



amounts, they are unable to give corresponding increases in the 

 yield. On the contrary, the yield from these arid soils under irri- 

 gation is no greater than that obtained from the soils of the humid 

 regions, which contain far less of these mineral plant foods, pro- 

 vided the season in the humid region is just right, or that irriga- 

 tion be practiced. Yields of wheat as high as 50 or 60 bushels per 

 acre have been recorded in our Eastern States, and, as far as rec- 

 ords go, such yields have never been exceeded under irrigation in 

 the arid regions where most favorable moisture conditions are main- 

 tained and the soluble salt content of the soils is much greater than 

 in the humid East. 



Exhaustive investigation of many types of soil by very accurate 

 methods of analysis, under many conditions of cultivations and of 

 cropping, in areas yielding large crops and in adjoining areas yield- 

 ing small crops, has shown that there is no obvious relation between 

 the amount of the several nutritive elements in the soil and the 

 yield of crops; that is to say, that no essential chemical difference 

 has been found between the solution produced in a soil yielding a 

 large crop of wheat and that in a soil of the same character in ad- 

 joining fields giving much smaller yields. The conclusion logic- 

 ally follows that on the average farm the great controlling factor 

 in the yield of crops is not the amount of plant food in the soil, 

 but is a physical factor the exact nature of which is yet to be deter- 

 mined. 



The results of the investigations also seem to indicate that the 

 actual quantity of water a soil can furnish the plant, irrespective 

 of the percentage of water actually present in the soil, has prob- 

 ably a very important influence on the yield. When the supply 

 of water is inadequate to the need of the plant, as the water is a 

 medium for the conveyance of nutritive solutions to the plant, it 

 may well indicate not only a deficiency in the supply of moisture, 

 but also of nutrient material contained in the water. It may be, 

 moreover, that with an insufficient water supply, and consequently 

 an insufficient food supply, fertilizers, by temporarily increasing the 

 concentration of the solution, may increase the food supply of the 

 plant; but as in this case also the obstructive factor is the purely 

 mechanical function of delivering water from the soil to the plant, 

 the problem is one to be handled by physical methods, and can not, 

 in the nature of the case, be solved by chemical methods alone. 



Again, while some variations occur in the composition and 

 concentration of the soil solution, in the case of the great majority 

 of cultivable soils these variations are within comparatively narrow 

 limits. The nature of the solution is but seldom permanently af- 

 fected by the addition of ordinary mineral fertilizers, and it seems 

 safe to say, therefore, that the concentration with respect to the 

 mineral plant food constituents per unit of solution is approxi- 

 mately constant. Considering the wide variations in the percent- 

 ages of water present in different soils, however, the amounts of 

 dissolved plant foods in them may be quite different. 



