jan. 15, 1921 Effect of Crops on Water Extract of Soil 667 



There is, therefore, no proof here, either as to the rate of translocation 

 or the distance through which the soil solutes may move by diffusion. 

 But since the losses of concentration of the soil solution appear to be 

 somewhat proportional to root penetration, it would seem probable that 

 the rate of movement of solutes through the soil is less than the rate of 

 growth of the roots of normal barley and beets. 



CONCLUSIONS 



The gain in specific resistance and the decline in nitrate content of the 

 water extracts of soils planted to different crops warrant us in extending 

 the conclusions heretofore drawn from observations of the effects of bar- 

 ley. It is clear that the phenomena noted are not peculiar to the barley 

 plant but are characteristic of all the plants tested and probably apply 

 to all chlorophyll-bearing plants which root in the soil. The extent of 

 the reduction of concentration observed is variable with different crops. 

 We may not put too much stress upon the magnitudes of these differences, 

 however, because of the obvious differences in growth habits and life 

 history of the plants considered. It is interesting to note, however, that 

 corn which is commonly regarded as a "gross feeder" in ordinary fer- 

 tilizer practice has increased the specific resistance of the water extracts 

 more rapidly and completely than the other plants. 



The second experiment sheds little light on the rate of movement of 

 solutes toward the plant roots. Inasmuch, however, as reductions in 

 concentration of water extracts of soil at a distance from growing plants 

 did not take place until that portion of the soil had become filled with 

 roots, it would seem that rapid and extensive movements of soil solutes 

 are probably not an incident of normal plant absorption. 



SUMMARY 



(1) The effect of crops of corn, horse beans, potatoes, turnips, and bar- 

 ley upon the water extract from a typical silty clay loam was studied 

 throughout the growing season. 



(2) All the crops discussed in this paper reduced the concentration of 

 the water extract during the height of the growing season. 



(3) The nitrate content of the soil was reduced to a very low figure 

 by all crops. 



(4) An experiment in which the concentration of the soil solution was 

 studied by means of observations of freezing-point depressions in the 

 immediate vicinity and at a distance from the plants showed that con- 

 centrations are not significantly reduced until the portion of the soil 

 sampled is filled with plant roots. 



