Sept 9, x 9 x« Deep Tilling and Dynamiting in the Great Plains 483 
One cycle of wetting and drying overcomes the effect of cultivation. 
As Mosier and Gustafson (4) say— 
The subsoil ran together as soon as it was wet and became approximately as it was 
before. 
It is frequently stated that deep tillage prevents run-off by facilitating 
penetration. Soil-moisture studies show that in the Great Plains pene¬ 
tration and run-off are determined more by the condition of the imme¬ 
diate surface than by the depth to which the soil has been cultivated. 
Run-off in the Great Plains other‘than that from the rapid melting of 
snow on frozen ground is from torrential rains in which the precipitation 
of a few minutes or hours is measured in inches. Beating rains of this 
character smooth and compact the surface so that the run-off is heavy. 
As the great volume of water that constitutes the run-off does not get 
beneath the surface, the condition of the subsoil is of no importance in 
determining its amount. The finer and smoother the surface has been 
made by cultivation the more easily and quickly is it reduced to a con¬ 
dition that resists penetration and facilitates run-off. The Akron, 
Colorado, soil, on which^the results of subsoiling have been especially 
unfavorable to the practice, is a good example of a soil from which there 
often may be heavy run-off from a smooth and compacted surface over- 
lying a very loose and open subsoil. 
Many cases have been noted in the course of these experiments where 
the amount of water that entered the soil from a heavy rain was greater 
on a dry, cracked stubble than on a plowed field. 
Under the semiarid conditions of the Great Plains, where production 
is determined by the quantity of water available to the crop, the amount 
of water that enters and is retained by the soil is not determined by the 
depth of cultivation, and consequently is not increased by an increase in 
such depth. Under more humid conditions, where rainfall is sufficient 
to fill the soil regardless of the amount that may be lost by run-off, 
the depth of cultivation can not add to the amount retained and so can 
not be a determining factor, in so far as this item is concerned. 
It must be recognized, however, that the possible combinations of 
conditions of looseness, fineness, and water content of subsoil and surface 
and the character and amount of rainfall are so many that they are seldom 
exactly duplicated, particularly in semiarid regions. Different combina¬ 
tions of these factors may give rise to different results, as is clearly shown 
both in soil-moisture studies and in the crop yields reported in this paper. 
The study of root systems and of soil moisture indicates that the effect or 
lack of effect of differences in the depth of tillage is accurately measured 
by crop yields. From the average yields obtained it may be safely 
assumed that under the soil and climatic conditions obtaining in the 
region where the experiments were conducted a combination of factors 
favorable to deep tillage does not occur often enough nor result in increases 
great enough to warrant its general practice. 
