50 
Research Bulletin No. 5 
On this type of soil we feel that two to three inches is suffi- 
ciently deep to make an effective mulch. Where the mulch is 
too shallow, thei^e will be enough circulation of air thru the 
mulch to cause considerable loss. On this soil almost any imple- 
ment that stirs the surface can be used if it does not pulverize 
the soil too finely. Where the surface has become so dry that 
the drag-harrow pulverizes, we prefer the disk. On still lighter 
soils the disk might pulverize too much and make some other im- 
plement preferable. The objections to a real fine or dust mulch 
are that it will blow and that it offers too great resistance to the 
penetration of water. 
The third means of escape of water from the soil, that of 
being drawn out by growing vegetation, is much more effective on 
this type of soil than surface evaporation (see Tables 14 and 17). 
Considering only the moisture content of the soil, the water used 
by any plant is lost. From the point of view of the farmer, only 
the water used by weeds is lost. This is, of course, a real loss, 
since the weeds have little value. The water that is taken up 
by the roots of the crops, passed thru the plant, and then exhaled 
by the leaves, has served its purpose in plant growth. It is for 
that purpose that we strive to store the moisture in the soil. 
DEPTH TO WHICH CROPS HAVE USED MOISTURE. 
The depth to which a crop will use moisture depends upon 
the root development of the crop, the length of its growing sea- 
son, its normal development, and the type of soil on which it is 
being grown. 
Some crops make more root development than others and 
naturally send their roots deeper into the soil to obtain water 
and plant food. An extreme example of deep feeding is seen in 
alfalfa. Other crops feed to a lesser depth. The length of the 
growing season also affects the depth to which water is used. 
A perennial crop, such as grass, will dry the soil to a consider- 
ably greater depth in a series of years than it will in one vear. 
It may be that there is some continued growth of the roots, and 
that they keep growing so long as they can find water; or the 
drying of the lower soil may be due to the water being brought 
to the roots by capillary action during the greatly extended time. 
Winter wheat, due to its longer growing season, develops a deeper 
root system than does spring wheat, and consequently uses the 
water from a lower depth than does spring wheat. 
During a very unfavorable season a crop may become so in- 
jured that it will not make normal development and consequently 
