72 BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
spring. They usually have a high content of potassium, and the 
phosphorus content is sometimes large. Their treatment, therefore, 
must be such as to overcome their peculiar difficulties and take 
advantage of their particularly strong points. 
Drainage. — Since large portions of these heavy clay soils were 
formed as deposits in standing bodies of water, they very commonly 
have comparatively level surfaces. They therefore frequently have 
poor surface drainage as well as poor under drainage. For general 
farming everything possible must be done to secure good surface 
drainage when the expense of tile is unwarranted. Tile drainage, 
however, is often necessary in order to permit the use of such land 
for crops requiring considerable tillage. This form of drainage for 
such land is usually profitable, even for staple crops. The expense, 
of course, varies, depending on the distance to an outlet, the presence 
of stones in the subsoil, and other factors. Ordinarily the expense is 
between $20 and $30 per acre. Since a tile system once carefully 
installed in clay soil will last almost indefinitely, the expense to be 
charged to the land is simply that of the interest on the investment, 
or from $1.50 to $2 per year. Indeed, the entire expense is very 
commonly recovered by the increase of crops in from one to three 
years. 
Tilth. — The most serious difficulty in the management of heavy 
clay soils results from their poor tilth. Such soils are apt to bake 
and form large clods, so that preparation of a good seed bed and the 
cultivation of the crop is difficult and involves much extra labor. 
This poor tilth is due to the fact that the films of water surrounding 
the fine grains draw the particles so closely together when they dry 
that they are held with considerable tenacity. This difficulty may 
be overcome to a limited extent by increasing the amount of organic 
matter. Humus and vegetable matter in such soils has the effect of 
lessening the tendency to form clods. Thus, after a heavy clay soil 
has grown a crop of clover, or has been in grass for some time, it is 
easier to retain a good tilth than if it is kept in tilled crops con- 
tinually. As before shown, liming of clays, especially with quick- 
lime, produces a flocculating effect upon the soil and so reduces the 
tendency to clodding and greatly improves its tilth. Another 
extremely important factor is the moisture condition when they are 
cultivated. As before stated, when such soils are plowed or other; 
wise worked in a wet condition, they have a marked tendency to 
puddle and run together in such a way that very hard and resistant 
clods are formed. It is extremely important to do all the work of 
tillage on such land when the soil is in just the right condition of 
moisture, so that the clods will break down in the soil. This condi- 
tion must be determined for each individual field and with a little 
practice can readily be recognized. Plowing clay land in the fall and 
