hastened, so that a large supply of plant food is prepared for the succeeding crop. Barley or clover, for instance, is often followed by a fall planted grain with an interval of some weeks between the harvesting of the one and the planting of the other. If the field be plowed immediately after the first crop has been removed, and cultivated frequently, the results will be beneficial in starting the next crop with a larger supply of nitrates and moisture than would have been present if the ground had remained undisturbed.

In the case of the fallow or cultivated crops previously mentioned, the process of nitrification is accelerated between the rows as it is in the bare fallow, but the growing plants appropriate the nitrates almost as rapidly as formed, and hence prevent loss of nitrogen in the drainage water.

CHAPTER IX

HUMUS AND GREEN MANURING

Humus Necessary to Soil Fertility.-Loss of fertility in a soil is in a great number of instances due to

The plants growing at the edges of lakes an i streams retain the soil whic is carned down from the adjoiing highlands. Sometimes muck or humus soils are formed in this way.

the rapid decrease in the amount of humus which it contains. Humus is the product formed by the partial decay of organic matter, and is the material that gives the rich, black appearance to some soils. It is formed in most cases from the plants which have previously.

grown on the field, and have later become a part of the soil. It may also arise from animal or vegetable materials added as manures. Virgin soils are comparatively rich in humus, but investigation has shown that continued cropping with no provision for maintaining the supply of humus may result in its being decreased from one-third to one-half in a period of not more than 15 years.

Humus is generally considered to be a necessary ingredient of fertile soils. To be sure, soils may be prepared containing no organic matter which will produce good crops under artificial conditions where the water supply, etc., are under complete control. Under field conditions, however, a sufficient supply of humus is of paramount importance.

Humus Increases Amount of Soil Water.-Humus increases the power of the soil to absorb and retain water, and, consequently a crop grown on a soil containing a fair amount of humus is less likely to suffer from drouth. The following table giving the amount of water held in a cubic foot of the different varieties of soil illustrates this point.

It will be seen that the quantity of water increases with the amount of humus present, the sand containing the least and the loam which has the largest percentage of humus, with the exception of the strictly humus

soil, containing much more. The organic matter in the high humus soils acts like a sponge to absorb the water, but at the same time holds it in such condition that it is available to the crop. The peats are examples

Organic matter increases the power of the soil to retain water. The soils in the glasses contain 0, 6, 12 and 20 per cent. of organic matter respectively from left to right. The tall cylinders show the relative amounts of water retained by the same weight of the soils.

of extreme humus soils, and as is well known may hold more water than is desirable.

Humus Improves Physical Condition of Soil.Humus is also valuable in improving the physical condition of the soil. Sandy soils are made more compact by its presence and better able to supply the crop with moisture. Clay soils, on the other hand, are made

more mellow by the addition of humus forming materials. Clay is likely to become too compact unless there is a certain amount of organic matter present to prevent it from getting into this condition. In other words, humus forms loam, for a sandy loam is simply a sandy soil well supplied with organic matter, and a

clay loam is clay which has been made light and mellow by the same material.

Humus prevents extremes of soil temperature. A soil containing a sufficient supply of organic matter does not respond to changes of temperature so readily as one deficient in humus. The latter warms up somewhat more slowly and retains its heat for a longer period.

Humus a Storehouse for Plant Food.-Humus is of importance because it is a storehouse for plant food, especially for nitrogen. It has been shown that most. of the nitrogen of the soil is present in the more or