GRASS IN NATURAL FORESTED AREAS

by Richard Bradfield

THE CLIMATE of the humid forested area differs from that of the natural grassland areas in the western part of the United States in several important respects. The average rainfall is higher and is better distributed throughout the year. Droughts are not so frequent, they do not last so long, and they are not so severe. The average humidity is higher and the average temperature in summer is lower.

As a result of these differences in humidity and temperature and some differences in wind movement, the amount of evaporation in the growing period is less in the forested area than it is in the natural grassland regions. Taken as a whole, the climate of the forested regions is better suited for the growth of grass than that of the natural grassland areas.

With good management, higher yields of grass can be obtained in the humid regions than in the natural grassland regions. The grassland area is characterized by short periods which are very favorable for the growth of grass, followed by periods in which the growth of the grass practically ceases. As a result, growth takes place in these areas for a much smaller proportion of the growing season.

Soils are also greatly influenced by climate. Forests and associated climatic factors tend to produce soils that are not so well adapted to grasses naturally as soils of the prairies.

There are two principal reasons for this. First, soils of the forested region do not as a rule contain so high a reserve of organic matter as prairie soils. The organic matter in forest soils is contributed largely by leaves that fall upon the surface. It is mixed to a limited extent with mineral soil material underneath by soil fauna. Because of its superficial position, a high proportion of the litter in forest soils may be lost by burning when the soil is cleared. Most of that remaining is concentrated in a relatively shallow surface layer. A high proportion of the organic matter in prairie soils is contributed by grass roots and as a result it is diffused through 2 to 4 feet of soil. Because of this difference in distribution, organic matter of forest soils is more quickly oxidized by soil micro-organisms than that of prairie soil when put under cultivation. Many such soils, when first cultivated, were cropped heavily to the most profitable cash crop of the region corn, cotton, tobacco. The fertility from decomposition of the accumulated organic matter was rapidly exhausted; crop yields soon declined; erosion was accelerated. The poorer farms were soon abandoned and many of them still are in that state.

The second reason that forest soils are not so well adapted to grasses as prairie soils is that the nutrients essential for the vigorous growth of grass are leached from soils of the humid region to a greater extent than from the soils of the prairie region. Because of the higher average rainfall, lower average summer temperature, and higher humidity, less water evaporates at the surface or is transpired by the grass and more of it percolates through the soil each year.

Studies made at several locations indicate that frequently from 25 percent to more than 50 percent of the total rainfall percolates through the soil. These percolating waters, enriched by carbon dioxide from the soil air and by other acids produced by the decomposition of plant and animal residues in the surface soil, dissolve large quantities of certain minerals required for the growth of grasslands and carry them beyond the reach of crop roots.

The amounts of some of the more important of these minerals removed from two soils that differ widely in natural productivity, Dunkirk silty clay loam and Volusia silt loam, are given in the table.

The Volusia silt loam referred to in the table is a very acid soil of very low natural productivity; the Dunkirk silty clay loam is a fairly productive soil that is acid in the surface but contains free calcium carbonate in the subsoil.

The absolute amounts of the various minerals that will be removed by percolating waters each year will vary widely, depending upon the nature of the soil, the amount and distribution of the rainfall, the nature of the vegetative cover, and several other factors.

In spite of all of these complicating factors, however, several general statements can be made with reference to the soils of the humid region. In all such studies, calcium is the predominating constituent in the leachate. Magnesium and potassium are usually present in fairly large quantities, but usually in much smaller quantities than calcium. As a result of this removal of basic elements in relatively large quantities each year, the soils become increasingly acid. In large areas in the forested region, this leaching of bases has proceeded so far, and the soils have become so acid, that certain desirable species of grassland crops cannot be grown without replacing some of these leached elements. This is usually done by the use of limestone and in many cases potash as well.

Most of the nitrogen lost is in the form of nitrates. It is also interesting to note that where the soil is covered with grass the loss of nitrogen in the leachings is very small because grass has the ability to absorb nitrates from the soil about as fast as they are formed. This is important when we consider the treatment of grass with nitrogenous fertilizers. In general, such nitrogen is used very efficiently by grass.

Another important lesson to be learned from the table is that but little phosphorus is lost from the soil by leaching. In no case has more than a trace (usually less than a part per million) of phosphorus been found in the Teachings from soils, even though they had been fertilized rather liberally with phosphatic fertilizers. The reason is that soils have an enormous capacity for fixing soluble phosphates. A part, at least, of the phosphate added to such soils, while not readily soluble in water, is nevertheless available to plants. Because of the small loss by leaching and the fact that added phosphates do retain a certain degree of availability for long periods, it is possible to add sufficient phosphate to the soil at the time of seeding grass to last for at least 5 or 6 years. Such applications usually give better results than more frequent top dressings.

While all soils of the humid region are subject to more or less leaching of the type I have described, their capacity to support a good cover of grasses today varies widely. This variation is due partly to differences in the original soils and partly to differences in the type of management they have received since they have been under cultivation.

One way in which we obtain valuable information regarding the soil conditions that are favorable for the production of certain types of grasses and legumes is to study the soils upon which the various types of grasses and legumes seem to predominate in old pastures and meadows.

In West Virginia

An interesting study of this type was made by W. H. Pierre and his colleagues in West Virginia a few years ago. West Virginia is near the center of this area, and the results of the study are rather typical of what might be observed in many other places in the humid regions of the United States. In the poorer pastures of the area, plants like povertygrass, broom-sedge, and weeds made up 50 percent or more of the total ground cover. An average of about one-fourth of the surface of such pastures was bare; only about 25 percent was occupied by the more desirable species of pasture plants. The cover of the more productive pastures was made up of Kentucky and Canada bluegrass, timothy, orchardgrass, white clover, and hop clover, with small percentages of the undesirable species. The study of the soils that produce these different types of pasture indicated that, in general, the soils that produced the higher proportion of undesirable pasture species were higher in acidity and lower in exchangeable calcium and the available phosphoric acid than the other soils.

If the pH value of the soil were 5.8 or above and an adequate supply of available phosphoric acid were present, it generally supported a good stand of Kentucky bluegrass and white clover. (pH refers to the degree of acidity or alkalinity of the soil. pH 7.0 indicates neutrality, higher values alkalinity, and lower values acidity.)

Eighty percent of the soils in the area studied were found to need lime in order to grow the desirable species of pasture plants satisfactorily. Of the soils having less than 10 pounds of readily available phosphorus as measured by Emil Truog's test, only 20 percent had a cover consisting of more than 50 percent of desirable species, while of the soils containing more than 20 pounds of readily available phosphorus, 72 percent carried a cover of the desirable species, Kentucky bluegrass and white clover.

This study indicates rather clearly that the first requirement of the soils of the area for growing desirable species of forage grasses and legumes is a more adequate supply of lime. A close second need in many parts of the area is for additional amounts of available phosphoric acid. In certain sections, the need for phosphorus is more acute than the need for more lime.

With these facts in mind, let us now approach the problem from a somewhat different point of view.