Proves Weedy Pastures Lack in Plant Food
ENEMY INVASIONS THREATEN not only our seashores but the agricultural center deep within our continent, for it is here—far inland—that weed enemies are a serious threat to the productivity of our pastures, thereby endangering our meat and milk supplies.
Defense against this form of unsuspected sabotage has not yet been fully organized. Some thought might be given to making our pasture herbages more than a step-child in the list of crops. Are we ready to admit that our soils are sick and for that reason no longer immune to weeds? If this invasion of weeds into our pastures is to be halted, it is essential that we think of treating our soils to make them healthy enough to grow healthy, desirable pasture grasses and these so abundantly that weeds can have no place in the competition.
A 55-Year Record
That healthy soils are immune to weed infestation, while sick soils are readily susceptible, is illustrated clearly by two plots on Sanborn field of the college of agriculture here, one of the oldest fields given to experimental studies of soils. These two plots have been in timothy now for the entire fifty-five years since the field was established. One has been given six tons of barn yard manure annually. The other has had no soil treatment but has been kept simply in this grass crop.
In order to help the latter maintain itself more distinctly in timothy, and to keep it from being completely overrun by all kinds of weeds that were always gradually creeping in, the soil was plowed and reseeded with timothy about ever six or eight years. The adjoining plot given manure annually and in good timothy without weeds was, of course, also plowed and reseeded at the same time. The last plowing and reseeding occurred eight years ago. Here is a form of permanent grass cover, maintained as a fine, long-season, pure crop with manure additions in one case and maintenance attempted only by reseeding in the other case with no soil treatments whatever. In this contrast there is offered the experience of fifty-five years to illustrate the immunity against weeds in the former case and complete infestation by weeds in the latter.
From Grass to Weeds
During the year 1943 the plot without soil treatment changed its flora and shifted from what had once been a timothy plot to a complete stand of broom sedge (or beard grass). This plant, that marks out unproductive soil areas by waving its brown, 2-foot wisps in the winter wind in pastures where cattle have eaten short all the other grasses but have left this plant untouched, is telling us that the soil is now at a low productive level. It is a crop animals really do not eat. It reports that as a pasture this soil has truly “run out.”
Why did this weed infestation attack only this one plot in the field of some forty plots? Why did not broom sedge take the adjoining companion plot where manure has been given annually? Surely the evil scatterer did not fail to put some of the same sedge seeds on the manured soil alongside the infested plot or even on the other plots of this old field. If we as humans can be “well fed and healthy” against infectious diseases, can’t we imagine that a soil, too, might in some similar manner be “well fed and healthy” and keep its timothy crop while holding out against infestation by weeds?
Food for the Microbes
Perhaps putting manure on annually is a form of giving the soil some extra chemical or mineral fertility to feed the microbes within it. Such additions give it more life. This greater life is an integral part of the soil’s internal physiological performances by which the mineral reserves of the soil are changed and delivered in larger amounts to the timothy plants that keep growing continually throughout the season. With vigorous and healthy timothy plants on the healthy soil, there is little chance for such soil to become infested even by such vigorous weed invaders as the broom sedge.
Perhaps manure contributes some animal products, some unknown compounds, or hormone-like substances that benefit the grass crop through the soil. In constructing what has been called the biotic pyramid by Mr. Leopold of Wisconsin, we have the soil supporting the microbes and above them the plants and they in turn supporting animals and then the humans on top. Perhaps there are not only contributions coming from below upward in this pyramidal structure, but possibly there are influences coming from above downward.
It is not beyond possibility that something of a hormone similarity from the animal manures may be helping the timothy to maintain itself and to smother out weeds that might start from chance seedings. At any rate the timothy receiving animal manure—and with it these mineral, chemical or microbial additions—has held its own ground against broom sedge infestation. In contrast the adjoining plot without manure was taken over so completely by broom sedge in a single season that this weed crop grew to seed stage and produced a tonnage of vegetation equivalent to 3,700 pounds per acre. With this weed, as A. W. Klemme’s studies show for others, tonnages per acre can be high even after the plot had been cut at haying time.
Merely going to grass is not the solution for our soil erosion problem, nor will it be the introduction of ourselves to a pastoral agriculture. Sanborn Field testifies that merely going to grass may mean going to weeds which the animals know better than to eat. Invasions by weeds are not so much a matter of seed introductions. They are a matter of declining and neglected soil fertility which must be corrected if our pastures are to carry through long grazing seasons with nutritious herbage.