Some Rates of Fertility Decline

Fig. 1. Continuous timothy since 1888 with regular applications of manure is still making good hay (left), but without manure it is taken by broom sedge (right). Upper photo–before blooming; lower photo–during blooming.

 

Just how rapidly the fertility of a soil is declining may be as baffling as the question concerning the opposite effect, how rapidly a soil can be built up. To the latter we can only reply, “Very slowly.” To the former, we must, unfortunately, answer “Very rapidly.”

As a consequence of the rapid decline in fertility and the slow rate of its restoration, the more productive soil areas under cultivation are shrinking. Land is being taken out of production faster than we desire. Some suggested rates of fertility decline are coming to us from farmer experience and from soil-crop studies. They are putting the rate of disappearance of the soil fertility more nearly on a time basis. They are giving answers in numbers of years by which we may well look into the future. Unfortunately, however, these numbers of years before yields will be put below continuance of economic production are much smaller than we like to have them.

Pastures Decline Rapidly

Perhaps one would not expect that pastures are recording very accurately a high rate of soil fertility decline, when so much emphasis is being put upon grass for protection against erosion. But pastures are usually the less fertile soil areas of the farm. It is for this reason that such soils are not put into tilled crops, but are commonly put to, or left in, grass. Naturally, we have no accurate measure, like bushels of grain, of the decline over the years in the fertility under grass. However, when one takes inventory of the increasing incidence of weeds–which incidence of a crop that the cow won’t eat is the reciprocal of the disappearance of good grass that the cow takes readily–there are accumulating data in terms of years to tell us how rapidly the fertility is being pushed down below the level needed for cow-satisfying herbage.

The incidence of broom sedge (Andropogon virginicus) is one of these indicators. Its advance over the country from the East toward the West has been considered alarming by some folks. Its increasing prominence in late autumn during its ripening and scattering of seed and its fuzzy whiteness that makes its name “Old Man’s Beard” very appropriate have caused extensive concern. That it is not eaten by the livestock and that it remains as a tall growth in the pasture through the winter tell us that its food value is so low that it will not even tempt animals. There is the suggestion, then, that in her refusal to eat broom sedge the cow is reporting on the rate of the decline of soil fertility. She is telling us that the soil, which once made feeds for her, is now making only bulk of no feed value according to her judgment.

The transition from a virgin soil supporting good permanent timothy to one with only broom sedge may be measured as years from the records of Sanborn Field at the University of Missouri. Two plots there have been in Timothy continuously since 1888. One of these had no soil treatment except that of being plowed out and reseeded when the overseer thought it was so foul with weeds that reseeding was necessary. It has been in this grass continuously with no more attention than the annual hay harvest. The other plot alongside, also in timothy, has been given six tons of manure annually. It has been plowed each time that it was necessary to plow the weedy companion plot in order to keep the tillage treatments of these two plots alike.

The plot given manure up to this date has not yet suffered from the incidence of weeds. On the plot with no treatment, the seeding of timothy lasts now scarcely two years before weeds take over. In 1945 the broom sedge had completely taken this plot. But the broom sedge had not crossed the border lines to the adjoining timothy plot alongside given manure, nor had it gone to the roadways at the ends in continuous bluegrass.

Even this timothy, now for less than 60 years, has exhausted the fertility to the point where reseeding fails to hold much of this crop past the year of seeding. Here was cropping to a grass agriculture for but a little more than half a century to tell us that even this much-publicized system of farming for a permanent cover to guard against the erosive effects of running water needs more than merely this special system of cropping. It is suggesting that any system of cropping must be undergirded by a regular and generous flow of fertility from (a) the organic matter in decay, or (b) the exchangeable store adsorbed on the colloid, or (c) the breakdown of the mineral reserves. It is directing attention to the fertility flow that is keeping well filled those assembly lines of agriculture hidden away within the soil.

Mesquite Crowds Out Grass

The westward march of mesquite (Prosopis glandulosa) across the Southwest Plains to occupy what was once considered good range pastures is another trouble, equally as disturbing as the weed problem in the fenced pastures. While the mowing machine and hormone sprays may be consolation to some folks worried about the weeds in the pastures, such tools and treatments can scarcely be feasible helps on the extensive range areas going to mesquite so completely and so speedily.

The Forest Service has cited the short time of 40 years in the Santa Rita Mountains, with their low annual rainfall, as the time required to exhaust the soil fertility by only grazing, and to push the soil’s productivity down from good range for cattle to mesquite brush. Here again the figure is near the half century mark. It is small, even under the livestock system of using the land, and is the lifetime of the fertility supply at the respectable nutritive level of the short grass and its limited production of vegetative bulk. 

Fig. 2. Grasses went out and mesquite took over on these ranges in the Santa Rita Mountains in less than a half century. Upper photo–1903; lower photo–1943. Photo Courtesy Forest Service.

 

The Decline of Tilled Soil

Under tillage, the decline of the fertility of the soil would be expected to be more rapid. But even under such treatment, one cannot arrive at the longevity of the productivity by considering only the system of land use. Here too, the flow of fertility from the assembly lines of the soil determines the number of years it can hold out under the cropping pressure. Once more Sanborn Field, but this time under continuous wheat, gives some duration figures as years from its plot with no fertility return–not even the straw–since 1888.

This plot demonstrated a gradual decline of wheat yields from 1888 onward for almost 40 years before its nearly complete crop failures became so evident. These crop failures have now become almost regular occurrences in alternate years since 1925. Here are some suggestions: (a) That the virgin supply of soil organic matter is almost completely exhausted, with the former store of actively decaying humus no longer helping much to make the seed crops, (b) that the mineral reserves in their breakdown are contributing at a rate too slow for annual crops, and (c) that the clay colloid is not restocked with exchangeable nutrients by the October seeding after the crop’s exhaustion of them in the preceding July harvest.

In the case of the continuous wheat, like the cases for the pastures and the ranges, the nearly half century of regular productivity was the limit given us for this prairie soil prevailing in northeast Missouri. After less than a half century this soil became what might be called an alternate-year bearer, or a regular biennial reproducer, because of the insufficient flow of the essential inorganic nutrients from the soil. The decline in the supply of soil fertility during but a half century has recorded itself as a failure in the crop to reproduce itself as even the equivalent of the necessary seed, and then only when seed from outside sources for this plot was used.

Do such records have any implications for tree crops as well as for grain crops when we remind ourselves that it is the older apple trees that become “alternate-year,” or “biennial” bearers of apple crops while young trees are annual bearers? When older cows become “shy breeders” can the decreasing soil fertility under their feed crops possibly be the indirect factors in the case?

Quite contrary to expectations and common claims for rotations in the upkeep of soil fertility, the crops in even the 6-year rotation are now down to failure on the plot given no soil treatment. This is true although corn, oats, and wheat are only three tilled crops out of six in the rotation, and each has been grown now only 10 times since the soil was put under cultivation. The clover has been failing for a quarter of a century. The timothy occupying the land for two years in the rotation has been little more than tickle grass. Rotation with no more help from the clover and sod during three years of the six was little or no different than the continuous wheat in the rate of decline of the soil fertility. Without soil treatments of manure, lime and other fertilizers, and regardless of whether continuous sod, continuous tilled cropping, or any length of rotation going as long as six years, the supply of fertility on Sanborn Field is approaching exhaustion in nearly half a century.

Fig. 3. The soil growing wheat continuously without treatment since 1888 on Sanborn Field, Columbia, Missouri, has become an alternate-year bearer during the last 20 years. The graph of the annual yields drops almost to zero in regular alternate years since 1925.

 

Task of Future Restoration

Reproduction by the plant from seed is not a matter of only the starch in it, except as this is reserve energy for the germ that is rich in protein and many other compounds. For the synthesis of these complex components representing potential reproduction, the fertility of the soil, more than the weather, is demanded. Troubles in reproduction in plants and animals are the reflection of the declining fertility supply in the soil. Shrinking lifetimes of our fields are the underlying causes of much that has not yet been traced to this as the cause. Better reproduction can come only as we minister to the soil, which is the source of the entire process of creation. If creation of food as plants and animals is to continue abundantly in the future, we can scarcely guarantee its projection there without being guided by the records of the past concerning the rates of decline of the soil fertility.