THE hypothesis that environment influences plant disease and insect damage is not new. In early agricultural literature, writers generally accepted the theory that the better the growing conditions for plants, the less the risks from disease and insects. From 1910 to the present time, however, it has been difficult to find this theory expressed in writings on agricultural subjects.
It is true that the era of soil depletion has been contemporaneous with the period in which diseases and insects have become most troublesome. This might be true, of course, without being significant; but there are very good reasons for supposing a connection to exist. Many farmers in the United States can remember when there were no Colorado potato beetles, San Jose scale, and other insects and blights now common. These same men can remember also that at that time their ploughs separated a zone of almost black upper soil from the yellowish subsoil. This black topsoil has now disappeared; at the same time many new insects have made their appearance, and those that were present before have become much more numerous. Plant diseases have multiplied in number and increased in virulence during the same period. We may well ask, then: Is environment (meaning the soil) a factor in their control ?
Certain human and animal diseases and parasites have long been thought of as environmental. Hookworms belong entirely to the South, particularly to the south-eastern section of the United States. Malaria can occur only where the Anophelene mosquito is present. Pellagra and other so-called deficiency diseases have usually been thought of as belonging to certain localities. It has not been difficult to connect such troubles with the environment in which they have been found.
In recent years the control of deficiency diseases has been much improved in most areas because of the more general availability of protective foods. Coincident with this improvement, however, there seems to have been a general decline in the nutritive value of foods produced on average land. The discovery of vitamins has brought this matter into better focus. When knowledge of vitamins was new, it was thought that certain foods contained an abundance of particular vitamins. Egg yolk was said to be rich in most of them. Now it is known that the vitamin content of the egg yolk is seriously affected by the food consumed by the hen. Butterfat was considered to be uniformly potent as a source of vitamin A. More recent discoveries show that the vitamin content of butterfat or cream is largely dependent upon whether or not the cow has access to a quantity of grass, or to the richly coloured foods which provide her with this essential vitamin. Neither the hen nor the cow can by herself create the various vitamins she transmits to the consumer of her products. The vitamins must be supplied to each animal through food.
These disquieting discoveries — that foods we have thought were always richly endowed with health-giving substances may themselves be deficient in certain instances—have shifted attention to the plants, which normally would be expected to supply vitamins to the animal. It becomes a complicated chain of cause and effect: milk—cow—hay, grain, or grass —soil. In other words, the blame for any deficiency goes back, in the last analysis, to the soil.
Then we discover that during the very period in which deficiency diseases are being decreased in the localities where they have been most serious, the area in which they occur seems to be widening. In the last few years certain of the deficiency diseases have been found in places where they were unknown before. And, at the very same time, we find that the soil — life's ultimate source — has declined sharply in its ability to nourish properly the plants upon which we depend.
Characteristically, Americans confronted by this dilemma of deficient foods have turned to the drug stores to buy vitamins. There is little doubt that the development of synthetic vitamins has served to postpone disaster for many people; but it seems unnecessary to pay for something whose value is not yet wholly unquestioned, when by properly modifying the environment in which our plants live, we can again build into our food-plants all the vitamin richness they once had.
The logic of such a viewpoint is inescapable, yet it has not been investigated officially to determine whether it may be true in fact. We have experimental data to prove the necessary causal relationship between the completely nourished cow and the milk rich in vitamins and other nutrients. We know through experiment that only good feed in correct proportions and quantities can nourish the cow properly. We are just as sure, with ample experimental proof to sustain us, that only a soil that is capable of supplying a sufficiency of plant nutrients in suitable combinations can create foods richly endowed with the elements needed to produce human or animal health. We have, in other words, all the necessary elements of logic for reasoning from the good soil to the best of health, or from the poor soil to the direst of disease in the animals which consume the products of the soil; but we have not assembled those elements into the necessary whole to arrive at the logical conclusion. Our agricultural reasoning is in much the same condition as was the passenger transportation of America before the existing railway lines had been grouped into great transcontinental systems. We ought to be able now to take the entire " trip " from starting point (good or poor soil) to terminus (good or poor health) without having to make the local stops.
In my test work and, subsequently, in field work, I discovered that soil conditions seemed to be factors in the extent to which plants were affected by diseases and insects. The evidence was so convincing that I watched carefully for verification of the idea in commercial crop-growing when the tests were repeated on a field scale. The results in the field fully confirmed the earlier deductions. I am sure that the existence or non-existence of plant diseases in certain fields is related to the condition of the soil, and that the incidence of insect damage is likewise related. No other conclusion seems possible from the amazing behaviour of insects and the absence of diseases in the crops I grew on land which had been prepared by discing in (or down) great quantities of green manure crops. Yet, despite this existing chain of experimental evidence proving the truth of every element in the necessary reasoning, we cannot accept such an unofficial decision as true until it has been proved so by properly supervised official tests. For such tests we shall look to the experiment stations which have been established for that purpose.
While the presence or absence of insects or diseases seems not to be necessary to this reasoning, an important purpose will be served by dependable knowledge of just how completely their behaviour indicates the suitability of the soil in which the plant is growing. If insects and diseases prove to be a perfect index, as they must if they are truly environmental, then an entirely new " soil-testing " method becomes available to the farmer. Whenever his crops become infested by insects or are attacked by disease, he may know immediately that further green manure treatment will be helpful.
Since in ordinary farming and horticulture the fight against pests of all kinds has partaken of the inevitable, how, it may be asked, can a method of soil preparation possibly result in a change for the better ? I had to find an answer to that question before I could accept the idea. The answer was difficult to find. No official experimentation by soil-experts had been carried out on land prepared by surface incorporation of great quantities of organic matter. All experimental plots had been ploughed, if there was much organic matter to be disposed of. Discing had been considered feasible only where there was little rubbish or crop debris involved. Experiment station results, therefore, supplied nothing directly bearing on the case.
To me it seemed necessary to assume that the soil in which organic matter in great quantities was decaying would be richer by the quantity of products of decay that had been accumulated in it. As organic matter decays, mineral plant-foods in the soil are released, as are also the additional elements which make up the organic compounds of which the living material had been composed. Depending upon the character of the organic material, decay may take place quickly or slowly. In either case, unless there are roots present at the time the decay products are set free, these used plant-foods are almost certain to be flushed out of the soil by the very first rainfall. The only certain way to prevent this loss is to have roots of growing plants always present when decomposition is going on in the soil. On land that is left undisturbed, Nature takes care of this. Roots are always present; therefore, no plant food is leached away.
On the farm, these salvaging roots may be those of beans, cucumbers or any other crop the farmer wants to grow. The roots of such crops will gratefully absorb all the products of decay they can get. It is a reasonable assumption that the soil solution that these roots pick up from decaying organic matter is different from that that they would find available in pure mineral soil where nothing is rotting. Decaying material in the soil enriches the soil solution, so that each unit of liquid can supply several times as much plant food as the same quantity of water from soil not enriched by decay. That is only common sense.
It follows that the more decaying material there is in the soil, the richer the solution that these roots pick up will be; the richer the soil solution carried in by roots, the richer in minerals the plant sap will be. From this point it is easy to assume that variations in the richness of the plant sap may affect the attractiveness of the plant for its customary parasites. A great proportion of minerals in the sap may result in its carrying less sugar, and a decrease in sugar content may easily make the plant sap distasteful. Possibly cucumber beetles, for example, could be starved for lack of palatable juices, even when their host plant is enjoying the richest possible food from the decay in progress in the soil.
Such a theory is not entirely without foundation in science, even though no specific research has been devoted to it. We do know that variations in internal juices of plants are produced by variations in the fertilizer treatment and in the available moisture of the soil. This fact was established in 1918 by Dr. Kraus and Dr. Kraybill, whose report (1) has been used by a generation of students as a reference work in horticulture. There is no question that changes in plant composition are produced by changes in the nutrients available in the soil. We cannot, of course, know how insects feel about having their favourite host plants overfed on minerals that originate from decay. We can only guess, from the fact that they prefer the scantily fed plants to those that are better fed, that the richer sap is less palatable to the insect.
If this theory is tenable, the human race is extremely fortunate. It becomes possible, because of this relationship between the insect and its food supply, to improve the human food supply by the very method that will starve the insects.
Apparently diseases yield even more completely to the environmental conditions which are most favourable to plant growth. I am unable to advance, as tenable, a reason for this. It appears, however, that the leaf surface of fully nourished plants is better fortified to prevent the entrance of infections. That there is a difference in texture of the leaf surface of well-nourished plants compared with those growing on thin land may easily explain their better resistance to disease. In this connection, the natural resistance of healthy, well-nourished plants becomes entirely logical.
It is reasonable to believe that insects and diseases thrive only in a suitable environment, just as do other living things. Further, it appears that the environment that is best for the disease and the insect is poorest for the host plant; and the conditions that favour the host plant's development are intolerable for insects and diseases.
Scientific men with whom I have discussed this theory are not inclined to agree, because they still think the type of soil would be an important factor and doubt that the experience would be the same under other soil conditions. My contention is that the one determining condition is the surface incorporation of great quantities of organic matter, that any type of soil so treated would bring similar results - provided other conditions were no less favourable than they were for my 1940 tests. (It can be said truthfully that the seasonal conditions from July 12th, 1940, to the onset of frost were such that many other plantings of beans had to be abandoned in the neighbourhood where my beans thrived.)
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