Balanced Soil Fertility: Recent Soils Research Suggests That a More Carefully Balanced Soil Fertility May Reduce Plant Pest and Disease Losses

Recent soils research suggests that a more carefully balanced soil fertility may reduce plant pest and disease losses.

Modern soil research suggests that the responsibility for disease and pest control, in many instances, should be placed with persons managing crops and their nutrition–and not with the chemist synthesizing more poisonous drugs to fight vermin and disease.

Nature grew most of our crops in healthy conditions before we took them over. When we found wild plants growing in excellent condition, or in what the ecologist calls “an ecological climax,” (a) they were in a pure stand, i.e., there were no contaminating weed crops, (b) they had been growing as the same crop in succession in the same location for many years, (c) they had accumulated much of their own residues as organic matter on and within the soil, and (d) they were free from diseases and pests.

These facts present a distinct paradox when in an extensive reference work, published no later than 1954, there appears the following statement: “It would be impossible to overemphasize the importance of crop rotations to control diseases, maintain fertility, prevent erosion, and maintain soil structure.”

Such contradiction between the crop management by nature, giving healthy crops without rotations of them, and man’s claims for rotations in respect to plant health, suggests that much is yet to be learned by us about nature’s success in growing crops with their healthy survival and without chemical wars on diseases and pests.

When plants in nature reach their climax where no soil fertility (neither inorganic nor organic) is removed and when our agricultural management removes these far in excess of their return, might we not theorize that depleted soils and accompanying poor plant nutrition result in plant diseases and insect attacks?

On the converse, then, we might theorize that properly balanced fertility levels would prevent these troubles and yield healthier plants.

An affirmative answer was given in some delicately controlled studies of legume-plant nutri – tion. In tests with fungus diseases of soybean research clearly demonstrated that attacks were highest on soils with low calcium levels. (Fig. 1). These attacks were prevented by higher levels of exchangeable calcium in the soil. This was the demonstration when all the soils were decidedly acid with a pH of 4.4.

Here is a striking example of how increased fertility cut fungus attack on soybean plants grown in pure quartz sand. There plants were given increasing amounts of a very acid colloidal clay (pH 4.4 saturated about 25% by calcium). As more of the clay representing more exchangeable calcium is put into the sand (visible from left to right in the clear glass containers), the plants prospered and showed less and less signs of attack by fungus-suggesting “damping off.” (Fig. 1)

 

Were this a case with no additional tests to expose soil fertility characters more clearly, we might have drawn the once common conclusion that the soils were all too acid to grow legumes. But as the sandy soils contained more of the same supply of acid clay and delivered more calcium to the legume plants, their growth varied from the miserable with severe fungus attacks to the excellent with no fungus.

Recent research methods indicate the presence or absence of the leaf-eating insect (Heliothrips haemorrhoidalis) varied with the levels of nitrogen and exchangeable calcium. When the soil supplied ten milligram-equivalents, or less, of nitrogen per plant, the thrips made their attacks. When the soil offered twenty, there were no thrips. When nitrogen supplies were low but calcium was plentiful (increasing through 5, 10, 20, and 40 milligram-equivalents per plant) there were less thrips injuries as more calcium was available in the soil for plant nutrition.

Less pests and diseases resulted from balanced soil fertility in these observations:

(1) A legume crop growing its self-protection against fungus by the help of more calcium, the commonly deficient fertility element in humid soils, and the one regularly associated with plants producing more protein.

(2) A non-legume in which nitrogen helped the plant protect itself against an insect pest, but better so when that nitrogen was balanced with calcium and with possibly other untested elements which calcium mobilizes into the plant.

(3) The grain with its extensive destruction by a grain-eating pest when grown with nitrogen fertilizer only, but grain with an ingrown protection (resistance, immunity, antibiotics, or what have you?) against those insects when phosphorus combined with nitrogen was the added fertility growing the plants.

There is the forceful suggestion that not one element of fertility, but all of them in balance and integration of their separate functions are required to grow plants healthy enough to ward off these diseases and pests. There is the additional suggestion that any other element as the limiting factor, or in major deficiency, might show up under test as the “cure” or the preventive in similar tests or demonstrations.

Here the observations cited, first, the calcium, then nitrogen, then phosphorus in a readily anticipated order when we grant that calcium is the major nutrient in the soil.

Following in common order, the use of potassium would come next in line as help for the plant’s growing its own protection. Then others would fall in line, too, based on the way they become limiting elements in the plant’s processes of growing whatever organic compounds through which it protects itself. 

Corn grown on soil with a low fertility level is more subject to insect damage while in storage than corn grown on more fertile soil. Here we see a small infested ear of corn that was grown on a low fertility soil. It was an easy mark for the grain borer. The ear at the left was grown on the same soil but with added nutrients. It was undamaged except for a few borer holes where the two ears were in direct contact for a period of time. The open-pollinated ear at the right was in contact with the infested one five months but showed only one sign of borer injury.

 

Such evidence indicates nature was able to produce crops without destruction from diseases and pests. It also suggests that while we may manage soils to give bigger crop yields, we have much to learn about soil management that nourishes plants well enough for them to protect themselves from diseases and pests without our poisons or medications.

It raises the question, also, of whether crops too deficient in their nutrition to grow their own antibiotics for self-defense would contain enough nutrients to produce healthful livestock.

Soil research faces a serious challenge to formulate balanced fertility as guarantee of less plant diseases and fewer crop pests, as well as a means of producing more vegetative bulk per acre.