Plants that are growing in soils with optimum fertility and experiencing little or no stress generally have significantly lower incidences of (herbivorous) pest damage. The plant’s natural resistance is typically higher and so is the incidence of predators. The reasons why increased predation occurs are not clear but trials have shown a notable difference in predator activity where fertility and biological activity are near optimum and stress is minimized. Fertility management, however, is not as easy as it may seem.
Luxury consumption of plant nutrients is rare in nature and can be as unhealthy to plants as it is to people. Available nutrient levels that are either long or short of optimum can cause stress to plants, and lower their resistance to pathogens and to insects. Insect bodies are almost 50 percent protein, the synthesis of which demands a good source of nitrogen. Herbivorous insects must be able to locate an abundance of relatively soluble nitrogen to survive. A 1983 study showed seventeen of twenty-three plots fertilized with soluble nitrogen resulted in an increase in insect damage. The fine line between too much and too little nourishment, however, is as important to understand for plant health as it is for human health.
Of all the essential plant nutrients, superfluous or insufficient levels of nitrogen and potassium in the plant seem to have the greatest effect on insect activity. Excess nitrogen can accumulate in plant tissue as soluble amino acids and nitrates-two forms of nitrogen that not only induce insect feeding but also egg laying. On the other hand, inadequate nitrogen can arrest plant growth and slow the movement of protein and sugars through the plant. These compounds can then accumulate in the leaves and stems. Production of defense compounds can also be suppressed. The weakened plant is typically more susceptible to insect attack. Optimum levels of nitrogen are not necessarily those that promote the greatest amount of growth, however. Researchers have recently discovered that when maximum plant growth is achieved, the production of allelochemicals (toxic compounds produced by the plant for self-defense) is significantly diminished. Moderately stressed plants seem to produce the greatest amount of defense compounds. It’s interesting that moderately stressed humans tend to be the healthiest, too.
Deficiencies or superfluities of potassium have an indirect effect on insect activity due to their relationship with nitrogen. Potassium acts in conjunction with nitrogen to accelerate the processing of soluble amino acids and nitrates into proteins by the plant. The less soluble nitrogen there is in the plant, the less attractive it is to herbivorous insects and plant pathogens as well.
Although no specific link has been made to other nutrient deficiencies and insect pest activity, it is reasonable to assume that all components of nutrition for both the plant and soil organisms are equally important. Micronutrients that are needed in only trace amounts, for example, play a major role in the creation of over 5,000 different enzymes necessary for life functions-many of these enzymes provide protection for the plant.
The soil’s physical condition, which includes structure and density, can have a big impact on the diversity of soil organisms that regulate fertility. Heavy, wet soils with little porosity can inhibit the existence of many beneficial aerobic organisms. On the other hand, sandy soils often lack enough moisture to support significant populations of beneficial organisms.
The balance of organisms can affect the availability of nutrients to plants and increase efficiency. Biological activity often corresponds with plant growth and synchronizes availability with need. Biological nutrient management can ensure that plants receive optimal fertility, but it cannot occur in environments that lack resources for soil organisms. The proteins, carbohydrates, and other organic compounds in plant residues, compost, and organic fertilizer are food for these valuable organisms.
Factors that we cannot control, such as extremes in precipitation and temperatures, also play a major role in the incidence of pest organisms. While the weather cannot be controlled, producing a strong, healthy plant in a rich and enlivened soil can increase the level of tolerance each plant has for extremes in the climate. Soils with adequate organic matter not only help accomplish this but also buffer extremely wet or dry climates, by increasing the soil’s water and air holding capacity.