The Effects of Soil Fertility

George L. McNew.

Many farmers and gardeners have observed that plant diseases have become more prevalent and soils less fertile than they used to be. Some have argued that the two phenomena are related and that diseases are more destructive because the plants have been weakened by poor mineral nutrition. A few have even maintained that there would be no serious disease problems if plants were grown in properly conditioned soil, but evidence obtained in hundreds of experiments throughout the world does not uphold this extreme viewpoint.

Soil fertility does affect the prevalence and severity of some plant diseases, but it is only one of several factors that predispose plants to infection by fungi, bacteria, viruses, and nematodes. We can make no sweeping generalizations about the effect of fertilizers on diseases because of the extreme differences in crop plants, their special nutritional requirements. the soil types upon which they are grown, and the diversity of the pathogens that attack them. Some diseases are severe on weakened, undernourished plants. Many others are most destructive when plants are growing vigorously.

If wheat on a moderately fertile soil, for example, is given an extra supply of nitrogen it probably will escape seedling diseases more readily, be more subject to pythium root rot, suffer less from take-all disease, and be more subject to infection by leaf rust and powdery mildew. Phosphorus and potassium fertilizers would have an entirely different effect on those same diseases. The addition of barnyard manure to wilt-infested cotton-fields will reduce the amount of wilt in places in Arkansas where potassium and nitrogen are deficient but will increase the severity of the same disease on the delta of the Nile, where nitrogen is readily available.

Such effects of soil fertility on plant diseases must be understood in this age when fertilizer practices are being changed so rapidly. More than 18 million tons of commercial fertilizer and 25 million tons of agricultural lime were used in 1948. That is about twice as much as was required before the Second World War. Less barnyard manure is being added to the soil each year. Not enough green manure from cover crops is being plowed under to maintain the organic matter content of soils on most of our farms. Under such conditions, both the balance of the various nutrient elements and the total supply of nutrients available to the plant can be expected to fluctuate appreciably from year to year and during the growing season.

THE PRIMARY CONSIDERATION in fertilizing soil is to use such materials, in combination with suitable crop rotations and other soil management practices, as are necessary to promote the maximum productivity of the plant. Disease control is a secondary consideration beyond the fact that one must avoid certain conditions, such as an excess of nitrogen or other available nutrients, a deficiency of Potassium, or changes in soil reaction that affect diseases of a crop.

Fertilizer materials that leave acid residues (such as those from ammonium sulfate, potassium sulfate, sulfur, and calcium sulfate) should be employed where neutral or alkaline soils favor diseases like potato scab.

Sodium nitrate, calcium phosphate, limestone, and similar materials that leave alkaline residues should be used where diseases such as club-root of cabbage and some wilt diseases are suppressed by alkaline soils.

Residues from organic matter are valuable for stimulating the growth of beneficial micro-organisms in the soil. They may destroy or prevent the growth of some plant parasites that are not well adjusted to living in the soil environment.

When diseases become a serious problem on properly nourished, productive plants, other control measures such as spraying, crop rotation, or use of disease-escaping varieties will have to be employed. There is no sound reason for starving the plant into an unproductive state in order to escape disease. If the plant is properly nourished and capable of full development, the disease control measures, such as spraying or soil disinfestation, are fully justified. They ire a form of crop insurance. They pay the largest dividends on productive soils. Often they are not worth applying to weak and undernourished plants.

Several plant diseases are influenced so seriously by soil deficiencies that much of the damage from them can be avoided by soil treatment. The outstanding examples, which are discussed here, are take-all disease of wheat, wheat root rot, Texas root rot of cotton, sugar beet seedling diseases, fusariam wilt of cotton, wildfire disease of tobacco, clubroot of cabbage, common scab of potatoes, bacterial leaf spot of peaches, and powdery mildew and rusts of cereals.

A deficiency of any plant nutrient may influence disease development. Nitrogen, phosphorus, and potassium, the primary elements, are mentioned most often. The secondary elements, calcium, sulfur, silicon, manganese, and boron also have been observed to exert appreciable influence on the prevalence of plant diseases.

Nitrogen is supplied in the form of sodium nitrate (Chilean nitrate), ammonium sulfate, urea, organic nitrogen, or anhydrous ammonia. It promotes vigorous growth and is essential for production of amino acids, growth regulants, and new protoplasm. Used to excess, it encourages rank, vegetative growth, delays maturity, and tends to cause thin cell walls. Fungi may penetrate the thin walls more readily than normal ones. Infected plants collapse more easily. Cereal plants lodge. Lesions on leaves elongate rapidly. Because nitrogen in a proper combination with other nutrients often speeds up growth of seedlings and roots, the plants escape severe damage from pathogens that develop slowly. But because nitrogen may prolong vegetative growth, leaves are exposed to infection over a longer season. The roots, water-conducting tissues, leaves, and fruits of plants that are supplied with nitrogen are more nutritious to most pathogens, which grow better in them than in nitrogen-deficient plants.

Phosphorus is applied as rock phosphate, superphosphate, ammonium phosphate, basic slag, or bonemeal. Phosphorus is essential for utilization of carbohydrates and for cell division because it combines with carbohydrate materials to form nucleic acids. Adequate supplies of phosphorus promote root growth and seed development. Applications of it are most beneficial against seedling diseases and certain root rots where vigorous development of roots permits the plants to escape destruction. It is essential for multiplication of viruses in host cells and may increase susceptibility to viruses and other disease agents if too abundant. Because of its use in building new cells, any imbalance with nitrogen may cause disease losses to increase.

Potassium (potash) is supplied to soils as potassium chloride (Kainit, muriate of potash), potassium sulfate, potassium nitrate, or wood ashes. Unlike other essential nutrients, it does not become a structural part of the plant cell. It is a mobile regulator of cell activity and promotes the reduction of nitrates and the synthesis of amino acids from carbohydrate and inorganic nitrogen. Potassium promotes the development of thicker outer walls in the epidermal cells and firm tissues, which are less subject to collapse.

A deficiency of potassium enforces the accumulation of carbohydrates and inorganic nitrogen in the plant. Eventually it retards photosynthesis and production of new tissues. More plant diseases have been retarded by use of potash fertilizers than any other substance, perhaps because potassium is so essential for catalyzing cell activities. It is unavailable in many soils of light texture because its salts are so water-soluble they readily leach from the soil.

Calcium is added to the soil as ground limestone, hydrated lime, gypsum, or calcium phosphate. It is essential for normal growth since it regulates chromosome development in cell division and is assimilated into the middle lamellae of new cell walls. It is therefore important for cell division and cell development. It also may neutralize acid byproducts of cell metabolism that could become harmful if not precipitated in an insoluble condition. Calcium also influences plant diseases indirectly by its effect on soil acidity, by neutralizing toxins produced by wilt-inducing fungi, and by affecting cell division in those diseases where abnormal growth of tissues is important. Its balance with potassium becomes a primary consideration in gall development because both materials contribute to the growth and division of cells.

Silicon affects the availability of potassium. It also may be combined with other materials to give the cell walls greater structural strength. The primary effect on plant diseases apparently is in the prevention of infection by powdery mildew, a disease in which the fungus develops externally and usually penetrates the host cell through the outer wall. Sulfur is oxidized to sulfates and thereby promotes soil acidity, which discourages growth and survival of some bacteria and fungi.

Soil reaction (the hydrogen-ion concentration, expressed in terms of pH units) influences the growth and persistence of some fungi and bacteria. The parasitic organisms that depend on delicate bodies, such as thin-walled swarmspores, may have difficulty in multiplying and infecting roots in soils at unfavorable hydrogen-ion concentrations. Hydrogen-ion concentration may change the ability of pine tree roots to resist invasion, but that, if true, is an exceptional case. Soil reaction also may affect the availability of essential nutrients in the soil and the biological balance between plant parasites and saprophytic soil-inhabiting fungi and bacteria.

Although organic matter contributes essential nutrients to the crop, it probably exerts more influence through the physical and biological changes chat it brings about in the soil. The carbohydrates and proteins in animal and plant byproducts provide nourishment to soil organisms that fix nitrogen from the air, tie up available nitrates in the soil, and frequently suppress plant parasites by antibiotic activity.

The humus from lignified plant tissues and other residual products promotes the massing together of soil particles and thereby improves aeration and water-holding capacity of soils. Organic matter may promote the growth of plant parasites or even facilitate their dispersal. Some of the decomposition products may increase the susceptibility of roots to invasion.

YOUNG SEEDLINGS of most crops are attacked by fungi that live in the soil or are carried on the seed.