Boron and Soil Fertility

Darrell A. Russel.

The modern American kitchen contains enough boron to produce 16 tons of alfalfa hay. This boron is in the enamel in freezers, stoves, refrigerators, sinks, dishes, and glassware.

But many thousands of acres of agricultural soils in the United States contain only enough available boron to produce a ton or two of alfalfa hay. That amount is the equivalent of the boron in an iron cookstove, an icebox, and a good set of china dishes.

Many crops are affected by a deficiency of boron in the soil. Sugar beets, alfalfa, and clovers are the most commonly affected of the agronomic crops. Celery, beets, cauliflower, apples, grapes, pears, walnuts, sunflowers, and asters are a few of the many vegetable, fruit, nut, and ornamental crops that may suffer from too little.

BORON WAS FIRST USED as a fertilizer about 400 years ago when borax (then called Tincal or Tincar) was shipped from central Asia to Europe. Not until 1915, however, was boron suggested as an essential element for plant growth, when P. Maze of France made this suggestion as a result of his work with corn grown in nutrient solutions. Katherine Warington, at the Rothamsted Experimental Station in England, provided the first proof in 1923 that boron was an essential element.

An inadequate supply of boron in the soil was shown to be the cause of heart rot and dry rot of sugar beets and marigolds in Germany by E. Brandenburg in 1931.

L. G. Willis and J. R. Piland, of the North Carolina Agricultural Experiment Station, were among the first in the United States to show that legumes, especially alfalfa, responded to borax fertilizer. The two agronomists published the results of their research in 1938. Boron deficiencies have been reported since then in 41 States and for 90 or more crops.

Most of the boron in soils is in the form of the highly insoluble mineral tourmaline. The total boron content of the soil varies between 20 and 200 pounds in the plow depth of an acre.

The total boron content of the soil is not a reliable guide to the adequacy of boron for crop growth, because less than 5 percent of the total may be available for the use of plants. The determination and measurement of the available forms of boron have been of concern since 1931.

AVAILABLE BORON occurs in two broad forms, inorganic and organic.

The inorganic forms (chiefly calcium, magnesium, and sodium borates) resulted originally from the slow dissolution of minerals containing boron. Soil micro-organisms and plants utilized this boron in their growth, transforming the boron to organic forms. Upon the death of the micro-organisms and plants, the organic boron was oxidized to inorganic boron.

The amount of boron available for reuse, plus that made available through the continued weathering of unavailable forms, is adequate for crop growth in some soils. The loss of available boron through crop removal, leaching, and reversion to unavailable forms, coupled with higher requirements for boron through better crop varieties and improved cultural practices, has resulted in an inadequate supply of boron available for crop growth on many agricultural soils.

Loss of boron through crop removal is unavoidable. Nevertheless such losses must be recognized, and the nutrients that are removed (boron, as well as the other essential elements) must be replenished eventually by the application of fertilizer.

Each ton of alfalfa hay contains 1 ounce of boron. A ton of sugar beets contains 2.5 ounces of boron. One hundred bushels of peaches contains 4 ounces of boron, while 100 bushels of corn contains only 0.4 ounce of boron.

The seriousness of boron losses by crop removal depends, then, on the kind of crop and on whether the crop is utilized on the farm and returned to the soil as manure.

Leaching losses are considerable, particularly in the acid soils of humid regions. Joe Kubota, K. C. Berger, and Emil Truog, working with soils in Wisconsin, found that the rate of boron movement in the soil was related primarily to soil texture. Boron fertilizers applied to soils that were uniformly light-textured throughout the profile moved to a depth of 24 inches, or deeper, in 6 months. Little of the boron moved below the 12-inch layer in the heavier soils.

Many other factors influence the availability of soil boron. One of the most important is the pH of the soil.

James A. Naftel, working in Alabama in 1937, discovered that small additions of boron to the soil counteracted the harmful effects of overliming and that the application of lime to the soil often results in a lower availability of soil boron.

R. V. Olson and K. C. Berger, in research in Wisconsin in 1946, investigated the effect of soil reaction on boron fixation. They found that fixation was closely related to the clay content and to the soil pH as changed by the addition of sodium hydroxide, calcium hydroxide, and hydrochloric acid. They demonstrated that clays fixed the most boron, silts fixed an intermediate amount, and sands fixed very little. The cations used had little influence on the boron fixation, but the alkalinity they produced resulted in fixation.

The fact that the boron that is fixed because of a rise in pH can be. made available again by lowering the pH to its original value indicates that a reversible chemical reaction is involved. This reaction is quite rapid.

The evidence suggests further that most of the boron is fixed by a soil mineral or group of minerals, the activity of which predominates in the clay fraction of soils but is also present in the silt fraction. The pH of the soil has a great effect, direct or indirect, on the ease with which this fixation occurs.

The organic matter also influences the fixation of boron. A. Maquori, G. Stradaioli, and E. Perici in Italy learned that organic boron is the most soluble boron in soils. The Wisconsin scientists, Dr. Olson and Dr. Berger, found that oxidation of the organic matter in soils caused significant increases in the amount of boron released to the available form as well as decreases in the amount of boron that could be fixed by the soil.

Dry weather accelerates the appearance of symptoms of boron deficiency in crops in soils low in available boron. L. P. Latimer, of the New Hampshire Agricultural Experiment Station, found that drought in June and July was the chief predisposing factor causing boron deficiency in apples. The actual role of dry weather in causing boron to become unavailable has not been ascertained.

R. Q. Parks, at the Ohio Agricultural Experiment Station, found that drying of the soil in the laboratory increased the fixation of boron. Boron deficiencies have been observed to be more severe in places in the field where the soil had dried out excessively in dry years, according to J. C. Walker and his associates at the Wisconsin Agricultural Experiment Station.

Dr. Berger has stated that it is doubtful whether the plow layer of a field will dry out enough in most years to cause appreciable fixation of boron. If boron were fixed to any extent, most soils would be deficient in boron; soils in and regions would be particularly deficient. That obviously is not so.

Dr. Berger suggested that the reason for the appearance of boron deficiency symptoms in dry years, as contrasted to wet years, is that most of the available boron is in the surface, or organic, layer of the soil. When this layer becomes dry, plants feed in the layer to a limited extent because of the lack of water. Instead, plants feed from the lower soil horizons, which are usually low in organic matter and low in available boron.

Boron deficiency in dry years therefore occurs because the supply of available boron has been reduced not by fixation but by the inability of plant roots to feed in the surface horizon. Some boron fixation may be caused by the drying of surface soils in extremely hot and dry weather, but it is doubtful if much boron is fixed below 2 inches.

THE GEOGRAPHIC EXTENT of boron deficiencies in the United States is difficult to describe. The greatest areas of deficiency are in the humid regions where soils are generally acid.

When Kenneth C. Beeson, of the United States Plant, Soil, and Nutrition Laboratory, mapped the boron deficient areas in the United States in 1945, the presence of boron deficiencies had been reported in only 31 States. A current map of the boron-deficient areas shows that boron deficiencies have been found in every State east of the Mississippi River and generally in the first two rows of States west of the Mississippi, as well as the Pacific States. Only seven States reported all their soils to be supplied adequately with available boron.

Dairy farms in particular are among the first farms to show deficiencies of boron in most areas. Alfalfa is grown extensively on many of them, and alfalfa has a high boron requirement. Unless the dairyman conscientiously returns manure to his fields, the loss of boron through crop removal may soon deplete the available boron.

The supply of available boron is exhausted rather quickly in places where vegetable crops are grown commercially. Yields of vegetable crops usually are high because of the large amounts of fertilizer used. Since several crops may be harvested from a field each year, the loss of boron by crop removal may soon mean deficiencies.

Orchards are established preferably on deep, well-drained, well-aerated soils. These conditions favor the loss of boron by leaching. Applications of nitrogen fertilizer, by increasing the tree growth, increase the boron requirement and thereby augment the deficiency.

SYMPTOMS of boron deficiency vary with the type and age of the plant, the conditions under which it grows, and the severity of the deficiency.

The first visual symptom generally is the death of the terminal growing point of the main stem. The lateral buds then produce side shoots, but the terminal buds on the shoots die also. Further rebranching may occur. This multibranched plant is often described as having a rosette appearance.

Further symptoms are a slight thickening of the leaves, a tendency for the leaves to curl, and sometimes chlorosis.