BORON -
AN IMPORTANT MICRONUTRIENT

Boron is unique among the micronutrients in that very small quantities are necessary for normal plant growth, and only slightly higher concentrations are toxic. With a number of plants the range between deficiency and toxicity is only a few parts per million. The actual amount of boron that must be applied to correct a deficiency varies with the method of application, the season, the soil type, the source of boron, and the crop.
Irrigation water may contain boron in amounts toxic to plants. The occurrence of boron in irrigation water and the deposits of boron occurring in the evaporated residues of surface soil water in some arid regions indicate that boron is not absorbed and held by the soil colloids. Additional evidence of this condition is that very slight applications of borox over that amount needed by the plant cause plant injury. In this respect, boron is quite different from zinc or copper, which are absorbed and held by the soil colloids.
The primary loss of boron from the soil is leaching. Because boron is not absorbed on the soil particles, boron moves with mass flow, and boron deficiency occurs following long periods of heavy-leaching rainfall. Leaching is also the technique used to remove excess boron from the surface soil and root zone. Boron availability is influenced by soil pH; the optimum range for maximum availability is between pH 5.5 to 7.0.
Most of the boron in soils is present in plant organic matter and soil microorganisms.
The accepted range of boron is narrow, with a deficiency occurring at less than 1 ppm in the soil. A toxicity occurs at levels above 5.0 ppm. Various crops differ on the level for deficiency and toxicity.
Plants deficient in boron exhibit abnormal growth at the growing points (meristematic tissue) with the apical growing point becoming stunted and dying.
In celery boron deficiency first shows as a brownish mottled bud leaf. The mottling is associated with brittleness of the stem with brownish stripes along the ribs. Crosswise cracks appear on the surface of the petiole and the tissue curls outward and turns brown.
Deficiency in cauliflower first appears as small concentric, water-soaked areas in the stem and in the center of the small curd. The small leaves around the curd may be deformed and stunted. As the deficiency continues, the outer and inner portion of the head may turn brown.
Boron deficiency in lettuce starts as malformation of the rapid-growing leaves, spotting and burning of the leaf tips, and death of the growing point of the head. Marginal growth ceases, and this results in folding back of the leaf tips.
Boron deficiency can be corrected with either a soil- or foliar-applied fertilizer. Care should be given when applying the boron since toxicity can occur from irregular distribution. On acid, sandy soil, applications of 10 lbs. of Borox per acre are sufficient.