Source: Dr. Joseph R. Heckman, Extension Specialist in Soil Fertility, Rutgers Cooperative Extension

Function of Boron in Plants
Boron (B) is essential for cell division in growing plant tissue; good pollination, fruit set and seed development; translocation of sugars and starches; synthesis of amino acids and proteins; nodule formation in legumes; and regulation of carbohydrate metabolism.

Boron-Deficiency Symptoms
Normal plant growth, from germination to maturity, requires a continuous supply of B. Once B is taken up and used in the plant, it is not translocated from old to new tissue. This explains why B-deficiency symptoms often start with the youngest growing tissues. In B-deficient plants, growing tips may not develop properly, or they may die back while the older leaves remain green.

Plant Tissue Analysis
Analysis of plant tissue gives a good indication of a plant's B nutritional status. Tissue analysis also helps determine whether B is in proper balance with other nutrients, or if it is present in excessive amounts.

Depending on the crop, the minimum B sufficiency levels are in the range of 5 to 30 ppm B in the dried leaf tissue. Mature normal leaves may have concentrations of up to 100 ppm B. Leaf concentrations above 250 ppm B are in the toxic range. For plant B analyses in general, mature leaves just below the growing tip on main branches and stems are suitable tissues for sample collection. Procedures, however, vary with growth stage and crop species to be sampled.

Soil Analysis
Soil testing, when used in combination with plant tissue analysis, is an effective way to predict need for B fertilization and to diagnose B nutritional problems. The problem may be deficiency or toxicity.

Most soil laboratories use the standard hot water extract for soil test B. Some laboratories use the Mehlich III soil test for B. This method is well correlated with the hot water method and the soil test levels have similar results. Soils are normally considered to have sufficient or adequate levels of B when the soil test level is greater than 1.0 ppm, as measured by hot water soil test. The minimum B level that is defined as adequate, however, varies with the crop to be grown. Crop sensitivity to B deficiency and soil characteristics is an important consideration.

Boron toxicity may be a concern when the soil test level is high. Soil B levels above 1.5 ppm have the potential to cause B toxicity in sensitive crops grown on sandy soils. Crops differ in the amount of B they require, and certain species (grain legumes, snapbean, small grain cereals) have a low tolerance to excess B. To avoid B toxicity, discontinue application of B if the soil tests are excessively high in the nutrient.

Occurrences of Boron Deficiency
Boron deficiencies occur over a much wider range of soils and crops than do deficiencies of any other micronutrient element.