Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209709
07/10/97
Secondary and Micronutrients for Vegetables and Field Crops Extension Bulletin E-486, Revised August 1994
By M.L. Vitosh, D.D. Warncke and R.E. Lucas Department of Crop and Soil Sciences Michigan State University Extension
Boron primarily regulates the carbohydrate metabolism in plants. It is essential for protein synthesis, seed and cell wall formation, germination of pollen grains and growth of pollen tubes. Boron is also associated with sugar translocation.
Boron requirements vary greatly from crop to crop. Rates required for responsive crops such as alfalfa, celery, sugar beets and table beets can cause serious damage to small grains, beans, peas and cucumbers. Boron deficiency may occur under a wide range of soil conditions. Alkaline soils have reduced uptake of boron due to high pH. Leached soils may be boron deficient because of low boron reserves. The soil types most frequently deficient in boron are sandy soils, organic soils and some fine-textured lake bed soils. Boron deficiency frequently develops during drought periods when soil moisture is inadequate for maximum growth.
Boron Deficiency Symptoms
Boron deficiency in crops causes a breakdown of the growing tip tissue or a shortening of the terminal growth. This may appear as resetting. Internal tissues of beets, turnips and rutabagas show breakdown and corky, dark discoloration.
Boron deficiency and leafhopper damage in alfalfa are often confused. Boron deficiency shows up as a yellowish to reddish yellow discoloration of the upper leaves, short nodes and few flowers (Vis. 27), (Vis. 28). Growing tips of alfalfa may die, with regrowth coming after a new shoot is initiated at a lower axis. Leafhopper damage shows up as a V-shaped yellowing of the affected leaves and may appear on any or all parts of the plant; the growing tip is usually normal and the plant may support abundant flowers. When the soil is dry and plant growth is retarded, both boron deficiency and leafhopper injury often occur in the same field.
Deficiency in cauliflower shows up as a darkening of the head and is associated with hollow and darkened stems. Hollow stem can also be caused by adverse weather conditions. Boron deficiency usually appears in small spots and may spread until the entire head is discolored. In sugar beets, the first symptoms are white, netted chapping of upper blade surfaces or wilting of tops. Later, if the deficiency becomes severe, transverse (crosswise) cracking of petioles develops, the growing point dies and the heart of the root rots (Vis. 29).
In celery, the first symptoms are brownish mottling along the margins of the bud leaves and brittle stems with brown stripes along the ribs. Later, crosswise cracks appear on the stems.
Acute deficiency in corn appears on the newly formed leaves as elongated, watery or transparent stripes; later, the leaves turn white and die. Growing points also die and, in severe cases, sterility is common. If ears develop, they may show corky brown bands at the bases of the kernels. Boron deficiency in corn has not been observed in Michigan.
Correcting Boron Deficiency
Crops grown in Michigan show a wide range of response to boron fertilizer (Vis. 6). The most responsive crops are alfalfa, cauliflower, celery, table beets and turnips. The boron recommendations for soil applications are 1.5 to 3 pounds for highly responsive crops and 0.5 to 1 pound per acre for medium responsive crops. Occasionally, certain deficient soils may require up to 5 pounds of boron per acre for cauliflower and table beets. The suggested rate for foliage application is 0.3 pound of boron per acre in 30 gallons of water for highly responsive crops and 0.1 pound for low to medium responsive crops.
The boron carrier most frequently used in fertilizer is sodium borate, which ranges from 10 to 20 percent boron. "Solubor" is a trade name for a sodium borate that is 20.5 percent boron. This compound is commonly used in foliar sprays or in liquid fertilizers.
Because boron is fairly mobile in soils, several methods of application can be used. Boron may be mixed with regular N-P-K fertilizer, applied separately on the soil, sprayed on the plant, topdressed (for alfalfa) or sidedressed (for row crops). Be sure to mix completely when boron is combined with other fertilizers. Segregation due to particle size differences is often a problem. Boron should never be used in combination seedings containing legumes and grass or small grains because it will injure the grass or small grains. Boron for the legume should be topdressed after the grass has become well established or the small grain companion crop has been harvested. Be careful when banding fertilizers containing boron near the seed or plants. Too much boron near the seed or plant may be toxic to young plants or germinating seeds.
Boron Toxicity
Boron toxicity on Michigan crops is usually limited to situations where boron-containing fertilizers are used at planting time on highly sensitive crops such as dry edible beans, corn, grass and small grains. Toxicity to crops has also occurred when sensitive crops were planted where fertilizers containing boron had been used earlier in the season. Similar problems may occur where sensitive vegetable crops are planted with high rates of boron in the starter fertilizer.
Unlike copper, zinc and manganese, boron is rapidly leached out of the soil or fixed in the soil so there is little potential for toxic carryover from year to year. Some waste waters used for irrigation may have high boron levels, but irrigation waters are not a problem in Michigan.
Boron toxicity is characterized by yellowing of the leaf tips, interveinal chlorosis and progressive scorching of the leaf margins (Vis. 30). In soybeans, the leaves may have a rust-like appearance. High levels of calcium may increase the boron tolerance of plants. Average boron concentrations in mature leaf tissues can be used to estimate plant boron status as follows: deficient-less than 15 ppm; sufficient-20 to 100 ppm; and excessive or toxic-over 200 ppm.
| Visual title - Visual size | Visual title - Visual size |
|---|---|
| Boron-deficient alfalfa - 26K | Boron-deficient alfalfa - 32K |
| Boron-deficient sugar beets - 32K | Relative response of selected crops to micronutrient fertilizers - 78K |
| Boron toxicity in navy beans - 33K |
