Michigan State University Extension
Soils & Soil Management - Fertilizer - 05209710
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
Molybdenum functions largely in the enzyme systems of nitrogen fixation and nitrate reduction. Plants that cannot fix adequate N or incorporate nitrate into their metabolic system because of inadequate molybdenum may become nitrogen deficient. The usual carriers of molybdenum are sodium or ammonium molybdate. These salts contain about 40 percent of the element.
Molybdenum is required in very small amounts. Normal tissues usually contain between 0.8 and 5 ppm; some plants may contain up to 15 ppm. Deficient plants usually contain less than 0.5 ppm. Certain non-responsive crops such as grass and corn may contain as little as 0.1 ppm. The responsive crops are clover, cauliflower, broccoli, lettuce, onions, spinach and table beets. Very few soils in Michigan show a need for molybdenum fertilizers. Those that do are fibrous peats, acid sandy soils and organic soils that contain large amounts of bog iron.
Molybdenum Deficiency Symptoms
Molybdenum deficiency in clover shows up as a general yellow to greenish yellow foliage color, stunting and lack of vigor. The symptoms are similar to those caused by nitrogen starvation. Early stages of the deficiency in cauliflower and broccoli appear as a marginal scorching, rolling or curling upward, and withering and crinkling of the leaves (Vis. 31). In later growth stages, the deficiency shows up as "whiptail," especially in the younger leaves. The leaf blade is often very narrow or non-existent. Older leaves show crinkling and marked yellow mottling between the veins. In onions, molybdenum deficiency shows up as dying leaf tips. Below the dead tip, the leaf shows 1 or 2 inches of wilting and flabby formation. As the deficiency progresses, the wilting and dying advance down the leaves. In severe cases, the plant dies.
Correcting Molybdenum Deficiency
Molybdenum deficiency can be corrected by seed treatment and/or foliar applications. For seed treatment, dissolve .5 ounce of the molybdenum compound in 3 tablespoons of water and mix with sufficient seed to plant one acre. Using excess water can cause the chemical to penetrate and injure the seed embryo. Mix the seed thoroughly and let dry. It is advisable to use a suitable fungicide dust to help dry the seed.
For foliar sprays, apply 2 to 3 ounces of the compound per acre. Use wetting agents in the spray when applying the solution to cauliflower or onions. For some cauliflower varieties, repeated applications at two-week intervals are beneficial.
Soil acidity has a marked influence on the need for molybdenum the greater the acidity, the greater the need for molybdenum. Research plots on a Montcalm sandy soil showed that liming from pH 4.9 to pH 6.7 increased the molybdenum concentration of cauliflower fivefold. In a Houghton muck, raising the pH from 5.4 to 7.2 increased the concentration of molybdenum more than threefold. Liming severely deficient soils, however, will not completely correct the deficiency.
Molybdenum Toxicity
Plants appear quite tolerant of high soil molybdenum concentrations. There is no record of molybdenum toxicity under field conditions. In greenhouse studies, tomato leaves turned golden-yellow and cauliflower seedlings turned purple. Animals fed foliage high in molybdenum may need supplemental copper to counteract the molybdenum.
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| Molybdenum-deficient cauliflower - 30K |
