From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Summer 1997, No. 1


Good soybean producers recognize that phosphorus and potassium are as important as molybdenum and nodule bacteria for the soybean nitrogen-manufacturing processes. Soybeans get most of their nitrogen through the nitrogen-fixing nodules on their roots, but only if molybdenum, phosphorus, and potassium are adequate, and if nodule bacteria are abundant and active. Molybdenum and bacteria, which form and live in nodules on soybean roots, work in partnership to provide the majority (70 to 90 percent) of the soybean plant's nitrogen requirements. Molybdenum enables the symbiotic nitrogen-fixing bacteria to convert nitrogen gas from the air into a form that plants can use.

As soil becomes more acid, molybdenum availability decreases. At soil pH values below 7.0, and especially below 6.5, soybean growers should consider treating seed with 0.2 to 0.4 ounces of actual molybdenum per bushel at planting. If soybeans have not been grown on a field within the last three years, seed should also be inoculated with the Bradyrhizobia bacteria which are specific to soybeans. This would especially apply to land that is coming out of the Conservation Reserve Program or any fields with a history of plants exhibiting pale-green colors, poor nodulation, and lower-than-expected yields. Care should be taken to ensure that live bacteria are used by checking the expiration date on the label of the inoculant container. In many southern soybean fields where soil pH is below 5.8, a molybdenum seed treatment alone has often increased yields from 2 to 6 bushels per acre, when phosphorus and potassium fertility were not limiting.

Research has shown that adequate phosphorus and potassium fertilization is important and that optimum response, especially to phosphorus, is not possible unless the plant has an adequate supply of molybdenum. For example, scientists at Auburn University observed no yield response to phosphorus unless molybdenum was adequately supplied, even where the soil pH was above 6.0. They measured yield increases with molybdenum ranging from 5 bushels per acre on plots which had historical applications of lime plus phosphorus, potassium and some micronutrients, to 15 bushels per acre on plots which had received phosphorus and potassium but no lime. Increases in leaf nitrogen concentration and 100-seed weight were also measured. Lime alone significantly increased yield by raising the soil pH and increasing soil molybdenum availability.

If lime is needed, it should be applied. If lime is applied this year on soils with a pH below 6.5 to 7.0, molybdenum should also be applied as a seed treatment at, or prior to, planting. If soybean seed are not treated with molybdenum at planting, growers can foliar spray about one ounce of sodium or ammonium molybdate per acre within a couple weeks after emergence to prevent or correct the molybdenum deficiency.

Apply phosphorus and potassium based on soil tests for high soybean yields and make sure that molybdenum does not limit the potential for profitable responses. At a cost of $2.00 to $3.00 per bushel of seed for a molybdenum seed treatment, and with soybeans priced at more than $7.50 per bushel, balanced phosphorus, potassium, and molybdenum nutrition can result in a very good return on the investment. Failure to provide a balanced supply of these nutrients will limit nitrogen fixation, reduce yields, and lead to lost profits.

Manage a soybean field as an efficient nitrogen-fixing factory. Apply phosphorus and potassium based on soil tests, provide adequate molybdenum, and inoculate seed with nodule bacteria where warranted. Sound nutrient management is profitable, environmentally friendly, and economically rewarding.


For more information, contact Dr. Cliff S. Snyder, Midsouth Director, PPI, P.O. Drawer 2440, Conway, AR 72033-2440. Phone (501) 336-8110.

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