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

Spring 1996, No. 7

RICE RESPONDS TO PHOSPHORUS AND POTASSIUM

Many rice growers in the Midsouth have traditionally not applied phosphorus fertilizer to their fields. Under flooded conditions, the phosphorus that is bound with iron and aluminum oxides is changed chemically and becomes more available to the rice plants. However, a recent study in Arkansas has documented a profitable response to phosphorus application on soils testing low in phosphorus and potassium. Phosphorus should be applied to low testing soils, especially where the soil pH is above 7.0. Each bushel of rice removes about 0.3 pounds of P2O5. Yields above 150 bushels remove more than 40 pounds of P2O5, and after several years, available reserves of soil phosphorus can be decreased if supplemental phosphorus is not supplied.

In fields which fail to green up after midseason nitrogen applications, the problem may be potassium deficiency. Arkansas research has also shown good responses to potassium fertilization on such fields. When a rate of 60 pounds of K20 per acre was applied as potassium chloride, the yield in one study was increased 10 bushels, from 172 to 182 bushels per acre. When the needed phosphorus was also applied at a P2O5 rate of 40 pounds per acre, yields were increased further to 198 bushels per acre. Phosphorus and potassium worked together to increase the rice yields.

There is some grower concern about the risk of increased “salting out”, or salt-induced stand reduction, with increased potassium fertilization. The Arkansas research has shown that some stand reduction may be possible during early seedling development as soluble salts wick near the soil surface with soil drying. However, even with some mild stand reduction, which did not prevent adequate plant populations, rice yields were increased as potassium deficiency was overcome. When the required phosphorus was applied with the needed potassium, stand loss was reduced and yields were improved.

Field observations and lab diagnoses by agronomists and plant pathologists have documented increased rice diseases and poor yields where phosphorus and potassium needs have been neglected. Adequate phosphorus and potassium nutrition promote:
Once soil test levels drop to the point that rice will respond to phosphorus and potassium, wheat and soybean yield losses have probably also occurred. If soybeans or wheat are rotated with rice on fields with low phosphorus and potassium tests, they can also benefit from fertilization. Some growers in the past may not have fertilized soybeans which were rotated with rice because of low soybean prices. With improved soybean prices this year, it may be time to reevaluate your soybean fertilization program...to benefit the soybeans and the rice grown in rotation.

High yielding rice varieties, which respond to higher nitrogen rates, require balanced phosphorus and potassium nutrition. Can you afford to apply the nitrogen necessary for high rice yields and risk poor returns on your nitrogen investment...because you short-changed the crop’s phosphorus and potassium needs? Plan now to get your crop off to a fast and efficient start and optimize your rice yields and profits with improved phosphorus and potassium management.
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