Rice production: Samples key to managing nutrients

While more Delta rice farmers are aware of problems related to soil nutrient deficiencies, taking corrective measures may not be so simple a task.

According to Tim Walker, assistant professor of rice fertility and production at the Delta Research and Extension Center at Stoneville, Miss., that's because soil chemistry is complex and often wrongly interpreted.

Walker was a speaker at the Southern Plant Nutrient Management Conference in Olive Branch, Miss.

“The problem we are seeing is in some of our heavier clay soils that are highly fertile in terms of phosphorus and potassium but over years have been continually mined,” he says.

In such cases, Walker says, nutrient deficiencies can result in significantly lower yields. Having insufficient soil nutrients has been a recognized problem since the 1970s, he notes, but only in the past four years has the problem become more serious and widespread, especially in cut field areas.

Walker, who did doctoral research on the topic, said that for this year's Delta rice crop, deficiencies reduced yields 10 percent to 30 percent.

“In one situation, we cut less than 20 bushels of rice to the acre,” he said. “And that is almost unheard of in a non-treated check situation.”

Walker said farmers too often miscalculate when they try to replenish the nutrients removed when harvesting the crop.

“The farmer is going to say, ‘I sent my soil sample and it reads that I have 50 pounds of phosphorus. So, if I am removing 25 pounds of phosphorus with this harvested crop, then I am cutting it in half for next year's crop,’” he said. “That is the logical way of thinking.”

But he cautions that “one plus one doesn't equal two” in a particular soil's chemistry, and stresses that a soil sample cannot measure the natural reserve bank of nutrients below the topsoil, nor can one predict how much nutrient will be replenished into the topsoil naturally, commonly referred to as weathering.

“We are measuring only a small fraction (of nutrients in a soil sample),” he said. “You have total phosphorus and you have available phosphorus.

“You pull off the available phosphorus with the plants, but the total phosphorus — due to the weathering process — is going to release nutrients from the total available pool. That is an equilibrium in soil chemistry and is a variable.”

Still, soil sampling is encouraged. Worse than adding the wrong amount of nutrients, Walker warns, is for a rice farmer to ignore the problem altogether.

“When a farmer reaches the point that his soils are depleted in nutrients, he has a yield response every year, and he has no choice but to do something,” he said. “Then you have to put nutrients out whether pennies are tight or not.”

Instead, Walker says, the remedy is to make a long-term investment by keeping check on the potential problem annually or every two years.

“If a farmer is putting out some nutrients every year, and for whatever reason his funds are low one year, he can skip a year or two and still be OK because he would not see a yield response.”

Walker recommends rice farmers get in the habit of conducting soil sampling during early winter and on a regular schedule.

He says consistency is the key. “If you are going to sample a field this year, and you do it in the fall, then make sure the next time you sample — whether it is next year or in two years — sample during the same time again. “You don't have to sample soil every year; in problem fields you can do it every two years; and in non-problem fields every three to four years.”

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