In this final installment of a two-part series on Alabama's long-term soil fertility research, Southeast Farm Press looks at how historical experiments are guiding the phosphorus and potassium recommendations of cotton farmers today.

Does higher cotton yield potential require higher rates of phosphorus and potassium? This is just one of the questions being answered by Alabama's long-term soil fertility research, says Charles Mitchell, Auburn University Extension agronomist.

Alabama's Two-Year Rotation tests — begun in 1929 — are located at six sites in the state and always have involved cotton, corn, sorghum, soybean or peanut rotations. The Rates of N-P-K trials, started in 1954, look at various rates of nitrogen, phosphorus and potassium on cotton.

“These long-term experiments already are in place, so we don't have to spend money putting in new tests to answer these soil fertility questions,” says Mitchell. “We can go in each year and answer these questions quickly. This data soon will go into a regional publication that all states in the Coastal Plain will use for soil testing purposes.”

Phosphorus fertilization, says Mitchell, is one of the oldest fertilizer practices used in the Southeast. The discovery of rock phosphate in South Carolina in 1867 and in Florida in 1887 culminated in a superphosphate industry in the heart of the old Cotton Belt, he adds.

“Many of the early experiments with phosphorus application to cotton in the Southeastern U.S. showed yield responses from up to 40 pounds superphosphate per acre. A 1965 survey found that most agronomists from Alabama to Virginia commonly recommended 60 to 80 pounds superphosphate per acre for upland cotton,” says Mitchell.

As a result of standard phosphorus recommendations through the early 1950s, many Alabama fields in long-term cotton production reached “high” or “very high” soil phosphorus levels and no longer were responsive to additional applications, he says.

The Auburn University Soil Testing Laboratory began service in 1953 to help cotton and corn producers identify those fields that no longer need additional phosphorus applications but could benefit from other soil amendments, notes Mitchell. However, soil test summaries for cotton samples from the lab since the early 1960s seem to indicate a gradual trend toward lower soil test phosphorus values, he says.

“More samples are testing ‘medium’ and ‘low’ and fewer are testing ‘high’ and 'very high' in phosphorus. Possible explanations for this trend include the use of less phosphorus fertilizers for cotton, deeper tillage which dilutes residual soil P, soil erosion, P fixation by soil minerals and the failure of growers or consultants to routinely test high-testing fields.”

A cotton soil fertility survey of 312 Alabama fields in 1991 revealed declining plow-layer soil-test phosphorus levels but identified only 10 percent of the fields that had leaf samples below the established sufficiency range for phosphorus. Only 9 percent of the plow layers tested “low” or “very low” in phosphorus, according to Mitchell.

Despite a strong research basis for soil testing in Alabama, producers and their consultants often question the validity of soil test interpretations, he says.

“No doubt, part of this is due to a wide range of interpretations available from different public and private soil testing services, opinions of consultants and competition for fertilizer sales.

“Emphasis on precision agriculture may have created expectations from soil testing beyond what it is capable of delivering. Nevertheless, as producers adopt new technologies, genetically improved varieties and new production practices, they expect and deserve periodic verification of soil testing interpretations from their public laboratories.”

Predicting cotton response to phosphorus isn't an easy job, says Mitchell. “But generally speaking, the public labs in the Coastal Plain are doing a good job of telling you whether or not you need to apply phosphorus. I'm not addressing the private labs, because they don't let us know what they're doing.”

Most Southern soil testing laboratories use a “critical value” in their soil test reports, he says. This, he explains, is the concentration at which 95 percent of maximum relative yield is achieved. Above this value, no phosphorus fertilizer is recommended because the probability of a yield response is extremely low. The lower the soil test value, the higher the phosphorus recommendation, he says.

“The critical values used for phosphorus on cotton by the Auburn University Soil Testing Laboratory were established in 1968 and have been verified and updated in numerous reports since then. Alabama's critical values appear to be in line with other state laboratories in the Southern U.S. that used the same extraction procedure for Coastal Plain soils. Alabama has a different critical value for the red clay soils of the Tennessee Valley.”

Many Alabama farmers, says Mitchell, want to know how long it takes to reduce soil test phosphorus in the soil. “Even though the soil test may indicate that they don't need to apply phosphorus, many farmers are reluctant not to apply any. Some farmers put it on for insurance — to make sure that levels don't drop below the ‘high’ mark. That's not a real bad practice, but it may be unnecessary.”

On several test plots in Alabama, researchers stopped adding phosphorus in 1982, according to Mitchell. “We didn't add phosphorus until 1998. We measured soil phosphorus during this time period, and those fields that tested high in the beginning remained high. They did not change over a 15-year period. Even though we were producing good cotton on those plots, soil-test phosphorus did not change — it remained high.

“You don't take off phosphorus from growing cotton or soybeans — it doesn't change at all. Those fields that were low in phosphorus stayed low in phosphorus. Those that were medium stayed at medium. That was the case in all of these long-term experiments throughout the state.”

Potassium nutrition of crops on the acid, highly weathered soils of the Southeastern U.S. always has been a concern, especially for cotton, which is susceptible to potassium deficiencies, says Mitchell.

“With increasing acreage and yields of cotton on these soils, new varieties, eradication of the boll weevil and new technologies for insect control, potassium nutrition is of renewed concern to growers.”

Current soil test calibration potassium values for Alabama soils still are accurate for modern varieties and yields, notes Mitchell. Plow-layer, soil-test potassium still is a very reliable tool for predicting the need for potassium fertilization on Alabama soils when other factors are not limiting, he adds.

“As soil-test potassium goes up, yield goes up to a certain point and then levels off. We don't recommend any more potassium above this critical level. If you're above that level, you won't get any response to additional potassium - you can save that money. If you want to put it out, it's not going anywhere. But, it's not necessary when levels already are high.”

Some growers, says Mitchell, say they are seeing potassium deficiencies in their fields. “Invariably, that cotton has a very high boll load. It's a high yielding crop, and we know from research that cotton roots stop taking up potassium at about mid to late bloom.

“The boll is loaded up with potassium in the burrs. It gets potassium from the leaves. We think that when roots stop taking up potassium, the potash moves out of the leaves. And, if you have heavy boll load, you get early defoliation.”

Foliar fertilization worked well in one year out of five in Alabama trials, says Mitchell. “It's difficult, with low cotton prices, to encourage producers to spend extra money on potassium nitrate for foliar applications, especially late in the season. If you begin early in the season — at first bloom — and continue with foliar fertilization throughout the remainder of the season, you might stand a one in five chance of getting a positive yield response.

“Generally, it's the physiology of the plant, and there's nothing you can do about it. Once potassium is in the soil, the plant has all that it can handle. More potassium won't increase the yield.”

Just as with phosphorus, many producers want to know how long it takes to reduce soil-test potassium in the soil, says Mitchell.

“Even if the soil test says you don't need to apply potassium, many growers still are reluctant not to apply it. About 100 pounds of potash will cost about $15 per acre. That's not a big investment when you consider some of the other cotton inputs. If it makes you sleep better at night, go ahead and put on an extra 100 pounds of potash. In some years, however, it might seem like a big investment.”

Just as in the phosphorus level experiments, potassium levels didn't change over a 15-year period, says Mitchell. If the level was high at the beginning of the experiment, it remained high.

“It won't change very much because potash is recycled. You won't take off a lot unless you're planting cotton behind a hay crop. A hay crop takes off about 40 pounds of potash for each ton of hay taken from the field.”

Potassium may leach in some of the deep, sandy soils found in south Georgia, says Mitchell, but it shouldn't move much in heavier soils. Hardpans, nematodes and saturated soils all can affect the uptake of potassium by cotton plants, he adds.

e-mail: phollis@primediabusiness.com