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Deep soil tests may show residual nutrients, save money

The high cost of fertilizer suggests that producers need to get back to the basics to improve nutrient management efficiency. “Get a soil test,” remains one of the most important activities farmers can perform to manage a fertilization program. Farmers may want to consider deep sampling this year.

Cotton and grain farmers may want to dig a little deeper this winter as they test soils for nutrient levels.

The high cost of fertilizer suggests that producers need to get back to the basics to improve nutrient management efficiency, says Mark McFarland, Texas AgriLife Extension sol fertility specialist.

Two factors make nutrient management more important this year. Those fertilizer prices, which hit historical highs last year of up to $1 per pound, in addition to failed crops on many acres, mean some farmers have valuable residual fertilizers in the soil.

Speaking at the grains session of the Blacklands Income Growth (B.I.G.) conference in Waco, McFarland says nutrient prices have leveled off a bit in recent months but “are still way up there. Nutrient management is extremely important. We can’t do anything about high prices. All we can do is look at how we can manage better.

“Focus on the basics—fertilizer type, fertility rate, application timing and application method,” he said, adding that selecting the proper fertilizer analysis to develop a prescription for each field is important.

Soil test

It’s been an Extension mantra for decades but “get a soil test,” remains one of the most important activities farmers can perform to manage a fertilization program. “Soil testing gives us the ability to develop a prescription system field-by-field,” McFarland says.

With the likelihood that many growers have nutrients carried over from failed or poorly yielding crops last year, a soil test is a good bargain for 2012 management. “Farmers need to identify carryover and naturally-occurring nutrients. Also, available organic matter breaks down and releases nutrients back into the soil.”

He says some crop fertility programs may require up to $200 per acre. “Residual fertilizer may be worth more now than when growers applied it because of the higher price,” he says. But it’s not going to reduce fertilizer expense if they don’t know it’s there. “It’s important to identify what’s already there and determine what else is needed for balanced nutrition. Growers need to avoid deficiencies, too.”

By the time producers identify nutrient deficiencies the crop has already suffered potential yield loss.

Typical soil test recommendation is to collect samples from zero to 6 inches deep. “That gives us a prescription for each field,” McFarland says. “It should be routine and it’s not complicated.”

But that may not be enough this year. Nitrogen is the nutrient growers are most likely to spend the most money for. “It’s soluble and very mobile. With rain, it moves below that zero to 6-inch area. And we know that crop roots will explore down to 24 inches to find nutrients.”

The question is: How deep do nutrients go into the soil and how deep can the crop reach it? McFarland and other Extension specialists set up trials to answer those questions and tested soil as deep as 4 feet.

“The results showed us that we have a lot of available nitrogen at depth. In some areas we found as much as 400 pounds per acre, and residual nitrogen at depth was consistent all across the state.

“So, can the crop use it?”

They set up a study and found that in most locations in soils that had sufficient nitrogen at depth cotton needed “little to no additional fertilizer to make yield goals. Out of 55 locations across the state, 33 had more than 100 pounds of nitrogen available at depth. Only 13 of those 55 locations responded to extra fertilizer.”

He says most growers will not sample to 4 feet. “But if they can get to 1 foot or 24 inches, they can take advantage of residual nitrogen. A good amount of nitrogen is available at that level.”

He said trials in grain showed similar results. Trials from 2010 showed residual nitrogen at 1 foot to 24 inches, resulted in the same corn yield response as if the grower applied the full rate of fertilizer, based on his yield goals.

“If we find 80 to 120 pounds of nitrogen in the soil, we cold back off by that amount and lose no yield and still save the money we would have spent for the extra fertilizer.”

Residual fertilizer saves money

He says 25 of 26 (96 percent) of the corn locations tested showed that using residual nitrogen plus enough to get to typical fertility rate produced yield goal. For sorghum trials, 14 of 16 locations produced yield using residual nitrogen as part of the total fertility program.

McFarland says some fields show more than $100 per acre in residual nitrogen available. “Typical is $30 to $35 per acre. Consequently, farmers may want to consider deep sampling this year. They may be able to reduce nitrogen expenses.”

But farmers can’t substitute the deep sample for the zero to 6-inch standard test. “They still need that to get the base,” he says. “Then they can sample deeper, as deep as they can go. The deeper you can test the more money you’ll save.”

He recommends sampling with two buckets, one for the shallow sample and another for the deep one.

He says phosphorus and potassium also may be available at depth.

McFarland says growers may check a Texas A&M website, to help calculate how much fertilizer they need.

He says timing fertilizer applications also affects efficiency. “Crops don’t need a lot of nutrients in the early stages of development. Consider delaying full fertilizer application, especially nitrogen, and apply as a sidedress. That gives you an opportunity to look at the crop and adjust fertility to realistic yield potential.”

He says phosphorus is also expensive and is different from nitrogen. “It is not very soluble and stays where it’s placed. So, if you apply it in the middles, it will not do much good this year. We need to put it close to the plant and incorporate it, preferably 5 to 6 inches deep, in the active root zone.”

He says 20 years of research have shown that if farmers place phosphorus 5 or 6 inches deep they can reduce the broadcast rate by half and achieve “the same or better yields.” With phosphorus prices ranging from 85 cents to 90 cents a pound, a half-rate can make a big difference.

Potassium deficiencies, McFarland says, are showing up in some Blacklands fields. Drought last year may have been a contributing factor because potassium needs moisture to move through the soil. He says it gets trapped between dry layers of clay. “It’s like an Oreo. The potassium is the crème center. If the outside layers are moist, it can seep through. If it’s dry, it’s trapped. Soil compaction also sets us up for trouble.”

McFarland says soil and foliar-applied potassium have not been successful. “We have seen no response and don’t see an extra application of potassium as necessary. Heavy soils typically have sufficient potassium; medium textured soils may be deficient. Soil tests may indicate that marginal potassium sufficiency may justify fertilizer applications.” Deep soil testing may help identify these deficiencies, he says.

McFarland says farmers should test fields where silage has been removed. “Silage takes out a lot of nutrients.”

He also cautioned farmers about nontraditional fertilizer products. “There are no silver bullets. We’ve looked at a few of these and have not found any benefits.”

He recommends that if farmers want to try something non-traditional they should look for scientific data to back up claims and then test only on small plots.





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