The price of fertilizer may have decreased some and stabilized from recent historic peaks, but it’s still expensive. So it makes sense for farmers to be as prudent as possible with this essential resource.

“Pay attention to the price of fertilizer,” says Mark McFarland, Texas A&M professor and AgriLife Extension soil fertility specialist.

McFarland kicked off the grain commodity session of the annual Blacklands Income Growth Conference recently in Waco, Texas, with a recommendation that farmers should determine the amount of nutrients available in their soils before they add more and should consider the value of various application techniques, the reliability of new and standard products, and the effect that pH may have on nutrient availability.

“The price of fertilizer is still up,” McFarland said. “It has stabilized somewhat, but compared to prices from the not-too-distant past, it’s still extremely high.”

Consequently, farmers can’t waste money on products they don’t need, application techniques that don’t make economic sense or by not applying what’s needed on a timely basis.

He said a fertilizer delivery system, the Exactrix, has been touted as providing a method with no nitrogen loss. Other claims included increased stability in the soil, keeping nitrogen in ammonium form, uniform delivery and potential to reduce nitrogen use by 30 percent to 40 percent.

Research performed in the Texas High Plains did not support those claims, he said. Tests comparing application with the Exactrix system to traditional nitrogen application showed “no response to method of application in either of the two years in trials.

“With new products, it’s often hard to find reliable, non-biased information. The Exactrix equipment is good, solid machinery but is expensive. Anhydrous would have to be very cheap to pay back the investment.”

Deep sampling

McFarland said a better approach for nitrogen management is to identify what’s available before applying nutrients. Nitrogen carryover from one year to the next may be more significant and more useful than some farmers might think.

“Nitrogen is soluble, so it moves below the typical 6-inch sampling depth. Much of that nitrogen below 6 inches is still plant-available.”

As nitrogen prices have risen in the past few years, interest in residual nitrogen has increased. McFarland recommends farmers look at nitrogen carryover just before planting. “Sample to depth,” he says. Sampling should include the typical 6-inch depth for a routine nutrient analysis.  But producers also should collect a second sample from 6 to 12, 6 to 18, or 6 to 24 inches. Significant amounts of nitrogen likely will be present in the deeper sample and can be credited to reduce to fertilizer nitrogen needs for the next crop.

“Deeper sampling requires good soil moisture conditions, but with a little extra effort a hand probe can go that far,” McFarland said.

Much of that residual nitrogen will be available to corn, grain sorghum or cotton. Tests across Texas have shown that crediting residual nitrogen measured to 24 inches as part of the annual fertilizer requirement produces yields equal to adding the full amount based solely on yield goal.

Multiple tests have indicated that many samples to depth included a minimum of 40 pounds of nitrogen, available to the plant, per acre. “There is economy to be gained in using that nitrogen,” he added.

Cotton will respond to residual nitrogen, as shown by tests on more than 100 sites across Texas. “Consistently, we could achieve maximum yields with little, if any, supplemental nitrogen. We did a lot of early work on cotton with funding from Cotton Incorporated. Over the last few years, we’ve also tested corn and grain sorghum. Preliminary data show very similar results.”

When residual nitrogen measured in soil samples taken to 6, 12, or 24 inches was deducted from the fertilizer application, corn and grain sorghum produced as much yield as a full rate of nitrogen based on the yield goal.  

Economics make sense

The economics, McFarland says, make sense with savings of $20 to $33 by crediting residual nitrogen, “with the same yield. Some values are even higher, depending on the events of previous years in a particular field.”

If drought limited the previous year’s crop growth, plants removed little nitrogen from the soil. A Nueces County field showed a $93 per acre advantage following two years of failed crops.

In research trials on corn, 28 of 29 locations—97 percent—produced equal yields with residual nitrogen credits, compared to application of recommended nitrogen amounts. For grain sorghum, 17 of 19 sites—89 percent—produced equal yields with credited nitrogen compared to recommended rates.

“Consider deep sampling,” McFarland said. “At 6 inches, sample for nitrogen, pH, phosphorus, potassium, calcium, magnesium, sulfur and micronutrients. At depth, sampling for just residual nitrogen is cheaper, just $4 per sample instead of $10.”

Farmers may gain some advantage, however, by testing for other nutrients at depth.  Potassium and sulfur found in deeper samples also can be credited and provide additional savings.

Foliar fertilization

Application method and timing may play a role in nutrient efficiency. McFarland said foliar fertilization may be a useful tool, under limited circumstances. “We have two situations where we might consider use of foliar fertilizer,” he said. “If we have a deficiency in-season, we may need to apply foliar fertilizer to salvage the crop.”

The other situation where a foliar nutrient application might be justified is to take advantage of excellent growing conditions to achieve “a bonus yield.”

With a deficiency, the application may be justified, he says; for bonus yield, it’s unlikely to pay. Pushing yield requires significant amounts of the primary nutrients, like nitrogen, and foliar applications are not as efficient as soil-applied fertilizer. High rates also may burn leaves, so lower rates with multiple applications are often necessary to get enough nutrient into the plant, and that increases the cost.

“Plants are designed to take in nutrients through their roots,” McFarland said. “So do the math, and most often you will find that the (bonus) application provides no real economic advantage.”

With micronutrients, the situation might be different, however. A zinc deficiency, for instance, may be corrected with foliar applications. “But soil application is more effective,” McFarland said. “A farmer can get three years of soil applied zinc for what one foliar application will cost.”

He says no option except a foliar application is available to correct iron deficiencies. “But even with foliar application we get no translocation through the leaves, so we have to make multiple applications to be effective. That takes more time and more expense. The cost usually exceeds the advantage, even with iron chlorosis.”

He addressed other claims that purport to help a crop with additional fertility. Products with a combination of nutrients, for instance, claim to provide yield boosts. In numerous tests across Texas, however, yield responses with these additions “were flat versus basic fertility programs. There was no deficiency to fix,” McFarland said.

“Unless a deficiency exists, foliar nutrient applications provide little or no value and are much less effective than soil-applied fertilizer. It’s better to soil test and avoid the need to use foliar applications as a last resort.”

Potassium

Potassium deficiencies, even in heavy Blackland soils that typically have adequate levels, have shown up more often in the past few years, McFarland said. “In many cases it’s drought related. Potassium uptake occurs through diffusion, so without adequate moisture, uptake slows or stops. When clay soils dry, they shrink, collapse and trap potassium in the clay layers. It can’t get to the roots.” Compaction also may play a role.

Liquid potassium offers a viable option. “With band injection, the yield response has been substantial,” McFarland said. “It is an advantage to place potassium in the active moisture zone.”

He recommends growers soil test to determine if potassium is adequate. “If it’s marginal, 125 to 150 parts per million, it may justify adding potassium. Deeper samples also may show available potassium. If it’s adequate at depth, yield response to an application is not likely.”

pH effects

Soil pH also affects nutrient uptake and fertilizer efficiency. A pH below 6.0 can affect yield potential. “As pH drops, solubility of nutrients also drops.” A good range is 6.4 up to 7 and McFarland recommends growers “stay well away from low levels. As pH goes below 5.5, aluminum toxicity can begin to be a problem.”

Some Blacklands soils show pH levels as high as 7.4 to 8, but some soils in the area may be as low as 5.4.

Again, McFarland recommends sampling deeper than the typical 6-inch level. “Sometimes the pH is low just in the surface 6 inches, which may not be a problem.  But if it’s low to depth, it could be trouble. Anything below 5.6 or 5.7 will show a response to limestone. Below 5.5 applying limestone will likely provide a significant advantage.”

“It’s a good idea to test to depth.”

Tillage may be an adequate option if soil pH is low in just the top 6 inches. Mixing soil from 8 to 10 inches may correct the problem.

McFarland cautions Texas farmers to make certain of what they are buying when they purchase limestone. “There is no limestone law in Texas, so it’s up to growers to know the quality of the product. Content and fineness make a difference. Percent of limestone—50 percent or 100 percent—affects the value and dictates application rate.”

With fertilizer prices still extremely high, McFarland says farmers can ill afford to waste any, apply more than they need, or to pay for any product that will not provide an economic advantage. The best tool remains a thorough knowledge of soil nutrient levels—based on soil testing (including sampling at depth).

Paying attention to what fertilizer costs, he says, should drive that message home.