As the cotton-planting season nears in Arizona desert fields, it’s important for growers to remember that soil fertility and crop nutrient management are critical to producing a good cotton crop.

“Our goal is to plant the variety and the correct amount of fertilizer constituent to provide the most profitable plant response while minimizing potential losses, avoiding excess use, and losses from runoff, leaching, and volatilization,” says Randy Norton.

Norton is the University of Arizona’s (UA) statewide cotton specialist, and director of the land-grant university’s Safford Agricultural Center located at Safford in southeastern Arizona.

Overall, he says fertilizers are applied to the soil to meet crop demands, promote optimum plant growth, and better crop quality and quantity.

“The objective of fertilization is to promote optimum plant growth. We’re trying to grow the best crop we can with the highest yield and the best quality,” Norton told central Arizona cotton growers during a pre-season UA Field Crops Clinic held in January at Buckeye in West Phoenix.

Tracking crop development

Norton says nitrogen (N) is the primary nutrient that growers should closely track in Arizona desert cotton fields. As the crop is tracked during the season, growers can make adjustments to their fertilizer plan as needed to deal quickly with field issues.

The best way to track cotton crop development, he says, is counting the nodes above white flower bloom (NAWB).  

“This is a very simple measurement that allows you to track where your crop is with respect to development.”

At first bloom, Norton says plant maturity should be at 9 to 11 NAWB, early bloom from 8 to 9 NAWB, peak bloom from 6 to 8 NAWB, and less than 5 NAWB at cutout.

“If the crop is less than 8-9 NAWB at early bloom - seven for example, there is obviously something stressing the crop and sending it through the fruiting cycle faster. Something needs to be addressed,” Norton says.

Finding the culprit

A good knowledge of fertilizer and soil testing are good ways to determine whether or not a crop is on track to meet production goals.

Overall, Arizona desert soils supply 16 chemical elements essential to plant growth, including N, phosphorus (P), potassium (K), calcium (Ca), sulfur (S), magnesium, boron (B), manganese (Mn), zinc (Zn), iron (Fe), molybdenum (Mo), sodium (Na), copper (Cu), chlorine (CI), aluminum (AI), and silicon (Si).

Norton says, “Soil is the cheapest and probably the most efficient source of fertilizers. Our soils are typically rich in nutrients and have the ability to supply most nutrients that the plant will need.”

High yields remove fertilizer elements from the soil so if levels are low then supplementation by fertilization can help restore depleted soils, Norton says.

Pre-, early-, and in-season-testing, plus examining crop response, are telltale signs to guide growers to make good fertility management decisions.

The nutrients most often supplied through supplemental fertilization include the macronutrients N, P, and K, plus the micronutrients S, B, Zn, and Fe.

Recommended fertilizer levels

Critical soil test minimum levels for N and P in Arizona desert cotton are 10 and 5 parts per million (ppg), respectively.

Norton says, “For nitrogen, if you are at 10 parts per million or below you might consider applying a pre- or early-season nitrogen application. Otherwise, I wouldn’t mess with pre-season nitrogen since most of our research has shown it’s an inefficient application since the plant doesn’t fully use it. By the time the plant starts taking up nitrogen, early-season applied N might be gone.”

Other critical thresholds include: P at 5 ppm; sodium bicarbonate extractable P – 5 ppm; K at 150 ppm - an amount Norton believes should be re-evaluated; Zn at .6 ppm; Fe - 5 ppm; Mn – 1 ppm; and B at .05 ppm.

Norton believes petiole tissue testing is an excellent method to determine current crop nutrient status.

It’s all in the timing

So when is the best time to apply fertilizer? Norton says applications should coincide with crop uptake and need. He recommends using the most efficient form of fertilizer, for example UN-32 for N since it contains three different N forms for use under different conditions and at specific times.

Also important is choosing the most cost-efficient form of fertilizer. Use the correct technique for the specific situation.

For example, if P is broadcast applied around the crop, it contacts or enters the soil which can cause fixation. Norton says a banded P application, especially in cotton, can be a more efficient application method.

Nitrogen use

Norton calls N one of the most dynamic nutrients in the soil-plant system since it undergoes various transformations. As it enters the soil, N can de-nitrified, lost through volatilization, and incorporated into organic matter, making it unavailable to the plant. N can also be released (mineralized) into the soil and available to the plant.

“N can go through a whole host of different transformations which can render it unavailable to the plant. This is why N is one of the most complex nutrients to manage.”

The cotton specialist says 50 to 60 pounds of N is required to produce one bale of lint in the Arizona desert. A four-bale yield requires 200 to 240 pounds of N.

Norton says it’s important to choose a realistic yield goal.

“If you have high nitrate in your irrigation water you definitely want to take advantage of it and incorporate it into the total amount you will have access to during the season,” he says. “Whatever amount is needed beyond N in irrigation water and soil residue can be applied through fertilization.”

Since peak plant N use is from first square to peak bloom, Norton says apply N during this plant growth window, especially under the plant.

“By applying N up to peak bloom, it’s available for the crop after peak bloom. N uptake by the plant drops off precipitously after peak bloom.”

Norton recommends split N applications during the optimum N uptake window.

“You are spoon feeding - applying the N as the plant needs it. It also allows the grower on the second application to adjust the N amount based on field conditions.”

Optimum N application timing is from first square (about 700 heat units since planting) to 2,000 heat units since planting (peak bloom). Split applications are recommended from around 1,100 HUs to about 1,800 HUs.

“The optimum application window from first square to peak bloom is really what to shoot for.”

Phosphorus

As shared earlier, the timing of a P application is less critical as the chemical is more stable and immobile in the soil. Perhaps more important is the P application technique used.

Norton says, “Apply the phosphorus as close to the crop root system as possible, concentrating and minimizing soil contact with the fertilizer to minimize fixation in soluble forms.”

As with N, banded applications are best for P, although the UA has conducted studies on injecting phosphorus directly under the seed row when listing the beds up with good results. Norton says GPS allows the grower to place it in the same place every year if needed which increases efficiency.

Potassium

According to Norton, 60-65 pounds of K2O/acre is required to produce one bale of lint. Growers should determine the amount of potassium required to achieve their yield goal. Arizona soils typically have sufficient potassium to meet crop demand yet deficiencies have been found.

Norton suggests testing the soil in the pre-season or early in the crop growing season.

“If you get into boll fill with high potassium uptake, the soil cannot keep up with the plant, especially in coarse-textured soils or following high potassium-consuming crops including alfalfa,” he noted.

Norton has found some K deficiency in cotton grown with subsurface drip irrigation (SDI). He believes it’s a soil exploration issue where the root system is more confined with SDI, compared to roots exploring the soil under furrow irrigation.

Joint K research

Norton and 17 other cotton belt cotton specialists are participating in a National Cotton Council Beltwide project studying K use across a wide range of cotton-growing environments. Cotton is being grown with zero to 160 pounds in two forms - K₂O per acre via liquid KTS and solid potassium sulfate.

So far, “We are seeing little yield response even in some low soil K situations. We are scratching our heads trying to figure out why we aren’t finding the response we expected,” says Norton.

The specialists plan to study soil mineralogy to see if that plays a role.