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Route 1: Fine-Tune Fertility

00:00AM Jul 21, 2008
Pamela Henderson Smith, Farm Journal Crops & Issues Editor

Let's face it, fertility may be the single highest input investment you will have for cotton. Nitrogen (N) costs this fall already have farmers reeling with sticker shock. Nitrogen Sources
Cotton nutrient management boils down to deciding two things: what you have and what you need.

The key to determining what you have begins with a good soil test, says Mississippi State University cotton specialist Darrin Dodds. "Fall is the best time to pull samples,” he notes. "You'll get the results in time to plan a soil fertility program. Correcting a soil fertility problem before planting is much easier and often cheaper than after your crop has been planted.”

Test results give you an indication of plant-available phosphorus (P), potassium (K), magnesium, calcium and a few other ingredients. You will also get an indication of whether you need lime in time to get it applied in the fall.

What you need in fertility is determined by realistic goals for the farm operation. Yield history is a starting point to project future yields. Dodds suggests using average cotton yields from the past three to five years and then tacking on 10% for a realistic projection.

"Since efficient nitrogen use depends on balanced fertility, address liming needs first based on soil test results,” he says.
Obviously every field and region are different. Here are several basic guidelines from cotton fertility specialists.

Lime. Cotton grows best in soil with a pH between 5.8 and 7.0. According to Mississippi State University studies, yield decreases didn't appear until soil pH dropped below 5.5 on sandy loam and silt loam soils and below 5.2 on clay loam soils.

Liming more efficiently uses native nutrients, as well as P and K fertilizers. For best results, incorporate lime into the soil several months before planting.

Impurities in rock formation make it necessary to consider what type of lime is used. Limestone is tested and assigned a calcium carbonate equivalent (CCE) rating. Most soil tests assume a CCE rating of 100.

There are two basic types of lime—calcitic and dolomitic. Either will work, but calcitic contains calcium carbonate and dolomitic contains calcium carbonate and magnesium carbonate. You will need dolomitic if soil tests indicate magnesium levels are low.

Nitrogen. Natural gas accounts for 75% to 90% of the per-ton cost of N fertilizer manufactured. With prices reaching record highs, it's tempting to cut back on N. The obvious cost of cutting back is yield loss. Too little N can delay flowering by increasing the time to first bloom and the interval between flowering on the same fruiting branch.

There's a price beyond cost for overapplication, too. Excessive or improperly late-applied N can delay maturity, reduce micronaire, increase vegetative growth, open the plant to more insect pressure and hinder harvest.

As a general guide, 50 lb. to 60 lb. of N fertilizer is needed to produce a bale of cotton on light-textured soils, 60 lb. to 70 lb. of N on medium-textured soils and 70 lb. to 80 lb. of N on clay and clay-loam soils.

Studies show mid-South growers don't get yield response beyond 100 lb. per acre, explains Bill Pettigrew, USDA-Agricultural Research Service plant physiologist in Stoneville, Miss. Regions with higher yield potential, such as Arizona and California, can often justify more N.
N is best utilized when applied close to the time of crop uptake.
Fast Facts
Proper management practices ensure fertilizers are used in cost-effective manners with minimal impact on the environment.
  • Use soil testing to assess current fertility status.
  • Set realistic yield goals.
  • Consider application methods and climactic conditions when choosing fertilizer sources.
  • Price fertilizers on the cost per pound of nutrient.
  • Maintain and calibrate all application equipment.
  • Avoid accidental application to surface waters.
  • Time application properly.
  • Control soil erosion and properly manage water flow.

Potassium. Elemental K is not found in nature.  Muriate of potash (0-0-60) accounts for more than 90% of the K sold in the U.S., and National Agricultural Statistics Service figures show, on average, potash rates have increased in the past decade.

This nutrient increases water efficiency, lends a hand in almost all of the plant's biological systems and affects fiber properties, such as micronaire, length and strength. Uptake increases during early boll set with 70% of total uptake occurring after first bloom.

Soil test recommendations will generally get you home if yield potential is less than two bales per acre. Mississippi State tests show increasing potash rates by 50% more than the recommendations if yield potential is higher or fields historically test low, regardless of use rates.

Pettigrew notes plants deficient in K are more susceptible to southern root-knot nematodes.

As a result, he was surprised to find the opposite with reniform nematodes. Three years of testing fields fertilized with K posted a 12% larger population of reniform nematodes than fields left unfertilized. "We think reniform nematode may be attracted to the more robust root systems of potassium fertilized plants,” Pettigrew says.

Phosphorus. Seedlings need P for early root development and early fruiting. It stimulates blooming, promotes seed formation and is the primary form of stored energy in the seed.

Watch for small leaves that remain dark green or dwarfed plants—it's a sign your crop is deficient. Other signs are delayed fruiting and maturity.

Apply to test and broadcast in fall if soil pH is between 6.0 and 7.0—otherwise wait until spring.

Sulfur. "Sulfur is immobile in the plant, so watch for pale green new growth. It can be confused with nitrogen deficiency. However, nitrogen shows up in older growth first, and then progresses to younger leaves,” Dodds says. Sulfur deficiency is more typical on sandy soils or soils with low amounts of organic matter.

Boron. Very water soluble, boron is often wrongfully blamed for problems. Deficiency is related to soil type, organic matter content and timing of lime. Watch for short, thick petioles on younger leaves with dark, thick concentric bands along their leaves. The gossypol glands line up, and flowers may be distorted. You may also see flower and boll shedding.

Consult university and Extension advice, as needs are very regional.  

Start Driving Cotton Yields

Now is the time to put the pedal to the metal on cotton yields. During the next few months we will bring you a series of straight-forward management articles aimed at reducing inefficiencies and getting the most mileage out of every input dollar. We'll cover the latest in managing fertility, variety selection, precise planting and weed, insect and disease controls.

The goal is to make each route intersect at a final destination of more bales per acre.

It's all part of navigating a systems approach to high-yield cotton. So get revved up. It's time to grab the wheel and go!

You can e-mail Pam Henderson at