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Agronomists say: Make fertility a top concern

FEED THE crop if you hope to make a good one. With conservation tillage, you may have to modify a few things with fertilization but the same basic principles apply as with conventionally grown crops.

Be sure to soil test so you know your fields' fertility needs. Before sampling soil, though, make sure you understand your soil testing facility's requirements. Sampling depth may vary for different tillage systems.

One example: for cotton, the University of Tennessee Soil Testing Laboratory requires that soil samples for nutrients and pH be taken at a depth of 6 inches; organic matter samples should be taken at a depth of only 2 inches for both no-till and conventional till fields.

In cotton fields, if pH is 5.5 or less, lime should be broadcast. “There may be some advantages to incorporation of lime if the pH is extremely low. Research has not indicated the need to till or incorporate if a proper liming program is followed consistently,” says John Bradley, Monsanto's conservation tillage specialist.

Whether phosphorus and potassium are surface broadcast or incorporated, the results should be similar. They can be applied either in the fall or in spring prior to planting.

“Fall application has some advantages. These include the convenience of avoiding a big rush at planting time, application equipment availability, price advantages and allowing adequate time for a higher degree of activation. Of course, fall application should be avoided if erosion is not under control, because the phosphorus and potassium will leave the field with soil particles,” Bradley says.

It's vitally important that cotton plants have a good source of phosphate. That makes liming doubly important. When pH levels are below 6.0 or above 7.0, the availability of nutrients to the crop is reduced.

Potassium deficiency can visibly hurt the crop, as well. Low potassium levels may stunt cotton plants and keep leaves from developing a normal green color. Bolls may be immature and fail to open properly. Lint yield and fiber properties will be reduced. Where potassium deficiency is a problem, think about adding 30 to 60 pounds of potash to soil test recommendations.

Research at the University of Tennessee's Milan Experiment Station shows that starter fertilizers get better response in no-till cotton than in conventional fields (Don Howard, UT Ag Experiment Station Research, 1992-1995). You can go with liquid fertilizers such as 10-34-0 or 11-37-0. Twelve gallons of 10-34-0 provides 15 pounds of nitrogen and 50 pounds per acre of phosphorus.

“A one to one ratio of nitrogen and phosphorus seems to produce best results. This means combining 11-37-0 or 1-34-0 with 28 percent to 32 percent liquid nitrogen resulting in a 15-15-9 to 20-20-0 final mix,” Bradley says.

To keep starter fertilizer from reducing seed germination, it should be offset, or placed 2 inches to the side of the seed zone and 2 inches deep. An attachment to the no-till planter makes this simple. Then, when seeds germinate, lateral roots reach the fertilizer zone, boosting plant growth.

With no-till cotton, nitrogen should be applied near planting time. Split applications and late-season applications tend to not improve yields.

“One of the key questions about nitrogen is what source is best: anhydrous ammonia, ammonium nitrate, urea or UAN, which is a mixture of ammonium nitrate and urea. Another critical question is how to best apply or place the chosen nitrogen source in the soil. The simple answer to these questions is that all of the nitrogen sources give equal response if they are placed below the soil surface,” Bradley says.

Since it's a gas, anhydrous ammonia should be injected 4 to 6 inches deep, then sealed with a special closing wheel. It's low-cost but distribution is limited, and caution must be taken when transporting and using it.

Ammonium nitrate contains 33 percent nitrogen and can be surface applied, broadcast, surface-banded or injected. It's safe, convenient and tends to not volatilize. It can be more costly, however.

Urea is 45 percent nitrogen, is easily available, and relatively inexpensive. When applied to the soil surface, though, it may volatilize and some of the nitrogen will be lost. You might consider shallow injection of urea to minimize volatilization.

“Losses are greater when urea is applied to wet soils followed by good drying conditions, warm temperatures, high rates, thick mulches or residues and higher pH ranges. Losses are minimized by rain or irrigation soon after application. Normally during cotton planting season rain occurs frequently and nitrogen losses from surface-applied urea are low. However, there is reason to be concerned if there is no rainfall two to three weeks after application,” Bradley says.

The old “bulldog soda,” sodium nitrate shows no yield or quality advantages over ammonium nitrate, in tests. (Don Howard, UT Ag Experiment Station Research, 1992-1995.) It can be surface applied or injected, and tends to be expensive compared with other forms of nitrogen.

Some conservation tillage cotton farmers like to grow their own nitrogen in legume cover crops. A good crop of hairy vetch or crimson clover can amount to 50 to 70 pounds of nitrogen per acre.

“Legume cover crops have their disadvantages, such as providing too much nitrogen, delayed maturity, increased boll rot and challenges associated with planting in a no-till culture,” Bradley says.

In addition to covering your bases with nitrogen, phosphorus and potassium, don't neglect micronutrients. Boron, in particular, can be a valuable micronutrient for cotton.

Conservation Tillage Tips

  • Don't do any fall tillage unless it's absolutely necessary to smooth severe ruts or break up soil compaction.

  • Don't run combines, pickers or strippers through wet fields. It destroys soil structure, contributes to soil compaction and developing a hardpan and makes conservation tillage harder.

  • Don't shred corn or grain sorghum stalks. You'll need them for residue cover. Try a chaff spreader for even residue distribution.

  • Don't cut cotton stalks too short. Instead, cut them about 6 inches to 8 inches tall.

  • Don't re-bed fields unless they're rutted, compacted or have a hardpan. If you do re-bed, do it in the fall.

  • Get a jump on weeds and grasses with a fall application of Roundup UltraMAX herbicide. It can be applied pre-harvest in corn, soybeans, grain sorghum or cotton, or post-harvest in corn and soybeans. Fall applications of Roundup UltraMAX are often most effective because vines, weeds and grasses are moving nutrients and moisture into roots for overwintering.

  • Complete field preparation in the fall. Then, come spring, all you'll need to do is burndown with Roundup UltraMAX and plant.

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