Proper Fertilization for No-till Cotton. Dr. John F. Bradley Conservation Tillage Specialist Monsanto Company If you're planning on growing no-till cotton this year, you may need to modify a few fertilization practices. Good fertilization begins with soil testing. All cotton fields should be sampled annually or on a regularly scheduled basis. It's a good idea to check your soil testing facility's requirements before you take the samples. Some laboratories require different sampling depths for different tillage systems. For cotton, the University of Tennessee Soil Testing Laboratory requires that soil samples for nutrients and pH be taken at a depth of six inches and that organic matter samples be taken at a depth of only two inches for both no-till and conventional till fields.
Surface broadcasting of lime is adequate for cotton if the pH is greater than 5.5; however, 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.
Phosphorus (P) and potassium (K) may be surface broadcast or incorporated with the same results. These elements can be applied in the fall or spring, just 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.
Remember, the availability of phosphate to the cotton plant is very dependent on a good liming program. When pH levels are below 6.0 or above 7.0, there is reduced availability of nutrients to the crop.
It's important to maintain healthy levels of potassium in the soil. Low levels can cause stunted plants and leaves which fail to develop a normal green color. Mature leaves are often mottled after turning light yellowish-green, then reddish-brown between the veins of the leaf before the discoloration spreads to the leaf margins. The tips and edges of the leaves curl downward. The leaves then become reddish-brown and are scorched and blackened by the time they are prematurely shed. Bolls are small and immature and may fail to open or only partially open. Lint yield and fiber properties are reduced. Again, the availability of potassium is influenced by soil pH. In fields where potassium deficiency symptoms occur, cotton growers should add 30-60 pounds of K subscript 2O to the soil test recommendations.
Response to starter fertilizers in no-till systems has been more consistent than in conventionally tilled cotton, according to research at the University of Tennessee's Milan Experiment station. Liquid fertilizers such as 10-34-0 or 11-37-0 can be used as starter fertilizers. An application of 12 gallons per acre of 10-34-0 provides 15 pounds of nitrogen and 50 pounds per acre of P subscript 2O subscript 5. A 1:1 ratio of nitrogen and phosphorus seems to produce the best results. This means combining 11-37-0 or 10-34-0 with 28 to 32 percent liquid nitrogen (UAN) resulting in a 15-15-0 to 20-20-0 final mix.
Starter fertilizer applied in direct contact to the seed can adversely affect germination. A popular application technique is called 2" by 2", which means the fertilizer is placed 2 inches to the side of the seed zone and 2 inches deep. This can be accomplished with a specialized no-till attachment to the planter. After germination, lateral roots work their way into the fertilizer zone.
Most questions concerning no-till cotton fertility arise around the subject of nitrogen application. Nitrogen, in any form, should be applied near planting. Research has shown little advantage to split applications and no advantage to late season applications. One of the key questions about nitrogen is what source is best: anhydrous ammonia (NH subscript 3), ammonium nitrate (NH subscript 4NO subscript 3), urea (NH subscript 2CO) 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.
Anhydrous ammonia is a gas and must be injected into the soil four to six inches deep and sealed with a special closing wheel in no-till. Anhydrous is usually the lowest cost of nitrogen but it has limited distribution through dealerships. Careful handling practices must be observed when transporting and applying this nitrogen source.
Ammonium nitrate, which contains 33 percent nitrogen, may be surface applied, broadcast, surface banded or injected. As far as volatilization, this is the best form of nitrogen, and it is extremely safe and convenient. However, it may be a more costly form of nitrogen. Ammonium sulfate has the same characteristics as ammonium nitrate plus sulfur, and it can be applied using the same methods.
Urea, 45 percent nitrogen, is also an inexpensive source of nitrogen and is carried by most dealers. If urea is surface applied, part of the nitrogen may be lost by volatilization as a gas. Losses are greater under the following conditions: urea 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. Significant loss will probably have occurred during this period, and additional nitrogen should be applied. The safest and best way to apply urea is to inject it into the top 1 to 2 inches of the soil with a coulter.
Sodium nitrate (NaNO subscript 3), historically known as `bulldog soda', can be surface applied or injected, but according to tests really shows no yield or quality advantages over ammonium nitrate at the same rates. Sodium nitrate is expensive but has its following with many cotton growers.
Another good source of nitrogen is legume cover crops. Cover crops such as hairy vetch or crimson clover established early in the fall can provide 50 to 70 pounds of nitrogen per acre. However, legume cover crops also have their disadvantages, such as providing too much nitrogen, delayed maturity, increased boll rot and challenges associated with planting in a no-till culture.
It's important to pay careful attention to nitrogen sources and potential for losses. Consider shallow injection equipment to minimize volatilization from urea forms of nitrogen. You may also want to consider splitting nitrogen applications to minimize losses from volatilization and to address environmental concerns. And finally, always make sure micro-nutrients are balanced and added only when necessary, especially boron.