Dooly County Extension

Cotton Fertilization

Cotton Fertilization

With cotton prices expected to be lower than normal this year, it is even more important to follow a sound fertilizer and liming program that is based on good soil sampling. Liming on time (at least 3 months ahead of planting) and where needed is the first step to assuring maximum uptake of both nutrients already in the soil and those applied as fertilizer. Variable rate liming based on grid or zone sampling may also be more effective and economical than “blanket” applications of lime. Nitrogen rates should be based on yield goals as explained below, and should be split applied to again assure maximum uptake and efficiency. Potassium needs to be applied at planting and possibly later as foliar to avoid what has become the most common nutrient problem in Georgia cotton in recent years. Ten pounds of sulfur needs to be included either at preplant or with sidedress N. And a half of pound of boron per acre is still the standard recommendation for cotton to assure proper pollination and fruiting and thus yield. More details can be found below, but following this basic plan should assure good economically produced cotton yields.

Lime

The official UGA recommendation or “target” pH (water) for cotton is 6.0. However, a field with an average pH of 6.0 may very well have large areas measuring below this target pH. Recent precision soil sampling techniques have indicated that this happens frequently. Therefore, growers using standard soil sampling techniques are encouraged to maintain their soil pH for cotton between 6.0 and 6.3. Liming to pH values above 6.3 may cause manganese deficiency problems in the Flatwoods soil region. However, this problem can be handled easily with applications of foliar Mn during the growing season. Liming to between 6.0 and 6.3 for all soil regions in Georgia is critical for proper uptake and utilization of nutrients that are essential for plant growth. Fertilizer use efficiency is also best in this range. In addition, toxic elements such as aluminum (Al) are kept unavailable when pH is above 5.5.

There are many factors that affect the soil pH reading obtained from soil testing. Possible reasons for seeing abrupt changes in soil pH include 1) sampling variability (spatial and depth), 2) rainfall amounts and 3) nitrogen fertilizer usage. Even so, changes of more than 0.5 in soil pH in one year should be considered suspect and call for resampling.

Dolomitic lime (that has 6 % or more Mg) is still a common liming material used on Georgia cotton and provides magnesium (Mg) as well as calcium (Ca) and a pH adjustment. The use of calcitic lime (less than 6% Mg) is becoming more popular in Georgia every year and may be used in cases where high soil Mg levels occur. If calcitic lime is used for consecutive years, soil test 12 Mg levels should be tracked closely with soil testing. As soon as soil test Mg levels start to drop out of the high range into the medium range, the use of dolomitic lime should be resumed. The reason for this is that dolomitic lime is the most economical source of Mg fertilizer. In addition, a good liming program should supply all the Ca that a cotton plant needs for high yields and quality. Calcium deficiency in cotton is very rare, and the need for foliar Ca applications or small doses of supplemental Ca applied to soil should not be necessary.

Phosphorous and Potassium

Phosphorous (P) and potassium (K) levels in soil should be maintained in the upper medium range as determined by soil testing. All of the P requirements should be applied preplant since it is relatively immobile in soil and is important to seedling growth. All of the K requirements should also be applied preplant on all soil types including Piedmont, Coastal Plain, and Deep Sand soils. Widespread K uptake and deficiency problems continue to occur in Georgia cotton every year.). This problem is also made evident by weak areas in the fields (usually in sandy washed out areas) and the presence of certain leafspots. Cercospora, Alternaria and Stemphylium leafspot have all been linked to potassium deficiency. These leafspot diseases are considered secondary to potassium deficiency and if potassium deficiency is avoided then these leafspots should not be an issue. The relatively new Corynespora leafspot, however, does not appear to be linked to potassium deficiency.

Split applications of K, especially half the recommended rate at planting and half at sidedress, have also not proven to be effective on Tifton type soils. In fact, in some cases this approach may lead to potassium deficiency before sidedress applications are made. Recent field trials conducted in Georgia have focused on additional soil-applied K during N sidedressing versus foliar K applications during peak bloom (first 4 weeks of bloom). Preliminary results from studies conducted on Coastal Plain soils indicate that foliar K may be more effective than sidedress K in improving yields. Research on Deep Sands is still needed to determine which approach is more effective. Currently, foliar K applications should automatically be considered on deep sands (more than 18 inches to subsoil clay), low K soils, high Mg soils, high-yielding conditions, short season varieties and especially, where severe K deficiencies and leafspot have been observed in the past. Two foliar applications of 5-10 lbs/K2O in each application during early bloom (first thru 4th week of bloom) should be considered in these situations.

Because current cotton varieties are relatively fast fruiting and early in maturity, this makes them more susceptible to K deficiency. In most situations, the best strategy to avoid K deficiency is to 1) soil test, 2) apply the recommended K fertilizer at planting, and 3) consider foliar feeding K during peak bloom.

Currently, there are a number of commercially available fertilizer additives that are designed to improve the uptake efficiency of P and K fertilizers. Research results with Georgia cotton showing consistent advantages to these materials have not been seen at this time and their widespread adoption is not recommended.

Nitrogen Management

Nitrogen is probably the most important fertilizer used on cotton, yet it is the most difficult to manage. Low N rates can reduce yield and quality while excessive N rates can cause rank growth, boll rot, delayed maturity, difficult defoliation, and poor quality and yield. Total N rates for cotton should be based on soil type, previous crop, growth history, and yield potential. Base N rates recommended by the UGA Soil Testing Lab according to yield goals are listed below.

Click on table to enlarge

Cotton Nitrogen

Yield goals should always be realistic, preferably based on past production records. For N rates above 100 lb/A, cotton should be highly managed in terms of insect control, plant height, and boron fertilization. Total N rates above 120 lb/A should only be needed on deep sands or in special cases of history of inadequate stalk growth or where excessive leaching has occurred. The N rates for 1250 and 1500 lb lint/A yield goals assume irrigation.

The total N rate should always be applied in split applications. Apply 1/4 to 1/3 of the recommended N at planting and the remainder at sidedress. The preplant or at planting N application is critical for getting the crop off to a good start and ensuring adequate N nutrition prior to side-dressing. Sidedress N between first square and first bloom depending on growth and color (toward first square if slow growing and pale green, toward first bloom if rapid growth and dark green). A portion of the sidedress N can also be applied as foliar treatments or through irrigation systems. No N should be soil-applied (either top dressed or through the pivot) after the 3rd week of bloom. Studies have shown that uptake of soil-applied N from by cotton roots is basically ineffective after this critical point.

There are a number of sidedress nitrogen fertilizer materials that can be used on cotton including liquid UAN solutions, ammonium nitrate and urea. UAN solutions are made up of urea and ammonium nitrate and often contain sulfur (e.g. 28-0-0-5). Ammonium nitrate is losing favor as a sidedress N source for cotton due to higher cost and burn potential. Urea is considered an alternative to ammonium nitrate but is known to be prone to volatilization losses. Volatilization losses can be minimized however by irrigating after a urea application or by use of a urease inhibitor that contains the active ingredient NPBT. Another liquid N solution that is gaining popularity as a sidedress N source for cotton is “19 %” or 18-0-0-3(S). These sources are derived from a by-product of the Attapulgite clay mining industry in southwest Georgia and are made up approximately 60 % nitrate and 40 % ammonium (no urea). Replicated, small plot research trials conducted between 2010-2013 indicate that 18-0-0-3(S) is comparable to 28-0-0-5(S) in terms of producing cotton yield. Feed grade urea is still the product of choice for foliar N applications later in the growing season. Controlled release nitrogen foliar products are also available but usually contain potassium and boron and are less concentrated in N.

Sulfur

The official UGA fertilizer recommendation for sulfur is 10 lb/A. Sulfur can be applied either with preplant fertilizer or with sidedress N materials such as 28-0-0-5 or ammonium sulfate. Sulfur fertilization is most important on sandy, low organic matter Coastal Plain soils. With less 14

S input from cleaned (“scrubbed”) power plant smokestack emissions and the recent trend toward high-analysis (S-free) fertilizers, including S in a cotton fertilizer program is currently very critical. Adequate S fertilization is also important where higher rates of fertilizer N are used. Since S deficiency symptoms are similar to N deficiency (yellowing) and the N:S ratio in plant tissue is a good indicator of S nutrition, a plant tissue sample greatly aids in diagnosis when low S is suspected.

Boron

Boron (B) is an essential micronutrient that is important to flowering, pollination, and fruiting of the cotton plant. The standard UGA recommendation of 0.5 lb B/A, applied in two 0.25 lb/A foliar applications between first square and first bloom, fulfills the base requirement for B. Single applications of 0.5 lb B/A can be used but include a greater risk of foliar burn. Foliar applications above the base recommendation of 0.5 lb B/a and up to 2 lb B/A (applied in increments of no greater than 0.5 lb B/A per application) may help move nitrogen and carbohydrates from leaves into developing fruit. Cumulative applications totaling above 2 lb B/A, however, may reduce yields and quality. The need for additional B above the 0.5 lb/A rate is best determined by tissue or petiole testing. Since B leaches readily through sandy soils, foliar applications have always been considered the most effective and efficient application method. However, on a typical Coastal Plain soil like the Tifton series, with normal rainfall and irrigation, preplant, starter, and sidedress soil applications are also be considered effective. If no B is included in preplant, starter, or sidedress soil-applied fertilizer applications, is foliar B alone (with no insecticide or growth regulator) worth the trip? Yes, especially on sandier soils and with irrigation or adequate rainfall.

Numerous B fertilizer materials are currently available. Most are either derived from boric acid or sodium borate and can be either in the liquid or wettable powder form. There are many “additives” used with these base B materials such as nitrogen and complexing agents designed to improve efficiency of uptake. However, extensive field testing over recent years has proven that all of the B fertilizers currently on the market are equally effective in terms of plant nutrition. Therefore, choice of B fertilizers should be made on price per pound of B.

In addition, at least one boron fertilizer currently sold in Georgia is recommended at application rates well below the recommended 0.5 lb B/A rate — in fact the labeled rate only provides 0.025 lb B/A !. As far as fulfilling the base recommendation for B, any boron fertilizer recommended at a rate that does not provide at least 0.25 lb B/a should be considered uneconomical !

 Manganese and Zinc

Manganese (Mn) and zinc (Zn) are two essential micronutrients that are routinely measured in soil testing at UGA and can sometimes be deficient in cotton. Both Mn and Zn are less available for plant uptake at higher soil pHs. Therefore, soil test results should be examined closely for the combination of low levels of Mn or Zn and high soil pH.

In order to minimize the chance of Mn deficiency on cotton, minimum levels of soil test manganese should be maintained with varying pH levels as shown in Figure 1.

Click on graph to enlarge

Cotton Magnaese

Notice on the graph, that if soil pH is at the recommended target of 6.0, soil test level of Mn should be at least 5 lb/A. At soil pH of 6.5 the soil test level of Mn should be at least 11 lb/A.

Even when the soil test level of Mn falls below the recommended level for a given pH, the result is not an automatic recommendation to apply Mn fertilizer. Instead, the crop should be monitored using tissue testing between first square and first bloom and foliar Mn can be applied if a deficiency is confirmed. Small amounts of Mn can also be added to starter fertilizer applications. Be sure to read and apply Mn and other micronutrients starter packages according to label to avoid burn and stand loss.

Large amounts of soil applied Mn (above 5 lb/A) are not considered to be economical. Therefore, in situations where soil test levels of Mn need to be built up, do so slowly and monitor the crop for deficiency using tissue testing. In essence, if a grower likes to maintain soil pH near the UGA target pH of 6.0, then soil test Mn should be built to and maintained around 5 lb Mn/A. If the grower likes to maintain soil pH at a higher level, say around 6.5, then the soil test level of Mn needs to be built to and maintained around 11 lb/A.

Cotton growers in the Flatwoods soil region are cautioned not to maintain soil pH above 6.3 to minimize the chance of Mn deficiency (peanuts and soybeans are also susceptible to Mn deficiency at this pH on these soils). If soil pH is maintained above 6.3 on these soils, tissue testing is recommended regardless of soil test Mn levels in order to avoid deficiencies. If a deficiency is detected in this situation, it can be corrected by foliar feeding Mn.

Soil test levels of zinc should be maintained between 2 and 8 lb/A (Mehlich 1 extractant) . Unlike Mn, if soil test Zn falls below this range, it is considered low and an application of zinc fertilizer will be recommended. The recommended Zn fertilizer can be applied with broadcast preplant fertilizer or more efficiently, with a starter fertilizer application. In the event that no zinc is applied to the soil even though recommended by soil testing, a foliar application of zinc can be made. Tissue testing in both cases, whether Zn was applied to

soil or applied foliar, is recommended. The tissue sample should be taken between first square and first bloom. Tissue sampling at first square is better than at first bloom in order to correct the deficiency before the crop experiences any possible reduction in yield.

Deficiencies of the other essential micronutrients including copper, iron, chlorine, and molybdenum in Georgia cotton are extremely rare.

Starter Fertilizers

Although starter fertilizers do not consistently increase cotton yields, they are an effective way of providing early N and P as part of an overall fertility program. Yield responses have been most consistent where soil P levels are low or when planting in cool, wet soils. The use of starter fertilizer is strongly encouraged for conservation tillage systems and in high yield situations. Even though yield responses may not be realized, other advantages include the development of strong root systems and the encouragement of early rapid growth for weed control with directed herbicide sprays.

Ten gal/A of 10-34-0 is probably the most common starter fertilizer treatment used on Georgia cotton. Nitrogen solutions (with or without S) and complete (N-P-K with micronutrients) dry fertilizer materials can also give good results. Recent research conducted in Georgia showed that the choice of starter fertilizer should depend on soil type and conditions. For example, on “red dirt” such as the Greenville series that has a high affinity for P, P-containing materials such as 10-34-0 should be used. On “stiffer” Coastal Plain soils such as the Tifton series that have medium to high soil test P, N-only materials such as 32 % N liquid can be used. On sandy Coastal Plain soils with histories of S problems, N+S materials such as 28-0-0-5S should be considered. An economic evaluation of this same research showed that in 23 out of 30 comparisons, starter fertilizer gave greater economic returns compared to the untreated check. Adding liquid micronutrient packages to liquid starter materials is also gaining in popularity. This may be a good way of providing recommended B, Zn, and Mn in an overall fertilization program.

The recommended placement for any starter fertilizer is 2 inches below and 2 inches to the side of the row (also referred to as “2-by-2″). No starter fertilizer materials should be placed in direct contact with the seed “in furrow”. “Dribbling” liquid starter fertilizers on the soil surface, 2 inches to the side of the furrow (to avoid possible leaching into the seed zone) has proven effective on sandy soils but does not work on “stiffer” soils. Avoid using starter fertilizer rates greater than 15 lb N/A, even in the 2-by-2 placement, in order to reduce the risk of “starter burn.” Under certain conditions — namely dry, sandy soil — even 15 lb N/A can burn cotton seedlings if not placed properly.

Starter fertilizers can also be applied in conjunction with herbicide applications by spraying narrow bands (3 to 4 inches) directly over the row behind the press wheel. Mixing liquids containing both N and P with preemergence herbicides can result in clogging of spray nozzles and 18

can decrease the fertilizer effect (or benefit) by spreading the material in a wider band. However, this may supply some needed N when no other preplant N has been applied. Rates should not exceed 20 lb N/A when this method is used.

 Poultry Litter

Managed properly, poultry litter (manure mixed with wood shavings) can be a valuable source of plant nutrients for Georgia cotton. The fertilizer value of poultry litter varies depending on a number of factors including moisture, temperature, feed rations, number of batches before clean-out, storage, and handling. However, broiler litter has an approximate analysis equivalent to 3-3-2 (%N – % P2O – % K2O). Based on this average, one ton of broiler litter contains 60 lb/A of N, 60 lb/A of P2O and 40 lb/A of K2O. Due to variability, it is recommended that litter be analyzed for nutrients by a reputable laboratory before application rates are determined.

Poultry litter on cotton should be managed to provide preplant P and K and a portion of the total N requirement. The remainder of the N requirement should be applied as commercial fertilizer at sidedressing. For example, 2 tons/A of poultry litter preplant incorporated followed by 30 to 60 lb/A of sidedress N (depending on soil type) is a good, basic strategy. This approach should avoid unnecessary P buildup and should not cause rank growth, boll rot, or defoliation problems typically associated with excess N. In addition, the availability of N from poultry litter, because it is an organic material, is less predictable than from commercial fertilizer. Therefore, side-dressing with commercial fertilizer N assures adequate N availability when the crop needs it the most. The amount and timing of N released from litter depends on a number of factors, including soil pH, temperature, sand content, and available moisture. As a rule of thumb, 60% (or 36 lb N/ton of litter) is made available for crop uptake during the season if the manure is incorporated into the soil prior to planting. Most of the remaining N in the litter (about 40%) is either lost or “tied up” during the growing season and should not be considered for carryover to the next crop. Since N availability from poultry litter can be highly variable, petiole testing is strongly recommended. Build up of soil P and Zn are long-term concerns for using poultry litter as fertilizer. However, at the 2 ton/A rate, there are no short-term concerns for poultry litter use in cotton.

The only situation where poultry litter rates above 2 ton/A should be considered is where problems with “black root” are suspected. Black root is isolated to poorly-drained Flatwoods soils and has not been that prevalent in recent years. Rates of 3 to 4 tons of poultry litter per acre have been shown to alleviate this problem dramatically. However, at the 4 ton/A rate excess soil P will build rapidly. Therefore, this solution should only be considered a short-term fix and not a long-term strategy.

 How early can I apply chicken litter for cotton ? In general, it is best to apply any base fertilizer nutrient (inorganic/commercial or organic like chicken littler) close to when a plant needs it, typically 2- 3 weeks before planting. Therefore, ideally, chicken litter would not be applied until around April 1 for May planted cotton. Timing of acquiring litter and availability of labor tempts growers to apply litter as early as December and January. This is not recommended since most of the N and some of the K can be lost before the cotton crop will ever be established (depending on soil type and rainfall). In addition, if a cover crop is grown, the cover crop will take up the nutrients from the litter and greatly decrease the availability to the cotton crop. If at all possible, delay applying chicken litter for cotton until at least February 1.

Other By-Products

As landfill costs and regulations increase, more by-products are becoming available for land application on row crops such as cotton. These by-products are not only from the agricultural sector (such as poultry litter), but also from municipalities and industry. Examples include gin trash, mushroom compost, yard waste, biosolids, dairy manure, composts, fly ash, and wood ash. These materials may have some value as fertilizers, soil amendments, or liming materials. They may be free or available at very low cost. However, great caution is needed when considering the use of any by-product to ensure it can be used, safely, effectively, and economically.

Before considering the use of any by-product material on cotton, investigate the properties of the material. Find out what value it has (as either lime, fertilizer, soil amendment, or a combination), if it is safe (for example, low in heavy metal content and free of any toxins), how much it costs, and if it will handle and spread easily. Fortunately, any by-product material to be used as a fertilizer, lime, or soil amendment in Georgia must first be approved by the Department of Agriculture. Since by-products are unique, they should be investigated on a case-by-case basis.