In-Season Nitrogen Management in Cotton

Dr. Rasel Parvej, LSU AgCenter Soil Fertility Specialist; Dr. Brenda Tubana, LSU AgCenter Soil Scientist; Dr. Josh Copes, LSU AgCenter Agronomist; and Dr. Syam Dodla, LSU AgCenter Soil Scientist

Nitrogen (N) is the most yield limiting nutrient in cotton. Maintaining a sufficient and balanced amount of N throughout the growing season is a prerequisite for successful cotton production. Both inadequate and excessive amount of N during the growing season causes problems and reduces cotton yield and profitability. Inadequate N decreases boll and lint production, while excessive N results in continuous vegetative plant growth that delays maturity, increases pest problems, and decreases harvest-aid effectiveness. In addition, excessive N increases off-site environmental impacts.

Nitrogen shortage during the growing season is a more common problem in cotton production than excessive N because N is highly prone to loss from the rootzone through volatilization, leaching, runoff, and/or denitrification. An adequately fertilized cotton field can have N shortage due to excessive rainfall during the growing season that causes water-logged conditions and initiates denitrification loss of N. Excessive rainfall or irrigation also causes leaching loss of N especially in coarse-textured soils with low organic matter and cation exchange capacity (CEC <10), such as loamy sand to silt loam soils. Runoff occurs when rainfall or irrigation rate exceeds infiltration rate. Runoff is laden with N (and other nutrients) and soil sediments that contribute to fertility of receiving waters. Volatilization loss is very significant for urea-based N fertilizers if not incorporated into soils. Therefore, monitoring N status during the growing season is very important for profitable cotton production.

Cotton in-season N status can easily be tracked by different methods such as crop sensing (vegetation indices), measurement of leaf chlorophyll content, petiole (leaf stalk that connects the leaf blade to the main-stem) testing for nitrate-N (NO 3-N) concentration, etc. Monitoring petiole NO3-N concentration is the best tool of tracking in-season N status in cotton. This tool allows producers and crop consultants to predict the yield potentials dictated by soil nutrition and environmental conditions and adjust in-season N supply, if needed, to help achieve the highest potential yield. In addition, it helps motivate producers not to apply excessive amount of N when planting and hope for the best, which often causes environmental concerns. Cotton petiole NO3-N concentration typically indicates in-season N need 7-10 days prior to the onset of N deficiency stress.

A successful petiole NO3-N monitoring program consists of several weekly petiole samplings starting at one week prior to first bloom and continue for 4 to 5 weeks after first bloom. At each sampling time, at least 20 uppermost recently mature leaves with petioles on the vegetative stem should be collected. These leaves are usually the 3rd to 5th leaf from the terminal (a quarter-sized main-stem leaf at the top of the plant should be counted as the 1st leaf). The petiole of each sampled leaf should be separated from the leaf blade, placed in a labeled paper bag, and sent immediately to the plant diagnostic lab for NO3-N, total phosphorus (P), and total potassium (K) concentrations. The total N concentration in leaf blade at first bloom is also important in interpreting petiole NO3-N concentration. Therefore, the leaf blade of the collected sample at first bloom should also be analyzed separately for total N concentration. Leaf blade testing is not required at other sampling times. The K concentration in the petiole is also valuable in monitoring in-season K nutritional stress since K is one of the key nutrients in cotton production.

Cotton petiole NO3-N concentration across the blooming period can be interpreted using the NO3-N concentration in Table 1 and 2. Table 1 consists of Arkansas interpretations and Table 2 consists of Georgia interpretations. The Arkansas interpretation offers higher sufficiency ranges of NO3-N concentration than the Georgia interpretation and should be used for cotton production in Louisiana. Petiole NO3-N concentration in both interpretations peaks before blooming and then gradually declines towards boll maturation. In the Arkansas interpretation, the sufficient petiole NO3-N concentration ranges from 10,000 to 35,000 ppm (1 to 3.5%) at first bloom and 1,000 to 5,000 ppm (0.1 to 0.5%) 6-wk after first bloom and the sufficient leaf blade N concentration ranges from 3 to 4.5% at first bloom stage. It is very important to note that these sufficiency ranges of petiole NO3-N concentration across the blooming stages are not the critical levels, but desirable ranges, and NO3-N concentrations below or above these desirable ranges do not directly indicate N deficiency or sufficiency as such because petiole NO3-N concentration is greatly influenced by plant stress caused by several abiotic and biotic factors. However, these sufficiency ranges may indicate in-season N status and incipient problems. Along with NO3-N concentration, P concentration in petiole during the blooming period is very important in understanding environmental or physiological stress that can influence petiole NO3-N concentration and hence the interpretation. For example, petiole P concentration at first bloom should be >800 ppm and a decrease of >300 ppm petiole P concentration from the previous week during blooming period is a good indicator of water stress and in-season N fertilization decision should be delayed until the next sampling results are received. For proper interpretation of petiole NO3-N concentration, it is very important to make sure that the observed petiole NO3-N concentrations have not been influenced by any stress during the growing season. Therefore, petiole NO3-N concentration should be interpreted and in-season N fertilization decision should be made based on physiological, environmental, and nutritional conditions including crop growth stage, soil moisture status, irrigation events, seasonal rainfall amount, fertilization history, leaf blade N concentration at first bloom, petiole P concentration trend during blooming period, fruiting load, internode length, number of nodes above the white flower, pest damage, heat units, cloud cover, etc.

Overall, monitoring in-season petiole NO3-N concentration is simply a tool that, in combination with other stress indicators, can help make better in-season N management decisions and maximize cotton yield and profitability. This tool is best suited for high yielding cotton fields with irrigation or in high rainfall areas where water does not limit yield. Care should be taken in making in-season N fertilization decisions for fields with a potential for late-season N mineralization from organic materials.

Table 1. Sufficiency ranges of nitrate-N concentration in cotton petiole during blooming period.

Sufficiency ranges of petiole nitrate-N ((NO3-N) concentration

Arkansas interpretation - ppm (mg/kg)

Time of sampling Minimum Maximum
1 week before 1 st bloom

Week of 1 st bloom 10,000 35,000
Bloom + 1 week 9,000 30,000
Bloom + 2 week 7,000 25,000
Bloom + 3 week 5,000 20,000
Bloom + 4 week 3,000 13,000
Bloom + 5 week 2,000 8,000
Bloom + 6 week 1,000 5,000

Table 2. Sufficiency ranges of nitrate-N concentration in cotton petiole during blooming period.

Sufficiency ranges of petiole nitrate-N ((NO3-N) concentration

Georgia interpretation - ppm (mg/kg)

Time of sampling Minimum Maximum
1 week before 1 st bloom 7,000 13,000
Week of 1 st bloom 4,500 12,500
Bloom + 1 week 3,500 11,000
Bloom + 2 week 2,500 9,500
Bloom + 3 week 1,500 7,500
Bloom + 4 week 1,000 7,000
Bloom + 5 week 1,000 6,000
Bloom + 6 week 500 4,000

Source: Mitchell C.C., and W.H. Baker. 1997. Plant nutrient sufficiency levels and critical values for cotton in the southeastern U.S. Proceedings of the Beltwide Cotton Conference, National Cotton Council, Memphis, TN. 1:606-609.

7/23/2020 1:40:02 PM
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