Careful attention to nutrient management is an important consideration in profitable cotton growing. In particular, providing adequate, but not excessive nutrition to a cotton crop is critical in order to realize the full genetic yield potential of the seed planted. In addition, because the primary nutrient sources currently used are increasingly expensive, petrochemical-based synthetic inorganic fertilizers, growers can not afford to apply more than is needed. Further, improper management of all applied nutrients, whether synthetic or from natural sources, has the potential to pollute surface or groundwater quality.
Southeastern SSP has funded approximately 29 studies dealing with some aspect of cotton nutrition. These include: leaf petiole and tissue analysis, use of starter fertilizer, applying nitrogen by side-dress or split application, foliar application of nutrients, application of poultry or hog manure, winter cover crops (e.g., rye or wheat), and, where nematodes are not a problem, planting winter legume cover crops to fix soil N.
We presented growers with a variety of ways to determine the fertility and lime needs of their fields and asked how frequently they rely on each one. Growers responded by circling numbers from 0 (not at all) to 3 (very much). We included questions on how often they conduct soil testing, leaf tissue and petiole testing. We also presented growers with a variety of methods for applying fertilizer and asked how frequently they use each one. Growers responded by circling numbers from 0 (never) to 3 (always).
Grower responses to items about the ways they determine fertility and lime needs of their fields are depicted in Figure F. The figure shows the proportion of respondents who indicate they rely on this either often or always.
Figure F - How Growers Determine Fertility and Lime Needs
Nearly all growers (98.7%) say they use soil sampling often or always. Substantially fewer growers frequently rely on leaf tissue testing (21.5%) and leaf petiole testing (22.8%). Frequent use of precision grid sampling (17.4%) and aerial photos (4.9%) was even less common. For growers who do use soil and leaf testing, Table 3 shows the proportion that conducts those tests at least once per year.
Table 3 - Annual Soil and Leaf and Tests
Grower responses to items about the fertilization methods they use are depicted in Figure G. The figure shows the proportion of respondents who indicate they use this method often or always.
Figure G - Fertilization methods
Applying nitrogen by side-dress was the most frequently used technique with 88.6% indicating they use this method often or always. Nearly two-thirds (64.3%) frequently use split applications of nitrogen and 62.1% apply starter fertilizer. Less than one-third (30.5%) frequently use variable rate application. Poultry litter is used by about one-quarter (26.2%) and nitrogen fixing winter cover crops was used the least frequently (14.2%) of the methods we surveyed.
Other reports have noted that soil sampling is a standard practice frequently used by nearly all growers81. This is despite the fact that some Southeastern states do not recommend soil testing, at least for Nitrogen64 and several studies found little benefit in determining N status from surface samples.63 Our results may have reflected use of soil tests for nutrients other than N. However, many states do recommend the practice and consider soil sampling a BMP when developing nutrient management plans. 99
While reliance on leaf testing (tissue and petiole) was relatively low, further analysis of our survey data is required to reveal if growers who use consultants rely on these methods to evaluate lime and fertility needs more frequently than growers who do not use consultants. If this is the case, the extent of these differences may be obscured in our survey as some growers may not be fully aware of the specific nature of the tests that consultants are conducting. In any case, relatively low use of leaf petiole testing may be appropriate given the impact of weather on the need for sampling. These data suggest that many cotton growers are knowledgeable and sophisticated in their use of tools of nutrient management. 69
The prevalence of N application by side-dress and split applications likely reflects grower understanding of effective and efficient ways to meet crop needs. These methods appear to limit the potential for loss through leaching, especially on sandy soils. 36,2 Use of starter fertilizer was also common. This is despite the fact that while it has been shown to increase plant height 22,28,55 its impact on yield has been mixed21,28,55,22. The lack of clarity concerning starter fertilizers effects on yield may be due to differing soil moisture conditions after planting.100
Growers make only limited use of poultry litter although there is accumulating evidence that this practice is sound and cost-effective. One study found that poultry litter applied at equivalent of 70, 110 and 150 lbs./A of plant available N increased lint yield to at least 915 lbs./A., and that litter at 2.8 Tons per acre is equivalent to an application of a standard inorganic commercial fertilizer85. Although broiler litter can reportedly support normal cotton growth in the absence of any significant other nutrient sources, under some conditions, litter may need to be supplemented with inorganic fertilizer. 96 Poultry litter as a nutrient source should enhance profitability, especially as fertilizer prices continue to increase.74
Foliar fertilizer applications were used by less than half of growers. Expanded adoption may not be at the grower's discretion as they may be forced into an integrated soil and foliar feeding program if and when nutrient management plans (NMP) become mandated.82
The least commonly used source of soil amendment was planting of N-fixing cover crops. Although use of winter legume covers is both feasible and economical65. lack of use may be due to pre-existing problems with nematodes, seed expense, and difficulties with establishment and termination.69
We conclude that prevalence of soil testing to determine nutrient needs is seen as a sound and cost-effective practice by growers and that that limited use of leaf or tissue testing as a supplement to soil testing is probably the appropriate approach. Future research to document that leaf tissue and petiole testing are cost-effective and have clear benefits may not be warranted. It is interesting that use of starter fertilizer is substantial given inconsistent yield effects and may reflect grower interest in strong early growth of cotton seedlings irrespective of final yields.
Substantial levels of adoption of several fertilizer methods suggests that SSP-research has enhanced grower profitability both by helping determine what practices work and which ones do not. Further field demonstrations containing economic analyses are desirable given increasing cost of inorganic (petroleum-based) fertilizers.
Where it is available, use of poultry liter appears to be economically beneficial. The limited use of poultry litter may reflect the limited availability of the material.