Reducing Phosphorus Fertilizer Inputs to Flue-Cured Tobacco
Phosphorus (P) is an essential nutrient for the production of flue-cured tobacco in North Carolina. A healthy crop of flue-cured tobacco will require 12-15 pounds per acre of the nutrient (1,2); however, most tobacco producing soils in the state of North Carolina have sufficient soil reserves to meet this demand (Figure 1). In addition to the reduced need of supplemental P, the cost of P containing fertilizer is now a serious concern to many producers. From 1990 to 2013 prices for all macronutrient inputs increased exponentially, with P typically being the most expensive (Figure 2). What follows is an overview of P management and recommendations from the North Carolina Cooperative Extension Service.
Behavior of Phosphorus in the Soil
Once applied to the soil, P is not very leachable due to its unique chemistry and the complexes it can form with other elements such as iron (Fe) and aluminum (Al). Soil pH plays the largest factor in how P reacts with soil particles, and in a tobacco production system, where the pH is quite often between 5.5 and 6.5; it is most likely to complex with Al. While this may not be extremely crucial to consider for many flue-cured tobacco producers in the eastern growing region of North Carolina, it is a concept that many in the northwestern Piedmont must consider. There are special circumstances in those areas where additional P fertilizers are required due to soil chemistry and pH. In these rare circumstances producers should collect soil samples for nutritional analysis prior to the start of the growing season. Growers, in all regions, should be aware that although P loss through leaching is not of major concern, losses through soil/water runoff and erosion are serious and can be of great consequence to the water bodies in which they enter.
Soil Phosphorus from Production Agriculture
For decades tobacco producers have supplemented crop nutrition with blended, low analyses fertilizers such as 3-9-9 and 4-8-12. The overuse of these products for such an extended period of time has resulted in over 80 percent of all tobacco producing soils in North Carolina having a high to very high P index (3). To further compound this issue, in many areas of the state, agricultural operations have been applying livestock waste to fields and crops as a cheaper alternative to synthetic fertilizer. Livestock waste is often very low in inorganic N and high in P; as a result larger amounts of the product must be applied to significantly offset the need for large amounts of synthetic N fertilizer. The long term implementation of this plant nutrition model has resulted in excessive quantities of P being applied to agricultural lands, most of which is still available in the soil profile for plant uptake.
Phosphorus Demand in Tobacco Production
As previously mentioned, a healthy flue-cured tobacco crop will only require 12-15 pounds of phosphorus per acre (1). Phosphorus uptake occurs season long with the majority taking place early in the growing season between transplanting and topping. Phosphorus moves through the soil from the point of application to the root zone for uptake via diffusion. Early season environmental conditions (cool temperatures and excessive moisture) often inhibit P movement and uptake. Cool weather conditions slow diffusion rates of P through soil pores making it difficult for the nutrient to reach a location suitable for use. Furthermore, cool and damp conditions inhibit the root growth of tobacco plants. Root growth can overcome some of the effects of slowed rates of diffusion; however, when the two issues are coupled, plants are observed to be very slow in their development. These plants will typically not exhibit P deficiency symptoms, but will simply be slow in overall growth rates. Often, warmer temperatures and a slight reduction in soil moisture will correct this issue.
Phosphorus Application for Tobacco Production
As previously mentioned, P fertilizer application is not recommended to over 80% of the tobacco producing soils in North Carolina. Producers should collect soil samples from fields destined for tobacco production and submit them to a testing agency for analysis. This process will determine the appropriate rate of P to apply. When soil analysis results dictate, do not apply P as it is costly and will not add any benefit to the crop. Growers reluctant to not apply P can apply 5 pounds per acre in a transplant water fertilizer. Research indicates that 5 pounds in the transplant water is equivalent to 40 pounds per acre band applied after transplanting (1). If transplant water fertilizer is not an option, P can be supplied in a sidedress application post-transplanting. When using blended materials, producers are encouraged to utilize materials with a low P analysis such as 6-3-18.
In conclusion, P is essential for the growth and development of flue-cured tobacco in North Carolina. In most tobacco production systems sufficient reserves of P are present in the soil profile and can be utilized for season long demand. In current times, P is one of the more expensive inputs required for crop production; fortunately, avenues do exist for reducing this cost. Collecting soil samples for nutrient analysis prior to the growing season will assist in making these management decisions and should serve as the basis of P management.
- Vann MC, Smith WD. 2014. Managing Nutrients. Pages 62-77, in: North Carolina State University:Flue-Cured Tobacco Guide 2014. AG-187. Revised ed. North Carolina Cooperative Extension, Raleigh, NC.
- Raper CD, McCants CB. 1967. Nutrient Accumulation in Flue-Cured Tobacco. Tobacco Science 10: 109.
- Anonymous. 2012. State Soil Test Summary-2012. Available from: http://www.ncagr.gov/agronomi/pdffiles/12statexgrp.pdf. Accessed February 13th, 2014.
- Nehring R. 2013. Fertilize Use and Price. Available from: http://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx#26727. Accessed February 13th, 2014
Figure 1. Phosphorus indices by flue-cured tobacco producing region in North Carolina (3).
Figure 2. Price trend of macronutrient inputs from 1990-2013 (4).