Potassium Fertility: An Overview of Nutrient Demand, Application Rates, and Deficiency

Introduction

                Potassium (K) is an essential plant nutrient for the production of high-yielding, high-quality flue-cured tobacco with a healthy crop requiring about 90 pounds per acre for optimum growth (1-3).  The tobacco plant is a luxury consumer of K (1), and, as a result, potassium application has historically occurred at rates 2-3 times that needed for maximum yield (4).  Excessive application rates of K2O have long been justified as means to prevent K deficiencies; however, deficiencies are still common in tobacco production and occur across a wide range of soil types and growing environments. 

                Potassium is highly leachable and nutrient reserves are often deficient or very low in coarse soil types.  For these reasons, K application rates vary across the flue-cured growing regions of North Carolina.  Recommendations for K fertilization rates vary based on soil type, residual soil potassium, application timing, and application method.  The current minimum recommendation for flue-cured tobacco production in North Carolina is 90 pounds per acre (2); however, growers often apply as much as 150-175 pounds per acre.  Recent studies indicate that this targeted application rate can be reduced on specific soil types under certain conditions.

In current times, the balance between producing high-quality leaf while maintaining strong profit margins has received increasing attention.  It is widely acknowledged that there is great potential to reduce the cost of tobacco production through a re-evaluation of the fertility programs that are implemented on-farm.  For a number of years, producers in North Carolina have been encouraged to explore alternative fertilizer sources, alternative fertilizer application rates, and alternative application methods.  Potassium fertilizers and potassium fertility management is no exception to this initiative.  What follows is a brief overview of potassium fertility and alternative approaches to management.

Potassium Demand

                As previously mentioned tobacco is a luxury consumer of K and will continue to absorb the nutrient until soil reserves become exhausted.  Furthermore, K accumulation in the flue-cured tobacco plant is far greater than any other mineral required for growth and development.  This was confirmed by Raper and McCants (1967) who found a nutrient accumulation ratio of 2:1 for potassium to nitrogen. 

                Potassium demand is supplied by K containing fertilizers due to the naturally low-fertility of the soils in which flue-cured tobacco is commonly produced.  The common flue-cured tobacco producing soils of North Carolina are characterized by having a medium to coarse soil texture, low natural fertility, very good drainage, and less than one percent organic matter; all characteristics that lead to an increased need for supplemental fertilizers. 

Potassium Application Rates & Methods

                The minimum recommendation for applied K is 90 pounds per acre; however, research at NCSU indicates that K rates can be reduced to 75 pounds per acre (3) on specific soil types without reducing yield and quality.  These soil types have a medium to heavy soil texture (loamy sand to clay), medium to high potassium index, and relatively shallow depth to clay (<10 inches).  For tobacco producers that do not have soil types with these specific conditions, the same principle is true; K application rates can be reduced by 15-30 pounds per acre, sometimes more, without decreasing yield or quality.  While recommended K rates may be less than the required 90 pounds per acre, producers should be aware that adequate amounts of K are often found in the subsoil.  Subsoil K can be utilized to supplement nutritional demands when the depth to clay is less than 10 inches.      

                Potassium application can occur through a number of methods at different times during the season.  The promotion of prescription type fertilizer programs, compared to the use of blended fertilizer programs, has created the opportunity to apply K at different times during the season.  For producers who do not apply P fertilizer, due to high soil P indices, in their fertility program, independent applications of N and K are favored.

Potassium application and management can occur through the following programs:

  • ·         Broadcast Application Prior to Transplanting

Potassium can be applied via broadcast application and soil incorporated prior to the formation of raised plant beds.  The major benefit of this approach is that from a fertility standpoint the only action left to take is N application post-transplanting.  This management strategy can be utilized as early as one month prior to transplanting by producers who use fumigation.  This management program is preferable for producers on medium to fine soil textures with a shallow depth to clay so that the risk of K leaching prior to transplanting is reduced.  However, this is an option for producers on coarse soil textures with a deeper depth to clay (10+ inches).  These producers must pay close attention to rainfall and account for any K that may have leached during a precipitation event.  Leaching adjustments will vary based on water percolation through the soil profile and on the depth to clay.  Leaching recommendations can be found in the “Managing Nutrients” section of the NCSU Flue-Cured Tobacco Production Guide.

  • ·         100% Sidedress at Transplanting

It has long been recommended that K application occur in a band or sidedressing application to improve use efficiency by placing the material closer to the root zone for uptake.  This concept is still true, but in many cases may increase the number of passes across a field post-transplanting.  When implementing a sidedress application, fertilizer should be placed about four inches away from the plant and at a depth of four inches into the plant bed.  Placing K fertilizer this set distance from the plant will reduce root injury from machinery while simultaneously placing it close enough for rapid availability and uptake. 

  • ·         Split Application: ½ Rate at Transplanting and ½ Rate at Layby

Split applications can occur with the same recommendations from the previous section; however, a split application provides the benefit of avoiding large amounts of K loss through early season leaching.  This management program is beneficial for producers on soil types that contain a large percentage of sand and a deeper depth to clay.  Furthermore, the split application helps to ensure late season availability of K and the avoidance of deficiencies.

                 All of the mentioned application methods and timings have been confirmed through research conducted in North Carolina and are suitable across the various growing regions/environments of the state.  Prior to determining a suitable application rate and application method, producers must be aware of soil texture, residual soil potassium, and depth to clay.  Strong potential exists to greatly reduce K inputs and, as a result, increase profit margins.

Potassium Fertilizer Source

                Potassium fertilizer source must be given careful consideration prior to application.  The two most common sources of K fertilizer are Sulfate of Potash (0-0-50, 17% sulfur) and Sulfate of Potash Magnesia or KMag (0-0-22, 11% magnesium, 22% sulfur).  Both materials supply sulfur, which is needed at a rate of about 30 pounds per acre, but KMag also provides magnesium which can be in low supply on coarse soil textures.  Ultimately, when choosing a K fertilizer source cost of material and nutritional needs should be considered.

                Potassium Chloride (Muriate of Potash, 0-0-60) should be avoided due to high chlorine content (40%).  Chlorine can be beneficial to tobacco but is toxic at rates above 30 pounds per acre.  The rate of potassium chloride required to provide sufficient K would also provide far too much chlorine and plants would suffer.  Furthermore, producers utilizing soil fumigants receive chlorine from these products which could further the toxic effects of the nutrient.  Typically there is enough chlorine present from fertilizers used in previous crops or from fumigants to supply what is needed during the growing season.

Potassium Deficiency

                Potassium deficiencies are common across the various tobacco producing regions of North Carolina and are typically observed during every growing season.  As previously mentioned, many producers over apply K to prevent deficiencies, and in most cases this practice is successful; however, cases have been presented where this was not successful.  Potassium deficiency is more often than not directly related to weather/environmental conditions more so than available K in the soil profile.  Deficiencies are typically observed during weather extremes: drought, excessive moisture, and extreme heat.  Potassium moves through the soil, from the point of application to the root zone, via diffusion and any condition that slows diffusion can lead to deficient symptoms.  Diffusion is promoted when soil moisture is adequate but not excessive.  If moisture is excessive then leaching is likely to occur to some extent.  Additionally, during periods of drought or less than adequate soil moisture the medium for diffusion (water) is not present and K movement to the root decreases.  Furthermore, any condition that reduces root growth has the potential to reduce K content within the plant because an increase in root volume provides more locations for K uptake to occur.  In many cases if soil moisture conditions can be improved or if additional root growth can take place deficiencies can correct themselves.  Additional applications of K fertilizer are recommended in extreme cases when leaching has occurred.

                Potassium deficiency is characterized by the yellowing of leaf tips and margins in mid-stalk positions (Figures 1-3).  If not corrected deficiency symptoms will spread to upper stalk leaves.  In extreme cases the chlorotic (yellow) tissue can turn black and fall off of the plant.  In these cases leaf yield, quality, and value can be greatly impacted.

Conclusion

                In conclusion, K is essential to producing the style of quality tobacco that North Carolina is known for.  Many producers have over applied K for decades but with increasing input costs and growing environmental concerns new considerations must be made.  Potential exists for producers to reduce K application rates and possibly explore alternative application methods where appropriate.  Producers who consider these management approaches must be aware of the specific soil type/texture in a given field, the residual soil K, and the depth to clay. 

References

1.     Raper CD, McCants CB. 1967. Nutrient Accumulation in Flue-Cured Tobacco.              Tobacco Science 10: 109.

2.     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.

3.       Vann MC, Fisher LR, Jordan DL, Hardy DH, Smith WD, Stewart AM. 2012.   The  Effect of Potassium Rate the Yield and Quality of Flue-Cured Tobacco. Tobacco Science 49:14-20.

 4.       Sims JL. 1985. Potassium Nutrition of Tobacco. Pages 1023-1043. In: RD Munson, ed. Potassium in Agriculture. American Society of Agronomy, Atlanta, GA.

Figure 1. Potassium deficiency in tobacco just after topping.k def 4

Figure 2. Above view of a flue-cured tobacco plant with K deficiency.k def 6

 Figure 3. Potassium deficiency in a field setting.k def 7

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