As many are probably aware, we are getting reports from multiple sources that the availability of potassium sulfate (sulfate of potash, 0-0-50) for the upcoming growing season will be rather limited. At the current time it is not known what factors have contributed to the fertilizer shortage or how long it will last. It is advisable that tobacco producers, fertilizer dealers, and crop consultants make other arrangements when sourcing or making recommendations for potassium (K) fertilizer applications in tobacco production.
When considering K sources for tobacco production, chlorine (Cl) content is always a primary concern due to the toxic effect it can have on the plant when applied at rates greater than 30 pounds per acre. Chlorine toxicity is a result of excessive chloride (Cl–) accumulation and will result in a decrease in leaf yield and quality as well as very poor smoke flavor. Homogenized tobacco grade fertilizers, such as 6-6-18 and 8-8-24, will contain no more than 2% Cl or nor more than one-half the total amount of nitrogen when nitrogen is greater than four percent. Homogenized fertilizers will typically blend various sources of K to meet crop based nutritional requirements. Additionally, non-homogenized K fertilizers, such as sulfate of potash and K-Mag, will contain somewhere between one and two percent Cl. Ultimately, both fertilizer sources (homogenized and non-homogenized) will not oversupply Cl when used at recommended rates.
Issues with Cl content in fertilizers are found when non-tobacco grade sources are used in substitution for those previously mentioned. The most commonly used K fertilizer, worldwide, is muriate of potash (potassium chloride, 0-0-60) because of its high K analysis and relatively cheap cost. Muriate of potash is not recommended for use in tobacco production, regardless of tobacco type, due to its high Cl content (45-47%). An example of this issue is as follows; 100 pounds of muriate of potash applied per acre will only supply about 50 pounds of K+ but will supply around 46 pounds per acre of Cl–. The end result of this application is that K is in short supply, while Cl is at a concentration too high for industry standards. This issue is further compounded when soil fumigants are used to suppress soil borne diseases and nematodes. The two most commonly used soil fumigants in tobacco production (Telone and Chloropicrin) both have active ingredients comprised of chlorine based compounds. It is safe to assume that these products will provide 10-15 pounds of Cl– per acre when used at recommended rates, which only furthers the need to give serious consideration to fertilizer source. Ultimately, to account for residual Cl– and Cl– added to the soils from fumigation, it is recommended that fertilizers applied to tobacco supply no more than 20-25 pounds per acre of Cl.
Alternatives to sulfate of potash do exist for tobacco producers that are having difficulty in sourcing the product. Those alternatives are as follows:
- Do not over apply K for soil building purposes; rather only apply the recommended amount based on a recent soil test. Over 60% of the tobacco producing soils in North Carolina have a high to very high K-index; therefore, soil reserves of K can be utilized to supplement nutritional needs beyond what is applied through fertilizer.
- K-Mag (Sul-Po-Mag, 0-0-22): K-Mag is an excellent source of K and also provides sufficient amounts of magnesium (11%) and sulfur (22%); additionally, the Cl content of K-Mag is also very low (less than 2.5%) so it is not a significant concern. The price of K-Mag is typically higher than sulfate of potash; however, price may not be a strong consideration if 0-0-50 cannot be sourced.
- 14-0-44 (potassium nitrate), 13-0-14 (sodium nitrate), 15-0-14: These products contain trace amounts of Cl and can be used for tobacco production; however, they are often more expensive than other sources of K. Producers should consider the economics of these products and the need for nitrogen prior to purchase.
- The use of nitrate-nitrogen based products will reduce Cl– uptake compared to ammonium-nitrogen based products. Chloride (Cl–) is needed to balance the positive charge of ammonium (NH4+) within the plant.
- Assuming soil pH is above 5.5 the conversion of ammonium to nitrate will occur at a rate where the majority of nitrogen uptake is in the nitrate form.
- 6-6-18 & 8-8-24: Many producers continue to utilize complete fertilizers in tobacco production. Homogenized fertilizers used in tobacco contain trace amounts of Cl and are suitable for tobacco production; however, producers must consider the economics of these products because they contain nutrients (such as phosphorus) that may not be needed.
- Muriate of Potash (potassium chloride, 0-0-60): Muriate of Potash can be blended with other potassium fertilizers as a last resort. It is critical that blenders and producers use appropriate blending ratios and techniques to ensure that Cl does not become highly concentrated. Forty-five pounds of Muriate of Potash will supply about 20 pounds of Cl and it is recommended that blending rates do not exceed this threshold.
- It is critical that growers are aware of this issue and that they inquire about the source of K used in their blended fertilizers. Assuming that the source is the same as in previous years can lead to a number of production related issues.