Potassium Fertilization of Warm-Season Grass Pastures in Florida

May, 2019
 Dr. Joao Vendramini, UF/IFAS Range Cattle Research & Education Center, Ona; James Yarborough, UF/IFAS Extension Orange County, Orlando; Dr. Maria Silveira, UF/IFAS Range Cattle Research & Education Center, Ona

Fertilization is a management practice with potential to minimize variation in bahiagrass production and quality; however, commercial fertilizers are the most costly input in most forage-based beef cattle production systems. Fertilizer prices have increased significantly over the last decades due to high energy cost and limited supply of fossil fuels. Nitrogen is routinely the main nutrient applied to warm-season grass pastures due to its great impact on forage production and nutritive value. However, repeated fertilization with N only may cause an imbalance of other nutrients in the soil, which may negatively affect forage production, nutritive value, and persistence.

Potassium is an important macronutrient for production and persistence of warm-season grasses. However, because of the limited ability of Florida soils to retain nutrients and often, limited use of potassium fertilizer, potassium deficiency can be an important factor responsible for poor performance and decline of pastures and hayfields in Florida. Potassium fertilization recommendations are based on soil testing results and, in mechanically harvested systems, the number of harvest during the growing season. However, in grazing systems potassium can be recycled through animal excreta and litter and generally limited amounts of potassium are exported as animal products. Potassium can also be lost from the system via leaching, particularly in lounging areas.

In collaboration with the Deseret Ranch, Deer Park, FL, the UF/IFAS Range Cattle Research and Education Center, Ona, FL initiated a number of greenhouse and field potassium fertilization trials to generate scientific information regarding potassium fertilization responses in grazed pastures. This information is expected to improve our ability to predict potassium fertilization requirements and also increase the efficiency of potassium fertilization in Florida.
An ongoing grazing study conducted in three locations at the Deseret Ranch was designed to evaluate different combinations of nitrogen and potassium fertilization. Treatments included three levels of nitrogen [,0 N, 1 application of 50 lb N/acre (spring), or 2 applications of 50 lb N/acre (spring and late summer)], and 2 levels of potassium fertilization (0 K2O or 50 lb K2O/acre (spring). The study sites consisted of three established bahiagrass pastures that exhibited either low or very low soil K levels at the start of the experiment. The experimental areas have been grazed and data have been collected every 6 weeks during 2 years. The results indicated that the additional nitrogen fertilization in late summer showed no increase in forage production. However, potassium fertilization resulted in a 20% increase in forage production in one location but no significant results were observed in the remaining two study areas.  Although K fertilization may not increase above-ground forage production, it may promote root/rhizome mass and, consequently; improve the persistence of warm-season grass pastures.

In addition, a greenhouse study was conducted at the UF/IFAS Range Cattle Research and Education Center to determine the critical tissue potassium concentration for bahiagrass and Jiggs bermudagrass. Despite the large number of studies published in the literature indicating average levels of tissue potassium concentration, particularly for bahiagrass, the critical levels below which bahiagrass and Jiggs bermudagrass production can be limited are not known. Treatments consisted of a factorial combination of 3 levels of N (0, 50, and 100 lb N/acre) and 4 levels K (0, 20, 40, and 80 lb K2O/acre) after every harvest. Plants were harvested at every 6 wk in 2014, and root and rhizome mass determined at the end of the experiment. Bahiagrass showed no significant effect of potassium fertilization on forage production and root and rhizome mass when no nitrogen was applied; however, when N was applied at 50 lb N/acre, bahiagrass production and root and rhizome mass increased by 100% as the potassium levels increased from 0 to 40 lb K2O/acre.  There was no conclusive relationship between potassium tissue levels and bahiagrass production. Similar to bahiagrass, potassium fertilization had no effect on Jiggs bermudagrass production and root and rhizome mass when no N was applied; however, Jiggs bermudagrass production and root and rhizome mass increased by as much as 300% as potassium levels increased from 0 to 80 lb K2O/acre. Jiggs bermudagrass production decreased when tissues potassium levels were below 1.7% (critical level). Potassium content in Jiggs bermudagrass roots and rhizomes decreased linearly with increasing levels of N fertilization when potassium fertilization was 0 or 20 lb K2O/acre. Conversely, potassium content in root and rhizome increased with increasing levels of N fertilization when 40 and 80 lb K2O/acre was applied.

It is evident bahiagrass response to potassium fertilization in areas with low soil potassium levels can be variable and may be related to factors such as other soil type, chemical properties, and nitrogen fertilization. Despite the lack of above-ground response, potassium fertilization can promote root and rhizome mass, which are important reserve organs responsible for pasture persistence. Identifying the critical tissue potassium levels will be an important diagnostic tool to guide potassium fertilization of pastures. There was no correlation between bahiagrass production and tissue potassium levels, thus, it was not possible to identify the critical level yet; however, data indicated that when Jiggs bermudagrass tissue potassium concentration is less than 1.7%, forage production can be reduced.

If you have any questions about this article, please contact Joe Vendramini, at jv@ufl.edu or 8637351314.

 

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