Phil Busey Agronomy
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Rapid decline of available soil potassium

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Rapid decline of soil potassium ppm

Soil potassium (K) ppm (mg/kg) by ammonium acetate extraction in untreated and sulpomag fertilized plots in sand sports field.

 

South Florida sports field soils are generally calcareous, sand, have high (> 7.0) pH, and have insufficient potassium (K).  For such soils the medium sufficiency range for turfgrass is reported as 75-175 ppm K by ammonium acetate extraction, appropriate for soil pH 7.0 and higher. (Carrow et al., 2004).

The example in the graph shows the rise and rapid decline of soil potassium after fertilizing a low K soil (blue circles, 39 ppm K by ammonium acetate extraction of sample from day 0) with 5 pounds K2O per thousand square feet (sulpomag, 20% K2O by weight). On day 6 after fertilization, potassium fertilized areas had 153 ppm K, followed by a decline to 131 ppm K on day 16, and to 44 ppm on day 45.

It is more difficult to sustain sufficient potassium in calcareous sand soil than other soils. A silt loam soil under alfalfa increased 1 ppm K per application of 5 kg K/ha, but in a calcareous sand green with pH 7.7, K increased only from 28 to 46 ppm by sodium bicarbonate extraction over three years with application of 400 kg K/ha/year (Johnson et al., 2003). If the latter soil behaved in a linear manner as the former soil, it would have had an increase of 80 ppm K during a year.

As another contrast, a Hadley silt-loam with pH from 6.0 to 6.3 was initially 119 ppm K by Morgan extractant, and after three years was 101, 154, and 210 ppm K, respectively, with fertilization at 49, 245, and 441 kg K/ha/year (Ebdon et al., 2013). Despite some question of time frame, the 91 ppm K increase is about the same as the 88 ppm K increase that would be expected. But a calcareous (pH 8.3) sand green fertilized four times with 0, 30, 60, 130, 190, and 250 kg K/ha/56 days, (about 0, 0.17, 0.34, 0.7, 1.1, and 1.4 mmol K/kg soil), initially had a linear rise of soil K to about 2.4 mmol K/kg by ammonium acetate extraction, under low irrigation, but had a net loss of potassium under subsequent high irrigation (Woods et al., 2005).

For comparison, the graph is based on fertilization with 202 kg K/ha. While the potassium increase is substantial, it is temporary. It would seem visually that the sports field would have to be fertilized at this high rate of K about once per month to maintain a sufficient level of soil K throughout a longer period.

A major potential problem in loss of soil potassium, particularly in low exchange soils, is the presence of Ca2+ as a competing ion in irrigation water and soil (Kolahchi and Jalali, 2007) causing the displacement and leaching of K+ from soil. Soil in the graph had average pH 7.9 and average 2542 ppm Ca2+ by ammonium acetate extraction and total exchange capacity (TEC) was 15.4 meq/100 g.

This presentation purposely avoids the larger questions of the sufficient level of potassium in soil, whether tissue analysis may be more appropriate for fertilization decisions, how to assess nonexchangeable potassium in the soil in meeting the needs of turfgrass (Ebdon et al., 2013), whether other factors need to be considered such as N fertilization and plant growth rate (Woods et al., 2005), as well as whether ammonium acetate extraction is the most relevant for plant fertilization decisions. For example, working with pinto beans in an extremely wide range of high pH soils in Iran, Hosseinpur and Zarenia (2012) showed that 0.01 mol/L CaCl2 extraction had the highest correlation coefficient with relative yield, 0.70 compared with only 0.44 for ammonium acetate extraction.

For a review of the effect or lack of effect of potassium on turfgrass plants, visit Micah Woods's interpretation:
http://www.blog.asianturfgrass.com/2014/06/turfgrass-and-textbooks-are-there-problems-with-potassium.html

References cited

Carrow, R. N., L. Stowell, W. Gelernter, S. Davis, R. R. Duncan, and J.Skorulski. 2004. Clarifying soil testing: III. SLAN sufficiency ranges and recommendations. Golf Course Management, January 2004, 194-198.

Ebdon, J. S., M. DaCosta, J. Spargo, and W. M. Dest. 2013. Long-term effects of nitrogen and potassium fertilization on perennial ryegrass turf. Crop Sci. 53:1750-1761.

Hosseinpur, A. R. and M. Zarenia. 2012. Evaluating chemical extractants to estimate available potassium for pinto beans (Phaseolus vulgaris) in some calcareous soils. Plant Soil Environ. 58:42-48.

Johnson, P.G., R. T. Koenig, and K. L. Kopp. 2003. Nitrogen, phosphorous, and potassium responses and requirements in calcareous sand greens. Agron. J. 95:697-702.

Kolahchi, Z. and M. Jalali. 2007. Effect of water quality on the leaching of potassium from sandy soil. J. Arid Environments. 68:624-639.

Woods, M. S., Q. M. Ketterings, and F. S. Rossi. 2005. Effectiveness of standard soil tests for assessing potassium availability in sand rootzones. Soil Sci. 170:110-119.