Exploration of Responses to Potassium in Western Uruguay

Several regions of Uruguay has recently shown K deficiencies and responses. This project looks to determine the different areas with K deficiency, evaluate responses in the main field crops, and provide information on K soil testing.

IPNI-2007-URY-2

14 May 2018

2017 Annual Interpretive Summary


This research is based on previous observations of potassium (K) deficiency and responses to K fertilization in field crops grown within the northwestern Uruguay region. Data from the initial field work, including 50 experiments under wheat, barley, maize, soybean, sunflower, and sorghum, indicated a critical soil K concentration of 0.30 to 0.40 cmol/kg [i.e., soil test K (STK) of 117 to 156 ppm]. Below this K concentration, there is a high probability of a positive yield response to K fertilization. Integration of soil survey data with this field work has allowed researchers to estimate that an area of 4 million ha is potentially K deficient in Uruguay.

Field work in the 2016-17 growing season included field experiments evaluating K response in soybean (two) and pastures (two). Soybean grain yield responses were verified at the low soil test K location (0.26 cmolc/kg), although without differences between rates of 60 and 120 kg K2O/ha. In the two pasture experiments, under low to medium soil test K conditions, responses in dry matter production were 9 to 19%, with larger impacts on grass production than on legumes.

A compilation of 110 studies, 50 experiments conducted through 2007 and another 60 field trials from 2007 through 2015, was carried out to correlate relative grain yields to STK. The STK ranged from 0.12 to 1.22 cmol K/kg and relative yields (yield without K application/yield with K application) from 54% to 105%. Crops responded positively to K application in 58% of the cases in soils testing <0.20 cmol K/kg, 43% of the cases when testing from 0.21 to 0.30 cmol/kg, 34% at soils from 0.30 to 0.40 cmol/kg, and 29% in soils from 0.40 to 0.50 cmol/kg. Only at three of 35 sites with soil testing above 0.51 cmol K/kg, was response to KCl application observed. Potassium increased crop yield more frequently for corn than for soybean. Critical levels were closer to the 0.3 to 0.4 cmol/kg range that was previously established.

Laboratory studies have shown that the Mehlich extractant acquires 11% less exchangeable K than the conventional ammonium acetate extractant used at Uruguay (Exch K-Mehlich 3 = 0.89 exch K-Amm acetate +14.4). Similar studies with several soils in Uruguay, showed that the sodium tetraphenylboron extraction receives 50 to 100% more K than conventional ammonium acetate extractions. The non-exchangeable K reserve is larger in soils with illite as the dominant clay.

Several B.Sc. thesis and graduate students are involved in this research project. Results of the project and K management strategies have been presented at several meetings organized by companies and farmer associations, as well as at international meetings.