Exploration of Responses to Potassium in Western Uruguay

A summary of the K research carried out at Uruguay by Facultad de Agronomia and IPNI with the support of CANPOTEX and activities by other institutions such as INIA, coops, and several university groups was carried out at the K symposium organized by IPNI EECA at the 2016 Soil Science Congress of Russia celebrated at Belgorod (Belgorod Oblast) on August 16-20, 2016.

Agriculture at Uruguay has historically developed in high K soils, under conventional tillage and crop rotations that included pastures, resulting in no K fertilizer recommendations. Initial studies in K response to fertilization were done for crops that have high-K requirements such as sugarcane, sugar beet, potato, onion, and cotton, for which some guidelines for fertilizer recommendations based on soil type were established. In grain crops, the first K studies were made in the 60’s, and K responses were observed in wheat grown in soils developed from cretaceous sandstones.

However, agriculture scenarios of Uruguay have changed during the last two decades: cropping systems have been intensified, showing a current index of 1.5 crops per year, resulting in soil K depletion. Research reported that soil test K (STK) under agriculture at Department of Soriano, in the western agricultural area, have decreased 40% and 44% at 0-7.5 cm and 7.5-15 cm, respectively, from the levels observed in the same soils without agriculture history. In addition, agriculture has expanded to marginal soils in the northcentral and eastern regions of the country, where low STK soils are common.

The K balances in Uruguay (application minus removal), have historically been negative due to the absence of K fertilization. Moreover, as soybean has increased in area in the last two decades, due to its high K requirements, K balance has become more negative; i.e., soybean exports for 2014 were 3.6 M t, implying a K removal of approximately 63,000 t of K₂O, considering an average grain content of K.

In the late 90´s and early 2000´s, field research by the faculty of Agronomy (UdelaR), INIA, and other organizations reported some cases of K deficiency symptoms in soils with low STK in maize and Lotus corniculatus L.. Moreover, the increasingly frequent occurrence of visual K deficiency symptoms, confirmed by plant analysis, lead to more specific studies which showed K response in several crops. A summary of more than 60 studies (under same tillage systems, and similar experimental design, rate, and K source), found a critical STK level of 0.3-0.4 meq/100 g (120-160 ppm; 0-20 cm depth), representing a breakthrough in K research in Uruguay.

Research in recent years has focused on the ability of STK to predict yield responses, and the variability of STK determinations. Quality and management of crop residues, may affect K distribution with soil depth, and it should be considered by soil survey/sampling and fertilizer recommendations. STK determination in moist samples showed a better adjustment with yield responses than dry samples. Evaluation of non-exchangeable K fractions (Na-tetraphenylborate extraction) improved the correlation between STK and yield response.

Future research and experimentation will have to focus on the relationship of K dynamics with soil mineralogy and physical properties, and changes in cropping systems and soil management history in the medium and long term. These studies would be useful to develop K fertilization guidelines. Potassium use efficiency depends on understanding of K dynamics in the soil-plant system, as well as crop and soil responses to soil fertility management.