Estimation of plant available potassium in loamy soils by extraction with magnesium sulfate solution

Abstract

The aim of the study. To find a method to estimate plant available  potassium for soil testing that does not require expensive flame photometry or atomic absorption spectrometry.

Location and time of the study. Southern parts of West and Central Siberia, 2016-2019.

Methodology. Comparison the potassium pool, extractable by 0,1M MgSO4 solution, with exchangeable potassium, extractable by 1M NH₄OAc solution (according to GOST 26210-91) by regression analysis of the contents in 100 soil samples of arable soils (Luvic, Haplic and Calcic Chernozems, and Luvic Phaeozems) with different granulometric composition.

Results. Exchangeable potassium showed the best correlation with the uptake of the element by plants and with yield in a variety of soil and climatic conditions.  It was found that the relationship between potassium availability indices KAI_Ac (potassium extracted by 1M NH₄OAc) and KAI_Mg (potassium extracted by 0,.1M MgSO₄) was satisfactorily described (R² = 0.88) by a simple linear function for soils with  texture classes from sandy loam to sandy/silty clay and well described (R² = 0.92) by a power function for silt loam, clay loam and sandy/silty clay. In 81% of the soils surveyed the difference between the calculated (according to the power equation) and actual KAI_Ac values was no more than 20%, and for 97% of the studied soils it did not exceed 30%. The cost of analytical equipment required for potassium analysis for soil testing is 7-60 times lower for KAI_Mg determination as compared with KAI_Ac.

Conclusions. The KAI_Mg potassium availability index can be used alongside the standard KAI_Ac availability index for soils with soil texture classes from silt loam to sandy/silty clay. Conversion of KAI_Mg into KAI_Ac can be made by the following formula: KAI_Ac = 8,.108 • KAI_Mg^0,793.