04-2020 High Mg Soils Impact on Soybeans Annual Report

RATIONALE/JUSTIFICATION FOR RESEARCH: Magnesium is a critical nutrient for crop development and grain production that is typically found to be in adequate supply throughout the Mississippi Delta. Typically, the percentage of readily available potassium is low and limited to that held on cation exchange sites on clay colloids and what is dissolved in the soil solution. High soil magnesium exacerbates the already low percentage of available potassium by super-saturating exchange sites with its double positive charge and essentially crowding out the potassium ions. This magnesium induced antagonism is known and generally addressed by increasing potassium rates in fertility recommendations. New knowledge suggests that high magnesium levels could be could be a more widespread issue than previously considered, with negative effects extending beyond that of simple potassium antagonism. High magnesium can also negatively impact soil structure and related functions. Recent studies point towards magnesium induced soil disaggregation as the underlying process. As clay particles disaggregate (i.e. disperse) under high magnesium conditions soil pore space decreases resulting in crusting and compaction thereby reducing soil aeration and water infiltration/internal drainage. In the field, these conditions are particularly expressed in medium to fine textured soils through symptomology such as: 1) prolonged soil wetness; 2) soft soil easily subject to compaction; 3) rapid transition from “wet” to “dry”; and 4) extreme soil “hardness” when dry. While this may or may not influence yield, it does significantly influence field operations.