Excess Phosphorus and Potassium in Manure-Amended Soils: How Much Is Too Much?.

Use of manure-based compost is common in intensive vegetable cropping systems. When compost is used to meet crop nitrogen requirements, phosphorus (P) and potassium (K) are typically over applied and accumulate in soils. While P is regulated as a pollutant, K is not. Both P and K are implicated in antagonistic interaction with micronutrients including zinc, manganese, iron, and copper. However, the levels of phosphorus and potassium at which antagonism occurs are not well documented, especially for high-pH soils. Field experiments were conducted at two locations in Montana in 2016. Both locations had soil pH above 7. Initial soil Olsen P and exchangeable K levels at Bozeman were 103 and 707 ppm, and at Corvallis 11 and 168 ppm respectively. Commercially available composted sheep manure was applied at 0, 17, 34, and 67 Mg ha^-1. Additional phosphorus and potassium was applied as nitrogen salts of both nutrients, with N-only and unfertilized control treatments, resulting in 12 unique combinations of elevated phosphorus and potassium from both organic and mineral forms. Swiss chard (Beta vulagris) was grown and harvested as leafy greens. We hypothesized an interaction among compost and fertilizer levels on crop growth or tissue micronutrient levels that would elucidate thresholds for levels of P and K antagonistic against micronutrient uptake. There was no effect of compost on crop yield at either location. At Corvallis, all fertilizer treatments increased crop yield equally, indicating nitrogen limitation. There were no significant compost-by-fertilizer interactions or effects of compost on crop micronutrient concentrations. There were, however effects of fertilizer P&K on crop micronutrient uptake. At Bozeman, K fertilizer decreased Mn and B uptake, and at Corvallis K fertilizer reduced concentration of Mn, B, Zn, and Fe. In no case did tissue micronutrient levels fall below published sufficiency levels.