Phosphorus fertility in Northern Great Plains dryland organic cropping systems
Maintaining phosphorus (P) fertility in northern Great Plains (NGP) dryland organic cropping systems is a challenge due to high pH, calcareous soils that limit P bioavailability. Organic P fertilizers, including rock phosphate (RP) and bone meal (BM) are sparingly soluble in higher pH soils. Certain crops species have demonstrated an ability to mobilize sparingly soluble P sources. Objectives of this project were to 1) evaluate the effect of green manure (GM) crops and organic P fertilizers on the P nutrition of subsequent crops, and 2) investigate P fertility differences between organic and non-organic cropping systems. A two-year cropping sequence was conducted on an organic farm in north-central Montana (mean pH=6.6; Olsen P=16 mg kg-¹). Spring pea (Pisum sativum L), buckwheat (Fagopyrum esculentum L.), yellow mustard (Sinapis alba L.) and tilled fallow were fertilized with 0, 3.1 and 7.7 kg P ha-¹ as RP, grown to flat pod stage and terminated with tillage. Winter wheat (Triticum aestivum L.) was grown on these plots in year two. Phosphorus uptake of winter wheat was enhanced (P>0.05) by RP following buckwheat only (P=0.02) at 7.7 kg P ha-¹ compared to 0 P. Results indicate buckwheat can enhance P in a subsequent crop. A greenhouse pot experiment in a low P soil (Olsen P=4 mg kg-¹) consisted of four green manures; buckwheat, spring pea, wheat, and a non-crop control fertilized with 7.0 and 17.5 kg available P ha-¹ as RP, 13.0 and 32.5 kg available P ha-¹ as BM and 10 and 25 kg available P ha-¹ as monocalcium phosphate (MCP). Green manures were harvested, dried, analyzed for nutrient content, and returned to pots. Pots were seeded with wheat. Phosphorus uptake in wheat following all crops was enhanced by MCP (P<0.05). Phosphorus uptake of wheat following buckwheat was enhanced by all P sources over the control. Buckwheat demonstrates the capacity to increase the availability of organic P fertilizers. Soil sampling of organic and non-organic no-tillage (NT) cropping systems was conducted in two separate studies to determine differences in P availability between management systems. Soil analysis determined available P tends to be lower in non-fertilized systems.