Green manure and phosphate rock effects on phosphorus availability in a northern Great Plains dryland organic cropping system

Maintaining or improving soil phosphorus (P) fertility is challenging for dryland organic producers in the northern Great Plains (NGP) of North America because of the region's high pH, calcareous soils combined with limited supplies of P-rich organic materials, such as manure. Certain inorganic P sources, such as phosphate rock (PR), are allowed on organic farms yet are sparingly soluble in neutral to alkaline soils. Green manure (GM) crops may improve availability of PR, yet little is known on their ability to dissolve PR in NGP soils. A 2-year cropping sequence study was conducted on a Montana farm that had been managed organically for 20 year. First-year crop treatments included spring pea (Pisum sativum L.), buckwheat (Fagopyrum esculentum L.), yellow mustard (Sinapis alba L.), and tilled fallow, fertilized with 0, 3.1, and 7.7 kg available P ha-1 as PR, and terminated at early pod stage with tillage. Following GM incorporation, winter wheat (Triticum aestivum L.) was sown in the fall and harvested at maturity. Aboveground P uptake of winter wheat was not affected by previous GM crop or PR rate, despite differences in GM P uptake amounts among crops and PR rates. Labile P fractions in the surface soil (0-0.15 m) were also not increased over pre-fertilization levels by PR rate or previous crop. The results suggest GMs have a limited ability to increase P availability of PR for a subsequent crop in this region. © 2011 Springer Science & Business Media BV.