Synergistic Effects of Rock Phosphate and Green Manure on Soil Phosphorus Availability.

Organic farms in the northern Great Plains (NGP) are generally too far from confined animal feeding operations for manure to be used as a phosphorus (P) nutrient source. Rock phosphate (RP) may be the only viable option for many large acreage organic growers. However, this source is generally not very plant-available in neutral to alkaline pH soils common in this region. There is evidence that some crops such as buckwheat, legumes, and some mustards can increase the availability of RP through rhizosphere acidification, yet there is limited field research on whether this effect extends to the following crop at economically viable RP rates. A field study was conducted on a farm in north central Montana that had been organically certified for 16 yr (mean 0-15 cm soil pH = 6.6, Olsen P = 16 mg kg^-1). Available (citrate soluble) P rates of 0, 1.5, and 3.7 kg ha^-1 as RP (0-1-0) were applied in early April, 2006, and tillage incorporated to 15-cm. Spring pea (Pisum sativum L.), buckwheat (Fagopyrum esculentum L.), and mustard (Sinapis alba L.) were subsequently seeded and grown to the first pod stage prior to termination with tillage. Tilled fallow was included as a control and each crop-fertilizer treatment contained four replicates. Winter wheat (Triticum aestivum L.) was subsequently seeded in all plots and harvested at maturity. Total aboveground P uptake in wheat following buckwheat was 15% higher (P=0.02) at 3.7 kg P ha^-1 than at 0 kg P ha^-1, and the P uptake difference between these treatments represented 45% of applied available P. Conversely, P uptake was not enhanced (at P=0.05) by RP fertilization following spring pea, mustard, or fallow. These results suggest that the buckwheat rhizosphere can dissolve sufficient RP in a neutral pH soil to increase available P for a subsequent crop.

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