Terry Rick1, Clain Jones2, Richard Engel2 and Perry Miller3, (1)Land Resources and Environmental Sciences, Montana State Univ., Bozeman, MT (2)Montana State Univ., Bozeman, MT (3)334 Leon Johnson Hall, Montana State Univ., Bozeman, MT
Most organic farms in the northern Great Plains are located too far from
confined feeding operations for manure to be an economically viable nutrient
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.