Juan Pablo Garcia, Charles Wortmann, and Martha Mamo. University of nebraska Lincoln, Keim Hall of 321, 5052 R Street Apartment 1220, Lincoln, NE 68504
Continual application of manure or fertilizer phosphorus in no-till systems can lead to high available P levels at the soil surface with very low P levels below 10 cm. This can result in increased P delivery in runoff to surface waters and in reduced plant uptake. Several one-time tillage operations, in an otherwise continuous no-till system, were compared for effectiveness in redistribution of stratified soil P and for crop P uptake. The research was conducted on two dryland sites in eastern Nebraska with a RCBD split plot arrangement of five tillage treatment main plots (moldboard plow, disk, continuous no-till, chisel plow at 20 cm, and either chisel plow at 30 cm or mini-moldboard plow), and two manure-P application sub-plots (0 and 200 kg P ha-1). The crop was soybean (Glycine max L). Total P (perchloric acid digestion) and Bray-P1 were determined for five depths to 30 cm. Mycorrhizal colonization was measured using direct hydrolysis, derivatization and extraction of fatty acids. Manure application increased total P and Bray-P1 in both sites at all depths. Bray-P1 in the surface 2.5 cm was reduced between 52% and 86% (mean=14.4 and 16.6 ppm) with moldboard and 30% (20.7 ppm) with mini-moldboard plowing, but increased 22% (mean= 36.3 ppm) by chisel plow and between 22% and 26% (mean= 36.2 and 144.6 ppm) with disk tillage. Neither manure nor tillage affected total P uptake at the V6 and R6.9 growth stages in vegetative and reproductive parts of the plant. Grain yield also was not affected by manure application and tillage. At V6 mycorrhizal colonization was reduced by manure application and by plowing. In conclusion, occasional tillage by plowing could be adopted to reduce P stratification, even though this practice may cause, at least a short-term, reduced mycorrhizal colonization.
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