RATE OF P UPTAKE BY ONIONS COMPARED WITH RATE OF PYROPHOSPHATE HYDROLYSIS IN ORGANIC SOILS
The rate of phosphorus uptake by onions was compared to the rate of pyrophosphate hydrolysis in mesic (MSM) and humic soil materials (HSM). Dry matter production and P uptake were very small from seeding to leek stage but increased considerably toward bulb maturity. Pyrophosphate half-life was 2 days in the HSM and 4 wk in the MSM, since addition of Ca(OH)2 markedly decreased pyrophosphatase activity and amounts of non-retained pyrophosphate in the MSM. By the time of high P demand by the crop, pyrophosphate was completely hydrolyzed in the HSM and most of it was already hydrolyzed in the MSM. These results suggest comparable availability of orthophosphate and pyrophosphate for onions grown in organic soils.Key words: Allium cepa L., onion growth, polyphosphates, Histosol, pyrophosphatase activity