Evaluation of Composted Pig Manure and Organic Fertilizer for Organic Onion Production in Paddy Soil

A two year study was conducted on the effects of composted pig manure applications on organically managed intermediate-day onion ($Allium$ $cepa$ L.) with mid-season production. The manure application rates were 0, 4.0, 6.0, and 8.0 $ton{\cdot}ha^{-1}$, and accompanied by organic fertilizer with 240 $kg{\cdot}ha^{-1}$ nitrogen for all treatments. There was a positive linear effect on the marketable bulb yield with increasing applications of pig manure. However, pig manure rates of 6.0 or 8.0 $ton{\cdot}ha^{-1}$ did not affect the marketable yields or bulb weights at harvest. There was no significant difference between pig manure application rates and mineral concentration or nutrient uptake in onion bulbs. Soil pH and electric conductivity (EC) were lower at harvest compared to preplant soil, while exchangeable potassium and calcium contents increased at harvest. However, there were no significant effects on soil pH, EC, and organic matter or exchangeable cations caused by different pig manure application rates. Mineralized $NO_3$-N content was not affected by pig manure application rates except at transplanting and 129 day after transplanting. Based on the results of this study, when organic fertilizer is applied at a rate of 240 $kg{\cdot}ha^{-1}$ N, 6.0 $ton{\cdot}ha^{-1}$ composted pig manure should be adequate for producing onions using an organic production system.