Correlations between soil physico-chemical properties and plant nutrient concentrations in bulb onion grown in paddy soil

• Correlations between soil fertility and plant nutrient contents in bulb onion.
• Changes in plant nutrients and nutrient uptakes at the initial bulbing stage and at harvest.
• Changes in soil physical and chemical properties at the initial bulbing stage and at harvest.
• Soil N or NO3N at the initial bulbing were negatively associated with bulb nutrients at harvest.

This study was carried out to evaluate the correlations between soil physico-chemical properties and onion (Allium cepa L.) plant nutrients in a long-term onion producing area over 30 years, during the growing season of 2010–2011. Soil and plant samples were collected from 16 onion growing fields. Each mineral content of onion plants showed different trends in conversion from the initial bulbing stage to harvest. Nutrient uptakes of leaf tissue decreased from the initial bulbing to harvest, while nutrient uptakes of onion bulb substantially increased. In soil, water content, nitrate-nitrogen (NO3N), and electric conductivity with high mobility decreased from the initial bulbing to harvest, but available phosphorus (av. P) or exchangeable cations with highly accumulated content did not change significantly. At the initial bulbing, soil N or NO3N and ex. K content were not positively correlated with each counterpart in leaf tissue, while av. P content was positively related with leaf P content. However, bulb N, P and K were not significantly correlated with each counterpart in the soil. Soil N or NO3N at the initial bulbing were negatively associated with bulb nutrients at harvest, especially Mg or soluble solid content. Soil av. P content at the initial bulbing showed strongly negative correlation with dry matter (DM), carbon (C), calcium (Ca), magnesium (Mg) and iron (Fe) at harvest. Soil ex. K content at the initial bulbing was solely positively related with the counterpart in bulb at harvest. Meanwhile, soil bulk density at the initial bulbing was positively correlated with DM, C, Ca, Mg, etc., in the bulb at harvest. In conclusion, the accumulated soil nutrients in a long-term onion growing area could negatively affect the bulb weight or mineral contents in bulb at harvest. Therefore, a new fertilizer recommendation program will be necessary for sustainable onion production.