Role of Phosphorus in Onion Production
Phosphorus is a component of nucleic acids (DNA and RNA) and essential for energy transfer within the plant.
Thus it has a direct effect on yield and quality.
Provided phosphorus is available, rates of around 45-90 lb/ac satisfy a high yielding crop’s requirement.
Trials in Germany have shown that placement of phosphorus within the root zone works better than when P-fertilizer is broadcast. This is particularly important in low-P containing soils, where yield is increased and crop maturity brought forward.
Growth, yield and grade quality improvements are commonly seen when using starter fertilizers in onions. The figure above showns yield increase in UK trials.
Sufficient availability of phosphorus is also important to balance potential losses in store through use of high rates of nitrogen. Phosphorus can be used to ensure top yields through use of high rates of N, yet at the same time providing little deterioration in crop quality.
Phosphorus effect at growth stages
| Stage | Phosphorus effect |
| Pre-Planting | Maximize root development and to supply reserves for season long growth and good bulbing |
| Bulb Fill | Encourage ripening, early maturity and a big bulb size |
See more on Onion Growth Stages.
General Guidelines for Phosphorus Application
Phosphorus is rarely required in large quantities. However, where crops are grown on soils with very low natural P levels, higher rates are required to boost yield and bulb size. It is important that phosphorus is available early season in order to encourage growth, particularly of the root system. Placement of phosphorus within the root zone has been shown to work better than when P-fertilizer is broadcast.
Phosphorus Deficiencies in Onion
Plant growth and establishment slows and rooting is adversely affected and stunted. Leaves are mottled green/yellow brown. Maturity can also be delayed and crops tend to have a thicker neck at harvest.
