- Fertilizers high in phosphorus can be added before the seeds or bulbs are planted to stimulate bulb and root growth. When planting allium seeds, high-phosphate fertilizer can be added underneath the row at a depth of about three inches. A layer of soil of about an inch is added on top of that, then the seeds and another thin layer of soil. When transplanting bulbs, the phosphorus fertilizer should be sprinkled in the bottom of a four-inch deep trench, covered with two inches of soil, then the bulbs planted on top of that.
- While growing, the allium plants require nitrogen-rich fertilizer. This can be applied starting about three weeks after they are planted, then every two to three weeks thereafter. The plants should be watered after fertilizing so that the soil remains adequately moist. Weed control is also very important as alliums do not compete well with fast-growing weeds. Mulching can provide some protection against weed development, as well as adding another source of fertilizer during the growth period. Once the neck of the onion begins to feel soft, fertilization should stop so that the bulbs dry correctly once pulled out of the ground.
- If the onions are not receiving enough fertilizer the leaves will start to look yellowish. If they do not receive enough phosphorus at the beginning of their life they will develop thick necks which don't store as well. Overwatering can leach nutrients from the soil, but alliums require adequate moisture, especially as harvest time approaches, so make sure to adequately and regularly fertilize to avoid problems with lack of nutrients. Once the allium leaves start to fall over, they will not require further fertilization.
