Osmocote Plus is another fertilizer with a great NPK ratio for onions. This formula uses a ratio of 15-9-12, which leaves your plants with an abundant source of nitrogen.
Onions require a high source of nitrogen. A nitrogen-based fertilizer (ammonium sulfate or ammonium nitrate) should be applied at the rate of one cup per twenty feet of row.
Mulch your onions with hay to keep down the weeds and trap in moisture. In late June to early July you'll start to notice your onion tops falling over. Don't be alarmed! This is part of their natural process and an indication that the bulbs have stopped growing and are ready for fresh eating or curing.
If you're short of nitrogen, add compost, aged manure, feather-meal, fish-meal or other high-nitrogen amendments. Incorporate them well before you plant, if possible. You can also add nitrogen later in the fall when the garlic or onion leaves are 4–6 in. high.
The 14-14-14 NPK balanced analysis makes Osmocote® Flower & Vegetable a great choice for garden vegetables, tomatoes, melons and berries. The original slow release, general purpose plant food. Suitable for both indoor and outdoor plants, including transplants and blending your own potting soil mix.
Can Osmocote® be used with edible plants such as vegetables, fruits and citrus? Yes. In fact, Osmocote® Flower & Vegetable Plant Food is formulated for just these purposes. Each granule contains essential plant nutrients such as nitrogen, phosphorous and potassium.
For outdoor use, sprinkle evenly across areas to be fed and work into the top 1-3 inches of soil. When transplanting annuals from flats or when re-potting, mix into the soil and growing media at the bottom of the hole before planting.
Feed your onions lots of nitrogen to increase bulb size. Because they are a root vegetable, many people believe that onions require lots of phosphorus, however, for increased onion size, nitrogen is your key nutrient. Each leaf that an onion plant produces means an additional ring on the onion.
Onions are heavy feeders. Blood, fish and bone or onion fertiliser is recommended prior to planting and during the growing season.
The best time to plant onion sets is in early to mid-spring. Onions need a rich, fertile neutral or alkaline soil in full sun. To prepare the soil for spring planting, dig in lots of compost or well-rotted farmyard manure in autumn and leave it to settle over winter.
Onions make great planting companions due to their ability to improve the flavor of their companion plants, as well as deter pests like aphids, Japanese beetles, and rabbits. However peas, pole beans, bush beans, and asparagus don't grow well when planted near onions.
You can use mulch around onions, as long as you don't bury them too deep. Use only 2 to 4 inches (5 to 10 centimeters) of mulch to avoid smothering the plants with too much heat or too little air. Also, wait to use mulch until the onions grow a bit larger. That way, you won't bury the small seedlings.
Onions are shallow-rooted and require constant moisture for good growth. If the planting does not receive one inch of rain each week, soak the soil thoroughly at least once a week. An inch of water will wet a sandy soil to a depth of 10 inches, a heavy clay soil to 6 inches.
The key ingredient is the Nitrogen, since both Phosphorous and Potassium tend to have high levels in soil testing but are not in a form that is readily available to the plant. Adding additional Phosphorous or Potassium beyond the desired amount will rarely harm the onions.
Season and NPK Ratio
When planting, an NPK ratio of 15-15-15 is ideal. A month or two after they've been planted, potatoes need lots of nitrogen, so a fertilizer with an NPK of 34-0-0 is the best choice.
Stop fertilizing when the onions start to bulb. (See Bulbing below.) Controlling weeds is critical to prevent competition for nutrients. An application of Corn Gluten Meal raked into the top inch of soil every six weeks during the growing season will prevent weeds from returning.
Bone meal acts as a great fertiliser for a few key reasons. The first is that it's a great source of Phosphorus which is used by new plants to produce strong roots and so is great for root vegetables such as onions, garlic, carrot and parsnip.
Nitrogen is one of the most important nutrients for onion plant growth and development. A typical onion crop will use about 150-200 pounds of actual nitrogen per acre during the growing season, with a majority of the nitrogen taken up after the plant has started to bulb.
If onions are planted too close to one another they will compete with one another, resulting in smaller bulbs. Whether direct seeding, transplanting seedlings or planting onion sets, onions should be spaced 3-4 inches apart to maximize growth.
Grow them in a sunny spot that has fertile, well-drained soil with a pH of 6.0 to 6.8. Improve your native soil by mixing in several inches of aged compost or other rich organic matter. Onions aren't great at taking up water, so it's important to keep soil moist so their shallow roots can drink up.
The problem with using too much osmocote is that ammonia might leach into the water column and affect fauna, especially if you disturb the substrate with planting/rearranging and so forth.
Easy to use slow release fertiliser specifically formulated for tomatoes and other vegetables, with added trace elements for optimal nutrient uptake. Also suited to annuals and flowering bulbs.