How to Fertilize Onions Under Plastic Mulch

Transplant young onion plants to stand four inches apart.

Plastic mulch encourages healthy microbes to grow in the soil. It helps maintain soil temperatures and moisture levels. Carbon dioxide builds up under the mulch, escaping through the holes around the plants, which encourages more vegetable growth. Onions have shown they benefit greatly from plastic mulch and they benefit from fertilizing the soils under the mulch.

Things You'll Need

• Spade
• 10-20-10 fertilizer
• Onion transplants
• Drip irrigation
• Plastic mulch
• Knife

Instructions

1. Prepare your vegetable garden for planting four weeks before the last average spring frost. Dig a trench four inches deep and 10 feet long with a spade. Spread half a cup of fertilizer with 10 percent nitrogen, 20 percent phosphorus and 10 percent potassium ratio to the bottom of the trench in a band. Cover the fertilizer with two inches of soil.

2. Transplant your onion plants every four inches along the trench. Lay down a drip irrigation system with one drip tube watering each plant. Spread the plastic mulch in a sheet over the row of onions. Cut holes in the plastic with a knife over each onion and allow the plant above the ground to fit completely through. Avoid cutting the drip system.

3. Cut upside down T-slits every foot along the plastic mulch. These only need to be large enough to fit your hand through to fertilize the soil over your onions. You know your onions need a nitrogen rich fertilizer if the leaves start to yellow. If the plants grow slowly and have thick necks, they may be low on phosphorus. If the leaf tips turn brown, they may need more potassium.

4. Spread a half-cup of fertilizer for every 10 feet of onions through the T-slits every three weeks after planting. Run the irrigation system right after you finish applying fertilizer. Do not fertilize under the plastic mulch after the necks of the onion plants get soft and the tops begin falling over, because the onions will be ready to harvest four weeks later. The extra fertilizer at the end of the season causes rot after harvest.

Tips & Warnings

• It is difficult to over-fertilize onions. If you have questions about which fertilizers' ratios include the nutrients you need based on the signs in Step 3, check out Planet Natural's explanation of Fertilizer Numbers.

References

• Growing Onions
• Plastic Mulches and Landscape Fabrics
• Food Gardening Guide for Onions

Resources

• Planet Natural: Fertilizer Numbers