Gardening Articles: Care :: Soil, Water, & Fertilizer

Soil Prep for Alliums

by National Gardening Association Editors

Onions will grow in practically any kind of soil but, one that's rich in decayed organic matter and humus and drains well is best. Heavy soils that stick together after rain will bake hard when the sun comes out, making it difficult for the bulbs to expand. Another problem with heavy soils is that water stays on the surface in puddles. That can drown plants! Southern gardeners can overcame this problem by making raised planting beds that are about 10 inches wide and four to six inches high. When it's time to plant onion sets in January, your raised seedbeds won't be too wet to plant. The planting beds drain well and leave the soil moist but not packed, soggy or impossible to work.

Adding Organic Matter

Adding plenty of organic matter to heavy soil will help to loosen it up and improve drainage. With light, sandy soil, the organic matter will help hold the water after rain or irrigation. If your soil has plenty of organic matter -- such as decayed leaves or compost -- worked into it, it will stay moist after a rain or irrigation. Organic matter acts like a sponge, holding moisture near the surface. That's good for onions, which have shallow roots and can't tap water or nutrients deep in the soil.

If you're wondering about soil acidity or alkalinity (pH), onions do best if the soil pH is between 5.5 and 6.5, a little on the acid side. They'll grow outside this range, but not quite as well. You can easily test your soil's pH by using a soil-testing kit, available at garden centers. Soil testing is also done by county cooperative extension agents.


Onions love fertilizer and they can take about twice as much as most other vegetables. Your onions won't mind it a bit if you add plenty of well-rotted or dried manure, compost or commercial fertilizer to the soil before planting. In addition, try adding a handful of bonemeal or rock phosphate, which is high in phosphorus, when you prepare your soil for planting. Phosphorus stimulates early bulb formation and root growth. It helps give onions a rapid start.

To determine how much fertilizer you should mix in your onion row before planting, this chart may help.

Onion Row/ 5-10-10/ 8-16-16/ 10-10-10
10'long x 15"wide/ 2 1/4 cups/ 1 cup/ 1/2 cup
25'long x 15"wide/ 6 1/4 cups/ 2 cups/ 11/4 cups
50'long x 15"wide/ 12 1/2 cups/ 4 cups/ 2 1/4 cups
100'long x 15"wide/ 25 cups/ 8 1/3 cups/ 5 cups

Consider this a minimum requirement. Also add a 1- to 2-inch layer of yearly compost or dried manure for great onion growth.

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