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.