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October 31, 2014

On Gardening: A garden is only as healthy as its soil

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Posted: Sunday, March 23, 2014 12:40 am

Most gardening publications suggest you locate your garden on well-drained soil. Well, what is “well-drained soil?” What if you have a wet-natured soil or clayey soil that dries out slowly? Or sandy soil that holds no water or nutrients?

An ideal soil is about one-half air space, one-half solid soil mineral with 2 to 4 percent organic matter. Ideally about half of the air space will be filled with water. Alabama soils, which range from coarse sands to heavy clays and contain less than 1 percent organic matter, create unique problems for vegetable gardeners.

Clay soils rarely drain well. If the soil is too wet, plant roots can’t get the oxygen they need. Yes, roots will die without oxygen, just like other organisms. Another consequence is that a wet soil becomes cold soil in early spring. More heat is required to warm water than soil. If the soil is well drained, it will warm up rapidly, allowing for earlier planting, fewer seedling diseases and perhaps a longer growing season.

Sandy soils, on the other hand, have a very low capacity for holding moisture. While sandy soils do provide oxygen for the plant roots, nutrients from fertilizers leach quickly with the water as it drains. Not only will there be no nutrients available for vegetable plants to use, but excessive leaching of nutrients is hazardous to nearby water sources.

Soil compaction is one of the biggest challenges of the highly weathered, low-organic matter soils of the Deep South. The tendency to overwork or till the soil often results in a highly compacted area inches below the ground, an effect that is confounded when the soil is tilled or worked too often or when it is too wet. To tell if soil is too wet to till, use the ball test: Turn the soil over with a garden implement, then take a handful of the uncovered soil and form it into a ball. If it starts to crumble, the soil is dry enough to be tilled. If the soil stays in a ball, several rain-free days are needed before the soil can be worked.

Super soil starts with compost

Lucky for vegetable gardeners, there are several management practices that can be done to improve the health or tilth of soil. The best way is to add organic matter or soil amendments. Organic matter is derived from living things — plants and animals in the environment. Common sources include composted pine bark, leaves, compost and cover crops.

Soil amendments are extremely beneficial to all soil types. In clayey soils, organic matter increases air space, and thereby drainage. Increasing the drainage dries out soil faster so it warms up faster in spring. Increasing air space also means plant roots are able to take in oxygen for a healthier root system. In sandy soils, organic matter increases the soil’s ability to hold moisture, which increases nutrients and decreases time spent watering and fertilizing, as well as the risk of fertilizers leaching into groundwater.

We may think of the vegetable garden environment as the plants and mulch we can see, but beneath the surface the soil is alive with microorganisms that create healthy environments for vegetable plants. In organic soils, microorganisms break down plant debris into nutrients for the plants, and organisms like earthworms feed off decaying organic matter, enriching the soil and the plant life that lives off of it.

Amending soil is not a one-time management practice. Due to our humidity and temperatures in Alabama soils rarely freeze, so biological breakdown of organic matter happens year round. The best way to amend soil is to apply 3-4 inches of organic matter on top of the soil in the fall. Mix the soil with the organic matter to a depth of 12 inches to create a loose, healthy soil that vegetable plants will appreciate.

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