Soil Preparation for Vegetables

Starting a New Garden Bed

Mixed Greens

Desert soil is often hard, compacted, and lacking organic ingredients. The soil must be reworked in new gardens to provide a suitable environment for vegetable crops. First double dig the dirt and remove any rocks and roots and smash any clumps. For clay soils that are rock-hard when dry, mix together equal amounts of river bottom (clean) sand, compost and dirt. If the soil is reasonably easy to dig when dry, omit the sand.

Compost is created from rotted plant material and often animal manure. Vegetable gardening requires the most animal manure. See Home Composting in the Desert. The manure of any herbivore will do. Manure from household pets is not recommended because they share some diseases in common with humans.

Having properly aged manure is very important. Many gardeners dump manure and/or compost with manure down-slope next to their garden and let it age in the rain for a year before mixing it into garden soil. This leaches out excess salts and evaporates ammonia in the manure. Turning the pile once a month also helps. Manure straight from the animal has too much nitrogen (too hot) and will burn or kill plants.

Water and Moisture Control

In southwestern desert regions, raised garden beds are preferred. This provides better moisture control, allowing heavy rains to drain from the garden bed quickly. The traditional raised bed has sides made of lumber 1-2' high. Larger garden areas can be composed of long rounded dirt rows, with a high center running the length of the 4-6' wide row. Low channels on the sides, lower at one or both ends, allow water to drain.

Preparation for the Growing Season

For existing garden beds, every year before planting, dig and overturn the soil one foot deep. This will allow you to spot and kill some of the insect grubs that may be over wintering in your garden bed. Then lay 2-3" of compost on top and mix it in to a depth of 8-12". The garden will now be ready for the next round of rotated crops. If you had a problem the previous growing season with mites, nematodes, fruitworms, diseases, or weeds, see Solarizing the Garden Bed, below.

Bagged soils for container vegetables can be purchased as a substitute for amended soil, but long term, compost with aged manure added to soil works best for vegetable gardening. Cheap bagged soil amendment often contains wood chips or even sawdust. Wood (but not shredded bark) is bad for vegetable gardens because the carbon/nitrogen (C/N)ratio is wrong (too much carbon) and it promotes excess fungal growth as well as starving the vegetables of nitrogen.

A second problem is that bagged soils with built-in fertilizer have the wrong formulation for vegetables. If you do use bagged soil, use a vegetable or tomato fertilizer with it. For tomatoes, available fertilizers are usually 18-18-21 (N nitrogen - P phosphorous - K potassium). For chili peppers, a 1-2-2 ratio such as 8-16-16 is recommended, but tomato fertilizer will do. Some formulations will list a fourth number, the amount of sulfur. Follow directions on the package for dilution and frequency. Avoid fertilizers where the first number (nitrogen) is higher than the second number (phosphorous).

Soil Acidity and Alkalinity

In desert valleys, the soil is often pH 7.5 to 8.5 (alkaline), and saline as well. In mountain regions it is usually pH 7.0 (neutral). Many vegetables grow best in slightly acidic to neutral soils because that is when soil nutrients are most chemically available to the plant.

A proven method to reduce alkalinity is to add lots of organic matter such as compost. Soil (elemental) sulfur can be used when the pH is 8.0-8.5, but is slow acting, and should be used in the fall before spring planting.

Mixed lettuce plants

Avoid using ammonium sulfate, a high nitrogen fertilizer, to acidify the soil. Tomato and chili plants, for example, grown in high nitrogen soils will produce lots of foliage but no flowers or fruit. That is true for many flowering and fruiting plants. Legumes (peas and beans) make their own nitrogen and do better without fertilizers containing nitrogen.

To make an acidic soil more alkaline, use ground limestone or wood ashes (from non-treated wood). One-half pound of limestone is needed for each three square feet of soil. Sprinkle the ashes or limestone on the ground and mix into the soil. Water thoroughly. Wait at least one month before testing and possibly adding more. Avoid using shells from shellfish, such as oyster, because they can contain lead and other heavy metals from industrial pollution.

Lime can also be used to make soil more alkaline, but it is stronger and should be used at the rate of one-half pound per ten square feet of soil. Mix into the soil and water thoroughly. Wait three months before testing or adding more.

Vegetables that tolerate pH 7.0 - 8.0 alkaline soils include asparagus, beets, cabbage, carrot, cauliflower, celery, lettuce, parsley, and spinach.

Vegetables that tolerate pH 6.5 - 7.5 near-neutral soils include beans, beets, broccoli, chili peppers, chives, corn, cucumber, grape, melon, peas, peach, pumpkin, radish, squash, and tomato.

Spreading composted mulch on top of a garden bed keeps the soil from drying out too quickly and also provides long-term soil acidification.

Solarizing / Sterilizing the Garden Bed

If you had a problem the previous growing season with mites, nematodes, fruitworms, fungal diseases, or weeds, solarizing is a cheap, easy, and environmentally friendly way to sterilize the soil. Using a clear plastic tarp and sunlight, many pests and weed seeds can be killed by high temperature. It will, also, kill earthworms if they cannot burrow deeper into the soil. Obviously, the fewer the clouds, and the higher the sun in the sky, the more effective this technique becomes. Do this two months before planting a new crop.

1. Rake and level the garden bed. Remove any plants, roots and rocks.
2. Water the bed evenly and deeply.
3. Cover the moist garden bed with clear, 1- to 2-mil, plastic tarp. This will help sunlight most fully penetrate the soil. The tarp should extend to the sides where it can be weighed down with a 2x4" frame or rocks. Do not use thicker tarp or black tarp because the sun must heat the soil, not the plastic.
4. Let the tarp stay in place for 8 weeks.
5. Covering the first tarp, lying on the soil, with a second tarp 3 inches above it, sealing in a pocket of air, will significantly increase the temperature and extend warming further into evening hours. A 2x4" frame can be used to provide the separation and seal the sides.

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Latest update: March, 2021