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Soil Preparation

Gardening in Tucson, Phoenix, Arizona and Southern California

Soil Preparation

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Soil Preparation for Vegetables

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.

In southwestern desert regions, raised garden beds are preferred. This provides better moisture control, allowing heavy monsoon rains to drain from the garden bed quickly. Large garden areas can be 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.

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 overwintering 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. For chili peppers, a 1-2-2 ratio is recommended such as 8-16-16, but tomato fertilizer will do. 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 8.0 (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. Elemental sulfur can be used when the pH is 8.0, but is slow acting, and should be used in the fall before spring planting.
Avoid using ammonium sulfate, a high nitrogen fertilizer, to acidify the soil if you are growing tomatoes, chilies or legumes. Tomato and chili plants grown in high nitrogen soils tend to be all foliage and no fruit. Legumes (peas and beans) make their own nitrogen and do better on their own.

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.

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

Digging Holes for Plants

To dig a hole in the hard soil of the desert, dig a circle several inches deep of the proper width, then pour in water. Wait until the water is absorbed, then dig out the mud and add more water. Repeat until the hole is the proper depth. Digging is best done after a long rain when the ground is soft.

Finished holes should be one to two inches shallower than the root ball and three times as wide. This allows roots an opportunity to grow before they run into rock-hard dirt. A wide hole also allows more rainwater to accumulate around the roots in our hot, dry climate and may reduce the need for frequent watering.

Fill the completed hole with water to check for drainage. If it does not drain, use a metal rod and hammer to pound holes through the bottom until drainage is attained. Or use a pickaxe to dig a chimney below the caliche or decomposing granite. Ensuring good drainage is the most important part of digging a hole. If you cannot get the hole to drain properly, fill it in and dig a hole in another spot.

In general, because of hard soils, dig the hole before you buy the plant. Do not buy more than one plant at a time without a hole already dug. This will prevent a plant from sitting around in a container with its roots getting baked by the sum.

To plant ground cover on a slope, dig narrow trenches perpendicular to the direction of the slope. The trenches should be the width of a pickaxe and eight inches deep. A trench will collect and retain more water than a round hole. The species of plant being grown will determine the distance between the trenches. Check for drainage, and if necessary, make a narrow cut from the center of the trench down slope to drain excess water.

Caliche or caliche clay is a compacted layer of soil and accumulated salts found in valleys that is nearly impenetrable. Mountains do not have caliche, but can have decomposing granite that can be mistaken for caliche. Decomposing granite is a variable brown / reddish-brown / white mixture. Thickness varies considerably. Digging into either material is best done after a long, heavy rain with a pickaxe and lots of patience.

Planting Depth

When planting any tree or shrub, the junction between the trunk and the roots (the crown) should be positioned 1-2 inches above ground level. This allows for a slight settling of the root ball. Never allow the crown of a plant to be below ground level. "Plant it high, it won't die. Plant it low, it won't grow."

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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