Gardening in Tucson
and Southeastern Arizona

Soil Preparation

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 animal manure. See Home Composting in the Desert. Manure of any herbivore will do, although chicken and sheep manure have the highest amount of potassium and are the most suited for tomatoes. Many gardeners dump compost 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.

For existing garden beds, mixing in additional compost every year before planting will keep the garden 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.

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. Bagged soils with built-in fertilizer have the wrong formulation for vegetables. If you do use bagged soil, use vegetable or tomato fertilizer with it. Vegetable fertilizer has a nitrogen-phosphorous-potassium formula of 10-10-10 while tomato fertilizer is usually 18-18-21. 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 Mulching

In Arizona 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). A proven method to reduce alkalinity is to add lots of organic matter. Elemental sulfur can be used when the pH is 8.0. Watering will reduce soluble salts in the soil, especially in raised beds.
Avoid using ammonium sulfate, a high nitrogen fertilizer, to acidify the soil if you are growing tomatoes or legumes. Tomato plants grown in high nitrogen soils tend to be all foliage and no tomatoes. Legumes make their own nitrogen and do better without added nitrogen.

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, chives, corn, cucumber, grape, melon, peas, peach, pepper, pumpkin, radish, squash, and tomato.

Spreading composted mulch on top of the soil 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.

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.

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

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.

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.

To plant groundcover 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 and variety 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.

Planting Depth

When planting any tree or shrub, the junction between the trunk and the roots should be positioned 1-2 inches above ground level.