The ideal vegetable garden soil is deep, friable, well-drained, and has high organic matter content. Proper soil preparation provides the basis for good seed germination and subsequent growth of garden crops. Careful use of various soil amendments can improve garden soil and provide the best possible starting ground for your crops.

Soil testing
Soil testing can be helpful if your garden is not performing as it should. Soil testing is costly for the home gardener since home test kits are of little value for Arizona soil. To obtain an accurate soil test a soil sample must be sent to a certified soils laboratory. County Extension offices have a list of labs available for your use. In general soils in Arizona will be lacking in nitrogen and phosphate. Regular applications of these two elements should prevent the majority of problems due to nutrient deficiencies. (Refer to Chapter 2, Soils and Fertilizers for more detail.)

Equipment
The type of equipment used to prepare your garden will depend on the size of the garden, your physical ability, time, and budget. Options include hand-digging with a spade or shovel, tilling with a power rotary tiller, using a small garden tractor, or a full-sized farm tractor.

Make basic nutrients and pH adjustments to the soil by spreading required fertilizers and sulfur to soils of high pH. In new garden spots, if present remove sod with a spade and use it to patch your lawn or put it in a compost pile to decay. Plow, spade, or rotary till the soil. Work only when soil moisture conditions are right. To test, pick up a handful of soil and squeeze it. If it stays in a ball it is too wet. If it crumbles freely, it should be about right. Excessively dry soil is powdery and clumpy and may be difficult to work. Take samples at the surface and at a 2 to 3-inch depth in several locations in the garden plot. If soil sticks to a shovel, or if when spading, the turned surface is shiny and smooth, it is still too wet. Working soils when excessively wet can destroy soil structure, which may take years to rebuild.

Plowing with a tractor when the soil is wet is especially damaging, causing the formation of a compaction layer that will inhibit root growth. Soils with adequate humus levels generally allow more leeway because of their improved structural qualities.

Tilling the soil
It was once assumed that gardens should be turned yearly with a moldboard plow, mostly for weed and pest control, or to incorporate nutrients or organic matter. While garden plowing is still a common practice, turning the soil completely over has been found to be detrimental in some cases, causing soil compaction, upsetting balances of microorganisms, and often causing layers of coarse organic material to be buried below the influence of insects and microbes which would otherwise cause breakdown of the material. Chisel plowing, which does not have this disruptive effect, is one alternative, but it is limited to sandy or loamy soils and many farmers who work gardens do not have chisel plows. In addition, gardeners in other-than-rural areas have trouble finding a farmer who will come to plow and disk the garden for a reasonable price (or at all). Rototilling most home gardens is sufficient, as long as plant debris accumulation is not out of hand. Rotary tilling mixes the upper layers of soil rather than completely turning the soil over, and the effect produced are generally desirable. One possible harmful effect of rototilling is the formation of a compaction layer just beyond the reach of the tines. This also occurs when a moldboard plow is used to the same depth every year, but at a somewhat deeper level. Use of deep-rooted cover crops or double-digging can do much to prevent or alleviate this problem when it exists. Small gardens can be designed using raised beds which may be worked entirely by hand if the area is small enough.

Gardeners often wonder whether to plow/till immediately following harvest or wait until just prior to planting. Working the soil following harvest has several advantages over the traditional of plowing just prior to planting. It allows earlier planting, since the basic soil preparation is already done when planting time arrives. Turning under large amounts of organic matter is likely to result in better decomposition when done earlier, since there is more time for the process to take place.

Insects, disease organisms, and perennial weeds may be reduced by killing or inactivating them through burial or exposure to harsh weather. The physical condition of heavy clay soils may be improved by the alternate freezing and thawing, which breaks up tightly aggregated particles. Also, moisture is trapped between the hills of roughly-plowed soil, so more moisture is retained than on flat, bare ground. Incorporation of rock fertilizers in the fall gives them time to become integrated with the soil and influence spring plant growth.

After harvest plowing alone is not recommended for hillside or steep garden plots, since soil is left exposed for long periods, subjecting it to erosion when the rains come. If a cover crop is grown to improve soil and prevent erosion, the ground will have to be tilled to prepare the soil for seed, and again later to turn under the green manure. Generally, most gardens must be disked or rotary-tilled prior to planting to smooth the soil.