Soil Amendments for the Garden - March 13, 2002
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

I know spring fever is here and everyone is dying to do something in his or her vegetable or flower garden. First, let me tell you something that you may not be aware of: Arizona soils can have abundant rocks, too much clay, too much sand, high alkalinity, excessive salinity, and little or no organic matter. Second, vegetable and annual flowers are not adapted to the desert environment and since we expect them to produce something within a growing season, we must provide them with abundant resources before planting. Given these facts, lets learn what we can do to overcome these hurdles and achieve greater gardening success.

As stated above, vegetable and annual flowers require regular (once or twice per year) inputs of soil amendments and fertilizers. A soil amendment is any material added to a soil to improve its physical properties, such as water retention, permeability, water infiltration, drainage, aeration and structure. The goal of soil amendment is to provide a better environment for plant roots. Fertilizers provide essential plant nutrients that improve growth and production of the crop plant. The correct combination of amendments and fertilizers can make the difference between success and failure in your garden. The remainder of this column will focus on soil amendments.

There are two broad categories of soils amendments: organic and inorganic. Organic amendments come from something that is or was alive. Inorganic amendments, on the other hand, are either mined or man-made. Organic amendments include sphagnum peat, wood chips, grass clippings, straw, compost, manure, biosolids, sawdust, and wood ash. Inorganic amendments include things like vermiculite, perlite, and sand.

It is best to have some knowledge of local soil conditions before adding any amendment. Some examples of trying to do the right thing with the wrong materials follow. Wood ash is high in both pH and salt, which will magnify common Arizona soil problems. Adding sand to clay soil will create adobe. Adding sawdust without other amendments can cause a severe nitrogen deficiency.

Organic amendments increase soil organic matter content and offer many benefits. Organic matter improves soil aeration, water infiltration, and both water- and nutrient-holding capacity. Many organic amendments also contain plant nutrients and act as organic fertilizers. Organic matter also is an important energy source for bacteria, fungi and earthworms that live in the soil. By feeding these organisms, you will increase bioactivity, which will, in turn, release more nutrients to the plant roots. Below are some common sources of organic matter.

As mentioned previously, wood products can tie up nitrogen in the soil and cause nitrogen deficiency in plants. Microorganisms in the soil use nitrogen to break down the wood. Within a few months, the nitrogen is released and again becomes available to plants. This hazard is greatest with sawdust, because it has a greater surface area than wood chips. If you plan to apply wood chips or sawdust, you may need to apply nitrogen fertilizer at the same time to avoid nitrogen deficiency. Some wood-based amendments are composted or have nitrogen added. If so, this should be indicated on the product label.

Sphagnum peat is an excellent soil amendment, especially for sandy soils, which will retain more water after sphagnum peat application. Sphagnum peat is generally acid (i.e., low pH) and can help Gardeners grow plants that require a more acidic soil.

Biosolids are byproducts of sewage treatment. They may be found alone or composted with leaves or other organic materials. The primary concerns about biosolids are heavy metal content, pathogen levels and salts. To avoid excessive levels of heavy metals and to ensure that pathogens have been killed, always choose a Grade 1 biosolid. While Grade 1 biosolids are acceptable for food Gardens, do not use them on root crops because they will come in direct contact with the edible portion of the plant. Do not use biosolids below Grade 1.

Fresh manure can harm plants due to elevated ammonia levels. To avoid this problem, use only aged manure (at least six months old). Since issues related to food safety have increased, so have concerns about manure. Remember that pathogens (such as E. coli) are another potential problem with fresh manure, especially on vegetable gardens. Compost manure for at least two heating cycles at 130 to 140 degrees F to kill any pathogens before applying the manure to vegetable gardens. Most home composting systems do not sustain temperatures at this level.

During composting, ammonia gas is lost from the manure. Therefore, nitrogen levels may be lower in composted manure than in raw manure. On the other hand, the phosphorus and potassium concentrations will be higher in composted manure. Modify fertilizer practices accordingly. Salt levels also will be higher in composted manure than in raw manure. If salt levels are already high in your garden soil, do not apply manures.

Soil texture, or the way a soil feels, reflects the size of the soil particles. Sandy soils have large soil particles and feel gritty. Clay soils have small soil particles and feel sticky. Both sandy soils and clay soils are a challenge for gardeners. Loam soils have the ideal mixture of different size soil particles. When amending sandy soils, the goal is to increase the soil's ability to hold moisture and store nutrients. To achieve this, use organic amendments that are well decomposed, like composts or aged manures. With clay soils, the goal is to improve soil aggregation, increase porosity and permeability, and improve aeration and drainage. Fibrous amendments like peat, wood chips, tree bark or straw are most effective in this situation.

If your soil has less than 3 percent organic matter (which is common in native Arizona soils), then apply 3 cubic yards (one cubic yard is 27 cubic feet) of your chosen organic amendment per 1,000 square feet. To avoid salt buildup, do not apply more than this.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: January 23, 2002
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