Garden Soil Amendments - February 25, 2015
Jeff Schalau, Agent, Agriculture & Natural Resources
University of 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 readers something 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.

Soil amendments are materials added to soil to improve its physical and chemical properties. Physical properties include: water retention, permeability, water infiltration, drainage, aeration, and structure. These improve the soil environment for plant roots. Fertilizers provide essential plant nutrients that improve growth and production of the crop plant. Other amendments, such as soil sulfur, alter soil pH and improve availability of many plant nutrients. The correct combination of amendments and fertilizers can make the difference between success and failure in your garden.

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 soil sulfur.

It is best to have some knowledge of local soil conditions before adding any amendments. A soil test can provide information about nutrient availability which fertilizers should be added. In most areas of Arizona, yearly additions of organic amendments are always necessary.

Most 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 during the growing season.

Undecomposed wood chips or sawdust should not be added directly to the soil. These materials are high in carbon, but lack the nitrogen necessary for decomposition to occur in a reasonable time frame. When nitrogen is in limited supply in the presence of uncomposted wood fiber, microbes tie up this nitrogen leaving little or none for plants. Under these circumstances, adding some nitrogen fertilizer will provide for the microbes to do their decomposition while leaving the rest for plant growth.

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 human 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. To minimize pathogens in edible garden crops, fresh manure should be incorporated into soil 120 days prior to their harvest.

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. Symptoms of excess salts include burning of leaf edges and poor growth.

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 pose challenges 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. Compost and/or aged manures help clay soils by improving soil aggregation, increasing porosity and permeability, and improving aeration and drainage. See the on-line edition of this column for additional soil testing and amendment information (see URL below).

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Additional Resources

Fertilizing Home Gardens in Arizona
University of Arizona Cooperative Extension

Ten Steps to a Successful Vegetable Garden
University of Arizona Cooperative Extension

Using Compost in Utah Gardens
Utah State University Extension

Backyard Composting in Utah
Utah State University Extension

Soil, Water and Forage Testing Laboratory
Texas A&M AgriLife Extension Service

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: February 17, 2015
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