Organic Matter for the Garden - March 19, 2008
Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County

If you grow annual flowers or vegetables, ample quantities of organic matter must be incorporated into the soil prior to planting each year. Omission of organic matter results in poor plant performance and/or crop yields. Gardeners that make compost have their own ready source of organic matter on site. Non-composting gardeners often rely on outside sources of organic matter that are purchased or hauled in from elsewhere.

Simply stated, organic matter is raw or partially decomposed plant and/or animal residues. Organic matter binds soil particles, granules and aggregates together. It aids water penetration and aeration of plant roots in clayey soils and increases moisture-holding capacity of sandy soils. It also provides some nutrients for plants and food for beneficial soil microorganisms. Organic matter must be replenished each year because our summer temperatures and soil alkalinity cause rapid decomposition.

Peat moss is a commonly available source of organic matter, but it does not usually contain essential plant nutrients like nitrogen and phosphorus. Most nurseries have dried peat moss in plastic bags or bales. It is highly variable in color (reddish brown to black) and texture (fibrous or non-fibrous) depending upon its state of decomposition. Water-holding capacity ranges from 150 to 3,000 percent of its dry weight. Some nurseries sell moist peat moss, so be aware that you may be purchasing water. The dry form is an economical way to purchase peat moss on a pound basis. However, dry peat moss can blow away if not incorporated into the soil immediately after application. Peat moss is mined from peat deposits and is usually considered a non-renewable resource.

Animal manures are another source of organic matter and contain relatively larger amounts of essential plant nutrients than peat moss. Animal manures should be dried, aged or composted before being used in garden or flower beds. Manure quality varies depending on animal species and its diet. Bagged steer manure often contains salts that accumulated from urine during confinement. Horse manures can contain weed seeds. This can be a major disadvantage and should be considered before using horse manure in a garden or flower bed. Well-composted horse manure can be weed-free.

Rabbit, goat, sheep, and llama manures are excellent for use in gardens and can be incorporated without composting just prior to planting. These animal species digest their foods completely and weed seeds will not be viable. Another advantage is that these manures are produced in pellets and have little odor which makes them easy to handle, gather, and transport. Cattle also digest their food completely, but it is usually not as easy to gather. Chicken manure is too high in nitrogen to be directly incorporated and must be composted. It is often mixed with bedding which should also be composted before use.

Wood shavings, straw, and dead leaves are certainly sources of organic matter. However, they require additional nitrogen to break them down. These materials are best composted with plenty of green waste or added nitrogen fertilizer before adding to the soil.

When using peat moss or well-decomposed compost, two to three inches can be applied to the soil surface and incorporated by tilling or hand spading. Manures are higher in nitrogen and it is possible to provide too much of nitrogen for “fruit producing” vegetable (tomatoes, squash, eggplant) and flowers. This can result in lots of foliage and little food or fewer flowers. Corn is an exception and requires larger amounts of nitrogen. Phosphorus fertilizer should also be applied when growing all vegetables and flowers.

Another option for organic matter is growing a cover crop that is turned under three to four weeks before planting. Cover cropping and green manuring is a larger topic and was covered in the October 12, 2005 Backyard Gardener column available on the web site (see URL below).

As you can see, there is more to soil preparation than simply tilling and planting. Organic matter additions are critical, but it takes some practice and an understanding of the materials available to you. Each gardener learns what works best for them through experience.

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 ext. 14 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: March 13, 2008
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