Any textbook on soil structure improvement will undoubtedly spend much of its pages discussing the importance of organic matter, and with good reason. Organic materials incorporated into soil will decompose into humus, which helps loosen soil for better water infiltration, and also stores water within the soil. In fact, for every 1% of organic matter content, each cubic-foot of soil can hold an additional 1.5 quarts of plant-available water (Sullivan, 2002). Living components (e.g. microorganisms, worms) of organic matter produce byproducts that "glue" together soil particles into aggregates, and their movement through the soil makes it more porous.
To determine the percent organic matter already present at a new or established site, request that soil tests also include organic matter evaluation. The University of Massachusetts Soil and Plant Tissue Testing Laboratory (www.umass.edu/plsoils/soiltest) can perform this assessment. A minimum of 5% organic matter in the soil is recommended for tree, shrubs, and herbaceous landscape plantings.
Incorporate organic matter in the form of dry, well-aged compost when preparing topsoil for new landscaping, at a rate of 135 cubic yards per acre (equivalent to a 1-inch thick layer) (Kujawski and Cupo, 1998). In open areas of established landscapes, work in aged compost at the same rate; otherwise, spread an organic mulch (e.g. aged bark or wood chips) on the surface of planting beds to a depth of 2 to 3 inches. Finished compost alone is not a good mulching material as it forms a surface crust which interferes with infiltration of rain or irrigation water. In lawns, leave mown grass clippings or topdress annually with up to 1/4 inch of dry, aged compost to get organic matter back into the soil.
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