Managing Soil Structure for Water Conservation in the Landscape
Soil quality is an integral part of water conservation. Well-developed soil allows irrigation water and rain to infiltrate rather than run off; it also has the capacity to retain the water that soaks in. Plant roots penetrate easily and deeply in quality soil and can reach water reserves held low in the soil profile. All of these attributes result in a reduced need for landscape irrigation. In addition, properly managed soil means reduced soil erosion, fewer fertilizer and pesticide inputs, and less chance of chemical runoff - all of which contribute to cleaner, healthier water resources.
Many interrelated factors go into building soil quality, and excellent books exist to explain them (for example, see Magdoff and van Es, 2000). By improving and maintaining one of these soil quality components, soil structure, landscape managers can promote water conservation in both new and existing landscapes. Building soil structure in new landscapes is generally easier than in established settings, simply because much of the work can be done before plants are installed; nevertheless it is entirely possible to build better soil in existing landscapes.
Structure is determined by the way in which individual soil particles bind together or "aggregate." This aggregation influences water movement into and through the soil. Healthy landscape topsoil is filled with various-sized aggregates and has a grainy, crumbly consistency; it contains more spaces for water passage and storage than poorly structured soil, which may be identified by very large chunks or "clods," or may have very few aggregates.
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 can perform this assessment. A minimum of 5% organic matter in the soil is recommended for tree, shrub, 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 ¼ inch of dry, aged compost to get organic matter back into the soil.
Since organic matter is continually breaking down, it must be continually added. Regular (every 2 to 3 years) soil testing will track changes in percent organic matter in the soil as well as monitor soil nutrient levels, which are affected by organic matter decomposition.
While adding organic matter is probably the most effective way to improve soil structure, there are other ways to improve or avoid damaging structure, and preserve communities of soil organisms. Till compacted soil on new sites to a depth immediately below the compaction layer to allow water penetration, then limit soil disturbance and compaction by designating paths for machine and foot traffic. Work soil when it is neither saturated nor extremely dry. Aerate established lawns by coring at times when plants are actively growing.
To protect water-retentive soil structure and prevent contamination of water resources by nutrient, pesticide, and sediment runoff, be sure to control erosion. In many cases, this can be accomplished through the very act of adding organic matter to soil, which results in larger soil particles less likely to be blown or washed away. Mulching around plantings also keeps soil in place, as does maintaining healthy turfgrass plantings.
With proper attention to soil structure, landscape professionals may realize another benefit besides water conservation - economic "conservation" - as less time and money is spent on mitigating problems related to water shortages. For more information about soil structure, organic matter, and other topics associated with soil quality management, consult the reference list below.
- Kujawski, R. and H. Cupo. 1998. Best Management Practices: Guidelines for Using Compost in the Green Industries. University of Massachusetts and Center for Ecological Technology. 24 pp.
- Magdoff, F. and H. van Es. 2000. Building Soils for Better Crops, 2nd edition. Sustainable Agriculture Network handbook series; book 4. Burlington, VT. 230 pp.
- Sullivan, P. 2004. Sustainable Soil Management. National Sustainable Agriculture Information Service.
- Sullivan, P. 2002. Drought Resistant Soil. National Sustainable Agriculture Information Service.
Written by: Jennifer Kujawski