Soil management eases pollution

In recent years, the concept of nonpoint-source pollution has come to the forefront of the public dialogue on water quality. This phenomenon describes pollution that results from sources that are difficult to identify.

Our complex water system, which includes precipitation, soil saturation, percolation of excess water through the soil profile, transpiration of water from plants and the evaporation of surface water, at times may make it difficult to detect the sources of environmental pollution.

Many people would be quick to point to industry or agriculture as the biggest cause of nonpoint-source pollution. But could it be that our efforts as home gardeners contribute to this pollution, as well? Alone, we each may have limited impact on our environment, but together do we pose a greater threat? In what ways could we make some simple modifications to our yard and gardening practices to lessen our environmental impact?

One cause of nonpoint-source pollution may be the washing of nutrients through the soil during leaching. Where do these nutrients go, and why does it matter? Excess nutrients can end up in our surface and groundwater. The nutrients, over time, may reduce the quality of our surface and drinking water, and may affect the environment of aquatic organisms.

The rate of leaching is determined by climatic factors (precipitation primarily) as well as soil type. In some regions of the United States, the potential for natural leaching of soil nutrients is high. In the Southeast, for example, frequent percolation of excess water makes the loss of nutrients through the soil profile a common phenomenon.

In more arid areas, or in areas that tend to receive less natural rainfall or irrigation, leaching of nutrients is less frequent. Generally speaking, here in the Northeast, leaching occurs at an intermediate rate – a rate somewhere between that of the Southeast and the more arid West. Even so, at certain times of the year when rainfall or snowmelt are high, leaching is the rule.

Soil composition has a great effect on the loss of nutrients through leaching. Without delving too deeply into the complex science of soil chemistry, suffice it to say that as a general rule, sandy soils have a higher rate of percolation (water passes through these soils with greater ease than through, say, clay soils) and thus have a greater nutrient loss.

In addition to a high percolation rate, sandy soils have a lower nutrient-absorbing power than clay soils. These characteristics combined – ease of water percolation and difficulty holding onto nutrients – make sandy soils a prime candidate for leaching.

Since soil type has potential to affect so greatly the loss of nutrients, improving our soil quality likewise can improve its ability to reduce leaching losses. Incorporating composted organic matter enhances the ability of a sandy soil to retain nutrients. This material may reduce the rate of percolation and may increase the nutrient-absorbing potential of the soil.

When using synthetic fertilizers and lawn amendments, keep several rules in mind, in regard to leaching: Don’t overfertilize. Apply fertilizer at a rate appropriate for the task at hand. More is not better. Follow carefully the manufacturer’s directions, and, whenever possible, use more stable alternative, natural fertilizers.

Don’t fertilize in fall if you can help it. Fertilizer left on bare soil over the winter will be subjected to the leaching action of snowmelt and spring rainfall. By the time your garden plants undergo active growth, much of that fertility will have washed away.

Apply fertilizer at the proper time and direct it at your crop. Sidedressing plants may better serve their needs than broadcasting fertilizer over ground not touched by plant roots. Incorporate the fertilizer into the soil. Some nutrients are lost through the atmosphere. Incorporating fertilizer may reduce the loss of those nutrients.

Keep a cover crop on your soil. In fall, sow a crop of oats, winter rye, or another cover crop of your choice. These crops will put to use fertility in the soil and reduce the leaching of those nutrients through the soil. While ephemeral fall snowstorms threaten to flush out nutrients from our soil, these plants offer a level of protection for the gardener.

Diana George Chapin is the NEWS garden columnist. Send horticulture questions to Gardening Questions, RR1, Box 2120, Montville 04941, or e-mail them to dianagc@ctel.net. Selected questions will be answered in future columns. Include name, address and telephone number.