Soil Testing Reveals Impact of Gardening Practices

An interesting duty of a Regional Specialist is to review soil test reports for the area. For Horticulturists, that involves lawn and garden samples as well ones by fruit, vegetable, turf and ornamental growers. Gardeners often take soil samples if they are having a problem, doing a landscape project, or when it pertains to an area of great interest— i.e., vegetables or roses. Usually only one sample is submitted, sometimes two, and rarely three or more. Have you ever wondered how much your soil might vary across your garden and yard?

As an avid gardener living at the same place for 10 years, I thought my yard would provide a good location to explore this issue. I’ve done what other gardeners with a range of interests might: improved the soil in the annual flower beds, started a raised bed vegetable garden, established numerous trees, planted small fruit and an orchard, hauled in dirt to make a berm to plant perennials, and built a patio with a rose garden.

For my one acre residence and yard, the soil is all the same type, Mexico-Urban land complex of 1 to 3 percent slope. A gardener might describe it as a heavy and poorly drained soil with a substantial amount of clay. To improve the drainage and texture I have focused on increasing the organic matter, especially in the raised areas for the annual flowers and vegetables, and for the roses, where the soil seemed especially sticky. The raised area with the perennials was purchased soil that came from ‘east of town,’ so might it test a little different? Around the trees I always mulch, and for the lawns I do as little as possible, and let the clippings drop. The table below provides the results on the most critical factors that would vary on a similar soil- pH, organic matter and the nutrients of phosphorus (P), potassium (K) and calcium (Ca).

The results match the amendment efforts. The lower levels of organic matter are in the lawn, orchard and perennials, where little to no effort was spent. And it is highest in annual flowers where the soil was consistently amended with peat moss, humus and bark for the longest duration. The MU Guide, G6955-Improving Lawn and Landscape Soils, advises a minimum of two percent organic matter for lawns and five percent for garden plantings. The only area needing a higher organic matter level would be perennials. Given there is lawn around the shade and orchard trees the 3 - 4 percent organic matter levelin these areas is reasonable.

The soil test reports recommended no potassium, phosphorus or calcium for any of the areas sampled. Lime was only recommended for the perennials. Thus the only soil needing lime and improved organic matter was the soil purchased to create a raised berm.

Nitrogen recommendations are based on organic matter. If it is suffi ciently high, then the natural breakdown (mineralization) of the organic matter will provide sufficient nitrogen. No nitrogen was recommended for seven of the eight areas. The exception was for the lawns, where some nitrogen is always advised as the growth and constant clipping of grass requires additional nitrogen to support lush growth.

Many gardeners do not submit soil samples, with time and expense being issues. As this case demonstrates, the cost of purchasing fertilizer is avoided. And the time spent on soil sampling is done in the off season, where fertilizer is spread, and often purchased, during the busier growing season. Early spring is an ideal time to take and submit a soil sample. If you haven’t taken a sample before, or done so in awhile, the publication Garden, Landscape and Lawn Soil Testing (http://soilplantlab.missouri.edu/soil/gardensoil.htm) may be worth reviewing.

Water quality emanating from urban landscapes is degraded when fertilizers are over-applied. Nitrogen and phosphorus are generally cited as the nutrients of most concern. For several samples phosphorus and potassium levels are rated very high or excess. This does not mean these levels will cause any harm to plant growth, but no further additions are required, and if applied at the wrong time (e.g., during rainy spring weather) might contaminate runoff. Two common sources of synthetic nitrogen available to gardeners are urea and ammonium sulfate, and should be considered when potassium and phosphorus are not recommended. However, fertilizers are often sold in combination to provide multiple elements, with the nitrogen, phosphorus and potassium percentage indicated by numbers (e.g., 12-12-12 for N-P-K). These should be avoided when fertilizing the lawn area in this example, as P and K both tested high. Organic fertilizers with nitrogen almost always contain phosphorus and potassium as well; blood meal is a commonly available exception. The MU Guide ‘Organic Gardening Techniques’ is a good source of information on organic fertilizers frequently used by gardeners (http://extension.missouri.edu/explore/agguides/hort/g06220.htm).

James Quinn,
Regional Horticulture Specialist, and
Manjula Nathan,
Director MU Soil Testing & Plant Diagnostic Service Laboratories