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What is good soil fertility?

Why are some gardeners more successful than others? Is it a secret family technique acquired generations ago, a degree in a horticulture science or just luck? More often than not a person's "green thumb" comes from an understanding of how plants grow and their growth requirements. The most fundamental element in the "green thumb" formula is with understanding the soil and a good place to start is with a basic soil test.

Most people pooh-pooh soil testing; not enough time; too scientific; just add complete fertilizer and its OK. All of these are sad excuses. Fact is, we overfertilize most everything and that ends up to be environmentally and economically wasteful. While many people think that soil tests only provide information to estimate the nutrient composition of the soil, this is only partially correct. Soil test results also characterize the growing conditions of the soil and its power to supply nutrients whether they are abundant or not.

Typically, there are four important results you should get from a soil test: pH (acidity/alkalinity), percent organic matter, phosphorous and potassium levels. Other elements can be analyzed, but these generally do not have a great deal of bearing upon the odds of growing good plants except in the case where you have unusual soils and have experienced repeated problems.

Soil pH influences the availability of nutrients to the root system. Values for most plants except rhododendron, azaleas and blueberries should be in the range of 6.0 to 7.0; slightly acidic to neutral. These other crops require more acidic soils of around 4.5 to 5.5. If your result is 7.5 or above, then sulfur can be applied to bring the pH down. Mix into the soil about 5 pounds of elemental sulfur per 1000 square feet to initiate this lowering. Water well and wait several weeks before applying more if indicated. Adding too much sulfur may jump the soil acidity and this can injure the roots. Liming the soil to make it less acidic is rarely needed in the St. Louis area and should only be done as recommended following a soil test. Some people still "sweeten" the soil as a matter of routine and many problems can result.

Organic matter most always refers to decomposing plant remains. As it breaks down, nutrients are leached away and can be taken up by plants. Typically, organic matter is not added for its contribution to the nutrient supply because it is so low. Organic gardeners, may mix hundreds of pounds per 1000 square feet to supply the proper amount for heavy feeding vegetables and fruit. The real benefit of organic matter is with the improved retention of soil moisture and drainage. Proper levels of organic matter can benefit any planting bed or turf area and should range between 2 and 10 percent or more.

Phosphorous is responsible for good root growth and support of the plant's energy systems. If you have used a complete fertilizer in the last 5 years, it is a good bet that the phosphorous level is high. Phosphorous does not readily leach through the soil so it stays where it was put. Newly installed turfgrass areas and early plantings of vegetables should be supplied with a starter fertilizer containing phosphorous.

Potassium is moderately mobile in the soil and in adequate levels assists a plant in vigor, disease resistance and water regulation. The capability of plants to store potassium leads to lower nutrient levels when grass clippings are removed and vegetable crops harvested. For this reason, the testing of potassium levels in the soil is important. If you find excessive levels, some burning of certain plants is possible.

The procedure for taking a soil test is quite simple. For turf, collect 8 samples at the 3-inch depth for every 1000 square feet. Scrape off the top 1 inch and mix in a bucket. Extend the sampling depth to 6 to 8 inches for flower and vegetable gardens and 12 inches for large trees. Soil samples can be taken to University Extension, a private soil testing company or the Center for Home Gardening at the Missouri Botanical Garden.