Understanding and Improving Soil Fertility

Daniel Gill, Koske, Thomas J.  |  4/19/2005 11:19:20 PM

Soil pH regulates many nutrients.

Good soil has good tilth.

Plants will produce best in a location when they have the proper fertility. Some plants are heavy feeders, some light feeders and some are in between. If you feed a light feeder as a heavy feeder, it does not mean you will get more flowers or fruit. In fact, you usually get less or maybe none at all. Specific plants needs are too numerous to tackle in a short article. This article will cover the three basic areas of soil fertility: Soil nutrients, pH and organic matter.

There are 12 to 14 soil nutrients considered essential to most plants. There are fourteen if you consider nickel and chlorine. The primary nutrients nitrogen, phosphorus and potassium are used in large quantities and are the three main numbers found on a fertilizer's analysis. Also used in large amounts are the secondary nutrients calcium, magnesium and sulphur. We often don't directly supply these, but sometimes we do when we add lime, etc.

Other soil nutrients are used in small quantities and are referred to as micronutrients. Although needed in small quantities, micronutrient availability is very important for normal growth.

All of the soil nutrients are required at the correct levels and in the correct forms. This is one place where the soil pH, or measure of soil acidity, comes into the picture. The soil pH levels can greatly affect the form and thus the plant availability of a soil nutrient. Most soil nutrients are very available between pH 6 and 7. Several nutrients become very available at higher pH and several arev ery available at pH of 5 to 6. These high availabilities may produce a nutrient toxicity to sensitive plants. Likewise, about six nutrients are much less available at pHs around 8 and above, and all but iron become less available around 5.5 and 5. Some plants tolerate high (alkaline) pH. Some acid lovers, like azaleas, thrive in a pH of 5.

Organic matter is the third factor in soil fertility. OM doesn't always supply nutrients organically, although the fresher materials may, but it usually makes the soil more friable and workable. The OM can help stabilize soil pH, but it is mainly known for its nutrient storing capacity. The nutrients that go through the soil can be held on the OM for later exchanges to plant roots. Too much OM may produce a soil that is too rich for certain crops, so once again, more isn't always better. Good soil test levels of OM are generally 2 to 3% for true soil.

The best combination of these three factors should be offered for best growth, health and fruiting. A good way to figure out what you have and what is needed is the laboratory soil test. Our LSU AgCenter's routine soil test gives you pH, phosphorus, potassium, calcium, magnesium, sodium (salt) and texture.

On the soil test form, list a crop or two and the fertilizer recommendations. Any required liming, etc. will be given with the results for these crops. Be specific if you can. For example, don't just say lawn; include the type of grass. For vegetable gardens, just state vegetable garden unless you have just one or two crops to list. See our LSU Soil Testing Web site for more information.

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