Chemical Composition

Chemical Composition of the Soil

One of the three components of balanced soil is the chemical composition. For chemically balanced soil, the essential elements, both macronutrients and micronutrients need to be in balance, the pH of the soil should match the need of the plants, the salt content of the soil should be low, and the soil should be free of contaminants.

Soil Test

Have your soil analyzed by a reputable soil lab before adding chemicals or amending the soil. Know your starting point. I believe this is a necessary first step whether you are growing ornamentals, turf, a home vegetable garden or for agricultural production. Many soil labs are equipped to test crop plants for soil chemical or mineral deficiencies. In addition to chemical deficiencies, soil labs will provide you with information about your soil texture, organic content, pH and levels of salts and the major nutrients. Not all soil labs are alike. Do some research on what particular labs do best and what information you need. Tests should be repeated a minimum of every three years.

Most soil labs will have instructions on their website for taking soil samples. Follow their instructions depending on the information you require, whether it's a general soil sample of your landscape, or for a specific area you think may have a problem. If the soil looks different from one area of the landscape to another, it is different and take separate samples.

Provide information on the form, particularly what type of problem has been encountered, if any. Include information of past fertilizing or amending. This is often helpful when the lab makes their recommendations.

Macro and Micro Nutrients

It is important to have minerals balanced and available for use. Plants require sixteen essential elements for normal growth. Carbon, hydrogen, oxygen and nitrogen make up the top four elements. There are five more macronutrients— phosphorus, sulfur, calcium, potassium, magnesium and seven micronutrients or trace elements—boron, copper, iron, manganese, molybdenum, zinc and chlorine.

Unless a soil test has been performed and you're aware of a particular mineral deficiency, avoid adding them. Instead, the addition of a variety of organic substances provides a balance of minerals and nutrients. A well-rounded diet is needed for optimum performance. Adding too much of one mineral often causes imbalances and therefore chemical deficiencies.

Rock dust and powders, green sand and lava sand are sometimes added to soil to improve the general mineral content of the soil. Exclusive application of chemical fertilizers throws the chemical composition out of balance.

Saline Content

Adding chemical fertilizers to the soil increases the saline content of the soil. Soils with a high saline or sodium content cause additional problems for the gardener. Saline soils either ties up or releases micronutrients to toxic levels, causes a disturbance in cell water equilibrium, degrades the soil structure, and soil organic matter is chemically destroyed under high pH conditions. Higher sodium levels further inhibit the ability of plants to uptake water and nutrients and cause the plants to dry out quicker.

The method for reducing salt content is to leach it out with water. But irrigation can also cause salt build-up in the soil. Our water supplies both from Lake Meredith (in the past) and at the lower depths of aquifers contain salts, so leaching out salts already in the soil with irrigation water is not helpful. Rainy seasons help to leach the salts out if your soil has good permeability. With heavy compacted clay soil, few options remain.

The best practice is to limit the addition of salts by not using salt-based chemical fertilizers and grow low water-use plants and natives, keeping irrigation to a minimum.

Soil pH

The soil pH is an important factor in the choice of plants. The pH scale, from 1 to 14, is a measure of acidity or alkalinity of your soil determined by the concentration of hydrogen ions in a water or salt solution. Acid soil would fall within the range of 1 to 7.0 pH, and alkaline soil from 7.0 to 14. Neutral soil has a pH of 7.  The optimum pH range for growing plants is between 6.3 to 6.8, slightly acidic soil. This pH range is also ideal for most soil organisms and bacteria.

In alkaline soils with a pH of 7.5 - 9.0, some macro and micro nutrients are not able to be absorbed. On the extreme alkalinity end, microorganisms are disrupted, interfering with the breakdown of organic matter and its release of nutrients from their action. Dry to arid soils are usually alkaline, sometimes accompanied with a high level of soluble salts (sodic soils). This encompasses nearly the entire southwest United States. Our Panhandle soils fall in the 7.5 to 8.5 range, generally speaking.

Our alkaline soil causes certain minerals to be bound up, unable to be used by certain plants. Iron, phosphorus, copper, zinc, boron, and manganese are the chief minerals that are unable to be taken up in alkaline pH conditions. Adding organic matter to the soil buffers the alkalinity and helps release these bonds. If your soil is highly alkaline, choose plants what require alkaline soil. Plants that require acidic soil will fail even with efforts to continuously acidify the soil.

Often, it is recommended to add peat moss to lower the alkalinity of the soil. However, this is not now recommended. Once peat moss dries out, it is is hard to re-wet in the soil and most peat supplies are not easily renewable. Shredded pine needles, organic composted cotton seed hulls and cottonseed meal is helpful in lowering alkalinity.

If your soil analysis recommends adding elemental sulfur or gypsum, follow their instructions. If large quantities are needed, split the recommendation and add half, followed with the second half several months apart. Soil organisms will need time to adjust to a drastic change in pH.

Soil Contaminants

During the heyday of "Better Living Thru Chemistry", chemical fertilizers, herbicides and pesticides may have been applied in such quantities to cause contamination. Contamination may be caused by arsenic, lead, mercury, nickel, cadmium, copper, fluorine, zinc, manganese and boron or other heavy metals from manufacturing processes. Lead is the most common heavy metal contaminant.

DDT, PCB's, EDB (ethylene dibromides), dioxin, petroleum, plastics and other chemical compounds could be present, and not detected in a routine soil lab analysis. If it is suspected the soil is contaminated, your Cooperative Extension Service or the Environmental Protection Agency may need to be contacted for soil sampling procedures, lab analysis, containment or removal.