Soil Oxygen: The Forgotten Element

PHYSICAL COMPONENTS. Healthy soils do not have too much air or too little air. Nor are healthy soils too wet or too dry. The balance of soil moisture and soil air are two physical components of the soil that landscape managers must control in order to be successful. Soil air and soil water must be in balance with the solid component of the soil. When soils are in balance, the air and water occupy about 25 percent each of the soil volume with the soil solids occupying the remaining 50 percent.

All too often in the landscape, soil water is allowed to occupy too much of the soil volume too often. This is frequently the cause for failure in irrigated shrub beds, lawns and sports turf. However, excess water can also result from poorly constructed, poorly draining soils. Other times, it is simply the result of an excessively wet season or two. The net result is too little air in the soil for the growth of healthy root systems. This leads to weak plants, which may eventually die. Or in the case of lawns and sports turf, the more desirable grasses are replaced with less desirable species or weeds that can tolerate the low soil air caused by excessively wet soils.

As is often the case with lawns and trees in the landscape, soil air may be limited by compaction. Compaction results as the weight of the soil solids increases per unit volume. This increase in soil bulk density is usually a result of foot traffic over time. However, it is very common on newly constructed commercial as well as residential landscapes from construction equipment. It is also the most common cause for thin, weak turf on recreational athletic fields. When too much of the soil volume is occupied by the solid components, the net result is again too little air in the soil for the growth of healthy root systems.

CHEMICAL COMPONENTS. Healthy plants require the correct balance of chemicals in the soil and especially the elements essential for plant growth. Fortunately, soil chemical tests are available to determine these components.

Plants have certain requirements for soil reaction or soil pH. Most plants do well in the pH range of 6.0 to 7.0, although certain acid-loving plants do best at a pH between 5.5 and 6.0. While most turf and ornamentals in the landscape cannot tolerate excessively low soil pH values, many are adapted to moderately alkaline pH values in the range of pH 7.8 to 8.3.

To maximize plant health, plant nutrients must be present in sufficient quantity in the soil and preferably in the correct balance. In many cases, the minerals making up the soil solids may be deficient in certain essential elements or the elements may be out of balance. This can be particularly troubling because excesses of some nutrients in the soil may cause deficiencies of other nutrients in the plant. The overuse of fertilizers, in either their mineral or organic form, may lead to an imbalance in nutrients.

Soil testing can be used to determine both aspects of nutrient availability. Having optimum levels of the macro-nutrients, the secondary nutrients and the micro-nutrients is important, not only to the plants, but also to the micro-organisms in the soil. In most cases, deficiencies or imbalances can be corrected with the proper use of liming and fertilizer products.

ORGANIC COMPONENTS. The organic matter in soils is made up of the living and dead organic components such as plant roots, fungi, bacteria, nematodes, protozoa, mycorrhizal fungi and micro and macro arthropods. Healthy soils have a complex order of living organisms and they benefit from the living and decomposing organic constituents in a number of ways.

The organic components are involved in nutrient absorption, nutrient cycling and nutrient uptake by plants. Soils with low organic matter content have a higher or more frequent fertilizer requirement to keep plant healthy and actively growing.

The end products from the decomposition of organic matter, called humates, help soils form aggregates that improve water infiltration, air holding capacity and root penetration into the soil. We are just beginning to understand how this complex organic pool creates suppression of soil-born diseases, insects and nematodes. In addition, this pool of living and decomposing organic material helps to bind and degrade toxic compounds and pollutants that injure plant roots.