Achieving Effective Soil Aeration

Constituents of a well-managed soil shown as percentages of volume.

The proportion to which the soil pore space contains water is referred to as the soil moisture content and is expressed as a percent of the total soil volume. In a soil that consists of 50% mineral matter and which has a soil moisture content of 30%, the remaining 20% of the soil volume will contain air. The moisture status of a soil after drainage has ceased is termed field capacity and is normally assumed to be at this point three days after saturation. It is not a quantitative measurement and the amount of water held at field capacity will vary in a soil, depending on its texture, structure, degree of compaction and soil temperature.

Balancing soil air and water
It is important for a soil to contain adequate moisture, but it is equally important that a soil contains adequate air-filled pore spaces. These air-filled pores provide routes for gas exchange with the atmosphere. This is termed soil aeration. Adequate soil aeration is needed to create a healthy environment for turfgrass roots and plant-beneficial microbes living in the soil. Turfgrass roots require oxygen for respiration and the beneficial microbes are also aerobic organisms. They consume oxygen in their respiratory processes and generate carbon dioxide (CO2). Efficient soil aeration is necessary to prevent soil oxygen depletion and the accumulation of excessive CO2 or other, toxic gases. As the rate of metabolic processes of organisms increase with a rise in temperature, the higher the temperature, the greater the demand for oxygen and soil aeration. Consequently, the demand for soil oxygen is at its highest during the summer months and at its lowest during the winter in the UK.
The ideal balance of water to air in the total pore space should be 70% water and 30% air. For adequate soil aeration, it is generally accepted that a soil should contain at least 10 to 20% air filled pore space for most of the growing season. If the air filled pore space is less than this for extended periods, the soil is considered to be depleted of oxygen, i.e. anaerobic. Waterlogged and anaerobic soils will result in turfgrass decline by inhibiting root respiration. For example, a rootzone containing 20% air filled pores would become completely anaerobic (without free oxygen) after 24 to 48 hours if gas exchange did not occur. A rootzone with less than 10% air filled porosity can become anaerobic within 24 hours.

Pore size influences water and air movement
Soil pores are generally classified according to their size. And it is pore size, rather than total pore space, that strongly influences the water and air content of a soil at field capacity. Macropores, those larger than 75m in diameter (1000m equals 1 mm), will readily drain and mainly assist water infiltration, percolation and soil aeration or gaseous exchange.

Mesopores, those between 30m and 75m in diameter, will lose some of their water during the three day period leading to field capacity. Mesopores allow water to move more slowly. They enable capillary water to move to roots and soil moisture to be redistributed within the soil. However, the importance of capillary movement should not be exaggerated as the water moves very slowly and generally only over short distances. Micropores, those less than 30m in diameter, do not readily assist water to move through the soil but retain water within it and serve as a storage reservoir. They will only lose their water through root absorption. Therefore, a soil that is dominated by micropores will retain far more water than the desirable 70 % of total pore space. A soil that is totally dominated by micropores smaller than 30m in diameter may have 100% of its total pore space occupied by water at field capacity. In such an instance, the soil water content at field capacity will equal saturation.

There must be an extensive and continuous network of macropores

Oxygen diffuses through water 10,000 times slower than it does through air. Consequently, water-filled pores such as micropores and many mesopores can easily become deficient in oxygen, causing problems to turfgrass roots and microbes. Macropores, on the other hand, are the major aeration pores and it is essential that a soil has an extensive and continuous network of these larger pores to ensure adequate soil aeration.

What is soil compaction?
Soil compaction is defined by an increase in bulk density and a reduction in total porosity. However, compaction does not affect all pores equally. Principally, there is a loss of macropores and a proportionate increase in micropores. By reducing the proportion of macropores in a soil, the potential for drainage and aeration are reduced. In all but the sandiest of soils, severe compaction can eliminate all macropores. Understandably, the most compacted layer within a soil is found in the top 100 mm where foot and vehicular traffic is most intense. It is this zone that most severely restricts gaseous exchange.

Compaction is probably the most serious soil problem that turf managers have to deal with.

Choosing the most effective equipment
Any mechanical soil aeration operation must increase the total macroporosity of a soil to be effective. All too often, equipment that is described as an aeration treatment fails to increase the number and extent of macropores. Instead, they may produce holes that merely enhance surface drainage by providing a by-pass route in surface layers through which surface water can escape to lower horizons. However, in so doing, the tines can destroy adjacent macropores and actually decrease soil aeration potentials. Solid and hollow coring tines, when operated in damp to wet soils, will at best only produce such surface drainage holes. Even where friction on removal of a tine causes a degree of heave, the disturbance is generally restricted to horizontal planes of weakness within the soil, e.g. at interfaces between soil layers or at rootbreaks.

Sisis Aer-Aid System in operation.

An implement that actually injects air, such as the Sisis Aer-Aid System, has the potential to increase the macroporosity of a soil by preserving existing macropores and enlarging additional mesopores. The mode of action can push air through the existing pore system, thereby relieving compaction and developing an extensive and continuous network of macropores. Preservation of existing macropores and the creation of additional ones is the secret of good soil aeration. The network of macropores must be extensive and continuous so that oxygen can readily diffuse to roots and soil microbes. When this is achieved, rooting can be more extensive, beneficial soil microbes will be more active and organic matter decomposition can be more rapid. Consequently, nutrients bound within the organic complexes are released and turfgrass plants are healthier. Irrigation becomes more efficient and black layer is avoided.

Early symptoms of black layer formation in fine sand.

As turf managers we must aid air movement uniformly throughout the rooting zone and not just limit it to sporadic vertical holes. The aim should be for whole aeration, not hole aeration.