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 within the soil. Turfgrass roots and beneficial microbes are ‘aerobic’ organisms; that is, they require oxygen for respiration. 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 75 µm in diameter (1000 µm equals 1 mm), will readily drain and mainly assist water infiltration, percolation and soil aeration or gaseous exchange.
Mesopores, those between 30 µm and 75 µm 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 30 µm 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 30 µm 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.
Choosing the most effective equipment
Classification of soil pores (adapted from Brewer, 1964)
Any mechanical ‘soil aeration’ operation must increase the total macroporosity of a soil to be effective. All too often, equipment that is prescribed as an 'aeration treatment' fails to increase the number and extent of macropores. A primary concept that is often overlooked is that any implement, on entry into a soil, will cause compaction and reduced aeration potential. Soil particles are pushed downward and laterally to accommodate the implement, thereby increasing the bulk density of adjacent soil and causing compaction.
Simultaneously, macropores are reduced to micropores and aeration porosity diminishes. It is inevitable and unpreventable. The extent of the damage will depend on the diameter of the tine or, as in the case of a hollow tine, the thickness of metal.
'aeration machines' only enhance surface drainage by creating a hole. They do little to increase the number or frequency of macropores.
Many so-c called
An implement can only have a positive effect on the soil, either during its brief period in the soil or during its removal from it. So, let us consider the various options and their effects. Also, let us consider how different soils and their moisture contents might influence the outcomes.
Vertical entry and removal tines
Any machine that is designed so that its tines enter and leave the soil in a vertical direction must rely entirely on the withdrawal sequence to disrupt the soil particles and produce macropores. In a moist to wet state, a soil is well lubricated and, in such a condition, any implement is going to withdraw with minimum friction and, therefore, minimal upheaval. Consequently, no compaction relief will occur.
The drier a soil is during withdrawal of the tine; the greater will be the upheaval and potential for compaction relief. However, 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 along rootbreaks. These types of implements are of limited value in achieving an improvement in soil aeration. Instead, they may produce holes that merely enhance surface drainage by providing a by-pass route through surface layers down which surface water can escape to lower horizons. The holes created will also accelerate evaporation from the soil and result in drier surfaces. But, the destruction of macropores on entry of the tine will have an adverse effect on soil aeration porosity.
The most effective machines of these types in achieving a degree of soil aeration are those fitted with needle tines that create a large number of small diameter holes at very high frequency. Generally though, this type of implement should be considered as ‘surface drainage enhancement’ cultivation systems; not soil aeration machines.
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