The science behind soil aeration. 23

This blog discusses the science behind soil aeration. You may also find our soil aeration case study and our flood management blog of interest.

Turf roots grow in small air pockets within the soil and in a compacted soil, these air pockets disappear. Soil aeration is caused by mechanical manipulation and modification of the soil and one of the main limiting factors to achieving optimal turf growth is a lack of aeration. Soil aeration aims to achieve three things:

1. Renew soil structure;
2. Aid in the management of organic matter and lastly
3. Allow gas exchange. Turf roots require oxygen to respire. In poorly aerated soils, there is not enough oxygen and roots struggle as they can’t breathe properly.

Allowing gas exchange through soil aeration is probably the least discussed benefit of this process but is seldom mentioned in the turf industry.

What shows you have soil compaction?

• Poor water infiltration;
• Turf diseases like Pythium sp;
• The presence of certain indicator weeds like knotweed or summer grass;
• A limited root system;
• Sour-smelling soil;
• Iron oxidation (red deposits) evident along root pathways;
• Surface water after rainfall or irrigation and
• Turf that tends to rapidly wilt under dry conditions.

Soil texture, soil aggregation, and bulk density all affect the amount of pore space and hence soil aeration.

The science behind soil aeration – oxygen

In compacted soil roots water, nutrients, and oxygen become limiting. All growing media are composed of a constantly changing environment. Nutrient movement and gaseous exchange, are constantly occurring. Consequently, any factor that influences one of these processes will severely affect the growth of turf.

The finer the soil, the more prone it will be to compaction, consequently leaving less space for oxygen. Denied oxygen, neither turf nor aerobic soil microorganisms can survive.

Soil equilibrium

This soil balance can be severely affected If soil compaction develops. For example, poor soil aeration or oxygen deficiency is a major factor that limits seedling establishment. Soil oxygen deficiency causes:

• Restricted root growth;
• Reductions in respiratory capacity;
• Limited carbohydrate accumulation and hormone synthesis and
• Limitations in water and nutrient uptake. Potassium uptake can be reduced by 55% in poorly aerated soil.

Within the soil environment, several biological processes are occurring. The majority of these are aerobic involving the uptake of oxygen and the production of carbon-containing compounds. Plant roots respire aerobically and therefore require sufficient oxygen supplies at root surfaces. In fact, it has been estimated that roots and soil microorganisms require 5-24 g of oxygen/m2/day. Oxygen deficiency in the soil occurs because of:

• Improper management resulting in soil compaction;
• Poor rootzone quality, such as heavy fine-textured soils or layered soils with inadequate drainage and
• Excessive irrigation, rainfall, or flooding.

As discussed later, waterlogging is detrimental to soil aeration. After downpours, floods, or excessive irrigation, water fills up the soil’s pore space, displacing the air and reducing the oxygen level nearly to zero.

The science behind soil aeration – Anaerobic Soil Conditions

Anaerobic conditions in the soil cause a series of both chemical and biochemical reactions. These include:

• A build-up of carbon dioxide;
• Denitrification (the processes by which nitrate is reduced to nitrite, then to nitrous oxide, and then to elemental nitrogen);
• Manganese reduction;
• Iron reduction;
• Sulphate reduction and
• The production of toxic compounds such as ferrous sulphide and ethylene.

This build-up of carbon dioxide in anaerobic conditions results in:

• Excessive watering;
• Increases in levels of some microbial activity and
• Root dieback.

How the soil atmosphere relates to soil aeration

In the atmosphere oxygen, carbon dioxide, and nitrogen comprise 21%, 0.03%, and 79% respectively. These percentages can differ drastically in the soil. Oxygen is less than 20%, carbon dioxide is up to 10- 100 times higher and there is about the same amount of nitrogen. The amount of air present in the soil is directly influenced by soil texture. Sandy soils contain 25% or more. In loam soils 15 – 20% and in high clay soils it can fall below 10% of the total soil volume.

In fine-textured soils structure also plays a significant role. Strongly aggregated soils with macroaggregates of the order of 5mm or more in diameter, generally have a considerable volume of macroscopic (interaggregate) pores. These drain very quickly and remain air-filled practically all of the time. These soils exhibit an air capacity of 20-30%. As the aggregates are dispersed or broken down by mechanical forces these pores tend to disappear. A strongly compacted soil can have less than 5% air by volume.

Soil Oxygen

In the turf rootzone, there is not a very great reserve of oxygen in the soil. For example, in the top 1 metre of a soil profile, there is a 3-4 day supply of oxygen present in soil pore space. Therefore, to sustain respiratory processes oxygen must be replenished and waste products removed.

The processes by which oxygen moves into the soil are the same as those for carbon dioxide removal. The oxygen moves from the bulk atmosphere into the soil and can move by mass flow, diffusion, or in water. The rate by which soil oxygen exchanges with atmospheric oxygen is the oxygen diffusion rate (ODR) and this directly influences the levels of carbon dioxide present. A low ODR results in increasing levels of carbon dioxide.

Mass flow occurs when a pressure gradient exists and involves the bulk flow of gas in a particular direction. This process can account for 5-10% of oxygen consumed in the soil. Gusting of wind can lead to sudden pressure increases at the soil surface and in turn, lead to small gusts entering the soil. Because it is localized and short term its importance is restricted to the top 2-3 centimetres.

The water solubility of oxygen is 0.028cm3 of oxygen in a cm3 of water. This may be important in stimulating a flush of activity but it is generally a negligible contribution to the transport process.

By far the greatest movement occurs by way of diffusion involving a concentration gradient and it is this process that enables oxygen to get down to depth. Given that a concentration difference exists and the thermal motion of gas molecules there is a tendency for gas molecules to move from high to low concentrations.

Flooding

Soil flooding impedes O₂ movement into soils, and so roots experience hypoxia (sub-optimal O₂) and anoxia (absence of O₂). Coupled with slow diffusion, the low water solubility of oxygen has a major negative impact on turf growth. One litre of air contains approximately 33-fold more O₂ than one litre of water at 20 °C. This restricted gas exchange when soil is waterlogged causes oxygen levels to rapidly, high levels of CO2 to occur in the root zone and phytotoxins to develop in reduced soils. These all impact on root metabolism, nutrient acquisition, and growth of roots and shoots. The image below shows what chemical changes occur over time in waterlogged soil and how organic matter and temperature also play a role.

When turf is submerged excessive growth commonly occurs. This is due to the accumulation of the phytohormone ethylene. Submerged plants tend to elongate excessively, as a physiological response in an attempt to maintain air contact until the floodwaters are too deep. Ethylene accumulation also triggers a loss of chlorophyll, leaf yellowing, the degradation of proteins, and the recycling of young tissues. This consequently means leaves are less fit for photosynthesis once flood waters fall away. Consequently, in order to give turf the best possible chance of recovering after flooding maintaining carbohydrate reserves is important.

Improving soil aeration

Thus it can be seen that soil aeration is extremely important to optimize the growth of plants. Often poor growth is an indirect result of its effects on gaseous exchange, nutrient availability, and drainage. Aeration is not just necessary in areas subjected to heavy traffic and it should be a regular maintenance operation over an entire site. Bear in mind the average walking human applies a ground pressure of 60–80 kPa, whilst a galloping horse will exert up to 3.5 MPa.

So how can you improve gas exchange in turf soil?

Many mechanical cultivation practices are now regularly carried out. Core aerification, vertidraining, and hollow or solid tine aeration help to maintain desirable root-zone physical properties and aid gas exchange in the turf root zone. However, all of these can potentially cause undesirable surface disruption. Consequently, air injection is increasing in popularity.  These work by tines penetrating the soil, and then injecting bursts of air. Claims made include an increase in infiltration and drainage and a reduction in soil compaction. This is all with minimal surface disruption. Does the research support these claims?

Trial work

1987 work on trees (i know its not the same but the principles are) looked at the soil physical affects of pneumatic subsoil loosening. The results showed:

• Reductions in soil bulk density;
• Increases in saturated hydraulic conductivity;
• Soil macroporosity increased and
• In sandy soils positive soil physical changes may occur.

2017 work in the USA compared new and traditional cultivation methods for their impact on playability on creeping bentgrass putting greens. Soil Air injection resulted in the least reduction of green turfgrass cover, no ball roll reduction from the control, and lower reductions in surface firmness compared to other methods tested. Hollow tine aeration had the greatest reduction in green turfgrass cover, lowest ball roll distance, and greatest reductions in surface firmness.

A UK four-month study in 2017 at the STRI, saw three trial plots set up in which one area examined the effects of the SISIS Javelin Aer-Aid with air injection. They concluded that the “The SISIS Javelin Aer-Aid was effective in reducing compaction,”

A 2021 study looked at the effects of air and sand injection on soil’s physical properties. They concluded that the incorporation of air-injection cultivation with hollow-tine cultivation shows promise for improving infiltration rates over hollow-tine cultivation alone.

There are currently two serious players in the Australian market. The Sisis Javelin Aer-Aid is sold by JT Turf and the Air2G2 is sold by Sustainable machinery. So, how do these stack up? From all reports, both do a pretty good job. As I see it I believe both have their place in the marketplace. Currently, the Air2G2 appears to be more widely available as it was the first onto the market here. The soon-to-be-launched upgraded Sisis Javelin Aer-Aid allows the injection of liquids through the tines and therefore should be a game changer in the turf industry.

Potential issues

As with most things these are not suitable to use everywhere.

• You need the right soil mositure content to get the best results. If the soil is saturated you won’t get fracturing and any benefits will at best be shortlived. Likewise, dry sand doesn’t work well either as the air blast just passes through the sand without causing any movement.
• Care should be taken if regularly aerating and use pre emergent herbicides. However, evidence does show that there is no significant impact of carrying our regular aeration.
• Lastly, they will not work well if the soil is too shallow.

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