Soil aeration. 23

The Science Behind Soil Aeration.

This blog discusses the science behind soil aeration and what happens with a compacted lawn. You may also find our soil aeration case study and our flood management blog of interest.

Our recently introduced Compaction calculator may also be of interest. This can help guide you as to the likelyhood of compaction occuring and how often to aerate.

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 (O₂) to respire. In poorly aerated soils, there is not enough O₂ 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 indicates soil compaction?

• Poor water infiltration.
• Turf diseases like Pythium sp.
• The presence of certain indicator weeds like Knotweed or Summer grass.
• A poor root system.
• Sour-smelling soil.
• Iron oxidation (red deposits) seen along root pathways.
• Surface water after rainfall or irrigation.
• 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 – O₂.

With a compacted lawn, soil roots water, nutrients, and oxygen become limiting. All growing media are composed of a constantly changing environment, and nutrient movement and gaseous exchange, are always occurring.

Consequently, any factor that impacts these processes will severely affect the growth of turf.

The finer the soil, the more likley you will have a compact lawn. This leaves less space for O₂ and when denied O₂, neither turf nor aerobic soil microorganisms can survive.

After flooding for example there are several chemical and soil physical processes that occur and emphasise the importance of aeration.

Soil equilibrium.

This soil balance can be affected in a compacted lawn. For example, poor soil aeration or oxygen deficiency is a major factor that limits seedling growth. Soil oxygen deficiency:

• Limits root growth;
• Reduces respiratory capacity;
• Limits carbohydrate accumulation and hormone synthesis and
• Limits water and nutrient uptake. Reductions of up to 55% in Potassium uptake can occur in poorly aerated soil.

In the soil, several biological processes occur. The majority of these are aerobic, and involve the uptake of O₂ and the production of carbon-containing compounds. Plant roots respire aerobically and so need sufficient O₂ at root surfaces. In fact, estimates are that roots and soil microbes require 5 to 24 g of oxygen/m2/day.

O₂ deficiency in the soil occurs because of:

• Soil compaction;
• Poor rootzone quality. This is the case in 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 too much irrigation, water fills up the soil’s pore space, displaces the air and reduces the O₂ level to nearly 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 N);
• Mn reduction;
• Fe 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 Affects Soil Aeration.

In the atmosphere O₂, carbon dioxide, and N comprise 21%, 0.03%, and 79% respectively. These percentages can vary in the soil. O₂ is less than 20%, carbon dioxide is up to 10 to100 times higher and there is about the same amount of nitrogen.

Soil tesxture influences the amount of air present in the soil.

• Sandy soils contain 25% or more.
• Loam soils 15 to 20%.
• High clay soils can be below 10% of the total soil volume.

In fine-textured soils structure also plays a major role. Strongly aggregated soils with macroaggregates of the order of 5 mm or more in diameter, generally have a considerable volume of macroscopic pores.

These drain very quickly and remain air-filled most of the time. These soils exhibit an air capacity of 20 to 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 O₂.

In the turf rootzone, there is not a great reserve of O₂ in the soil. For example, in the top 1 metre of a compacted lawn, there is a 3 to 4 day supply of O₂ present in soil pore space. This means that respiratory processes need to replenish O₂ and remove any waste products.

The way by which O₂ moves into the soil are the same as those for carbon dioxide removal. The O₂ moves from the atmosphere into the soil and then moves by mass flow, diffusion, or in soil water.

The rate at which soil O₂ exchanges with the atmosphere is the oxygen diffusion rate (ODR). This influences the levels of carbon dioxide present, with a low ODR resulting in an increase in carbon dioxide.

Mass flow occurs when a pressure gradient exists. This involves the bulk flow of gas in a particular direction. This process can account for 5 to 10% of the total O₂ used in the soil.

Wind gusts can lead to sudden pressure increases at the soil surface and so small gusts can enter into the soil. Because it is localized, and short term, this only effects the top 2 to 3 cm.

The water solubility of O₂ is 0.028 cm3 of O₂ in a cm3 of water. This may be important in stimulating a flush of activity but it only contributes a tiny amount to the transport process.

By far the greatest movement occurs by way of diffusion involving a concentration gradient. This process that enables O₂ to get down to depth. So given that a concentration difference exists, this means that there is a tendency for gas molecules to move from high to low concentrations.

Flooding

Soil flooding limits 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 O₂ has a major negative impact on turf growth.

One litre of air contains approximately 33x more O₂ than one litre of water at 20 °C. This restricts gas exchange when soil is waterlogged, and causes O₂ levels to fall rapidly.

At the same time high levels of CO2 to occur in the root zone and toxins form. These all impact 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 under water excessive growth occurs, due to the build up of ethylene. Submerged plants tend to elongate which is a attempt to try and maintain air contact until the floodwaters are too deep.

Ethylene also triggers a loss of chlorophyll, leaf yellowing, the degradation of proteins, and the recycling of young tissues. This means leaves are less prepared to photosynthesise once flood waters fall away.

So, in order to give flooded turf the best possible chance of recovery, it is important to maintain carbohydrate reserves before flooding occurs.

Improving Soil Aeration.

You can be seen that soil aeration is extremely important to optimize plant growth. 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. It should be a regular maintenance operation over an entire site. Bear in mind the average walking human applies a ground pressure of 60 to 80 kPa, whilst a galloping horse will exert up to 3.5 MPa.

How To Improve Gas Exchange in a Compacted Lawn?

There are several options to carry out aeration on a compacted lawn. Core aerification, vertidraining, and hollow or solid tine aeration help to maintain healthy root-zones and aid gas exchange in the turf root zone.

Also, contrary to the general perception, core aeration does not have a negative impact on pre-emergent herbicides.

However, all of these methods can cause surface disruption. Consequently, air injection is becoming more popularity. This works 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.

• Reductions in soil bulk density.
• Increases in saturated hydraulic conductivity.
• Increases in larger soil pore space.
• In sandy soils positive soil physical changes may occur.

2017 work in the USA into new and traditional cultivation methods. Soil Air injection results in the least reduction of green turf cover, no ball roll reduction from the control, and lower reduction in surface firmness in comparison to other methods. Hollow tine aeration has 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 to examine the effects of the SISIS Javelin Aer-Aid. The conclusion was that the “The SISIS Javelin Aer-Aid was effective to reduce compaction,”

A 2021 study looked at the effects of air and sand injection on soil’s physical properties. The conclusion is that the use of air-injection cultivation with hollow-tine cultivation shows promise to improve infiltration rates over hollow-tine cultivation alone.

Air Injection for Compaction Relief.

There are currently two options on the Australian market that you can use on a compact soil . 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, they both have their place in the market.

Currently, the Air2G2 appears to be more widely available as it was the first onto the market. The soon-to-be-launched Sisis Javelin Aer-Aid however, allows the injection of liquids through the tines and therefore will 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 moisture content to get the best results from any soil aeration program. If the soil is too wet you won’t get any soil fracturing. This means that at best any benefits will be shortlived.
• Likewise, dry sand doesn’t work well either. With a dry sand the air blast just passes through the sand without causing any movement.
• Care should also be taken if you regularly aerate and use pre emergents. However, evidence does show that there is no significant negative impact on these with regular aeration.
• They will not work well if the soil is too shallow.
• If you have limited access to a site these may not be practical.

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