Magazine: Soil Aeration
By Jerry Spencer in Consultancy on 6th Nov 2007 10:30
With the widespread use of fertilisers and irrigation more and more it is aeration which has become a major limiting factor to the attainment of optimal growth. It seems likely that root systems are commonly restricted in extent by the progressive decrease in aeration that occurs down a soil profile. Poor aeration can decrease the uptake of water and induce early wilting.
In the case of growing media ‘air space is the per cent volume of media or media component that it is filled with air after the media has achieved container capacity’. Air space is affected by container height i.e. the taller the container, the more drainage and therefore more air space. For a given bulk density, moisture content and container size, air space is equal to the total porosity minus container capacity. However the principles discussed apply to both turf and nurseries.
When soil becomes compacted, roots can suffer from a lack of water, nutrients and oxygen. As pressure increases on managers the need for maintaining a well aerated media has increased accordingly. All growing media are a constantly changing environment with nutrients being moved, gaseous exchange constantly occurring and water being in a constant state of flux. Any factor that influences one of these processes will severely affect the growth of plants.
If soil compaction develops it can severely effect this equilibrium.
Poor soil aeration or oxygen deficiency is a major factor limiting seedling establishment. Oxygen deficiency in the soil can occur because of improper management such as compaction; poor media quality, such as heavy fine-textured soils or layered soils with inadequate drainage; excessive irrigation, rainfall or flooding; usage of excessively small containers for transplant production.
Inferior stand establishment can occur due to the inhibitory effects of low aeration on root elongation, proliferation, viability, respiratory capacity, carbohydrate accumulation, hormone synthesis, and water and nutrient uptake. In fact poor soil aeration affects potassium uptake more than any other major nutrient with levels of uptake being down to only 45% of normal!
Within the soil environment there are a number of biological processes occurring. The majority of these are aerobic involving the uptake of oxygen and the evolution of carbon containing compounds. Plant roots respire aerobically and therefore require sufficient oxygen supplies at root surfaces.
Anaerobic conditions in the soil induce a series of reduction reactions, both chemical and biochemical. Included in these are denitrification (the processes by which nitrate is reduced to nitrite, then to nitrous oxide and then to elemental nitrogen), manganese reduction, iron reduction and sulphate reduction. Some of the products of these processes are in fact toxic to plants such as ferrous sulphide and ethylene. The associated build up of carbon dioxide in anaerobic conditions can occur as a result of excessive watering, soil compaction (reduced porosity and increased bulk density) and increased levels of microbial activity.
In the atmosphere oxygen, carbon dioxide and nitrogen comprise 21%, 0.03% and 79% respectively. In the soil these percentages can differ drastically with oxygen being less than 20%, up to 10-100 times more carbon dioxide and about the same amount of nitrogen. High carbon dioxide levels result in root die back.
The amount of air present in a soil is directly influenced by soil texture. In sandy soils it is of the order of 25% or more, in loamy soils it is between 15 and 20% and in clayey soils that tend to retain the most water 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 which drain very quickly and remain air filled practically all of the time. Hence such soils exhibit an air capacity of 20-30%. As the aggregates are dispersed or broken down by mechanical forces these pores tend to disappear so that a strongly compacted soil can have less than 5% air by volume.
A comparison of the amount of oxygen present in the soil with existing respiration rates reveals that there is not a very great reserve of oxygen in the soil. In the top 1 metre of a soil profile there is around a 3-4 day supply of oxygen contained in soil pore space. Therefore in order 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 carbon dioxide etc move out. 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 localised and short term it is really localised in significance and importance being restricted to the top 2-3 centimetres.
The solubility of oxygen in water 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.
Thus it can be seen that soil aeration is extremely important in order to optimise the growth of plants and that often poor growth is an indirect result compaction and its effects on gaseous
exchange, nutrient availability and drainage.
e-mail: jerry.spencer@etpturf.com.au
web: www.etpturf.com.au
ReferencesFoth, H, D, Fundamentals of Soil Science, 1994.Reed, D, W, A Growers Guide to Water, Media, and Nutrition for Greenhouse Crops, Ball Publishing, 1996Huang, B, and Nesmith, S, D, Soil aeration effects on root growth and Activity, ISHS Acta Horticulture 504: VI Symposium on stand establishment and ISHS Seed symposium
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With the widespread use of fertilisers and irrigation more and more it is aeration which has become a major limiting factor to the attainment of optimal growth. It seems likely that root systems are commonly restricted in extent by the progressive decrease in aeration that occurs down a soil profile. Poor aeration can decrease the uptake of water and induce early wilting.
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.
