The air content of a potting mix can significantly influence the growth and health of container plants.
A potting mix must supply the roots of a plant with air as well as water. The importance of air to the health of container grown plants is often underestimated.
The air content of a potting mix is measured as the air-filled porosity (AFP). This is the volume of air held in a mix after it has been thoroughly wet and drained expressed as a percentage.
The Australian Potting Mix Standard (AS 3743-2003) requires the AFP of a Premium grade mix to be at least 13%v/v. This is a minimum value for healthy growth of plants in a well-managed nursery and is not necessarily what is optimum for maximum growth and disease resistance.
Image: Air-filled porosity guidelines for nursery container media.
As a general rule, the AFP of a mix should be as high as possible within the constraints of irrigation and climate. This is because the AFP will decrease with time as the organic components decompose and the mean particle size reduces (see below image).
Image: Fine particles from decomposition have clogged the mix in the bottom of the pot.
The AFP and the water holding capacity (WHC) of a mix are inversely related and so increasing the AFP by 10%v/v will generally reduce the WHC by an equivalent percentage.
This means the advantage of having more air is eventually lost when it becomes too difficult to maintain water supply to the crop (Figure 1). Obviously, a higher AFP can be used where watering is well managed.
Image: Root length increases with AFP until water becomes limiting.
Choosing a mix with too high an AFP will simply expose weaknesses in the irrigation system and management and decrease plant uniformity. This is because when the scheduling is based on ensuring the drier areas receive enough water, the wetter areas will receive too much and growth may be restricted by nutrient leaching and by water logging.
Image: Poor uniformity attributable to oxygen depletion.
Choosing a mix with too low an AFP will mean that the irrigation schedule will need to be tuned to avoid waterlogging the wetter areas. Consequently, plants in the drier areas will not receive adequate water for growth or to flush excess salts from the root zone.
Clearly, it is important to match the AFP of a mix to the way water is managed in a nursery.
Why is air needed?
The air in a potting mix is the main source of the oxygen needed by roots for respiration.
Respiration is the chemical process that releases energy from carbohydrates synthesized in the leaves. It is effectively the reverse of photosynthesis:
Energy (light) + Carbon dioxide + Water ® Carbohydrate + Oxygen
Carbohydrate + Oxygen ® Carbon dioxide + Water + Energy
Respiration fuels growth and maintenance of cells. Energy is also needed for active uptake of nutrients from the soil solution and for their transport within the plant.
Without adequate oxygen (anoxia), root growth slows, nutrient uptake and distribution is disturbed and eventually roots and then the whole plant dies.
In the early stages of anoxia, root cell membranes begin to leak sugars and essential elements into the soil solution. This injury weakens plant defences and attracts water borne pathogens.
Roots also lose some ability to exclude toxic ions such as sodium, chloride and manganese.
Low oxygen can also cause changes in the soil microbiome which are unhelpful for the plant.
Soil microbes must also obtain energy from carbohydrate by respiration. They are growing in a light free environment and so most cannot photosynthesize. Their food source is organic matter either contributed by the plants or by the potting mix.
When there is adequate oxygen (aerobic), the by-products of respiration are water and carbon dioxide which are relatively harmless to plants.
However, when there is a shortage of oxygen (anaerobic), different microorganisms dominate the soil microbiome. This group obtains energy from carbohydrate by anaerobic respiration and the by-products of this process are often toxic to plants. They include alcohol, methane, nitrous oxide, hydrogen sulphide (rotten egg gas) and manganese.
The Australian Potting Mix Standard toxicity test is intended to ensure potting mixes have been composted aerobically and do not contain these toxins at dangerous levels.
When oxygen is limiting, even aerobic microorganisms can become a problem as they compete with plant roots for the small amount present in the soil solution.
Oxygen depletion in the root zone is accelerated by warm conditions because this increases biological activity and demand for oxygen and because the solubility of oxygen in water decreases at higher temperatures.
Image: Oxygen close to the root is consumed by plant and microbial respiration.
Symptoms of oxygen starvation (anoxia)
- Chlorosis of new leaves. The symptoms may be due to iron deficiency but the cause can be inadequate oxygen supply to roots.
Image: Iron deficiency symptom can be caused by anoxia.
- Pale green foliage and uniform yellowing of old leaves. These nitrogen deficiency symptoms can be caused by denitrification and root damage under anoxic conditions.