The potting mix is a foundation stone of nursery profitability.
A good quality potting mix can:
Shorten the production cycle by minimising stress.
Reduce the cost of water and fertiliser.
Improve plant health and natural resistance to pests and diseases
Increase uniformity and reduce throw out percentages
Improve the quality and after sale performance of plants.
Reduce the cost of production.
These benefits are possible because potting mixes have a major influence on the supply of water, air and nutrients.
Water has many unusual physio-chemical properties that make life possible.
Why is water important?
Water is essential for:
Photosynthesis – Energy collection and storage
Respiration – Energy release from carbohydrates
Nutrient availability in the soil solution, root uptake and distribution within plants
How is water absorbed?
Plants absorb water through the roots and to a lesser extent through the leaves. The growth spurt exhibited by plants after rain is not because of nitrogen in rain water as this is minor. Rather it is because the conditions for tissue expansion are suddenly improved by water absorbed through the leaves.
No energy is directly expended to absorb water. The flow of water into the plant is a function of the water potential difference between plant sap and the soil solution. Water loss through the leaves (transpiration) and tissue expansion drive this process.
Salts including fertilisers in the soil solution and the physical properties of the growing medium work together to restrict the availability of water.
Potting mixes retain water as a film that clings to the surface of particles and fibres. Some of this water is held too tightly to be available to plants. As a rule of thumb, water that can be removed at <10kPa suction is considered plant available.
Water holding capacity is not a good guide to how much water is available in a mix. This is determined largely by the pore size distribution of the mix. Water is held more tightly in mixes with fine particles because the pores are smaller. Mixes with a high air-filled porosity (AFP) have more large pores and so are likely to have more plant available water.
Water absorbed by roots moves towards the plant tops through specialised nonliving tissue called the xylem. Nutrients absorbed by the roots are transported in this water.
Water and nutrients tend to move preferentially to the mature leaves during the day light because these leaves are larger and lose the most water (Figure 1). However, at night when transpiration slows, more water and nutrients move to newer tissue because their cells are still expanding. This movement is driven by an osmotic effect called root pressure. The daily changes in water flows within a plant profoundly influence the supply of calcium to growing points and also the rate of plant growth.
Plant growth has two phases, cell division and cell expansion. When water supply is restricted, new cells do not fully expand and both leaves and plants will be smaller. When water supply is generous plants will be taller, softer and more susceptible to mechanical injury and damage from pests and diseases.
Plants can be hardened off with a controlled moisture stress. Alternatively, growth can be speeded up by ensuring that water is not limiting, particularly through the night. Irrigation should be done before sunset to prevent moisture remaining on the leaves as this will encourage disease.
Figure 1 Water movement to plant tops is largely driven by transpiration during the day and root pressure at night
What happens when there is too little water?
Water stress reduces photosynthetic efficiency. Plants conserve water by closing leaf stomata but this restricts the supply of carbon dioxide needed for photosynthesis and also reduces evaporative cooling. Leaf temperature may then rise above the optimum for photosynthesis (Plate 2). As plants begin to wilt, the leaf angle becomes less favourable for collecting light.
Figure 2 Leaves are evaporatively cooled and so heat up if not supplied enough water
Respiration is the chemical reaction that releases energy stored in carbohydrate for cell maintenance and growth. When stressed plants heat up, respiration rates rise using energy that might otherwise go into growth. Respiration also consumes oxygen and this may become limiting for roots.
When the potting mix dries out, roots produce a chemical signal that slows the growth of tops. This will even happen when only a small part of the root system is affected. For example, when the top third of the mix in the pot becomes too dry. The effect is like that of a growth retardant chemical such as paclobutrazol causing stunting and turning leaves a darker green colour. It takes 1 to 2 weeks for plant growth to recover from this setback.
Root elongation will also slow and this reduces nutrient uptake because the area behind the growing point is where most nutrients are absorbed.
As a potting mix dries out, the residual water is held more tightly. The change in availability increases the tension on the water column within the stem. At some point, the column will break and a gas bubble will appear in the xylem vessel. This produces a clicking sound that can be heard with a sensitive microphone. Listen here https://www.youtube.com/watch?v=uWL0EoZh09w
Once the column breaks, the plant loses some capacity to move water to the tops. If enough xylem vessels are damaged, the plant may never recover from the stress. The intensity of clicking has been used in research as a measure of moisture stress in plants.
The failure of the water column under tension sets the maximum height a tree can grow. It also explains why in a drought, trees tend to die back from the top branches.
What happens when there is too much water?
Waterlogging is an issue of too little air not too much water. The condition is more correctly called hypoxia and will be discussed in more detail in the next issue.
The main visible symptoms of hypoxia are root death, wilting and yellowing of leaves followed by disease.
The first reaction by most people to wilting is to provide more water which simply makes the problem worse. Hypoxia is a common reason for death of indoor plants.
Too much water can also cause oedema. This is the condition where the intercellular space in tissue fills with fluid. It is unlikely to happen during the day or in hot windy conditions because the transpiration losses are high. Long periods of very humid weather and water on the foliage at night are most conducive.
The symptom appears as sunken lesions in leaves (Plate 3) which initially look like small bruised spots. These then dry out and have a corky appearance. The lesions provide a foot hold for infection by a range of foliar pathogens.
Plate 3 Over watering can cause wilting (Left) and spotting on leaves (oedema) (Right)
A good quality potting mix must rewet easily and hold enough plant available water to prevent rapid drying between irrigations. The mix should make irrigation management easier.
The mix must also drain quickly to prevent hypoxia when plants have been over watered.