Watching a trolley full of plants being loaded into the truck bound for a retail nursery or landscape project provides a grower with an incredibly rewarding feeling. Nurturing your crop involves protecting the plants from pests and diseases, frost or heat damage, drying out and over-wetting. It’s been said “if growing plants was easy everyone would be doing it”.

Ideally, a grower could purchase a potting mix containing all of the nutritional requirements for the plants being grown so they could effectively ‘set and forget’. In reality much more effort is involved in getting the plants ready for sale, and it is important to get as close to self-sufficient as possible to limit the amount of work you have to do along the way.

When designing a potting mix blend, the manufacturer should ask a number of questions in order to match the mix to the grower’s requirements. Some of the factors that need to be established are the type of containers being used, for instance hard-wall or air-prune, the size of the containers, the species of plants being grown, the irrigation method used in the nursery and the growing environment, for example, in a poly tunnel, glasshouse or outside.

Once all these factors are known, the manufacturer will then set about designing the physical properties of the mix. By selecting different ratios of ingredients such as various grades of composted pine bark, coir fibre, sand and peat moss, the manufacturer can determine properties like air-filled porosity (AFP) and water holding capacity (WHC).

“The physical properties of the mix can significantly impact nutrient uptake”

Potting mixes are often categorised as either ‘open’ (high AFP but low WHC) or ‘closed’ (low AFP but high WHC) or somewhere in between. Not only does this determine how long water will remain in the pot, but also how much fertiliser will leach out before being absorbed by the plant. Many plants would benefit from a very open mix were it not for the fact that they would require much more water and fertiliser to obtain healthy growth.

Nutrient leaching can cause issues further down the line, if recycling the water, with increased algae growth in dams and an increase in salts which causes issues with irrigation water. It is also poor business practice to pay for fertiliser only to have the elements flushing away and not being taken up by the plant.

Nutrient leaching is drastically reduced by the addition of wetting agents, such as SaturAid. Wetting agents, or surfactants as they are often referred to, assist in holding water within the pot by breaking the surface tension of water and enabling it to hold on to the particles within a potting mix. This means that the nutrient held in the water also hangs around longer, therefore is more likely to be taken up by the plant rather than making its way straight out the bottom of the pot.

Another bulk ingredient that can assist in nutrient uptake is organic humate. A humus-rich material, humate acts almost like a magnet, dramatically improving the media’s ability to hold on to valuable elements such as calcium, magnesium and potassium.

Once the physical properties of the mix have been decided, it’s time to turn our attention to the nutrition profile of the blend. This will involve using a combination of controlled release fertiliser and soluble, short-term nutrition.

Pine bark, even once composted and mature, will continue to feed on iron. In order to avoid the potting mix competing with the plant for this important element, a form of iron is added at the time of blending as well as being used in the composting process.

Quite often, to encourage early growth, a fast-acting form of nitrogen is incorporated either in the form of nitrate nitrogen or ammonium. This ammonium will be converted into nitrate via the process of nitrification.

Nitrification is the name given to describe the oxidation of ammonia firstly into nitrite, and then into nitrate by bacteria present in soils and potting mix (see diagram below). In the coming months, we’ll be featuring a series of articles in Groundswell that will look more closely at the role of micro-organisms (including bacteria) in growing media, so I won’t expand on this process just now.

Many blends will also include lime which raises the pH of the mix. Pine bark and peat moss are quite low in pH (4-5), and many plants require a higher pH (5-6), therefore, the lime is crucial for creating a viable environment for growth. Lime also provides important elements like calcium and magnesium in varying levels depending on the type of lime being used. Gypsum can also be used to introduce these elements without having an impact on pH.

Finally, a controlled release fertiliser (CRF) such as Green Jacket will be chosen based on a few factors, one being the time the plants will be in the pot. CRFs are available in a range of longevities, ranging from a few months to over a year.

When considering different longevities it is important to also consider the rate you should use. For instance, the same amount of 3-4 month CRF will deliver a much greater release per day than the same amount of 12-14 month CRF. This is due to the 12-14 month having a thicker coating as it needs to be releasing nutrition well down the track.

Controlled release fertilisers differ in their make-up of major nutrients. Generally speaking, they contain a combination of nitrogen, phosphorus, potassium and sulphur. Many of the micro nutrients required for healthy plant growth are added to the potting mix prior to composting making them available immediately after potting, so there is rarely a need to add more of these in the short term, provided the mix has been well composted. However, some CRF brands do include micro nutrients.

Many plants will flourish with a general purpose CRF, however low phosphorus options are available for phosphorus-sensitive plants, and small-prill or mini options are available for use in small tubes or cell trays.

It’s important to be involved in the development of your potting mix blends, and to understand the ingredients that are added. This will give you a much better chance of keeping on top of issues and will ultimately result in better plants.

To have a potting mix designed specifically for your needs, contact the team at Australian Growing Solutions on 1800 709 588 or send us an email to sales@agsolutions.net.au.

Walk into any supermarket and you will be bombarded by products claiming to be ‘organic’. From breakfast cereals to cleaning products, rump steak to bananas, we are experiencing somewhat of an organic boom.

But how does one know if the product is actually organic?

What does organic even mean?

Many consumers are unaware of the guidelines surrounding the use of the term organic, and nearly everyone is confused when it comes to what constitutes organic.

The best way to tell if a product is organic is to look for a well-known accreditation stamp on the label. There are six certification authorities in Australia such as NASAA and Australian Certified Organic, and products sold in this country must be certified by one of these organisations in order to use the term ‘certified organic’ on their packaging.

In simple terms, a product sold in Australia that simply has the word ‘organic’ in the title does not need to be certified organic, and therefore, will not necessarily contain sustainably sourced ingredients or be manufactured in an environmentally responsible fashion.

However, if a product claims to be “certified organic” it must be certified with one of the six certification authorities. In fact, it is illegal to make this claim without backing it up.

Below are the logos of the six certification authorities in Australia.

So, now that you know how to identify a certified organic product, what does the term ‘organic’ actually mean?

To adequately answer that, I would need to write a novel, so let’s look specifically at compost and potting mix, bearing in mind that potting mix is a type of compost.

Much of what goes into compost is organic in nature. Raw materials such as pine bark, coir fibre and some green waste are, in their raw state, perfectly suitable for use in organic products.

However that’s not enough for the finished product to be classed as organic.

If a natural product is processed using chemicals or synthetic fertilisers, it is no longer organic, regardless of where or how it was sourced.

Many standard potting mixes on the market consist of mainly composted pine bark. In order to compost the pine bark, synthetic fertilisers are added at the beginning of the composting process.

With organic composts or potting mixes, these fertilisers are not allowed to be used; therefore the composting process takes longer and happens a little differently with organic mixes.

The general principle is that a product must be sustainably sourced and must not contain things like synthetic fertilisers or other chemicals if it is to be sold as a certified organic product.

Controlled release fertilisers are common in general potting mixes, but again, unless they are certified organic, and very few are, they cannot be used in organic products. This means on-going feeding is more important with organic mixes than with standard mixes. There are many good off-the-shelf fertilisers and plant foods that are certified organic, but very few that are designed for commercial use. However, with the increasing popularity of organic products, this is expected to change quite quickly.

The use of many herbicides and pesticides are also restricted when growing organic produce. Using a certified organic product guarantees that these chemicals were not used in the process of manufacturing or growing your material.

Why use certified organic compost or potting mix?

Plants absorb nutrients from the soil in order to grow. If your soil contains elements that are not natural, either from the growing media or from fertilisers, then the plant will be made up of these nutrients and will therefore not be classed as organic. Increasingly, people are more aware of this and are choosing environmentally responsible products to use in their gardens, home, and also their businesses. In fact, there are many organic nurseries and food producers popping up all over Australia.

In summary, if you wish to purchase an organic product, always look for the words “certified organic” and check the label for the logo of the certifying body, as well as the certification number for that particular product which will be written under the organic certifier’s logo.

If you are looking for an organic certified compost or potting mix, call Australian Growing Solutions and find out more about their Commercial Organic Mix.

Suitable for use as both compost and a potting mix, Commercial Organic Mix is perfect for anyone looking to grow organic produce or needing a soil improver to add life back into poor soils. Not only is this mix certified organic, but it is produced to the same high quality as commercial grade mixes supplied to many of the best growers around Australia, meaning you will get healthier plants, more fruit and you will require less fertiliser.

In this third article published in Groundswell magazine on potting mix properties, Geoff Cresswell explains how the physical, chemical and biological properties of a potting mix influence plant nutrition.

Plants require 16 elements for healthy growth and to complete their reproductive cycle (Table 1). These essential elements, are divided into major and minor elements based on the amounts required. In addition, there are other elements that are considered beneficial to plant health including silicon, aluminium, cobalt, selenium and sodium. Of these, silicon is possibly the most helpful for plants grown in soilless media because they are generally low in plant available silicon. Silicon increases plant resistance to a range of pests, diseases and environmental stresses including moisture, temperature, wind and nutrient disorders.

Table 1 Elements essential to plant health

No one essential element is more important to plant health than another. However, the supply of nitrogen normally limits growth most in horticultural systems. This is because plants require more of this element and because it is readily leached from potting mixes and consumed by microorganisms that decompose organic matter.

Plants absorb most nutrients from the soil solution through the roots. This means that the fertilizer must be either water soluble or readily broken down into ions by microbes to be plant available.

Solubility is important because nutrients must pass through a cell membrane in roots to enter the plant. This membrane is permeable to water but not to nutrient ions and most large organic molecules. If it was, the cell contents would leak out causing the cell to die.

The low cell membrane permeability prevents nutrient ions from freely moving into the cell at normal soil solution strengths. This property is essential because it allows plants to regulate the nutrients it absorbs through roots.

Uptake of nutrients across root cell membranes is controlled by electric pumps that are tuned to individual nutrient ions (Figure 1). These transporters are like key holes in the cell membrane formed by folded proteins. The holes have a shape and electrical charge that only allow ions with a specific size and charge (key shape) to pass through.

This micromachinery allows the plant to control the uptake of essential elements and to exclude elements that are potentially damaging such as sodium and chloride. It also enables nutrients to move into the plant against a concentration gradient (uphill).

Figure 1 Transport mechanisms for nutrients across the root cell membrane

Physical properties

Absorption through a transporter uses energy derived from respiration. This chemical reaction is fuelled by carbohydrate and requires oxygen. Roots get oxygen from the root zone which is why the air-filled porosity (AFP) of a potting mix is important.

Nutrient uptake is impaired when roots do not receive enough oxygen to fuel respiration. This is why plants in water-logged media develop nutrient deficiency symptoms. The main signs are pale green foliage and yellowing of the oldest leaves which are caused by nitrogen deficiency. Yellowing of new leaves which is caused by a trace element deficiency such as iron. Scorching and spotting of older leaves caused by the unregulated uptake of ions such as manganese and chloride.

Key points

The potting mix must supply enough oxygen to maintain active nutrient uptake. An AFP >15%v/v is adequate for most plants.

The potting mix must drain fast enough to prevent water-logging in wet weather.

Chemical properties

All nutrient ions have either a +ve or a –ve charge (Table 1) and so plants must excrete a counter ion (H+ or OH-) when the nutrient is absorbed to maintain a charge balance.

Table 1 Electrical charge on nutrient ions absorbed by plant roots

This balancing response alters the pH of the soil solution and this, in turn, influences nutrient availability (Figure 2). The optimum pH range for most plants (5.5-6.5) is where the trace elements are most available for uptake (except for molybdenum).

Figure 2 Influence of pH on the availability of plant nutrients

Because plants generally absorb more cations than anions, the pH of a potting mix usually falls over time. This change is accelerated when urea or ammonium is present in the fertilizer. Urea does not have a significant charge but is converted by soil microorganisms into ammonium and so behaves like a cation.

Not all plant species lower the pH (Figure 3).

Figure 3 pH of seedling mix after growing different plants under the same conditions https://www.ces.ncsu.edu/depts/hort/floriculture/plugs/ghsubfert.pdf

Nutrient supply is a function of the concentration of an element in the soil solution and how well the concentration can be maintained or buffered in the face of root uptake, leaching and competition from microorganisms.

Supply is also related to the available water content of a potting mix. Mixes that hold more water also hold more nutrient in reserve.

Cations resist leaching because they are held on negatively charged sites on surfaces. This property of a potting mix is referred to as the cation exchange capacity (CEC) and is mainly due to the presence of humified substances. These are products of organic matter decomposition and so CEC is increased by composting.

Modern potting mixes have relatively little anion exchange capacity. This means that nitrates, phosphates and sulphates are more susceptible to leaching than the cations. Anions will carry cations (usually calcium) with them in the drainage water and this accelerates acidification.

Buffering of nutrients can be improved by using slow release and controlled release solid fertilizers, by regular top dressing with soluble solid fertilizer, by liquid feeding and by increasing the CEC.

Key points

The pH of a potting mix should be less than 7 and greater than 5. Some plants can grow well outside of this range but most will not because the pH either reduces the availability of an element (iron) or increases it to the point where it becomes toxic (manganese).

Lime or dolomite should be added to a potting mix to counter acidification. Coarser grades are less susceptible to washing out of a potting mix.

The CEC of a potting mix should ideally be >100meq/L.

CEC can be increased by composting the organic components (pine bark, sawdust) and by adding products such as zeolite, diatomaceous earth or clay (vermiculite). Relatively large amounts of these additives are usually required to produce a significant increase in CEC and the cost may not justify the benefit.

Controlled or slow release fertilizers should be used to minimise leaching of the anions.

The EC of a potting mix should be <3dS/m (1:1.5v/v). Higher than this, and plant growth may be inhibited by osmotic stress particularly when the mix dries out between irrigation events.

Organic properties

Recent research indicates that plants can absorb organic forms of nitrogen such as amino acids, peptides and urea. However, this organic contribution is small relative to the inorganic forms of nutrients.

It now seems that plants may also:

Obtain some nitrogen by engulfing and consuming living and dead bacteria.Release enzymes from roots into the rhizosphere to break down proteins into smaller molecules that are then absorbed to supply nitrogen. Some native plants excrete organic acids to dissolve soil minerals.Absorb proteins into roots where they are degraded by enzymes to release nitrogen.

The organic properties of a potting mix can influence plant nutrition by reducing or by increasing the availability of nutrients.

Nitrogen drawdown is the process where microorganisms out compete plants for nitrogen dissolved in the soil solution. Drawdown is most severe in mixes that have not been adequately composted. Other nutrients (phosphorus and trace elements) can also be competitively removed this way by microorganisms.

Soil biology can also improve the availability of nutrients such as phosphorus and the trace elements. One way is by excreting organic molecules that act like chelating agents.

The physical properties of a potting mix will change with time due to the decomposition of the organic fraction which will reduce the particle size. This will in turn reduce the average pore size and so impact the air-filled porosity (lowered) and the water holding capacity (increased).

Potting mixes should be designed to perform well over the life of the crop which means that biologically stable materials should be used. Bark and sawdust should be composted as this improves a potting mixes stability and potentially provides a range of biological benefits to a crop such as disease suppression and stress tolerance.

Key points

Organic fertilisers need to be broken down by microorganisms to provide soluble nutrient forms that are available to plants.

All potting mix ingredients should be adequately composted to minimise nutrient drawdown, improve physical stability and to enhance the numbers of beneficial organisms.

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:

Photosynthesis

Energy (light) + Carbon dioxide + Water ® Carbohydrate + Oxygen

Respiration (aerobic)

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.

Image: Low oxygen (right hand pot) increases risk of ammonium and nitrite toxicity and gaseous losses nitrogen and nitrous oxide.

- Ammonium toxicity. Ammonium can accumulate when oxygen is low especially when the mix is wet and cold and ammonium and urea based fertilizers are used. Ammonium toxicity will kill the fine roots of plants and increases the risk of disease losses.

Image: Plate 4 Deeper roots can be inhibited by ammonium toxicity. Left fed with nitrate fertiliser and right only urea.

Manganese toxicity. Manganese can become more plant available under anaerobic conditions. Toxicity symptoms include iron chlorosis of new leaves and marginal yellowing and brown spots on older leaves.

Image: Manganese toxicity can be caused by anoxia.

- Wilting. Water logged plants will often wilt when the sun is shining brightly even though the mix is very wet.

- Distortion of new growth (epinasty). The problem is caused by Ethylene produced in the mix under anaerobic conditions can cause hormone like symptoms - twisting of shoots, stretching and abnormal growth of leaves.

- Death of root tips. This stops the roots from elongating and causes secondary roots to proliferate for a brief time. Injured roots have brown growing points.

- Root disease. Roots damaged by low oxygen are more susceptible to the full range of soil pathogens.

Bottom line

Ensure the AFP of a mix is as high as can be managed. Consider what has been recommended as a minimum for the nursery application and the limitations of irrigation management in the nursery.

Only use potting mixes made from organic materials that have been adequately composted and are biologically stable. Be wary of mixes containing large amounts of sawdust, rice hulls, raw bark and wood chips

Wilting, pale green foliage and yellowing of older leaves and iron-like chlorosis of new leaves may be signs of oxygen starvation. Check this possibility before supplying additional water or fertilizer.

Only use mixes that comply with the Australian Standard for Potting Mixes AS 3743-2003.

Australian Growing Solutions are Australia's leading manufacturer of professional grade growing media. They produce material to the highest standard and can advise you on the right mix for your crop.

For more information, contact AGS on 1800 709 588 or sales@agsolutions.net.au.

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 diseasesIncrease uniformity and reduce throw out percentagesImprove 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

Water has many unusual physio-chemical properties that make life possible.

Why is water important?

Water is essential for:

Photosynthesis – Energy collection and storageRespiration – Energy release from carbohydratesNutrient availability in the soil solution, root uptake and distribution within plantsCoolingGrowth

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)

Bottom line

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.