THE PHYSICAL ASPECTS OF A POTTING MIX FOR CAMELLIAS

by Michael B. Thomas
Plant Science Dept. Lincoln University

Introduction

Last issue I wrote about the supply of nutrients and fertilisers for a potting mix for camellias. Some recipes were given in this article but it is helpful to know why you use different media and how they can be used to provide different properties to the mix. Other considerations can also be taken into account such as cost and availability. So choosing the right materials for a mix can be very important and hopefully this article will help provide camellia growers with some principles to avoid those sick looking, container plants.

Drainage from Containers

The one KEY principle above all else is to know that plants in pots need extra drainage (aeration in the mix) compared to plants in the open ground. As soon as you put a plant in a container there is a danger of waterlogged roots. If you take 'an average garden soil' and put it into a pot, it will immediately be more prone to being excessively wet and lacking aeration. The reason for this reduced drainage (or aeration) is that the base of the container cuts across the profile of the mix just like a clay pan in some soils; water saturates from the base upwards. In consequence of this blocked drainage the plant suffers root death, and if it survives that, then it has to try and live in the top layers of the pot where there is only a limited volume of material supplying nutrients and surface drying is very likely to cause further problems to the roots. So never use garden soil alone for a mix. Some inquiring minds may be asking why the drainage holes do not allow the water to move downward as via the drainage channels of a soil profile. The answer is that air blocks the movement of water under low pressure. A good example of this are airlocks that can occur in hot water pipes. No hot water comes out the tap because a bubble of air can prevent water movement.

The other main principle to be aware of is that particle sizes in the mix influence pore sizes and it is the size of these air spaces or pores in the mix that control how much water or air will be held. Small pores tend to hold water only and large pores primarily hold air. Clay has tiny pores so it holds a very high amount of water whereas large stones hold very little. A mix will consist of solids and pores only, so all we need to do is keep the solids to a minimum and provide a good balance of small and large pores. The small pores will provide water holding and the large pores will provide the necessary aeration i.e. the drainage that is so vital for a container plant. So the essential feature of good potting media is to have more aeration than if using straight soil and this is done by using light open materials like peat and bark which provide a very high pore space. With these type of materials you can pour on as much water as you wish but they remain sufficiently open that even when saturated a good mix can contain 20% air [by vol.] or more while a garden soil under the same regime may only contain 5% air.

Stability of the Ingredients

Materials that will decompose in the potting mix can give rise to two problems:

Decomposing organic matter means that available nitrogen is used up by micro-organisms and the plant can suffer from N deficiency.
Materials that are breaking down have particles that are getting smaller so that drainage and aeration decline at the same time.

Fresh raw sawdust is one of the worst materials to use because of these two factors. However fresh sawdust can be used for short term mixes and media for cuttings it can provide a cheap open medium. Leaf mould is another material that may be cheap and freely available but its breakdown within the mix is a distinct disadvantage depending on proportions used and type of material. Mixes with significant media breakdown will be hard to manage because feeding is more complicated; you need to spend more on fertilisers and watering may be difficult to measure as drainage declines.

Selecting the Media

Virtually all commercial potting mixes are based on peat and/or bark, and usually sand is added. Peat and bark are excellent stable materials and are freely available. They contain very high pore space and good water holding capacity, if particle sizes are not too large. Sand needs to be added to provide some weight and also increase the water holding capacity especially if it is fine sand. Yes, sand can decrease the 'drainage'! This is because the heavy sand particles compress the mix and the small particles fill in between the large peat or bark granules to make small pores, which hold more water and less air than larger ones. A good rule of thumb would be to make-up a mix with a high proportion of peat or bark and then add up to 50% fine sand. The bark should be composted pine bark with most of the particles with a diameter of between two and five mm. Fresh fine grade pine bark can readily be composted by making a low flat stock pile of bark, keeping it moist, adding 2.5 g/1 of Nitrolime, and turning the mix one or two times over a period of two months.

Comments on Some Recommended Mixes

80% Composted Pine Bark + 20% Fine Sand: This mix has high aeration and good water holding capacity. Bark mixes are often very suitable for plants that need very good drainage like rhododendrons. Fine sand is important to increase water retention.
60% Peat + 40% Coarse Sand: The availability of good quality peat supplies in New Zealand allow mixes to be made up with excellent properties. Peat tends to hold more water than bark and so coarse sand needs to be used. The water holding capacity can be increased further by adding up to 50% coarse sand which adds weight and compresses the mix so that it holds more water. This could be suitable for tub plants.
60% Peat + 40% High Quality Soil: This mix is very suitable for home garden mixes especially if the soil has been sterilised and is high quality topsoil with good structure. The soil in this mix compared to the sand in the previous one, provides improved water retention and nutrient supply. This mix is especially suitable for tub plants where watering and feeding needs can be high. Soil is fundamentally a composition of sand, silt and clay plus organic matter, and therefore it provides an excellent range of pore spaces as well as a source of nutrients. However unsterilised soil will contain weed seeds and so if it is used in the mix it will be desirable to use equal parts peat and sand for the top layers to minimise weed problems.

These mixes can be stock piled (as related in the last Bulletin) and then the fertilisers added. Mixes with high water retention are probably the most appropriate for camellias in containers for the home, while topdressing with slow release fertilisers and repotting needs to be done early in the spring before growth begins. Container-growing is more demanding than open-ground growing, but a flowering camellia in a wooden tub or attractive pot can be a great reward for all that effort!

Illustrations:
THE VALUE OF A GOOD POTTING MIX

In each photo, the camellia on the LEFT has been grown in a soil mix with poor aeration and drainage; it shows unhealthy shoot and root growth along with iron deficiency. Root volume is meagre and plant has a straggly appearance. In each photo, the camellia on the RIGHT has been grown in an open mix; the roots have grown right down to container base and are healthy and extensive. Foliage on this plant is stocky, bright green and vigorous.
Photos: M. B. Thomas.

Illustrations will be given later since my scanner is defect. K.P.

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Issue No. 124 - JUL 1996 Volume XIX No. 5.