Potting Mixtures
Over recent years developments in potting mixtures have changed radically in commercial enterprises, mainly in the area of increasing freedom from pests and diseases and providing a medium which is essentially free-draining but able to hold the nutrients necessary for good growth.

Large enterprises allow scale of development which can provide different mixtures for different plant species needs.

U.C. mixtures (peat and sand) do have great versatility, but we have found them generally unsuitable for the crop range; the low buffering capacity of nutrients can lead to problems unless cultural understanding is excellent.

The standard mixture is now by volume:
2 parts coarse river sand
1 part peat (50/50 Tableland and German)
1 part loam top soil (Krasnozem)

We find however that it is desirable to have a higher percentage sand for some species, for example, mango and higher content of organic matter (preferably leaf mould, bagasse, etc.) for others such as Rambutan, Lychee, and Longan. However the standard mixture is reasonably satisfactory for the whole plant range providing container size is regulated. Fertilizers added to the potting mix prior to steam sterilization (pasteurisation at 80°C) per cubic metre of mixture.

Dolomite4 kgZnS04200 g
Superphosphate3.2 kgMgS04600 g
K2S04800 gFeS04200 g
KN03800 gMnS0420 g
CuS0480 gBorax (10%)4 g

The majority of plants handled do not favour a high pH hence a relatively low content of lime/dolomite compared with the J.I. mixture.

Potting and Watering
With a wide range of plant species under care, the critical consideration is for regulation both of potting mixture drainage characteristics and soil volume, giving moisture retention characteristics suitable for the particular species. One should assume that staff carrying out the watering have knowledge of the plant requirements but a 'fail safe' mechanism must be built into the operation to prevent over-watering when staff changes, staff rotation, etc. occur.

The simplest mechanism is soil volume. For example, mango does not tolerate a constant high soil moisture content. From the initial stage of individual seed sowing or breaking up and potting on polyembryonic clusters, the container size is kept small and second-stage repotting occurs only when the plant begins to suffer from transpiration exceeding daily water input.

On the other hand, carambola (Averrhoa carambola) has such a fast growth rate and high transpiration that container size must always be kept relatively large compared with plant size.

The potting operation should be carried out with some knowledge of the particular species requirements.

Plants usually suffer from 50 to 80% root loss during bare-rooting, both from destruction of root hairs and pruning of lateral and tap roots. This together with above-ground plant reduction reduces water demand during re-establishment to minimal levels. Drying out of the medium is thus extremely slow, and is actually a function of temperature and evaporation. Excessive water during re-establishment is extremely detrimental.

Our rule of thumb is to pot into approximately one-third to one-half of the soil volume of the previous container and thereafter only to repot when the plant is extremely vigorous and is starting to show some water stress between waterings.

The shape of containers is also important: those with a high height-to-width ratio drain faster than squatter types.

Automatic or semiautomatic trickle or sub irrigation watering systems are generally too dangerous to use in most operations where there is a mixture of species and plant ages. Unfortunately, the hand-held hose is the only practical solution.

Watering patterns need to fit seasonal fluctuations and when evapotranspiration is less it is better to increase the interval (eg. shift from daily to 2-day schedules) rather than to attempt to maintain less water each day.

Further, one very heavy watering each week or fortnight is useful to assist with flushing out excessive salt accumulation. Watering patterns following decapitation of rootstocks following grafting must also be drastically changed, again usually best attempted by vastly increasing the interval between waterings.

B.J. Watson,
D.P.I. Kamerunga

DATE: July 1983

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