The major consideration is the growing medium - what type - how to manage. Over time the medium will shrink as the organic matter decays, especially if sawdust is in the mix). An important factor is the pH of the mix and how to maintain a desired level according to the plant's requirements. Most potting mediums commonly available are composted pine bark and sand. You can amend these with components such as clay balls, Perlite, Vermiculite, etc. Mixes conforming to the Australian Standard, such as Yates-Hortico, Nu-erth, Peates, etc have trace elements incorporated - about two years supply. The REGULAR mix has a pH from the range of 5.3 - 6.5 and no Nitrogen, which must be added at potting. The PREMIUM mix alleviates the necessity to add fertiliser for one month. Some mixes, and particularly one widely-distributed brand, are hungry for nutrition, robbing nitrogen and resulting in yellow plants. Pine Bark takes quite a while to break down. It is stable for many years, until the tree fills the container with roots from whence the mix will be sand and decomposing old roots, maintaining quite a good balance for the plant.
At the Bi-Centenary Conservatory we specified a Mt Compass sandy loam with a pH maintained at around 5.9-6.4. pH above 5.6 reduces iron availability dramatically. Many tropical trees originate from deep, well-drained, leached soils and have most of their roots close to, or in, the mulch level. The leaching occurs as pure rainwater dissolves alkaline Calcium and Magnesium, leaving the soil decidedly acidic (pH 5-6) as the alkaline components are precipitated to lower topography. In the Adelaide region, the Hills areas are predominately acidic while the plains are mostly alkaline. In the Coorong area, you can see gypsum (Calcium Carbonate) strata exposed. Generally plants that are of lush or tropical origin are acid-loving, while plants from dry, arid regions prefer alkaline soil environments. The pH in your container should be tailored to a particular plant. For some plants, over pH 6, iron availability and also copper and manganese may be too low for the plant. So, how do you maintain a desirable pH level? What influences changes?
WATER. Use of rainwater or good quality dam water will gradually reduce pH by washing or dissolving out the calcium. The use of Adelaide tap water will tend to raise pH. One litre of 'Adelaide Tap' is equal to 100 mg of limestone because of the calcium and magnesium, which can be good for some plants if balanced with a good fertiliser. The influence of water on pH is relatively minor compared to that of fertilisers.
FERTILISER. Most nitrogen fertilisers are in the form of ammonium rather than nitrate, including urea which is quickly converted in the soil, making the soil more acid. Greenkeepers use ammonium sulphate which is the most acidifying and which also kills the worms. Counteracting this acidifying effect, as mentioned previously, is the regular addition of Adelaide Tapwater, so monitoring the pH over time is necessary.
MEASURING pH. The CSIRO origin kit becomes very expensive and it would be worth checking out the 'Manitek' pH Kit at Target stores which is a fraction of the price. Electronic meters such as the 'pHep' (pH 'Electric Paper') fits in your pocket, and if looked after, can give reliable readings for many years.
Mix one part volume of moist mix to one and half parts demineralised water. Let sit for 5-10 minutes and insert electronic probe. Take care, as a sand scratch on the probe can ruin your $80+ investment.
pH TOO LOW. Sprinkle ground dolomite on top of pot or 1/1 mix of hydrated lime and dolomite. The amount depends on the volume of the container. To raise pH 4.9 to pH 5.9 try 2-3 grams per litre of pot.
pH TOO HIGH. Add no more than 2 grams iron sulphate/litre of water (more could kill). To acidify from existing pH 5.5-6 (no more than 6) use ammonium fertiliser or dusting sulphur. No need to dilute. Sprinkle on ½ gram/litre of pot. Check pH after about two months. Summer is more rapid, while in winter nothing happens. If pH is up to 7, it is very difficult to remedy without killing the plant. If pH is high, iron uptake will be reduced and adding the common black iron chelates is useless as the iron is destroyed by the high pH. A foliar spray is useful if not too high strength.
SATURATION. The saturated level in a pot is constant. A tall container increases the 'dry' or moist area above the saturated area while a shallow pot will be mostly 'wet'. A shallow pot should have a coarse mix while a taller pot could have finer material which holds the water higher. A 12" pot should have 13-15% air space (measured by immersing the pot and measuring the freely-drained water volume as compared to the pot volume). Most soil physicists say a minimum of 105 Air Field Porosity otherwise viscous suction overcomes gravity. It may be necessary to locate the pot away from frequent rains, especially in times of low temperatures and low evapotranspiration.
SOIL WETTERS. Some organic mixes are water repellent and a good soil wetter can be beneficial used once a year at about 4 ml of concentrate/litre of mix. Be warned that some brands are useless. Aquasol and Wettasoil are among the recommended brands. An Australian Standard potting mix should not be repellent.
DATE: November 1994
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