Dynamics of Media Piles

By Rohan Davies, Australian Growing Solutions

How you store and handle your growing media can have a great impact on the performance of your crop. In this article, Rohan Davies of Australian Growing Solutions details the best practice techniques to improve your growing results.

When buying potting mix, there is a lot to consider; start-up nutrient, slow-release fertiliser, controlled release fertiliser, porosity, water holding, nitrogen drawdown levels, just to name a few. Some of these considerations become more relevant when you buy more media than you can use in a short period of time based on your potting schedule.

Here are some factors to consider when ordering a load of potting mix.

Timeliness

Buy enough, but not too much, “enough” generally equates to the volume of media you will use within a few weeks. If you are left with excess, it is best to use this up as quickly as possible.

Time, along with the other considerations can impact on the quality and performance of your media. Ordering larger loads can often reduce costs due to freight efficiencies, so try to condense potting into batches so you get through as much mix as possible in a short period.

Temperature

Always aim to store your bulk pile of media under 1.2 metres in height. This helps to maintain air porosity and reduces opportunities for temperature build ups due to media degradation. High media temperatures will hasten nutrient release and may cause excessive media EC levels. Consider an irrigation flush when first using the media to eliminate this problem.

Rainfall

Always cover your potting mix pile with a tarpaulin or a cover made from light coloured material. The light cover reduces the impact of heat absorbance from the sun compared to a dark cover. Excessive rainfall can cause bulk media to turn anaerobic and leach nutrient because of the potential to rapidly increase the release of nutrient from controlled release fertilisers due to a high osmotic potential.

What is Electrical Conductivity (EC)?

In water, EC is a measure of its ability to conduct electricity. When looking at water in its natural state, i.e., rainwater, it has a low EC. If we bring nutrients or ions into the water, the EC rises, meaning the water is better at conducting electrical current. EC is usually measured in micro-Siemens (µS/cm) and readings generally fall between 30 µS/cm to 2000 µS/cm. Seawater, for example, will have a reading of approximately 50,000 µS/cm, rainwater typically around 60-90 µS/cm and a typical nutrient solution 1000 – 2500 µS/cm.

Having multiple potting media bays allows the nursery to have a number of different media blends to best suit particular plant requirements. Ensure you keep the pile below 1.2 metres in height.

Fertiliser is more evenly distributed throughout the media when blended through a batch or inline blender, like the one AGS used to blend the growing media pictured above.

Factors Affecting Media EC

There are several factors that affect media EC or soluble salts: water quality, starter charge, media components, fertiliser type and rate, watering technique, environment, and age of plant roots.

Most commercial mixes have a starter fertiliser charge added to promote growth during the first three to four weeks following sowing or transplanting. When you take an initial measurement of media EC, you are measuring the starter charge along with the lime or gypsum. Fertilisers are a type of salt, so the fertiliser you choose, how much you feed, and the frequency of feeding all determine the media EC as you are growing the crop. How you water the crop will also influence EC levels. If you are heavily feeding the crop and do not flush on a regular basis, salts will build up in the container, causing damage to roots.

Environment can also affect media EC. Cool temperatures may keep growers from watering to run off often enough which could see the build-up of soluble salts. Low light levels and high humidity can also cause similar problems, in addition causing the plants to stretch and become soft. Finally, the older the root system, the lesser the effect soluble salts will have, lowering the chances of burning roots and causing root rot.

Dibbling vs Incorporated

AGS recommends the use of controlled release fertiliser in many situations and recommends the controlled release fertiliser be incorporated in the growing media rather than dibbling or topdressing. Having the fertiliser added at time of blending, ensures an even distribution of prills throughout the pot. This has several benefits. Firstly, it ensures there is an equal spread of nutrition throughout the rootzone which provides even root development, and secondly, it eliminates toxic concentrations of prills in one spot. These concentrations, or “hot spots”, can cause burning to the root fibres, and in some cases, can limit root development in that part of the pot.

Bulka bags can be a convenient option, but there are downsides to consider.

Having fertiliser on the surface of the mix by topdressing provides the perfect conditions for weeds and liverwort as there is a large amount of nutrition and moisture close to the surface. Topdressing is a great way of providing a plant with a boost prior to sale or to get it through until potting up, but it is not recommended for the initial feed.

Fertiliser is more evenly distributed throughout the media when blended through a batch or inline blender, like the one AGS used to blend the growing media pictured above.

Bulka Bags

Some nurseries opt to purchase their potting mix in bulk bags. This option does help keep the mix dry and protected, but there are some downsides. Bulka bags are made from plastic, and while there is an effort to recycle or repurpose the bags after use, they do eventually enter the waste stream. Bulk bags make it possible to move the mix around without the need for a front-end loader, and most nurseries have a forklift available. However, there are extra costs associated with filling and shipping bulk bags. Ultimately, nurseries will use whatever works best in their business, but it is worth reviewing your processes on a regular basis to assess the efficiency and safety of your potting operations. Weighing up the cost of new equipment versus the savings and potential productivity gains that you could deliver your business.

Australian Growing Solutions and the Nursery and Garden Industry of Victoria will be running some workshops in August exploring these points and more. Keep an eye out for the details.