Developing the Nutrition Profile of Your Potting Mix

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.