Growing Media - Part 1

By Joseph Farrugia

Long before mankind walked the Earth, trees, shrubs and all kinds of woody plants covered the land masses of our planet. Trees grew in every conceivable place wherever falling seeds could find enough soil to anchor themselves to the ground and wherever there was enough water to help them survive.

The basic ingredients that these land colonisers needed to flourish were Oxygen, Carbon Dioxide, water, sunlight, nutrients and last but not least soil. Long after mankind appeared, roughly about four thousand years ago, the people of Mesopotamia realized that by providing these basic ingredients, they could grow trees in containers wherever they fancied. This new knowledge led to such wonders as the hanging gardens of Babylon which are thought to have been created for Amytis, the king’s wife who missed the green hilly landscape of her homeland. A new form of science, based on newly acquired knowledge on seeds, tree habits and fruit requirements evolved around these “artificial” habitats.

In this article, we are going to discuss the latter requirement of wooded plants, namely soil.

Soil (or Mineral Soil)

Soil is the product of millions of years of erosion of all types of rocks by the continuous actions of wind, water and temperature variations. Water pours down as rain and runs from higher to lower ground levels, erodes the soft exposed rocks and forming sand and sludge in the process. In dry weather conditions, this sand becomes airborne and with the help of wind, grinds yet more rocks into finer and finer particles. The continuous fluctuation of air temperature between daytime and night-time and between the hot and the cold seasons, forces the exposed rock surfaces to contract and expand thus forming crusts which eventually flake off. (This last process is very visible on the sided of our bastions which have been enduring this natural process for hundreds of years.) In low lying areas, this sand accumulates and eventually becomes sandstone or soil. Where this soil is rich in nutrients and where favourable climatic conditions prevail, trees take root and flourish.

Since those early days, man has mimicked these natural conditions and used stone or earthenware containers filled with natural soil to cultivate there arborous initiatives.

The problems associated with soil are various. Soil undergoes a continuous process of breaking down into finer and finer particles. This is especially true for soils used in containers where the roots of trees are faced with a finite supply of soil. Roots relentlessly encircle and penetrate the particles to obtain the much needed water and nutrients for the tree’s survival. In a short period of time, this soil turns to sludge. Sludge has the unfortunate property of being compact, and thus prevents free drainage of water and does not allow oxygen to get to the roots of the tree. Moreover compact dry soil is very hard to rewet and can easily starve the tree of water. Restricted water movement inside the container (or clogging), also prevents the leaching away of root excretion which ultimately leads to poisoning. Soil does not buffer nutrients (which will be dealt with in further articles) and without continuous external input from its carer, the tree will eventually die. To partially alleviate this difficulty, soil can be amended with additives such as sharp sand, gravel and any other material which circumvents this inherent deficiency. But the basic problem remains the same.

For use with seasonal green plans or as a short term solution, mineral soil is an accepted alternative. But for longer term arboral care, we have to look elsewhere for a more appropriate medium. This is what we call a Soilless Substrate.

Soilless Substrates

As the name implies, these are substrates which do not consist of mineral soil.

Let us now take a look and the terms used. People talk about soil, aggregate, compost, peat, growing medium, substrate and potting mixes as though they were one and the same thing. One should always use the correct terminology to describe the material in question.

The new approach towards potted material in Soilless substrates is to have the basic material plus additives. Some basic materials used are:
• Sphagnum Peat.
• Compost
• Coir (or coco-peat)
• Rockwool
• Expanded clay (Hydro-pallets)
• Akadama

The more common additives for these basic ingredients are:
• Sharp sand
• Gravel
• Composted bark
• Perlite
• Vermiculite
• Dolomite Lime
• Wetting agent
• Clay or Argilla
• Pumice

These two lists are by no means exhaustive, but are certainly the most common. Each of the basic ingredients and additives is a means to an end, and the end product, or the wellbeing of our tree is what ultimately matters. In later articles we will be taking a closer look at some of these products and how we can make the most of their properties.

Click here to continue reading Part 2