Archive Article: August, 2009
Soil. Who doesn’t love it? It’s the basic element of mother earth, the holy grail of every organic garden and even if you are not an organic cultivator you must admit, we need it every day.
Soil, once defined by J.S. Joffe in 1949, is a natural body consisting of layers (horizons) of mineral constituents of variable thicknesses. Each layer differs from the parent materials in their morphological, physical, chemical and mineralogical characteristics. So, what type of good, rich organic soil does one need?”
To answer this question we should first understand how a plant works. Let’s start with the parts of the plant: leaves, flowers, stems and roots. Leaves are the plant’s kitchens; food making goes on in the leaves, during the on-going process of photosynthesis (where the energy from sun or artificial lighting is transformed into energy the plant can use).
The flowers are the plant’s reproductive organs, helping the plant generate baby plants (or blossoms). The stems are the support structures of the plant. Stems hold up the plant, but also conduct food and water throughout.
Materials like perlite and shredded bark are chunky enough to create air space, letting the roots have oxygen, even when the soil is wet.
Last but not least, the roots are an integral part of the plant’s body. Root holds the plant in the ground. As well, they provide a food storage area and nutrient absorption for the plant.
Now let’s add a little bit of environment: plants are using sun for energy during photosynthesis; they are using special cells filled with green pigment called chlorophyll to convert dioxide and water into a sugar called glucose; and they are using the glucose (a form of carbohydrate) for energy.
So knowing what different types of plants need is to know what type of rich organic soil you should use. So now you want to understand soil, correct?
Well, I can’t offer you a university degree in soil science but I can explain a few soil factors in basic terms.
Bulk density
The weight of a given volume of soil, which includes the pore spaces. It can be easily measured by gently pressing a small cylinder into the soil, removing the core and weighing after drying to remove the water contained in the pore space. An average value would be 1.3 g per cm3.
Water-holding capacity
The water-holding capacity of soil is a very important agronomic characteristic. Soils that hold generous amounts of water are less subject to leaching losses of nutrients or soil applied pesticides.
Absorbency
The ability of soil to absorb and hold water and other liquids. Some materials such a peat are quite water repellent and should be moistened before planting and allowed to dry out completely.
Friability
This has to do with the texture of the soil. It must be easy for the roots to move through the soil. One way to measure this is by sticking your finger into the soil. If this can be done easily, the roots can grow through the soil.
Air filled pore space
10 to 15 per cent of your soil mix should be air. These pore spaces are the ones through which water drains and air enters the root zone. Materials like perlite and shredded bark are chunky enough to create air space, letting the roots have oxygen, even when the soil is wet.
That gives us an insight into the composition of soil. However, to take some soil from the earth and put it in a pot indoors or in a greenhouse will not work as the contents are just a small part of the greater whole. Nutrient can be made available to plant roots in natural soil from many meters away for instance. Confined in a pot this is not possible. Therefore, soil mixtures or composts were developed to contain the same structure and to emulate the natural environment as closely as possible. If you've shopped around for soil mixes, you've probably noticed several ingredients that are listed in their contents. We summoned up the most common ingredients:
Sphagnum/Peat
Peat is organic soil that consists of rotted plants, organic minerals, water and air. It differs from muck (yes, muck is a technical term used by soil scientists) because in muck the organic material is decomposed beyond recognition. Sphagnum comes from bogs around the world nowadays and is often mined from environmentally renewable sources, and it's usually over 90 per cent organic. It is also extremely aerated. It's a bit lightweight, so usually bark or sand is added to avoid the "topple factor," the technical term for when your plant tips over. When it's wet, sphagnum absorbs water, but when it dries out, it can actually repel water, which is the kiss of death. Peat must always be mixed with wetting agents, and is usually mixed with some of the items listed below.
Composted Bark
Put bark in a big pile and allow it to rot for four months. When it reaches 150°F in the middle on the pile, it's done and you've got yourself some composted bark. Composted bark is actually much heavier than peat. If composted correctly, bark can resist disease. On the other hand, new bark and fine particles can cause problems. It helps to have properly sized chunks of bark, which will help to aid in porosity.
Vermiculite
Vermiculite is a silica rock, kind of like mica, that expands into little hollow kernels when heated quickly at a very high temperature. It's usually added to potting mixes because it's light, it absorbs several times its weight in water and it aids porosity.
Perlite
Perlite is a volcanic ore that, when heated, pops open like popcorn. Perlite doesn't absorb water (it holds it on its surface) so it hurts your mix's total water retention. What it does do, however, is make more air-filled pores, so you can be sure roots are getting enough oxygen.
Coir
Coir is a waste product of the coconut processing industry. It is a shredded form of the outside of the coconut shell. It's often used as a substitute for peat in potting mixes for plants that need moist soil, because of its superior water holding ability. Coir is usually sold in a compressed brick form that expands with water. One drawback - it can be high in salts because of the processing methods used to make it. So it should be soaked in water and washed before being added to the mix.
Knowing these things will ultimately make you a more educated consumer when deciding what soil mix is right for you and more important your plants. After all, “knowledge is power.” Also remember your soil mixtures, with care, can be infinitely reused. By removing the top surface of used soil and removing the roots, fresh ingredients can be added to replenish the soil health and away you go again.
Isn’t soil great! Till’ next time.
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