Here I will give all the basic information on soil and how to improve your soil for gardening.
Soil Formation and Soil Horizons
Soil is formed by the erosion (breakdown by water and wind) of rocks and bedrock. The deposition of organic material increases the disintegration of the rock and allows an environment for microorganisms and plants to colonise the soil. This process continues until the soil can be distinguished into several layers (soil horizons) with different physical and chemical properties.
Keep in mind that the combination of horizons and their relative thickness varies between regions (even around the house). Each horizon is divided into more layers, where different processes contribute to the overall profile of the soil.
The topmost layer contains all the organic material (O-horizon) or humus, which is a dark brown complex organic component derived from the decomposition of plant and animal remains.
Humus improves the soil by:
- Adding nutrients
- Retaining water
- Improving mineral reservoirs
- Reducing leaching
The Topsoil or A-horizon contains a mineral and humus mix. These nutrients are able to move freely along with water and air within the soil structure. The A-horizon is darker in colour at the top of the layer, becoming increasingly paler as the bottom layers leach nutrients to horizon B. This is the main area of plant root growth and is the most important to maintain.
Nutrients, including clay, humus, minerals and water leach from horizon A to B (Subsoil). Leaching is a natural process whereby dissolved nutrients in the water moves into lower layers of the soil. Resulting in a greatly varying profile of horizon B and larger plants’ roots can reach into this layer. Leaching is good as it builds nutrient and water reservoirs in the soil for larger plants to access (for instance during drought when the top layers remain dry), but leaching can become a problem, diverting all nutrients away from plants during erosion.
The two lowest horizons (C and D) form part of the ‘parent material’ (calcium carbonate CaCO3 and magnesium carbonate MgCO3) of the soil. It is progressively breaking down into the top horizons of soil, due to the micro-organism, animal (worms, nematodes etc.) and plant root activity in the O-B horizons of the soil. Most of the mineral content (inorganic portion) of the soil is derived from these two horizons (C and D). There is no organic matter or plant roots within this area.
Interesting soil profiles
Some soils do not contain a B-horizon as the activity of earthworms and other decomposing organisms prevents the B horizon’s formation. Thus the B-horizon becomes and extended layer of the A-horizon, known as Chernozem soil. This larger A-horizon is ideal for gardening and agriculture as this type of soil profile supports the major grain growing belts around the world.
– Latosol –
Another interesting soil system is that of tropical rain forests. Latosol soils have thick O, A and B-horizons due to the large amount of organic matter deposited into the soil from the forest leaf litter. The soil is maintained by the forest roots and their removal results in the breakdown of this system. Thus these soils are not suitable for agriculture.
– Podzol –
Boreal forests (colder climates) have a Podzol soil profile, where the A-horizon is bleached and layers of organic, iron and aluminium forms at the bottom of the A-horizon. The soil is not suitable for agriculture due to the cold climates, but is good for forestry and wood production.
– Savannah –
Savannah grasslands have ferruginous (iron containing) soils. There is a thin organic layer followed by a hard cemented layer of laterite in the top part of the A-horizon. The laterite is formed by the leaching and capillary action of minerals between seasons. This is the type of soil I have, and it is ideal for grass cultivation for grazing, but not ideal for gardening. Later in the article I will provide the details on how I am slowly converting this red soil into a more vegetable friendly soil.
The soil is characterised by its soil structure, fertility, water retention and biodiversity. Understanding the soil properties allows one to use and manage the soil to increase crop yield and minimise soil damage.
The ideal soil is quite obvious in terms of its properties:
- Retains water and does not dry out quickly
- Contains a large amount of nutrients
- Large air pockets for plant roots and soil organisms
- Soil community (soil animals)
To identify this ideal soil, is to look for soil with the consistency and appearance of ground filter coffee you put in you filter coffee machine each morning . This means as gardeners, we need to work towards a Chernozem soil profile in our vegetable beds, which is the ideal soil type for growing vegetables and fruits.
My Sad Soil Story…
Since I started to garden, I really did not care about the type of soil (sand, clay or loam) or pH (acid, neutral or alkaline) of the soil that I had, because I knew that slowly it would change and that I can provide the ideal environment for the plants that required it when needed. I started with typical red savannah soil. It had a thin layer of sand of 5 cm and then had a thick layer of red clay that would compact when it got wet. After digging in 30 cm into the soil I thought that I had hit bedrock, but I was the laterite layer, typical of Savannah soils. I hacked my arms off at the rock hard soil, feeling as if I were a miner instead of a gardener, and would hack some more to absolutely no avail.
Then we got compost to just kick-start the process. The problem was that the commercially produced compost contained high amounts of manure that was not properly decomposed. Only after planting I realised this as some plants got chemical burn (browning and die-back of large parts of the plant). The lettuces were first to signal this, starting to die within 2-3 hours. We dug the compost in well with the red soil in a 40:60 (compost: soil), which prevented further chemical burn.
I did not want to spent lots of money on improving my soil, so we started the wormery and added the liquid from the wormery to the garden at 1-2 monthly intervals (or how much was produced in that time…). The strategy that improved the soil the most was adding kitchen waste. Any spare kitchen waste (anything except meat, sauces, dairy) that the earthworms could not handle, I dug a hole in the ground, plopped about 1-2 L of kitchen waste into the hole (collected from the kitchen in a plastic container until full, 3-5 days’ worth – so it won’t start to rot) and then cover it up. Depending on the season, the waste would decompose by virtue of the decomposing organisms in the garden (3 months in winter and 2-4 weeks in the summer). This draws earthworms and micro-organisms (such as fungi, bacteria) to your garden creating a soil community to assist in the waste decomposition and rehabilitates the soil, creating air pockets and increasing water retention. The soil community will help to breakdown the laterite layer and provide more planting/rooting space for the vegetables.
The best soil status ‘indicators’ are your root vegetables, such as beet and carrots. Mine had a lot of roots (hairy appearance) and almost no carrot or beet set. This indicated the huge lack of potassium in my garden. So we searched for cheap yet ‘bio-available’ potassium to add to the soil. After some research we came upon good ol’ wood ash. We burned a whole lot of acacia branches (so basically untreated, raw wood) that were dried and not in use and had a braai (barbecue) at the same time. Then I had 7 L of wood ash and another bucket full of burned wood pieces. I added this each time I planted new vegies, and now I am getting lovely big and sweet carrots and beets (no more hairs). – Sprinkling the ash in top of the soil before watering and burying the bigger pieces around the veg roots.
It took a year of working in kitchen waste, adding wormery-liquid fertiliser and wood ash to the soil to get healthy and happily producing veggies. So that is basically my strategy for soil maintenance and it is easy and inexpensive way to improve what you have instead of buying large amounts of compost (that would essentially be cheaper to buy the veggies from the market than plant in the expensive soil) and working my way to establishing a Chernozem type soil in my garden.
We started some home composting, basically dug a big hole into the soil and add your garden/kitchen waste. We don’t follow any specific strategy such as ‘hot or cold’ composting, it gets turned when we have time and the chickens scratch through it every day. Adding about a year’s worth of compost to your soil (about 3-5 wheelbarrows) gives it a good kick. Matured home made compost is super high in nutrients and helps a tremendous amount with the overall soil structure- the chickens keep it clean from critters, but unfortunately due to it not going through ‘hot composting’ we do get weeds.
Green manures are very good to incorporate into your soil rehabilitation program as well (especially during the winter months here in SA when most of the garden remains unused). Green manures grow and break up the hard soil structure and add nutrients to the soil while they grow. Afterwards the plants themselves are added to the soil to provide additional nutrients and organic matter.
I will be posting details on each method of soil rehabilitation over the next coming months. Good soil is essential for plant health, resistance and production, these posts will include, Composting (including Instant Compost), Vermicomposting, Green Manures and Comfrey growing for adding nutrients to your soil.