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SOIL   -   WHERETO OUR ECOLOGICAL FOUNDATION  ?

 

By Dave Rushworth

If we ever find new land without the greedy stamp of humanity on it, the land usage allocation should be kept out of the hands of Politicians and left to the Soil Scientists.  They would allocate areas and define usage of areas according to Soil Types.  In many instances we have sprawling townships on rich 'agricultural' soils and farmers trying to grow crops on poor 'skeletal' soils. We irrigate sand and we try and build on unstable mud.

The study of Rocks and Soils is an involved but extremely interesting subject. There is not space here to go too deeply into discussion. Suffice it to say that it takes hundreds of years to form only a few centimetres of soil from rock, under natural conditions.  In the central lowveld / escarpment area our main Rock Types are - Granite, Basalt and Dolerite. The Granite produces light coloured, sandy soil. The Basalt a dark grey / brown clay soil and the Dolerite extrusions a red / brown clay soil.

Soil is a combination of rock particles and organic (plant and animal) matter.  The more organic matter the 'richer' the soil.  The 'parent' rock defines the mineral content and the pH (acid/alkaline) features while the organic matter defines the food 'energy' content and acidity of the soil. Particle size will define the porosity and therefore the 'leaching' qualities of the soil - that is the movement of minerals and fine particles by water within the soil.

Sand soils will be porous enabling minerals to be leached out of them while clay soils will be much less porous enabling them to retain their minerals. Water will drain through sandy soils resulting in a deeper water table, more suited to deep rooted plants such as trees, while clay soils will retain moisture resulting in a higher water table more suited to shallow rooted plants such as grasses.  Animal species adapted to these plant types will result.

 

In each handful of good soil there will be (as a general example) about eight million working bacteria.  As they 'work' they release gasses.  Therefore good soil 'smells good'. There is no smell to dry sand.  That wonderful smell you get after the first rains are the gasses released by soil fungus and bacteria.  These soil bacteria require, like you, moisture and oxygen for their survival.  If either is missing the bacteria lie dormant or die.   Excess soil heat, as in a 'hot' fire, sterilizes the soil by killing all the soil bacteria and it takes ages to replace them so that the soils can 'work' again.

Soil is nature's 'car-breaking factory' where all the various 'Life Forms' are taken apart and reduced to reusable minerals and 'energy particles' by 'macro-organisms' (insect life) and 'micro-organisms' (bacteria / fungi) all of which need oxygen and moisture.

Hot, dry soil - no moisture.  Compacted or 'capped' soils - no oxygen.  No working macro or micro organisms prevents animal droppings, leaves and other organic matter from being recycled - a blockage in the mineral and energy cycles. This effects plant growth and animal food sources.

I have mentioned, previously, the value of retaining plant cover to cut down soil temperatures and maintain moisture content.  We ignore soils surface cover to our peril.  I emphasise this point because lack of soil cover, apart from high temperatures, will result in accelerated soil erosion.  The top layer (or horizon) of soil contains the 'rich' organic matter which, if removed by wind or water, will leave a relatively lifeless lower horizon, on which plants will struggle or fail to grow.  Effective surface cover and viable soil is our lifeline to water catchment and plant production.  Despite this knowledge there is much careless land management.

The following is an example of what is happening to our area.  A few years ago, I collected a bottle of water from the flooded and muddy waters of the Oliphants River at the Mica bridge, blow the junction of the Blyde River. Flying over the junction one could easily see that the Blyde, although muddy, was much cleaner than the Oliphants.  So the sample of water taken was 'cleaner' than one that might have been taken above the junction.

The well shaken water was decanted into a one litre bottle and left to settle over a few weeks. The water was siphoned off and the 'silt' left to dry out.  When it was completely dry, the silt from One Litre of river water was weighed at .6 of a gram.  It didn't appear excessive until I started calculating the silt load of the flooded river.  You might like to check my calculations but I think you will get the picture.

Point six of a gram to One Litre equals 600 grams per cubic metre (cu.m.).  Figures for the Oliphants River were not available but the figure for the water going over the Blyde Dam at peak flood level was around 2000 cu.mecs. - that is 2000 cubic meters per second.  That means SIX TONS of soil per 1000 CUBIC METRES of water - or 12 tons of soil per 2000 cubic metres - PER SECOND.   SEVEN HUNDRED AND TWENTY TONS OF SOIL PER MINUTE.  -   FORTY THREE THOUSAND TWO HUNDRED TONS PER HOUR - AND A STAGGERING ONE MILLION THIRTY SIX THOUSAND AND EIGHT HUNDRED TONS OF SOIL PER DAY.

That is the peak flood of the Blyde River.  Imagine what the peak floods of the Oliphants River and the Blyde together carry away. It must be something like TWO MILLION TONS OF SOIL PER DAY (in peak flood ).  As water slows down in flat patches so it drops the sand load in the river bed.  When it reaches a dam wall it has time to settle and drops the finer material.  No wonder the Oliphants Barrage has silted up almost completely, even with sluice gates to assist.  Much of this soil is from Sekukhune area, judging from silt inspections further up river.  I am afraid that most of the claimed lands are already in Mozambique.