The Zimbabwean Gardener Issue 12 Autumn 2015 High res | Page 28

Soils

Gardening with our soils

By Andy Roberts

Understanding how soil works is an
important part in gardening, as you will be
able to improve your plants growth.
What makes a good soil?

While different plants have different requirements, they all
need the following elements in soils:
1. GOOD AERATION - With the exception of rice and
mangroves, all plants like a well aerated soil. This is
necessary for good root growth and soil organism
health.
2. GOOD STRUCTURE - The way the soil particles clump
together is called structure. Clumps leave spaces
(pores), which gives good aeration and drainage.
Excessive cultivation destroys this and breaks down
organic matter.
3. Good water holding capacitY - Plants can only
take up nutrients in solution in a very simple form
called ions.
4. A high organic matter content - Essential for a
healthy soil; your soil is alive!
5. And a slightly acid nature
What soils do we have in Zimbabwe?

We have all types of soils in Zimbabwe, from the heavy
black ‘cotton’ clays of the lowveld, to the red clays of the
Mazowe Valley, and the sands of Hwange. Most of the
country is not blessed with the fertile clays, which make
gardening so much easier. Certain horticultural crops are

grown more easily on sandy soils; carrots and onions are
both easily over-fertilised on heavier clay soils.
What nutrients do plants need?

Plants need a number of nutrients to grow well; all
are naturally present in Zimbabwean soils in different
ratios and as gardeners, we need to know which ones to
supplement. A number are required in relatively large
quantities, and are known as macronutrients. These are:
nitrogen (N), potassium (K), phosphorus (P), calcium (Ca),
sulphur (S) and magnesium (Mg). The micronutrients are:
zinc (Zn), boron (B), manganese (Mn), molybdenum (Mo),
iron (Fe) and copper (Cu). These are necessary in very low
amounts, so are also known as trace elements and, with the
exception of boron, which is often deficient in Zimbabwean
soils, these are not usually found in the compound
fertilisers we buy. The locally available compound fertilisers
contain varying ratios of macronutrients. Micronutrients are
available in the various foliar fertilisers.
What is soil fertility?

A soil’s fertility is its ability to supply required nutrients to a
plant over an extended period of time. It is directly related
to the amount and type of clay in the soil. Clay particles
are very small (diameter of 0.002 – 0.0002mm), so clay soils
have a huge surface area for holding onto the nutrients.
Clay particles are negatively charged, so attract the positive
ions that constitute most of the nutrients the plant requires.
Known as the cation exchange capacity (CEC), this is the
measure of the soil’s fertility; therefore, the more clay
particles in the soil, the more nutrients that are available.
Sandy soils are less fertile and will require more, but
smaller, applications of fertiliser than the heavier clays in
order to produce good results.
What physical properties are important?

High clay soils drain less well than those with a high sand
content. This is rarely a problem in Zimbabwe, as getting
enough water to the garden is usually a much bigger issue.
High clay soils are easily recognized by their greasy feel
when wet; test this by taking a small sample of soil, wet
it (spit will do), and rub it between forefinger and thumb.
Adding organic matter to heavy soils will assist drainage,
and to sandy soils will, for a while, help retain moisture.
Once fully broken down, this effect will be reduced.
What is the role of organic matter in soils?

Organic matter can be broadly classified as stable
or active. Stable organ X