Gardening with our soils in Zimbabwe

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 organic matter is of ancient origin, comprised of complex organic compounds, and is resistant to microbial action. It contributes to the soil fertility (CEC) and its ability to hold water. Of more concern to us gardeners is the active organic matter, which is derived from recently deposited organic matter (within 2 years):
1. It is susceptible to microbial action.
2. It is an energy supply for micro-organisms.
3. It contributes nitrogen, phosphorus and sulphur.
4. It is useful for improving drainage and aeration of heavy clays soils and reducing the capping behaviour of some soils.
5. It is vital for the overall health of the soil ecosystem!
6. It can improve soil structure.

How do we modify soils in Zimbabwe?
All soils in Zimbabwe will benefit from having organic matter added. This can take the form of compost/manure, mulch or a green manure crop. Green manure is simply a crop such as maize, lucerne or sun hemp that is cut before it flowers and then chopped into the soil. In winter, wheat or oats can be used. If using maize, sow at approximately double the normal density and cut before it tassels.

What chemical properties are important?
The acidity of a soil, measured by its pH, is important for the availability of certain elements to the plant. Extended use of chemical fertilisers will make the soil acidic, and can cause certain elements to become toxic and others deficient. It’s a good idea to lime the garden once every two to three years if living where the water supply is neutral to acid. This may not be the case where the water is alkaline, e.g. from the Chinhoyi aquifer which can cause iron deficiencies. If in doubt, get your soil analysed.

By Andy Roberts

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