Healthy soil makes for happy plants
When it comes to a healthy garden, your soil plays a vital role.
WWithout healthy soil, no garden will thrive. However, copious amounts of compost and fertiliser are not always the answer. It is important to know what type of soil your garden has so you know how to treat it and what to plant in it. And it’s a simple process to have your garden’s soil tested professionally. Dr Pieter Raath, a soil scientist at Stellenbosch University, explains...
1 Test your soil
For a mere R250 you can get your soil tested at any agricultural laboratory. This test usually indicates the chemical composition of the soil, the amount and type of nutrients it contains and whether it is brackish (has an excess of salts), acidic or alkaline. Certain tests will indicate whether the soil is sandy, loam or clay. If you’re not keen to have your soil tested professionally, its colour is a good indication of what type it is: the redder the soil, the more clay it is; the darker the soil, the more organic matter and nutrients it contains; and the lighter the soil, the more sandy it is. >>
2 Types of soil Sandy soil
Sandy soil does not retain water well which means it dries out quickly. Not only must it be irrigated more frequently but because the nutrients leach (flush out) more quickly, the soil soon becomes acidic.
One of the advantages of sandy soil is that it doesn’t become excessively waterlogged, so it’s better aerated. Plants in sandy soil get fewer diseases, especially root diseases. Adding large quantities of compost will dramatically improve the water-holding capacity and nutrient content of the soil.
Loam
Loamy soil contains reasonable amounts of clay but enough sand to drain well. It therefore has the advantages of clay and sandy soil without any of the drawbacks. Some loam may still become compacted if it gets too much water. Add organic matter regularly to counteract this problem.
Clay soil
One of the biggest disadvantages of clay soil is that it doesn’t drain well, especially if it’s also brackish. It is also often hard when it dries out and can be very dense, which impairs root development. The advantages of clay soil are that it is naturally fertile, has good water-holding capacity and needs to be fertilised or irrigated less regularly.
Create good drainage by adding gypsum; it will also help to rinse out the salts. Large quantities of compost will further help with drainage and will improve the structure.
3 Acidic or alkaline?
Unfortunately, you cannot tell from the colour of your soil whether it is acidic or brackish. Soil pH (an indication of whether it is acidic or alkaline) usually differs from region to region in South Africa. The soil in the Western Cape and KwaZulu-Natal is, in general, acidic, while the Northern Cape and parts of the interior have alkaline soil. Your local nursery will be able to tell you what the soil pH is in your area. If you want to know the exact acidity or alkalinity of your soil (it is important for certain plants), get the soil’s pH tested by a laboratory.
ACIDIC Soil acidity can cause a number of problems, especially poor root development and poor absorption of nutrients, which then leads to weak growth. Acidic soil can be treated with lime but it’s important to calculate the correct dosage or you may apply too much lime which will create alkalinity problems – this is where professional analysis is useful.
ALKALINE Alkalinity creates problems with the absorption of trace elements and phosphate. Without these, plants cannot grow properly and will look unhealthy. It is difficult to rectify soil with a high pH, but on a small scale you can incorporate pine needles, use ammonium nitrate as a fertiliser or even add vinegar to the water when irrigating. However, this is a slow process that may take years.
Many plants happily cope with high alkalinity and because alkaline soil usually drains well, your plants will thrive – as long as you regularly add small amounts of phosphate.
4 Grey water
With many regions of South Africa affected by drought in recent years, gardeners have been forced to use buckets of grey water to keep their plants alive. Unfortunately, long-term use of grey water is not good for your soil because it makes it brackish and impairs drainage. In addition, the soap in the water almost causes a type of soil “sterilisation” because micro-organisms find it difficult to survive or function, resulting in weak roots. Even biodegradable soap products have a saline content that can make soil brackish and contaminate groundwater.
It’s better to use grey water as little as possible – only once every two weeks in the same spot – and to alternate with fresh water.
However, recycled water is still a godsend in extreme conditions. If it’s the only means at your disposal to keep your garden alive for a while and your soil quality is negatively affected, it is essential to improve its drainage and structure. Large quantities of compost will return ‘life’ to the soil and also provide nutrients for microbes.
5 What about earthworms?
Earthworms are not essential for your soil – plants will still be healthy and beautiful without them. However, they offer many advantages: because earthworms are constantly loosening the soil and aerating it, they considerably improve the overall structure of the soil. They also create tunnels through which oxygen and water can move easily. In addition, they work plant material from the surface down into the soil and deposit their castings – all of which provide nutrients for plants.
However, it’s not a case of simply buying earthworms and adding them to your garden. The worms that are sold commercially don’t occur naturally in most soil in South Africa and the chances are slim that they will survive – especially if the soil dries out a bit and there’s not enough compost. It’s possible to attract earthworms to your garden by ensuring that there is sufficient organic material in moist, welldrained soil. Alternatively, start your own worm farm composter. >>
Sandy soil copes better with grey water as it is less affected by the salts. Don’t use grey water on heavy clay soil.
6 Compost is best
According to TC Kohler of Reliance, it’s a good idea to work compost into your soil about twice a year: 30–50mm deep in flowerbeds and 50–100mm deep in vegetable beds. After you’ve worked in the compost, you also need to add mulch to retain moisture in the soil and to suppress weed growth.
COMMERCIAL COMPOST
Buy from a reputable supplier and make sure you know what’s in the compost – rather avoid any chemical additives. A good compost should be dark in colour, have a coarse texture and smell like soil (no sharp aromas).
HOMEMADE COMPOST
Avoid the following in your home compost heap: • Pet droppings – they contain parasites and micro-organisms that are harmful to vegetables. • Tea and coffee bags; unless you remove the synthetic bags, because they don’t decompose. • Citrus peels and onions – they retard the decomposition process and are harmful to earthworms and micro-organisms. • Fish and meat scraps because although they will break down into compost, the smell will attract rats, mice and other domestic creatures (and will make your neighbours unhappy!). • Burnt wood and wood ash can be added in small amounts but the sulphur they contain may make your soil acidic.
7 To dig, or not to dig?
Pieter says digging is the best way to loosen compacted soil but don’t dig it over if your soil is in good condition. The exception is when you apply fertiliser twice a year.
When you dig it over, add organic matter immediately and leave the soil as is to stabilise – in other words, don’t walk on it. This “breather” will help the soil to not become compacted again too quickly and the organic matter will decompose properly.
Don’t apply too much water since the loosened soil particles will not adhere to each other to form soil structure, and the soil will then become compacted again.
8 What about fertiliser?
Be careful not to over-fertilise your soil, warns Pieter, because too much of a good thing can be detrimental. If your soil’s phosphate and potassium content is sufficient, you can apply nitrogen two to three times a year.
Fertiliser mixtures consist of inorganic fertilisers manufactured for a special purpose, he explains, and the composition is shown in the relationship between the main plant nutrients – nitrogen (N), phosphorus (P) and potassium (K). “Take the ratio 4:2:4 (20) as an example. The number in brackets denotes that the entire mixture contains 20% nitrogen, phosphorus and potassium in the ratio of four parts each nitrogen and potassium to two parts phosphorus.”
Consult your nursery about the specific needs of the crop for which you want to buy fertiliser.