Your soil structure plays a vital role in the success of your lawn and in many cases some improvement work is required to get the most out of it. But first of all let’s have a look at what makes up a soil so that you can get a better understanding of how it all works and why it’s important to carry out any improvements as opposed to just bunging down a few rolls of turf and then wondering why it’s not performing to its best. To understand soil, you need to know how it is made, and what functions its components perform. These are;
Soils vary a lot because they have different proportions of these components arranged in different ways, and this in turn affects the way they behave. It is important to know what sort of soil you have as this will tell you what you may need to do to improve it to get the most out of your lawn and of course plants. To analyse your soil, you need to have a good look at it. Dig some up and feel it in your hand. Make a note of what colour it is, and how the colour changes with depth.
Now carry out a ‘ribbon’ test to find out what sort and size of particles your soil contains – sand, silt or clay – and in what proportion. Pick up a handful and work it together, adding a little water at a time until it is moist. Note the different textures. Sandy particles are course and easy to fell, while the smoother particles are clay, silt and humus. Now squeeze the sample and try and form it into a ball. If it contains a fair proportion of clay and humus this should be easy, but if it contains a lot of sand it will keep breaking up.
Next, gently squeeze the ball to lengthen it into a strip or ‘ribbon’ and see how long you can make it before it starts to break up. The more clay it contains, the longer the ribbon will get and the more plastic it will feel to the touch. Soil ribbons can range in length from about 5mm – 75mm, with sandy soils being shortest, loamy soils in the middle and clay soils the longest. While soils that contain a lot of clay or sand may need work to improve them, loamy soils are generally ideal. These contain a mixture of sand, silt, organic matter and clay, providing the right balance of nutrients, oxygen, water and drainage which are just what your lawn needs.
This relates to how the different soil particles are arranged, the aggregates (clumps) they form and the spaces between them. Well-structured soils will have aggregates firmly bound together, which will help maintain the structure when the soil is wet, but at the same time be loose enough for roots to move through freely in search of water and nutrients. Well-structured soil will also have plenty of air spaces to provide oxygen, allow water to penetrate and facilitate drainage. Poorly structured soils are often hard and compacted, don’t accept water easily, have few air spaces and are subject to erosion. In simple terms they’re bad news for you and your lawn and need to be improved.
Your soil’s pH is another important factor, but one that is often overlooked in the eagerness to get your lawn down, but it can greatly affect nutrient intake and so your lawn’s health and appearance, so testing your soil’s pH and making any adjustments plays an important role in your soil preparation stage and is best done before you lay your new lawn.
The three main soil types are sandy, loam and clay, with many soils fitting in between these three, a sandy loam or clay loam to give a couple of examples. For general gardening and lawn purposes you can’t beat a loam soil, it’s got all the good gear and the right structure. If you are lucky enough to have such a soil you won’t have to do much soil improvement work at all, perhaps just a touch of fluffing here and there, and fine tuning of the levels. By now you should have a good idea of the type of soil you have. Ideally you need around 150mm of good soil for a successful lawn, at a stretch you might get away with 100mm, but getting away with it is one thing, a good lawn is another, so in some cases you will need to bring in some new soil.
‘Turf underlay’ soil mixes are specialist soils sold at good landscape suppliers and some nurseries, containing around 80% sand, and 20% black soil. They provide good drainage from the sand and some nutrients and water holding capacity from the soil. At the same time, they are easy to spread and so perfect for fine tuning the levels before laying the turf. Even if you have a good soil, a thin layer of turf underlay really helps to get everything spot on. If your soil is pretty ordinary and/or you need to build up your levels, then this is the soil mix to order. River sand is also excellent for levelling and turfing over.
Water drains quickly from sandy soils, which is good in some ways as you are unlikely to have any drainage issues, however as the water drains it takes essential nutrients with it. Sandy soils also dry out quickly due to their open, free draining structure and can become hydrophobic (water repellent).
Adding organic matter and a soil with some clay content will improve the nutrient levels and help with water holding capacity due to the clay. You need to mix the organics and new soil into your existing soil thoroughly. This can be done manually with a mattock or hoe, a rather lengthy and labour intensive process, but one that’s ok for small areas, especially if you get a bit of help.
For larger areas a rotary hoe does the job beautifully. You can rent these from hire shops, but familiarise yourself thoroughly with the controls and operating techniques to avoid any accidents. Another option is a mini skid steer loader like a Dingo or Kanga Loader with the rotary hoe attachment. If you have a large area, then a Dingo or Kanga Loader with the 4 in 1 bucket will help here too. Once you get the hang of these machines they are good fun to operate and make light work of the job, especially if you need to move the gear up hill. For the bigger jobs a larger machine with an operator may be the go.
At the opposite end of the scale from sandy soil are clay soils. These hold water and nutrients very well but don’t drain as well as sandy soils, especially if compacted, a common situation for lawns with a clay soil base. Adding organic matter and gypsum and then cultivating to mix these through the soil will work wonders, opening up the soil’s structure, improving the drainage and oxygen levels, and making the nutrients more available.
To work clay soils successfully, the moisture content has to be right. If it’s too dry it will be hard and very difficult to cultivate. If it’s too wet it becomes too slippery and boggy and you’ll probably do more damage than good, not just to the soil but also yourself as you slide around and fall over while trying to hang on to an out of control rotary hoe. A very dangerous situation to be avoided at all costs.
How much organic matter and gypsum you add depends on the soil. For light clay about 1Kg/square meter of gypsum is the usual application rate. 2kg/meter for heavier clays. A layer of about 20-30mm of organics should be plenty before cultivating through the soil. In some cases, with a light clay soil for example, all you may need to do is spread the gypsum and then cultivate before bringing in and spreading your turf underlay soil mix.
There are situations – particularly on new housing developments – where most, if not all, of the original topsoil has been stripped away, leaving hard clay that is pretty much useless. You might get some kikuyu to grow in patches, but you’ll have all sorts of problems for years. The best way to deal with this is to bring in new soil and raise the entire area at least 100mm but preferably 150mm. Drainage is a major concern here, so before introducing the new soil, contour the existing clay, sloping it in the direction you want any water to go which will hopefully reduce any pooling and bogginess. It’s important also to mix a layer of the new soil into the existing to facilitate drainage before building up the levels further. Raising the levels may well require some edging or retaining to keep everything in place.
A common scenario is to have a decent depth of good soil that has become compacted due to heavy foot traffic over time, or in the case of newly built houses, been neglected and abused during the construction process. Often, all that is needed here is to cultivate the base, perhaps with some gypsum, and then level with a thin layer of underlay.
Like with anything around the home and garden, if you have concerns or are unsure about things, get some advice from a professional landscaper or turf grower before going ahead.