Soil preparation is like baking a cake. If you don’t prepare it correctly you’ll end up with a mess and a disappointing end result. It’s really important to prepare the soil well before planting to ensure your plants grow well.
For those who are lucky enough to inherit an established garden with great soil, or are starting a new garden in an area with great topsoil, you may not need to do much at all. But for those who have a garden in an area with pretty average soil, you’ll need to modify the soil to ensure the end result is a great looking garden. Soil preparation is particularly crucial if you have purchased a block of land in a new residential estate, which has had some form of excavation and may not have had a good amount of topsoil put back on, or even worse has not had any topsoil put back on.
Soil provides plants with physical support, nutrients and contains water reserves. Soils are mostly made up of gravels, sand, clay, organic matter from broken down plant material and micro-organisms and may sometimes have stone or rock, such as coffee rock, limestone or basalt.
Here in Willow Grove in West Gippsland we have acidic yellow and grey dermosol soils, which are strongly acidic. In our backyard we have a dark grey-brown shallow topsoil with a yellowish and very slippery clay subsoil underneath. The red ferrosols around Warragul and Thorpdale, which are red in colour because of the higher levels of iron oxide present, are known for their good structure, high nutrient levels and good water holding capacity and are commonly used for agriculture and horticulture (Victorian Resources Online, vro.agriculture.vic.gov.au). If you are in Victoria and are interested in the soil type in your area there are maps available at Victorian Resources Online.
With any new planting there are always likely to be some casualties, but the likelihood of a failed planting will dramatically increase if the soil condition is poor. Here are a few pieces of info about soil that are worth thinking about before you start planting to ensure your garden will be green and plentiful:
Soil pH – Is your soil acidic or alkaline? A pH<7 is acidic, 7 is neutral and >7 is alkaline. A simple pH test kit from the hardware store or nursery is cheap, simple to use and will give you a good indication. But be aware that soil pH may vary in different parts of your garden and at different depths of soil. The pH is closely linked to nutrient availability and majority of plants will grow in soil with a pH in the range of 4.5 to 7.5.
There are some plants which grow better in acidic or alkaline soils. For example Blueberries grow best in acidic soil with a pH of 4-5.5 and Azaleas and Rhododendrons pH 4.5-6. There are various things you can add to the soil to raise or lower soil pH.
Soil fertility – The macro nutrients critical for growth are Nitrogen (N), Phosphorus (P) and Potassium (K) which you will often see as a percentage expressed as the N:P:K ratio. Other macronutrients include Magnesium (Mg), Calcium (Ca) and Sulphur (S). Micronutrients which are required in smaller quantities include Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Molybdenum (Mo) and Boron (B).
Soil nutrient levels are simple to modify because you can easily add fertilisers. However, without sending off a soil sample to a lab for a comprehensive analysis, you won’t know exactly what nutrients are present in what quantities. Keep an eye out for signs that the plants are suffering from a nutrient deficiency, such as leaf yellowing. There are also some plants like citrus and roses, which respond well to regular fertilising throughout the year to keep them healthy.
Soil structure and texture – what the soil is made up of and how it structured are not easy to alter like soil pH and nutrient levels. In soils with good structure, plant roots can utilise more of the soil to obtain the nutrients and water they require.
It is best when the soil has some ‘crumbing’ which means that it binds into crumbs and has a mix of pore sizes between particles. Gaps are present between the crumbs and the gaps are important because they allow for aeration and good drainage. As an example, a very sandy soil doesn’t crumb and won’t hold its shape when it is squeezed in your hand. A very sandy soil with low amount of organic matter will have very little water and nutrient holding capacity. At the other extreme is a heavy clay soil. If there are lots of smooth clay particles the soil will hold water and nutrients, but the negative is that it is slow draining and compacts easily. One of the simplest tests for soil texture is a test called the ribboning technique, which is done by placing the soil in your hand and mixing in a small amount of water to see how the soil responds. There lots of instructions on the ribboning technique online, such as the Department of Primary Industries NSW.
There are various soil conditioners on the market to improve soil structure. Adding organic matter in the form of composted plant materials, including manures, can assist in improving soil crumb.
Soil water – some trees use up so much ground water that it will be difficult to grow anything underneath. Often the soil underneath will become dry and compacted as tree roots get larger and expand into the soil space.
Sometimes soil is water repellent (hydrophobic) and will need a soil wetter added. If you water the garden and there is a lot of water running off the ground instead of soaking in it often means the soil is very compacted and/or hydrophobic. In hydrophobic soils the top centimetre or two is often wet after watering or rainfall, but when you dig a little deeper it may be bone dry. Some soils have more of a tendency to become water repellent than others. Soil that has had a lot of pine bark mulch laid on top of the soil can also become water repellent.
Another problem is waterlogging with too much soil water, in which case water fills the pores/gaps in the soil with water and prevents the plants from taking up oxygen. In this case, the drainage needs to be improved to allow some aeration around the plant roots, or they will sit in too much water and may eventually rot. It may be a case of not only improving drainage, but also improving soil structure by adding compost.
Organic matter – It is worth building a compost heap or investing in a compost tumbler because the number of beneficial microorganisms can be significantly increased and soil structure can be improved by adding composted organic matter.