The secrets and the satisfaction of improving soil
By Sandi Pullman
A good-quality soil is a vegetable gardener’s greatest asset and the trick to growing healthy vegetables is all in the soil preparation. The more organic matter and fertiliser you put into the soil, the tastier your vegetables will be. Most vegetables are treated like annual plants: they live and die within 6–8 months. For them to grow so quickly, the topsoil needs to be packed with goodies — compost, animal manure and blood and bone. Other factors that influence your success are the soil’s texture and structure. Most of the soil’s nutrients are located in the first few inches of topsoil. Australia is notorious for poor topsoil, the depth of which varies from backyard to backyard. Gardeners around Australia have to work hard at improving their topsoil. It’s important to do so because that’s where most of the oxygen and moisture reside and thus most of the plants’ roots. Plant roots only venture into subsoil (the next layer under topsoil) if there is oxygen.
Soil is made up of organic matter such as leaf and animal materials plus minerals from weathered rock, creating lots of differentsized particles. In our gardens we find three types of soil: sand, clay and silt (usually found on flood plains). Each type has good and bad characteristics and there is no perfect soil. Sandy soil has large particles and good drainage but low nutrient levels and no structure. It’s easy to dig, has good pore space (gaps between the soil particles) and lots of oxygen. Clay, on the other hand, has small particles, lots of nutrients, bad drainage and pore space, and low oxygen levels. It can be really frustrating to dig when the spade just refuses to go in! Silt can have great nutrient levels but very poor structure, depending on what has washed down during a flood. Sometimes silt sets like concrete and is a nightmare to work with.
The size of the particles is known as the texture or feel of the soil. Sand has a rough, gritty feel and clay has a smooth, sticky feel. Soil structure is how the particles are joined together, the size of the particles, the chemical composition and how much organic matter is in the soil. Clay has better structure than sandy soil. The aim of all gardeners, whether growing ornamentals or vegetables, is to be constantly improving the structure of their soil. Well-structured soil holds moisture, has good nutrients levels, good pore space and drainage, and is easy to work with. Another facet of good soil structure is that microorganisms such as our good friends, the worms, can move about easily and aerate the soil.
You may feel overwhelmed by the complexity of soil, but don’t. The way to improve all soil problems is the magic mix of compost and animal manures. They are the mortar between the soil particles and they can fix most problems. If you feel energetic over the weekend, you can dig it in yourself to a spade’s depth or if that sounds like hard work, especially with heavy clay, spread it over the top and let the worms dig it in for you. The improvement in soil structure won’t happen overnight but, over time, it will happen. Each time you plant a new crop it’s imperative to improve the soil. If you are going to buy in soil, be careful, as many soils aren’t sterilised and may contain weed seeds. To be sterilised, the internal temperature needs to get up to at least 60oC. This will kill all the bad pathogens and weed seeds. When ordering soil from a supplier, make sure you stipulate that you want to grow vegies. You want a mixture that is very open, well-draining, enriched with organic material and has some clay content so that it holds moisture.
The next step to understanding your soil is knowing its pH. Remember carrying out pH tests at school? Well, it’s a similar process. You can buy kits at your local nursery that will tell you what your soils pH is. If the soil turns yellow it’s on the acidic side (4–6); if it turns green/grey it’s neutral (6.5–7.5) and if it turns purple (8–9.5), the soil is alkaline. It’s crucial to understand and stabilise pH because, if the soil is neutral, all the nutrients are available to the plants, but as it moves towards acidic or alkaline, many of the nutrients become unavailable to the plants, even though they are still in the soil. For example, if soil is alkaline, the trace element iron becomes unavailable, causing iron deficiencies in the plants.
So it’s important to research what different plants require and grow those with like needs together. Many vegetables like a neutral pH — for example, squash and cauliflower. Cucumbers and carrots, on the other hand, prefer a pH of 5–6, while peppers and okra like 6–8. If all this sounds complicated, an easy way to manage the pH is to create separate beds and grow the vegetables with the same requirements together. Beans, radish and zucchini require 6–7; carrots and cucumbers 5–6; tomatoes, pumpkins and onions 5–7. Creating a good-quality soil is all about adding material that improves its structure. So if your vegies aren’t growing well, ask yourself the following questions: Did I prepare the soil with lots of organic matter? Are there worms in my soil? If the answer is no, our magic solution is to work in lots of compost and animal manure. It will fix the problems over time.
Tip: To improve the structure of clay, try adding gypsum. It loosens up the particles and improves the drainage. Warning: if your soil has a pH of less than 5.5 it’s not advisable to use gypsum as it can be toxic to plant roots.
Tip: To lower the pH of your soil, add sulphur; to raise the pH, add gardener’s lime. Adding compost will also help restructure your soil’s pH and be a lot cheaper
Tip: Create an Excel spreadsheet and note what you grew in each bed and how you ameliorated the soil. You will thank yourself next year when you can’t remember without your handy spreadsheet.
A test to try at home: Get some soil from your garden and put it in the palm of your hand. Add a little water and try to roll it into a sausage. If it falls apart, you have a pure sandy soil; if it rolls a bit and then breaks up, there is some clay present; and if it rolls into a good sausage, you definitely have a clay soil. If you can’t roll it at all but it is sticking together, you have very heavy clay.