As we get deeper into spring, we’re sowing, planting, mulching and digging – but which compost do we need for each job?
In general, quality compost should comply with the Australian Standard for Soil Conditioners and Mulches (AS4454). Don’t go off how the compost looks or how much it costs, look for compost products that are supplied with clear product descriptions, including information on how they should be applied in your specific situation. The Compost Australia Leaf Mark certified composts (based on AS4454) are designed for certain applications and are supplied with standard product information sheets.
Adhering to other certification schemes, such as organic certifications (BFA and NASAA), demonstrates the manufacturer is committed to improving product quality.
Garden material, which has been poorly composted or not composted at all, can come with the risk of introducing pests and pathogens into the area. It also may behave less predictably than composted material.
Here’s what you need to know about the different styles of compost available.
This is the most versatile compost, which can be used to dig in and enrich beds and borders, as well as plant up containers. There are many that contain blends of ingredients, including plant foods, which can feed your plants throughout the season.
Some gardeners argue that plants grow better in peat-based products, but others insist that if you adapt to the properties of peat-free, you should be fine.
Peat-free composts generally retain water better, which is great in hot summers, but can lead to rotting plants in wet winters. So, add grit to your peat-free compost before planting to enhance drainage and water your plants little and often during the summer, rather than completely soaking them once a day.
Check the wording on the bag – if it doesn’t say ‘peat-free’ then it generally isn’t. Marketeers may use wording such as ‘environmentally friendly’ and ‘organic’, but that doesn’t necessarily mean it’s peat-free.
Acid-loving plants including camellias, azaleas and rhododendrons are best planted using ericaceous compost. If you are planting them in a flower bed that naturally has alkaline soil, they are likely to suffer, as sooner or later the ericaceous compost will lose its efficacy and the original soil make-up will seep through.
It’s often better to choose plants that will like your existing soil, rather than trying to adapt unsuitable plants using specific composts. If you love azaleas but don’t have acid soil, consider planting them in pots using ericaceous compost.
Seed and potting compost
Seed compost has the lowest amount of nutrients, which encourages the best germination and growth of tiny roots. Seed compost is much more in tune with the needs of a developing seed than a general multi-purpose compost.
Low nutrient levels don’t affect the plant growth because individual seeds already contain a store of food to feed the developing plants.
Spent mushroom compost
This is generally cheaper than other composts and is often used as a soil conditioner or to mulch a bed. It is called ‘spent’ because it is compost left over from mushroom farming.
It has a high lime content, so is ideal for the vegetable garden as crops, including brassicas, grow best when the soil is not acid. However, don’t place it near acid-loving plants such as azaleas and rhododendrons.
It’s advised to use it in moderation, alternating with well-rotted manure or garden compost, which should help balance the alkaline nature of the mushroom compost. It’s also not suitable for fruit crops, which need a more neutral or acid soil.
Animal manure is a fantastic soil conditioner, but it must be well rotted before adding to the soil, or the concentration of nitrogen will scorch young plants.
If you are offered fresh manure, you’ll need to create a space to allow it to rot for at least six months before spreading it across the soil in spring, a few weeks before planting. Break up any lumps by raking it and mix in some topsoil.
What makes a quality compost?
It complies with Australian standards: look for an icon on the bag saying something like ‘premium certified product’. If you’re looking online, it will say ‘Certified to meet the Australian Standard for Premium Composts’ in the description.
It contains high levels of organic matter and nutrients: organic matter is essential for healthy soil. Australian soils are typically low in carbon, so a quality compost containing carbon can help to improve soil condition, and soil fertility. Quality compost also adds important nutrients into the soil, which are vital for plant growth.
It’s free from weed seed and diseases: compost that complies with the Australian Standard for Soil Conditioners and Mulches (AS4454) legislation will not contain weeds, pathogens or contaminants as they have been through a full composting process. Therefore, they will not put your landscape at risk.
Do you spend much time in the garden? What did you plant first this spring?
– With PA
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