With a ban looming for peat based composts, home growers are increasingly trying out coir based composts. Many are hitting problems.
I’ve had a number of emails recently about problems with coir based seed and potting composts. This email from a newsletter reader in Devon is a good example.
I tried this year to use coir instead of traditional potting compost that may contain peat. This was purchased as a compressed block and reconstituted by adding water to produce a wheel-barrow full.
I planted various seeds or small plants in it and all flourished initially but then rapidly died off, sometimes with blackened tips to the leaves. This ranged from pea and bean seeds to small plants like Mimulas that had self seeded from last years crop. The common denominator was the coir compost.
The same seed for peas and broad beans planted in traditional potting compost had no troubles at all.
Coconut coir, also known as coconut fiber, is a natural fiber extracted from the husk of coconuts. It is becoming a widely used substrate in gardening and agriculture, especially as a soilless growing medium and in seed and potting composts.
As a medium most home growers and commercial horticulturalists agree it is an inferior product compared to peat but as peat will be unavailable to amateur growers in the UK by 2024 we’re going to have use alternatives or move to another country.
Problems with Coir
Contains sodium nitrate and chlorine
Coir also has high levels of sodium nitrate and chlorine, which can be harmful for plants if not flushed out regularly.
To flush out the sodium nitrate and chlorine from coir, you need to rinse it with plenty of water before using it. Some sources recommend using boiling water to dissolve and flush away the salts more effectively. Often this has been done at source before the coir is shipped here but not always.
Peat can hold 10 to 20 times its weight in water, while coir only holds an average of 8 to 9 times its weight. Admittedly coir can be easier to re-wet if it dries out but seedlings may well be killed by being dried out.
Coir based composts require close attention to ensure they don’t dry out and more frequent watering. This may cause problems as over-watering washes out nutrients.
Micro-Nutrient Deficiencies in Coir Based Composts
One issue with using coconut coir as a soil amendment is that it can tie up micronutrients like iron and copper. This means that these nutrients may become less available to plants growing in coconut coir based potting composts.
Iron and copper are essential micronutrients that plants need in small amounts to grow and develop properly. When these nutrients are tied up by coconut coir, plants may suffer from deficiencies. These deficiencies can result in stunted growth, yellowing of leaves, and other problems.
Some other micronutrients that coir may tie up are calcium and magnesium. These are essential for plant growth and development, especially for flowering and fruiting stages.
The reason coconut coir can tie up micronutrients like iron and copper is that it contains high levels of lignin, a complex organic polymer that is difficult for plants to break down. As lignin decomposes, it can release organic acids that bind to micronutrients like iron and copper, making them unavailable to plants.
Add amendments to coconut coir based composts to help release micronutrients.
To address this issue, gardeners and horticulturalists can add amendments to coconut coir based composts to help release micronutrients. For example, adding organic fertilizers that contain chelated iron and copper can help make these nutrients more available to plants. See below.
The ban on UK commercial growers using peat doesn’t come into effect for a number of years. However, the growers are under huge pressure from the retailers to have the magic words ‘Peat Free’ on their products.
In part, it was to help the commercial growers cope with the deficiencies of coir that prompted S-Chelate to develop their micro-nutrient supplements using chelation.
Chelation keeps the micro-nutrients bioavailable at a wide range of pH levels in coir-based growing mediums.
For the home grower they have two products. S-Chelate-O Cultiv-8 which is an organically approved mix of 8 micro-nutrients designed as a supplement. The other product is S-Chelate 12 Star which is a complete fertiliser including the main macro-nutrients (NPK)