If your car starts to misfire, you might suspect the fuel. But if your seeds won’t germinate or your seedlings fail to grow and thrive, you may blame your lack of green fingers. But what if the fault lies with a bag of dodgy compost?
At Which? Gardening every spring we buy over 40 brands of sowing, cutting and multipurpose compost that you’d find in garden centres and DIY stores.
We then test it by sowing seeds, potting on seedlings and plug plants, and growing bedding plants and potatoes in containers, just as you would at home.
But this year we decided to do something different: we sent the worst performing composts to a lab for analysis. And we didn’t just have a rootle under the car bonnet, we called in a crack team of mechanics to diagnose the problem.
Lab analysis
The lab analysis examined the exact nutritional content of each compost and the results were staggering.
Some had almost no fertiliser, some had way too much and in some the fertiliser had excessive amounts of some nutrients and nowhere near enough of others.
Plants need 12 nutrients to grow. The most important is nitrogen which is vital for growth and photosynthesis. Without enough of the other nutrients the plants wouldn’t be able to carry on the many, complex processes needed to thrive.
We found that two composts had very variable amounts of nitrogen.
In some bags of Westland Multipurpose Compost and Levington Original Multipurpose Compost there was enough fertiliser for plants to grow tall and healthy. But in another bag of each of these there wasn’t much more nitrogen than you would find in the peat they were made from, leaving the plants small and struggling. Our expert assessor thought that the full dose of fertiliser may not have been added to these composts.
In J Arthur Bower’s Multipurpose Compost there was also very little nitrogen. The woody green waste used in the compost had led to high chloride levels, which mops up nitrogen leaving the plant starved.
We expected to see the same thing in Vital Earth’s Chelsea Mix All Purpose Compost, which is made with composted green waste. But here too much nitrogen seemed to have been added to compensate, which is just as bad for the plants as too little.
A healthy balance
Too much of a good thing is as unhealthy for a plant as it is for us. Composts with a lot of fertiliser will draw water out of the plant and burn the roots, leaving crisp, lifeless plants that are unlikely to survive, let alone thrive. Fertiliser levels were far too high in Gro-Sure Peat Free All Purpose Compost with 4 Month Feed and our plants barely grew in this compost.
Our tests are designed to find the best composts for your plants, so it’s disappointing to discover some composts that are just not up to the job. Thankfully, all the composts we’ve mentioned here, apart from Gro-Sure Peat Free All Purpose Compost with 4 Month Feed, will be withdrawn from sale or available in new formulations next year.
Do you think your plants have suffered at the hands of a dodgy bag of compost?
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