/ Home & Energy, Shopping

How can you spot a bag of dodgy compost?

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?


The article in a sense shoots itself in the foot as it identifies the variability in the bag’s contents as the primary problem not the name on the bag. Re-formulation is not going to stop the problem of insufficient or to much nitrogen – it is the way the bags are filled that needs attention.

Perhaps a stronger criticism is that this article does not provide any clue whatsoever to the incidence of of “bad” compost bags. Is it 1 in 5 or 1 in a 1000? This makes a very large difference to the value of the research.

Could Which? give an idea as to how many bags of each brand were bought and the incidence of poor components and/or mixing so the discussion can be based on robust findings?


It was interesting to read the test results. The best composts are unobtainable in our area as the garden centres only sell those on which they get the biggest mark-up, and they usually have discounts for multi-purchase of multi-purpose composts so they sell well and therefore become the “most popular”.

I think it would be a good idea to put the date on product reviews. Although the Composts review gave the timeframe for the tests in the text, I believe the date of publication should be always be given. Reference to “this year” in the Intro is confusing because it is obvious that the research was carried out last year, but that might not immediately be obvious to a reader later in 2016.


I agree there is a lot of low quality compost about that makes little difference to plant growth . Apart from my garden ,in which I allow bushes/trees to shed their leaves on the ground as compost (doesnt look nice but the earth likes it as well as worms ) I have various tropical indoor plants that for years didnt grow that fast, It wasnt until I had some water left from the bottled water I now drink continually, due to the amount of pain I suffered from kidney stones, that I used that instead of tap water to water them . The results were dramatic this wasnt some 6 months and an inch growth they seemed to thrive on it , so much so I didnt need any additives in the soil to help it. The name of the water — Highland Spring from the hills of Perthshire ,Scotland -organic society approved land – among the chemical make-up of the water is- Nitrate – 3.1–Potassium-0.7–sodium-5.6-sulphate-5.3–Magnesium-10.1 as well as Calcium/Bicarbonate/chloride all at per-mg/L . The difference was amazing I could almost hear the plants sing with joy !

Kate Gorham says:
13 January 2017

Hi Duncan To save you the expense of bottled water for your indoor plants, try using rain water. If you don’t have a water butt, you can leave any empty containers Glass jars or Plastic, outside to collect rain water. Generally speaking rain water is best for plants and some, to name one, Blueberry bushes must be watered during dry spells with rain water not tap.


I could spot a bag of dodgy compost when toadstools started to grow beside my recently repotted peace lily. Ah well, I should have known. I bought compost for vegetables (it said so on the “tin”) at Poundland for a pound. You get what you pay for and pay for substance misuse as well. But so far my peace lily doesn’t appear to mind, so neither will I.


Perhaps I should have, being a member, read the survey first however I think Conversations should have enough information that non-subscribers can feel confident in joining in.

I am very pleased that Which? has commissioned laboratories to scientifically analyse the products as in-the-ground tests must have a lot of variables – other than the variable of the compost manufacturers production process. A major advance in testing and I think Gardening Which? are doing a grand job to highlight the differences in compost quality.

At the back of my mind I have a nagging concern about whether this used to be done regularly during the last three decades and was lapsed at some point – am I imagining things?

A couple of points

” This is a peat-reduced compost containing 58% Irish peat, along with green waste and coir. Green-waste compost is notoriously variable in quality, although we didn’t see much evidence of this across the four batches of this compost.”

It would appear that the sample size is four bags.

[Sorry Dieseltaylor, we’ve removed the compost ratings from your comment as these are exclusive for Which? members only. I hope you understand. Thanks, mods.]

There seems to be a discrepancy between the term peat-free and a 50% peat content rating.


What about the bags that have introduced Jananese Knotweed from recycled materials
There were several instances locally of this and it was no where near as straightforward to get sorted as it first seemed
Personally I dont use the stuff but I am after a 60 or 72″ howard rotavator that still turns as our old one onlyy fitted a grey or 35 fergy and their gone


I just wanted to clarify a few points on our testing.
Which? Gardening tests composts every year and has been doing so for many years. In the January/February issue we test composts for sowing seeds and raising young plants, ie seedlings and small plug plants. In the March issue we will be covering growing bags and in the April issue we will report on composts for container plants.
As John Ward rightly comments, we carry out the tests the summer before we publish, so we sow seeds in April, and plant up the young plants and container compost tests in May. Young plants are assessed after 6 weeks and containers grow on for five months.
We buy composts in four locations around the country to be sure that we buy batches of compost that have been made a different points in the compost making cycle. Manufacturers start to make compost in September and the process runs through to spring.
From these batches we grow either three pots or five pots where there is only a single plant in a pot. We would love to grow plants from more samples, but this would be impractical.
This is the first year we have sent samples to a lab for analysis. In the past we have based our findings on the size, colour and flowering of the plants we have grown, as is standard in the compost industry. However I wanted to know exactly what was causing the problems and so commissioned the extra analysis.