Last year I volunteered to take part in a growing trial for Garden Organic. The aim of the trial was to assess the suitability of Persian Clover (Trifolium resupinatum) as a green manure crop, against the more regularly-used Crimson Clover (Trifolium incarnatum).
In my last update, back in October, I suggested that it wasn’t looking good for the Persian variety. The year-long trial period has now come to an end, and I have to say the situation really hasn’t improved.
Results
Here’s a photo of the Persian trial patch, taken a couple of weeks ago:
As you can see, the clover has all-but disappeared, and the Poa annua grass, Ranunculus repens creeping buttercup and other assorted weeds have moved in en-masse.
By contrast, the Crimson clover patch is at least showing some evidence that clover was once sown there:
Again, there’s a fair bit of weed growth in evidence, but the clover straw remains and will at least be usable on the compost heap.
Conclusions
On the evidence of the above photos, it’s very tempting to suggest that Persian clover isn’t such a great green manure crop after all. However, there are a few provisos that need to be taken into account before drawing any firm conclusions.
1. Sample Size
Obviously, this is just a single result from one particular site. A rather windy, weed-prone site at that. Garden Organic will be collating results from all their participants and I’ll be very interested in seeing the aggregate view in due course.
2. Duration and Parameters of the Trial
Garden Organic asked participants to run the trial for a whole year, specifically to assess whether the Persian clover would degrade better than the Crimson, which does tend to leave straw behind. On the evidence of my own trial I’d say that yes, it does. The problem being that it degrades too quickly, leaving bare soil behind for weeds to colonise.
But that’s not how green manures tend to be used, at least not on a small to allotment-sized scale. Rather, the crops are grown to the point just before they set seed – or earlier if the ground will be needed for sowing or planting sooner – and then either dug into the soil (or maybe shallow-rotovated, or just cut back and allowed to decompose on the surface, if you practice a no-dig methodology – discussions are ongoing). The idea is that their green growth provides organic matter to improve soil structure, and eventually degrades to release nitrogen and other minerals.
That being the case, there may be an argument for Persian clover being sown, as long as it was dug in at the appropriate stage. However, based on the update I posted last August, I’d have to say that, on our site at least, Persian clover is too slow to germinate and develop to be of any real use. It would still allow annual weeds to establish and potentially set seed before it had established enough ground-cover foliage to prevent the weeds from getting started.
To summarise: I, personally, wouldn’t use Persian clover as a green manure on our site. Not when the stronger, faster-growing Crimson variety does a much better job of quickly covering the ground and puts on a lot more soil-boosting foliage in the process. Digging in the Crimson clover at the appropriate time would avoid the issue of tough straw lingering and being slow to decompose in the soil.
2017 Trials
This year, I’m running another trial on behalf of SoilFixer.co.uk, testing the efficacy of two of their soil improvement products. Details of the initial setup can be found here. I’ll also hopefully be taking part in a wildflower growing and bumble bee counting project for Garden Organic. Details to follow.