The digestate may be the unsung hero in this process.   Our attention tends to be focussed on the biogas, but the truth is that at this scale it’s only ever going to be a modest amount, perhaps able to supply 80% of the gas used for cooking for a family of four.  On the other hand the digestate production can be quite significant, and some have suggested it may even be worth more than the gas.

To explore this we have been carrying out growing trials – applying digestate as liquid fertiliser to crops to see if there is a benefit.

Raised beds at the Bridport Community Allotment. Two beds were kindly made available by the Orchard Group. They were side by side and watered every two days. An average of 18 snails were removed on each visit.

We had 500 litres of digestate left at the end of last year, with an N:P:K analysis of 4.5:0.3:2.0 kg/te, and calculated that a dilution factor of 10:1 would be appropriate.

The growing trials were kept very simple in the belief that to be convincing the benefits had to be obvious.  In all cases two crops were grown – a trial and a control.  As far as possible the two were treated in exactly the same way with the trial receiving an additional weekly feed of digestate.  The soil used was ‘as found’ – well composted garden soil in the pots and grow sacks, and black alluvial soil (to die for!) in the raised beds.

The crops were mint, shallots, garlic, lettuce, salad onions, pak choi, mustard, wild rocket, potatoes, mange tout and flax.

Planting scheme for single raised bed. From the top and left to right, flax, mangetout, salad onions, pak choi, mustard, lettuce and rocket.

Growing sites were pots, grow sacks, and raised beds.  First to be planted were the onions, garlic and mint root cuttings on 27th January.  The potatoes went in on Easter Sunday, and the salads and mangetout were bought as seedlings and went in on 11th April at the same time as the flax seed.

The first to show a benefit was the mint:

Groups of pot grown mint. The fed mint was greener and more vigorous, but who would want to encourage mint to grow?

Onions garlic and potato exhibited greener or more vigorous growth, and everyone agreed that the salad beds were indistinguishable.  The Rocket, Pak Choi and Mustard bolted early, looked pretty and delivered little.  The potatoes were situated in the long grass, and one group suffered heavily from slugs and snails.

The potatoes were the first to be lifted and weighed, followed by the shallots and garlic, and successive pickings of mangetout, with the following results:

  • Potatoes: the treated crop suffered slug damage and weighed 27% less.
  • Onions: when the total weight was normalised for the number of onions in each crop the untreated crop was 5% heavier.
  • Garlic: the treated crop was 12% heavier
  • Mangetout:  The treated crop showed a benefit of 65% by weight.

In conclusion it seems hard to improve on a fully developed soil.  The mangetout was an obvious winner and something of a surprise.  The received wisdom seemed to be that as this was a nitrogen fixer it would fix all it needed, but apparently we were able to provide it with more.   Possibly the imbalance in the digestate, with the virtual absence of phosphate, meant it had no impact on the onions and garlic.

During these trials the digestate was also fed to rather tired looking house plants which are all now looking much better.

Hoya Carnosa doing very well following a fortnightly plunging in digestate solution.

Maybe they would have done so anyway with the increased level of attention, but I’d prefer to attribute the benefit to the nitrogen!

So overall the results seem rather inconclusive.   It would appear that we need to pay more attention to the chemistry, examining for trace elements, and vary the feedstock to achieve the results we want.  As a starting point we have our comfrey bed of 100 plants which have not been used this year.

And the flax?  I’m afraid I’m going to have to leave you guessing!