Does anaerobic digestate benefit soil health?

This year, Patrick Allpress joined a group of farmers working with Agri-Tech East to test the benefits of digestate for the soil beyond the acknowledged contribution to N, P and K.


What do you farm?

We currently have 1,600 acres under our control, either owned or tenancies, and then on an annual basis we hire in another 1,200 acres. We specialise in leeks and onions and are mostly non-organic but we do grow some organic leeks for our contract with Sainsburys and Waitrose.

What were your motivations for joining the trial?

In 2014, we installed our 500 megawatts AD plant. It produces 12,000 cubic metres of digestate a year and was designed to fit the farm it sits on (880 acres).  

What have you done so far?

It’s quite a slow process, so we’re planning for the trial to last three or four years. On our farm, we’ve got two different trial sites. Each site has five different applications. We’ve got: a cover crop with no digestate, no cover crop with digestate, a cover crop with digestate, no treatment at all and a cover crop with a shot of microbial products.

What are you hoping to find out?

We know about the indices but we don’t really know what it’s giving us compared to using inorganic straight fertilisers. We’ll look before and after application, and then we’ll check the following crop to see if there is any different activity in the soil to do with the digestate.

To measure the next crop is going to be the difficult bit, you can measure it in two ways: yield or root density. I’m hoping where the digestate has been spread with a cover crop, it’s going to put more back into the soil. It’s really our longer aim: putting more carbon back into the soil and increasing biological activity, meaning we can grow better crops.

What would success look like?

I think people have a hunch that digestate might do their soil some good but at the moment there is no measurement of what good is. They know it’s got N, P, K and the indices adjust overtime but it’s still not a cheap product, and straights are probably cheaper. So, I want to know if it’s putting something else into the land, besides the N, P and K, which helps the soil biological activity.

Has it been useful working with other farms?

Yes, as each farm has different soil types. Our soil type is an organic peat that is quite gravelly and sandy, whereas other farmers in Norfolk are mostly on sandy loams. 

When it comes to co-operation, it is very useful discussing the trial with other farmers, as everyone has a different approach. We’re not competing against each other, instead we’re testing the science and growing our knowledge. We also worked with an excellent PhD student to put more science behind the measurements we were using.

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