Improving our soils at HCF; some more thoughts and ideas.

Over the last 150 years many of the world's best agricultural soils have lost between 30-75% of their carbon, and this has contributed billions of tons of CO₂ to the atmosphere.  1/10 of all CO₂ emissions since 1850 have been from farm soils! This has had a major impact on climate change! Many of us will remember learning at school about the creation of the American dust bowls from the prairies in the 1930's. Most of the agricultural soils in southern England, especially those situated on the South Downs have less than 2% soil organic matter and could easily become dust bowls! Around 30% of the world's farm soils have been abandoned in the last 4 decades because of poor soil.

A recent study from the US has clearly shown that organic agricultural practices build healthy soils by storing away appreciably larger amounts of carbon and for longer periods than typical agricultural soils.  Probably the most compelling findings was that on average, organic farms have 44% higher levels of humic acid, (a component of humus) that sequesters carbon over the long term, than soils not managed organically. (Misiewicz, 2017)

So we ask the question; what practices are most important for building soil organic matter while at the same time ensuring good harvests of organic vegetables at HCF? Here are three more suggestions.

1. Minimise soil disturbance

There is an increasing body of evidence that suggests that digging, rotivating or tilling on the larger scale is utterly devastating to the soil if practiced five or six times a year. First it "slices and dices" many of the beneficial soil organisms like worms and destroys their habitat. As we saw in our worm survey on 29th March 2019 there are a group of earthworms, the epigeic worms that live in the surface litter and help to break it down. These actually form quite a large portion of thrushes' diets, and soil ploughing may be partially responsible for their massive population decline of more than 50% between 1970 and 1995.(Lusher, 2019). The anecic worms that are the drainage worms that make deep vertical burrows which help water infiltration and deep plant rooting are also disturbed or destroyed by frequent digging and their numbers decline leading to poorer drained soil. (Chambers, 2019)  

Soils which are tilled mechanically or repeatedly dug and raked to raise a mechanical tilth are often great for planting seeds in. The problem is that it destroys soil structure. It breaks up the soil crumbs or aggregates (particles held together by glomalin) breaking them to a powder. This tilth may only last a few days or weeks before settling back into a packed, consolidated and homogeneous structure with little pore spaces and structure. If heavy rain falls the surface can become a hard pan and be compacted afterwards and resistant to further water infiltration. 

A second problem associated with digging is that it turns the soil, introducing air containing oxygen to pockets in the soil which enables bacteria to thrive. Bacteria increase in numbers if the conditions are right: good levels of oxygen, moisture, nitrogen and carbon. If the soil is rich in nitrogen (has been manured or fertilised), the bacteria will take in as much nitrogen as they can, but they need about 20 times as much carbon as nitrogen. So the bacteria pull as much carbon from soil organic matter as they can, excreting the bulk of it into the atmosphere as carbon dioxide. Once all the easily digestible soil organic matter (SOM) has been used they will then turn to any humus available in the soil and more slowly break that down! This has been giving a yield boost to the farmers on the South Downs, which is what they want, but it is causing the humus levels in the fields on the Downs to fall dangerously low. So as we can see after just a few seasons of tilling the soil becomes carbon depleted.

There are other disadvantages to lots of digging or rotivating: dug or rotivated soils release far more nutrients than the soil plants can use and these leach through the soils or are washed out by heavy rains.  Finally digging stimulates weed seed germination. Weeds love disturbed, bacterially dominated soil because their goal is to grow quickly, set seed, die and get another generation of weed seeds into the soil.

Charles Dowding began practicing a "no-dig" method of soil cultivation in 1983 which seems to obtain good results on an allotment scale (Dowding, 2019).  By not digging a good biological tilth has developed over time. Biological tilth is the kind of crumb structure that good organic soils becomes as the fungal mycorrhiza produce glomalin which causes particles to aggregate together in little clumps roughly 1 cm across. This is often first found in a soil around the roots of healthy plants. It is resistant to weathering and degredation by digging, while at the same time allowing water to enter easily, but holding onto the water so it doesn't drain though too quickly and take the nutrients with it.

To understand better how Dowding's "no-dig" method works watch this 19 minute video Spring in the No- dig garden at https://mail.google.com/mail/u/0/#inbox/KtbxLxGgHQSXRTZNcHgjvZmXGbnJKXDlCL?projector=1 

Initially we may want to consider a "minimal dig" policy and reduce the use of rotivators on the soil.

2. Sheet Mulch in the autumn

We dealt last week with adding a few centimetres of an organic material to a crop as it is being planted out or adding a composted manure or mulch in the autumn before sheet plastic is used to cover the plot. What I am suggesting here is rather different.

As a teenage I remember buying dry tubifix worms to feed to my tanks of tropical fish. But then I discovered I could grow my own worms by placing bread on damp soil in a margarine tub in our shed and a few days later under the bread would be loads of worms! It saved me my pocket money! Sheet mulching is a bit like that on a bigger scale.

In Rwanda 35 years ago we made a deep bed full on any unwanted organic matter: old grass rugs, chicken droppings, sorghum and maize stalks, sheep and goat shed straw and poo, chopped down banana stems, old leather sandals and shoes, paper and wood shavings. Eventually it formed a bed about 20 cm high. We covered it with cardboard and then left it to the microbes to break it down and the worms to mix it for a few months. Then we grow all sorts of wonderful crops on it for several years. Might we not consider making a sheet mulch bed somewhere on the farm by collecting all sorts of unwanted organic matter over the summer: cotton tee shirts, paper, cardboard boxes, coffee grounds, garden weeds, leaf mould, sawdust, straw, urine and biochar? We could then make a deep bed in the autumn, to rot down over winter and grow in next year? By making one sheet mulch bed a year for several years we could slowly move to reduced digging this way!

 3. Focus on building soil resilience.

One of the features of climate change is that the seasons are becoming more unpredictable. Since we began at HCF we have had wet winters, dry winters, very cold late springs, warm early springs, droughts in summer, really damp summers, typical autumns and indian summer autumns! We have had to become flexible with planting dates and careful about hardening off crops grown in the polytunnels. We have lost whole sowings of crops to freak weather and we've lost plots full of potatoes to blight in damp summers. As we are becoming more resilient to climate change so we need our soil to be more resilient: able to cope with water stress, flooding, frost, snow, humidity, heat and sunshine.

Studies show that organic growing systems get around 30 percent higher yields in periods of drought than con­ventional systems due to the increase of SOM and its ability to capture and store water for crops. What causes this higher yield? We know that organic matter holds anything between 4 and 10 times its own weight of water. This is partly because organic matter acts like a sponge and partly because organic matter particles have a charged surface that attracts water so that it adheres to the particle surfaces. So if we can increase our SOM at HCF it is most likely that our soil will be able to hold more water and maybe the plants will be able to keep going without being watered for another 3 or 4 days. This could help a crop survive through a dry spell.  Similarly, after a very heavy period of rain the soil might be able to hold more water and so reduce runoff to flooding rivers and the loss of precious crop nutrients.

So if we are to build soil resilience at HFC we may need to be more proactive in putting organic material such as manure, compost, mulch, crop waste, progro, leaf mulch, wood chip, straw, bio-char and other sources of organic carbon onto our fields and try to avoid letting the majority of it turn to carbon dioxide within a few months by digging or rotivating.

What are your thoughts?