"Feed the soil, not the plant" is an old mantra of organic farmers that is as relevant today as it was 50 years ago"
Building soil carbon is relatively straightforward: minimise carbon losses to the atmosphere, and maximise additions of carbon to the soil. Preventing carbon losses is commonly overlooked but is of critical importance. Soil carbon is converted to carbon dioxide by oxidisation, the most common causes being deforestation, erosion and cultivation.
On my farm on the Isles of Scilly, I grow a range of organic fruit and vegetables using both mechanical and manual cultivation. My overall strategy is to minimise the depth and frequency of cultivation, and use cover crops and plastic mulches, reducing erosion and exposure of my soil to oxidation.
In Oxfordshire, Julian Gold grows arable crops on the 800 hectare estate he manages. But he is very serious about looking after his soils, and has been working hard to reduce chemical inputs and increase soil carbon whilst maintaining profitability. He uses satellite guided tractors that only drive over a fifth of any field, minimising tractor tyre pressure and soil compaction. No ploughs or rotavators are used, only shallow discs and harrows. This has led to a significant increase in earthworm populations and improved soil quality.
Soil carbon and climate change
Agriculture is a major contributor to carbon emissions, but the impact of farming on climate change can be reduced. Farming and forestry are almost unique as industries that could absorb more carbon than they release. The atmospheric carbon that could be absorbed in well managed soils is extraordinary. Soil carbon expert Rattan Lal estimates the potential for soil carbon sequestration across the world as "equivalent to a draw - down of about 50 parts per million of atmospheric carbon dioxide by 2100". This amazing figure proves that fixing carbon in soils is one of the few practical means we currently have to actually reduce global atmospheric carbon dioxide levels. Building up soil organic matter is a win-win situation for the fight against climate change as well as soil health and crop yields, and must become the focus of farmers everywhere.
The next step maximises carbon inputs to your farming system. In temperate areas the main ways are adding compost, manure, biochar, green manure and cover crops.
Rob Richmond is a dairy farmer in Gloucestershire who has increased soil organic matter at an extraordinary rate whilst maintaining high milk yields. He studied how to increase soil carbon on a worldwide tour, and adapted practices he witnessed on his own farm. Rob talks about three types of organic matter, green, brown and black. Green carbon includes lush cover crops, which are good food for soil bacteria. Brown carbon includes crop residues, mature cover crops and animal manures that become stable organic matter. Black carbon is the most stable form, including mature compost and biochar, and has a very important role in soil stability.
My own farm is next to the sea. I apply large amounts of seaweed, an excellent source of organic matter for my dry sandy soils. Like many organic vegetable growers, green manures are also an important part of my crop rotation, with a quarter of my land at any one time being under leguminous (nitrogen fixing) plants like clover, or non-legumes such as mustard and phacelia.
A diverse crop rotation builds good soil structure as it allows variations in cultivation requirements, nutrient demands and plant rooting depths, as well as providing opportunities for introducing green manures, and breaking up pests and disease cycles. Vegetable grower Iain Tolhurst in the Thames Valley has an extremely diverse rotation and needs to buy no manure and fertilisers. At least a quarter of his farm is covered at any one time with a two year green manure such as alfafa, and large amounts of organic matter is added when it is ploughed in. It's worth noting that perennial crops, such as fruit and nut trees, are also inherently better for soils, requiring little or no cultivation and sequestering carbon through their root exudates.
Improving soil health
Soil ecosystems are extraordinarily diverse and resilient, yet poorly understood. There are thousands of species of bacteria, fungi and insects in healthy soils, some beneficial to plants, others harmful.
Martin Howard farms 160 hectares in the Tamar Valley, and has seen life breathed back into his soils by a combination of minimising soil compaction from overusing his farm machinery, increasing soil aeration, and introducing beneficial bacteria and fungi using root drenches. He sows a diverse range of forage species, and applies compost and manure, and has seen steady improvements in soil structure, pasture productivity, animal health and yield. A well-functioning soil ecosystem is better able to turn organic matter into stable soil carbon, so a healthy soil is one that is better able to sequester carbon.
Measuring organic matter
Treat each field separately.
Measure in spring or autumn, avoiding hot, cold, dry or wet extremes
Measure at least a month after any cultivations. Take a sample core 30cm deep using a soil auger or spade but removing the top 5cm that may contain undecomposed organic matter.
Walk a W shape across the field, taking up to 25 samples in each field, mixed thoroughly in a bucket.
Remove weeds, stones or lumps or organic matter, and put about 0.5kg of this well mixed soil in a plastic bag, labelling it clearly with date and field number / name.
Send your soil sample immediately to an agricultural laboratory for soil organic matter analysis, asking for measurements by 'loss on ignition' with results to two decimal places.
Repeat same time the following year.
We recommend doing this every spring or autumn. Different fields may show different trends, so the farm as a whole must be considered by adding up measurements from all fields. With this, you can see whether your farm management practices are losing, maintaining, or building soil organic matter, and you can target management changes to individual fields. With an organic matter increase of 0.1% (e.g. from 4.0 to 4.1%) an extra 8.9 tonnes of carbon dioxide will be sequestered per hectare per year. This shows the huge potential of changing farming practices to climate change mitigation, while also improving soil health, yields and profits.
Rob Richmond has seen a significant increase of organic matter and improved soil structure by applying compost, growing diverse and deep-rooting grass swards, and 'mob stocking'. This is where a large herd of livestock intensively graze a small area of tall grass right down in a few days before being moved onto the next patch. Rob describes how, under the right management, pastures can sequester carbon dioxide at a rate of 20 tonnes per hectare per year. He uses a complex mix of forage species including clover, vetch, and alfafa that grow robustly, are good companion plants, and allow him to graze and rest his land for optimum efficiency. Furthermore, his soils retain more water and his cows are healthier.
Via Giri Kumar