I’m using a wet day to take the time to commit to writing my thoughts on soil fertility and soil building.
I’ve not made a secret of my belief that the ‘digging is bad for the soil’ argument is too simplistic. I liken it to the way that you warn a child about fire by repeatedly saying ‘HOT’. Eventually your child will need to know how to make and use fire for themselves. In gardening terms the argument that digging is harmful is fine as a general warning, and for people who are happy not to delve deeper into soil and its’ workings, but once you have a decent knowledge of the subject it is possible to be more specific.
The arguments against digging condense down to:
- It harms soil structure.
- It ‘consumes’ humus and organic matter.
- It harms soil organisms.
I actually agree with most of these to a degree, but all can be countered.
Harming Soil Structure
Good soil structure can only be harmed if it already exists. If it doesn’t then creating a better structure is needed. Soils that might be improved by digging include heavy clays, some silts, and any soil compacted by heavy machinery, or by repeated cultivations at the same depth creating an impenetrable layer.
Loss of Organic Matter
Organic matter is consumed more quickly when more oxygen is introduced to the soil, including by digging. However the nutrients contained in organic matter are only made available to plants once they are consumed by microbes. Therefore the more that is consumed, the more available plant food there will be. Provided that organic matter is added to the soil at a rate that replaces or exceeds that which is consumed, the net result is an increase in plant nutrients. One of the aims of creating a good soil structure is to improve the movement of soil gases (air) within the soil, so it does seems a bit contradictory to use increased aeration as a argument against digging.
Harming Soil Organisms
If it were possible to kill bacteria by removing oxygen, it would be easy to sterilise everything by immersion in water. The reality is that bacteria do not die when conditions are unfavourable, they go dormant, and in the dormant state they can still be eaten by microvores, releasing nutrients for plant use. Providing that the soil is worked properly, double digging without inverting the top layer, most soil organisms will be relativelyunharmed. Repeated cultivations will harm mycorrhizal fungi, but much of the damage can be mitigated by minimising disturbance, and by maintaining a constant supply of active plant roots throughout the soil profile.
The problem with no dig
With a decent soil structure, and lots of earthworms, not digging is a good option. My concern is the way that it is done. Most no dig systems use compost as mulch. Charles Dowding recommends a six inch layer initially to suppress weeds, followed by an annual application of two inches. In How to Grow More Vegetables, John Jeavons writes (and has proven) that a 1/2inch layer of compost incorporated into the soil produces a high level of sustainable soil fertility. This equates to 4 cubic feet of compost or 2/3rd of a wheelbarrow each year for a 100sq foot bed. In Growing Green, the authors recommend using a wheel barrow every second year depending on crop rotations, for a similar sized space.
Jeavons is attempting to grow his own biomass from within the growing space, whereas the level of compost used in a mulch type system requires the importation of huge amounts of organic matter, effectively ‘consuming’ considerably more land to produce food, and at least four times as much organic matter. That fertility needs to be replaced, and is likely to be done using chemicals, reducing overall sustainability, somewhere further along the supply chain.
When to dig
I would dig if I felt that the soil was too compacted for the majority of crops that I grow. A simple test is to take a short bamboo cane and push it into the soil. If it penetrates more than 12 inches (18 is ideal), without considerable force, there probably isn’t any benefit to digging/double digging. If the soil is penetrable, but there is a shortage of earthworms, there may be some advantage to incorporating some of the organic matter into the soil, but I’m not sure.
My own developing system
The way that my own system is heading comprises of the following:
- Opening of the soil structure to allow penetration by plant roots.
- Filling the newly opened pore spaces with plant roots.
- Maintaining a permanent plant cover with active roots.
- Growing crops with a high biomass and carbon content.
Opening of the Soil Structure
It is possible to open compacted soils with some plants. My own experience suggests that chicory is effective in my clay, with roots reaching 12 inches or more. However this is nowhere near as deep as roots can potentially reach, and if root run is limited, so is the amount of soil minerals available to plants.
My plan is to double dig the vegetable beds, but not exactly as Jeavons suggests. He spreads ½ an inch of compost onto the soil surface prior to commencing. This means that it will remain in the upper layer of the soil. I want to get organic matter deeper into the soil profile, not to become plant food, but to keep channels open for plant roots, until they are able to get there and do the job for me. For this I am putting high carbon material like straw and chipped wood into the bottom of the trench before mixing them in. I am adding a little less compost to the upper layers to compensate for this. This is being done a little later than I would like (October) this year, as I am a bit behind. Ideally I would do this in late August so that the subsequent growth of plants will be more rapid.
Filling the pore spaces
If soils are dug late in the year, and then left unplanted, soil nutrients can be leached, and the smaller soil particles (clay and silt) can be washed down through the soil creating a hard pan. So I am planting straight into the prepared beds, using plants like cereal rye, winter wheat, broad beans, chicory, and red clover. These are all relatively deep rooting, with the rye and beans suitable for planting late in the season. The chicory and red clover are best sown by September, or left until the following Spring. The rye in particular is great at mopping up nutrients, and actively grows at temperatures as low as 3 C. A sowing of White clover then aims to produce a weed suppressing ground cover.
This is the first step in a rotation.
Maintaining Plant cover
This is proving the hardest part of the system to design. My system aims to continue with a few years of winter grains, planted immediately after harvesting of the previous crop, into a layer of defoliated clovers and chicory. These do need to be thinned, and defoliated regularly in order to make nutrients available to the crops, and to stimulate additional growth. The problem comes later, when I plan to grow potatoes, followed by Alliums and roots. The onion part is fairly simple. Early potatoes can be followed by leeks and late carrots and beetroots. Late potatoes will need to be followed by a winter green manure and then spring sown roots. The green manure is problematical, as it needs to be sown late. Beans/clover will potentially provide too much Nitrogen for the following crops. Growing mustard or spring oats, which will be winter killed are potential crops, but a dead crop isn’t necessarily helping to maintain mycorrhizal fungi. An early maincrop potato variety may be the answer, allowing a wider choice of green manures to be used.
Another option is to keep all of the plants that grow well with an intercrop in one rotation, and those that do not in another. This means accepting a less than ideal solution, and just getting on with it.
Growing high carbon/biomass crops
The Grow bio-intensive system of Jeavons relies on utilising 60% of the growing space to produce high biomass crops. These include grains, broad beans, and sunflowers. All are crops relatively high in calories, and most are also crops that need a lower level of soil fertility. By growing taller varieties of grains and beans, I aim to produce all or most of the organic matter that I need, in order to maintain my soil fertility. This is accomplished by concentrating the fertility for the crops that need it, and following a period of increasing fertility with crops that need a diminishing level of fertility, until starting to build fertility again.
How might this build Soil Fertility
My rotation is likely to be:
Broad beans and cereal rye – winter wheat – rye or winter oats – winter wheat or spelt or emmer – Corn and winter squash – potatoes – onions and roots – repeated. This doesn’t include greenhouse/polytunnel crops or salads.
Before starting the rotation again, I intend to test my soil to see whether the structure has improved beyond the point where further digging is needed. The latter may be a problem as the final plants in the rotation, especially the alliums, don’t have a particularly extensive/deep rooting habit. I cannot predict that and will have to wait and see.
Before anybody goes out and copies this, especially the rotation, I am only at the early stages of testing my thoughts. Potential problems include inadequate fertility for the high yielding crops, especially corn, and potatoes. I also need to produce effective ways of defoliating clover grown amongst cereal crops. However the system has potential, and will certainly keep me busy for the next few years.