Friday, 7 October 2011

Thoughts on digging or no digging strategies.

I have mixed feelings about this debate.

The modern fashion for using raised beds to grow vegetables can be seen as a method of avoiding digging.  The no dig system seems to have been developed, like so many other gardening techniques, by trial and error.  Then the amateurs that developed it become evangelical and the gullible see it as a some magical method; following the method's disciples' recipes and instructions as if there were no other ways of cultivation.

It is just multiple layers of this and that built up within  planked containers to make  higher than ground level growing areas.  And lots and lots of paths.

I do not think that there is anything magical in the materials that are used to develop these beds.  Indeed filling a raised bed with commercial multi purpose compost or ordinary garden soil seems to get good results.  However, there seems to be a consensus that you start off with a layer of newspaper to suppress weeds and then cover  with compost or other organic material such as straw or hay in layers with some addition of chicken manure.  The layers are built up until the correct height is obtained and a surface layer of compost or soil tops it off.  Certainly it is a very  intensive organic matter form of gardening.  I should be really enthusiastic about this system of growing.

It is a quick and relatively easy way of cultivating an area of weed infested ground because weeds are suppressed by the newspaper in the raised bed and paths are covered in weed suppressing membranes.

It also emphasises the importance of creating and maintaining a high level of soil organisms such as nitrogen fixing bacteria and mychorrhizal fungi.  Again this is something that I agree with and would like to promote on my allotment.

The use of mulches and green manures should enable the beds to reach a high level of fertility and this might be maintained over many years. That is; leaching is reduced and chelating humin, fulvic and humic acids will form complexes that will trap minerals in the soil structure.

Thirdly you do not have to step onto the bed to cultivate it having access from paths constructed around the beds.  This means that the soil is never compacted and is always aerated and well drained.

There is some debate that says that the soil is rarely disturbed in nature and has not evolved to cope with being cultivated to the extent it is on an allotment.

So why am I apprehensive about following the fashion?

I have raised my whole allotment about 600 mm above the original ground level and this does indeed seem to improve the drainage.  However, I have dug down quite deeply too and broken up the subsoil to quite a depth and this might be the main factor in improving the drainage.  Also, I have added a lot of organic matter in the form of logs, branches, leaves and branch shreddings to the subsoil in a kind of Huglekulture and that too might have improved the drainage of the allotment. (Oh yes, and Huglekulture does not have to be constructed on the surface of the soil.)

Adding large amounts of organic matter does seem, anecdotally, to improve the fertility of the soil.  I would conjecture that this is some part due to the provision of carbon for free living nitrogen fixing bacteria such as Azotobacteraceae.  These bacteria are fairly ubiquitous and once they have a carbon source will multiply rapidly whether the organic matter is on the surface or mixed into the top soil.  I doubt very much if digging would severely deplete the numbers of bacteria in the soil especially if it is combined with the addition of organic matter.

I can see that the repeated addition of inorganic fertilizers in place of organic amendments would indeed reduce the number of soil organisms by reducing the amount of carbon available to the heterotrophic soil fauna and flora. However, there are few gardeners now that will use inorganic fertilizer to the extent they were used in the past.  

I would also suggest that a sizeable proportion of soil organisms can survive digging because of their relative size. It is the macro organisms such as worms, soil centipedes, etc. that would indeed be affected by digging. Yet some of these macro organisms may be pathogens and we might want them to be removed from the soil.  Such animals as slugs, snails and pathogenic fungi would fall into this category.  It is noticeable the difference in the number of worms in an undisturbed soil compared with cultivated soil. This is a major disadvantage of digging.  I would suggest though that any cultivated soil would have a reduced number of worms.

In one text book I read recently, there was a chapter on raised bed, no dig growing system giving chapter and verse about how good it was to grow in this way and went on to say that after making a raised bed why not plant green manure to start with...

Now, on several gardening, forums people are going to extraordinary lengths to grow green manure while not digging it into the top soil.  Some suggest strimming and leaving the tops on the surface to rot down or putting a membrane or cardboard over the surface to kill off the green manure.  Others are suggesting that the green manure is hoed off and the tops put onto the compost heap.

I find this incredibly unpractical and self defeating.   The green manure is there to add carbon to the soil in a form that is readily available to microorganisms.  Any nitrogen fixed by legumes will be removed by hoeing off and putting on the compost heap.  Covering or strimming are just adding slug and snail encouraging complexities unnecessarily.

Single digging, slicing off the green manure to put at the bottom of a shallow trench seems to me to be the simplest  and most practical option of mixing the green manure into the soil.

This introduces another important part of using digging as a strategy in the vegetable garden.  Digging done well will mix the soil.  Rotavators do this particularly well.  Any organic matter will be mixed relatively evenly throughout the top soil making it available to organisms throughout the profile.  When it has finally broken down into humus, this will enable chelation and possibly enable plant roots to access the nutrients trapped in it.    There is a great deal of evidence that top soil with large amounts of the black, oily liquid we call humus coating  soil particles is particularly fertile.

Apart from nutrients obtained from decayed organic matter there is also a source of minerals that can be obtained from the break up of stones within the soil by weathering.  I am not sure whether digging can be classified as a weathering factor but other factors such as wind and water seem to have an effect through friction.  I would suggest that digging also breaks up soil minerals by the action of friction releasing nutrients from stones and possibly rocks in the soil.  Digging also exposes the soil to the action of frost and freezing and thawing effects not only breaking up compacted soil but also releasing nutrients from stones in the soil.

There are some times that cropping means that effectively you are digging over the soil.  For example, taking out potatoes.  How on Earth do you do that without digging quite deeply down into the soil?  And then how do you ensure that you have removed all the little potatoes that will undoubtedly survive the winter and begin to grow again right in the middle of another crop and possibly harbour pests and diseases to infect the new years potatoes?  The only way I know is to dig over the potato bed very carefully. The same argument could be put forward for carrots, parsnips and other root vegetables.

There are some that grow potatoes under straw or membranes and harvest from under these.  Well I cannot justify covering the area of potatoes that I grow with membrane or straw for that matter.  Small crops on little beds may make it a little more practical but why add these complexities unnecessarily just so you can say that you don't dig?

The argument that the soil is rarely disturbed in nature is an uniformed view of how nature works.  If you had seem the damage that badgers do to a lawn you would never subscribe to this notion.  Foxes looking for worms on my allotment are continuously digging holes as are cats when they do whopsies.  Take that up to larger animals such as wild boars and deer and we can begin to see that the soil has been continuously turned over and this has happened from the earliest times.  Indeed dinosaurs could make a serious mess of soil structure.  That is why plants called ephemerals evolved; to exploit disturbed ground.  These are plants that we commonly call weeds.

I am really confused about why walking on the soil is such an anathema to no diggers especially as most of the ground in a raised bed system is covered in very compacted, membrane covered, soil paths.

Compaction of the soil is not to be encouraged but I made temporary paths along the sweet pea rows this year compacting the soil quite a lot.  This did not stop the weeds from growing - unfortunately.  Alongside my allotment is the trackway along which cars are continuously being driven up and down.  I mow the trackway regularly to make sure that the weeds do not take over.  Compaction does not seem to have affected the growth of these plants.  There is obviously some relationship between the optimum amount of water and air that should be in the soil and this might be affected by compaction, however to actually squeeze out all the air and water from soil is incredibly difficult - unless you are growing on a clay soil.

I have seen videos on YouTube - old gardening videos where the soil is consolidated by treading along the sowing lines before raking and the drill taken out.    This is what my grandfather and father used to do and I copy them even now when preparing a seed bed.  My seeds still germinate as did my grandfather's and father's.

I do have paths on the allotment; one I made myself and three that were inherited.  After having thought a great deal about them, I dug out the top soil and replaced it with stones covering them with subsoil and putting concrete slabs on top.  This meant that the top soil from under the paths could be added to the growing beds and I would have drainage channels across the allotment under the paths. There is no way I can reach the whole of the beds from the pathways though and I have to tread on the soil to reach their centres.  Does not seem to affect how the plants grow.  If you doubt look at the allotment photographs for April, May, June and July.  I do try to keep off the soil if I possibly can because it makes it easier to work the soil when it is necessary.  I would not like to hoe up the potatoes using compacted soil. But this is all I will concede -  keeping off the soil makes it easier to work

Digging does not prevent you from using green manures and mulches.

I am not rejecting a no dig raised bed system for growing, however I would not advocate that it is the panacea for all growing problems.  There are benefits to digging and sometimes it is the only effective procedure for preparing the soil for crops.  Sometimes, when the soil is clean from the previous crop, the ground will only need hoeing and raking over to make it suitable for the next crop.  I think that this is what I will do after all the brassicas are taken out next spring and I will plant peas and beans in the ground.

So I might be a grumpy old man stuck in my ways and clinging to the past but I see no advantage in making things too complex and creating work for myself.  With the large areas of crops that I grow, I will use the best method of preparing the soil and sometimes that will mean digging.

1 comment:

  1. I think you're confusing two different things, like a lot of people. A no dig system involved no, or to be realistic, minimum, digging, and usually a lot of mulching. I find it's effective as long as the perennial weeds are kept at bay. There's a lot less weeding, and it's easy to pull out what does grow. A few inches of mulch disappears in a few months, while, on my plot at least, the same amount of organic matter dug in was still there, apparently unchanged, when I dug it again a year later. All the organic matter helps to build up fertility.

    The other thing is raised beds. These are worthwhile if you suffer from waterlogging, as I do in winter, or if you have trouble bending. Otherwise I regard them as a waste of time.

    I don't think there's any such thing as a cure for all ills. No-dig is still a lot of work, with a formidable amount of mulch to barrow every year, and a certain amount of digging when roots have to be lifted.