Soil texture

A few years ago I took on an allotment. It was a new site and had been operating only a year when I started; I took over a plot that had proved too much for the first tenant. At the start it had been shallowly ploughed, turning the grass over but not effectively burying it. The first occupant had attempted to cultivate less than a quarter of the plot, then given up.

When I started, in April 2013, the first thing I did was to dig it from end to end, burying all the weeds about a foot down. As fast as I was able to prepare ground, I was getting crops into it. I dug fast enough to crop 3/4 of the plot in that first season, the rest I finished too late to plant any more.

I quickly became acquainted with the characteristics of the soil and while I dug it over again in the second season, by the third I had adopted a no-dig approach. It was clear to me that preserving soil structure was key to continuing success and that the nature of the soil made it especially fragile. I made some assumptions about the nature of the soil but at no point did I check that those assumptions were correct. I have never tested the soil for nutrients or for its physical properties. The same holds true for our garden, I have become familiar with the character of the soil without ever attempting to quantify it in any way.

I recently bought a book by Robert Pavlis called “Soil science for gardeners”. In a section on identifying soil problems, he gives a method for analysing soil texture that I had the equipment to carry out. Soil texture refers to the relative amounts of sand, silt and clay in a soil and has a huge bearing on how the soil behaves in respect of holding water and nutrients, and how it should be managed. I thought it was about time I checked my assumptions and found out what I had. There are many aspects of soil that can be tested but most are expensive to do accurately enough to be useful so are only carried out for larger scale agricultural or horticultural operations.

The method is called the Jar Method.
Put a sample of soil (with rocks, roots and mulch removed) in a flat bottomed jar. Fill the jar 3/4 full with water, add a pinch of table salt, give it a good shake then let it settle.
After one minute the sand will have settled, mark the height on the jar. After one hour the silt will have settled, mark that height on the jar. After 24 hours the clay may have settled, it may take 48hrs, mark that level. Work out the relative proportions of each.
Somewhere along the line I acquired a large glass measuring cylinder of a type familiar from chemistry lessons at school. It would be a perfect vase for tall grasses and has served as such in the past, but here was an opportunity to put it to proper use. Moles had been busy alongside my plot, I took some of the soil as my sample.

While admitting that no such thing exists, Pavlis describes an ideal soil as having 40% sand, 40% silt and 20% clay. Mine seems to be coming out at around 50:50 sand:silt, except that since I was unable to sieve it such that only particles below 2mm diameter were included, part of the sand fraction is technically rocks. My finest sieve has a 3mm mesh and that is what this mix has gone through. It could be 40:60 sand:silt. There is a tiny amount of clay but it cannot be more than 2-3%. It’s just a film on top of the silt layer.

The “sand” fraction is not sand as commonly understood. Beach sand or building sand is made up of roundish particles of quartz. What I have is small pieces of the local mudstone/slate bedrock. The silt will have also been produced by breakdown of the bedrock into even finer particles but clay is produced by a different mechanism which doesn’t seem to have played out in my soil.

Clay has a far greater water and nutrient holding capacity than either sand or silt, as well as being an agent in binding soil particles together into aggregates. In the absence of clay, organic matter will perform the same functions but lacks the permanence of clay and must be continually replenished.

I have always been aware that my soil dries out surprisingly quickly after rain and that it’s structure; the aggregation of its basic material into crumbs, is fragile. I am aware that it doesn’t hold onto nutrients well. These characteristics have always suggested a silty soil but even so I am still surprised at the almost complete absence of a clay fraction. I think I have been doing the right things to manage it to best effect; not digging, keeping the organic matter level up, protecting the soil surface, staying off the soil when it’s wet. I can and should do these things better. A green manure crop that grew all winter but was easily killed in spring would be a very useful addition to my armoury, protecting the soil surface, holding onto soluble nutrients and adding to the organic matter.

Pavlis describes aggregation as a very critical part of good soil. He says that when you have good aggregation you will grow good plants and when you don’t the soil will perform poorly. Creating good soil is all about improving aggregation.

I shall carry on to the end of the book, then read again those parts that seem most relevant. There is a greater understanding of the soil ecosystem now than there was when I started gardening, especially regarding the role played by micro-organisms. I seem to remember the worms getting much of the credit for doing good things in soil; now it’s bacteria and fungi that are getting recognised as doing a lot of it, as well as the plants themselves.

Growing vegetables is quite a long way removed from anything you would find in the natural world. I am quite surprised that nature has thrown up many annuals as it is hard, at least in the UK with its moist temperate climate, to see many natural niches they would fit into. Even so, I increasingly find myself looking to nature to inform what I am doing in the garden, even when I am doing something very unnatural. I don’t want to feel I have to choose between a scientific or a natural way of doing things, it seems like a false dichotomy. The more that science uncovers, the more remarkable the natural world becomes. It’s had a very long time to hone its methods and when you think that it came up with photosynthesis and eyesight and brains and butterfly migration with no help from humans at all, it is due a monumental amount of respect.

Reference: Soil science for gardeners, working with nature to build soil health. Robert Pavlis. New Society Publishers.

7 thoughts on “Soil texture

  1. You could have some of my clay, but maybe I should just keep adding compost, etc, and let the very many worms work on the soil. I’m fairly happy with mine. Always interested to read your articles.

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  2. I thought this article was extremely interesting, than you. I fully intend to test my soil too, I suspect there is virtually no clay in it either. It always feels slight amazing that stuff grows as well as it does anyway!!

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    1. It is amazing what plants will grow in and how good nature is at taking a substrate that we would struggle to grow anything in and turning it into fertile soil. Like Fraser Island, pure dune sand growing 100ft trees.

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  3. Excellent post – thanks! Where I live (and work) there’s a complete change in the soil within a very short distance.
    The higher ground sits on a sandstone ridge (which runs roughly north to south through the whole of Birmingham) and has very sandy soil which sounds similar to your allotment.
    The lower ground is thick clay (which sits on Keuper Marl mudstone, if I understand the geology correctly!). The clay is so much harder to work with and prone to water logging, but ultimately suits many plants a lot better – roses being an obvious example!

    I ought to follow your example and investigate further. Thanks again, that was a great read!

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    1. Realising I only had the Geological Survey map for the area next to ours, I found their website and you can access all their maps online for free, which is excellent. After shaking my sample a second time and letting it settle, I ended up with a bit of a clay layer, perhaps 5-10%, which was perhaps stuck to the coarser stuff at first? It’s mostly very uniform around here, except where lava intrusions have done a bit of metamorphosis.

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