Market gardener Max Johnson wrote this piece for us on taking a closer look at your soil, and how to get the most out of it in the short and long term.
If fields are worked with machinery while soil is too damp, it can result in compaction, which in turn means worse drainage, reduced ability of plants to access nutrients, and can even result in denitrification.
Soil life such as earthworms, bacteria and fungi are vital in soil structure and nutrient availability.
pH affects the ability of plants to absorb various nutrients, and that different crops prefer different pH levels.
Picture: Paul Flynn with field lab attendees. CREDIT: Neil Ferguson
With the wind battering down at Locavore’s Left Field farm in Neilston, this was a suitable day to blow out the cobwebs and get back to the elements.
Perfect, then, that today’s Field Lab was focused on one of the most fundamental topics: soil.
Growers and farmers across the range of professionalism, experience and scale gathered to hear Paul Flynn, the Soil Association’s Crops and Soils Advisor, take a closer look at soil and explain how we can get the most out of it in the short and long term. This was combined with a visit to Left Field to learn some practical ways to get to know our own soils.
Moisture content and compaction
We learned to listen to your soils rather than the market pressures pushing you to work your ground too early or too hard. For example, if fields are worked with machinery while the soil is too damp, it can result in compaction, which in turn means worse drainage, reduced ability of plants to access nutrients, and can even result in denitrification.
Of course, this risk is not as severe with no-till/no-dig approaches in which use of compaction-causing machinery is minimal or non-existent.
In remedying damp soils, Paul urged the use of green manures, which stop rainwater from reaching the ground and allow lots of water to leave the soil through transpiration. On the practical side, we learned how to carry out a basic water infiltration test.
A water infiltration test for your soil
A quick method is to dig a hole one spade deep and one spade wide, pour a bucket of water in, and then time how long it takes to drain. A second fill is normally made before timing, but on clay soils this may take a long time! Typical drainage rates vary from 30mm per hour to 1-5mm/hour.
Done properly, a cylinder is inserted to eliminate water movement to the side. On farm this can be as simple as a piece of down-pipe.
Picture: Locavore market gardeners Louis, Floortje and Beth
Soil structure and soil life
We then looked at soil structure and life, which are key not only to drainage issues but also to plant health and productivity. While quick fixes for a poor soil structure (such as lime and gypsum) were recommended, in the medium to long term Paul outlined how soil life such as earthworms, bacteria and fungi are vital in soil structure and nutrient availability.
Farmers should aim for ten worms per spade, alongside healthy soil bacteria and fungi populations.
A good tip for gaining healthy soil life populations was to ensure there is always organic matter on the soil and to minimise rotavator use (which kills fungi and two thirds of earthworms with every pass!). At Left Field, Paul then demonstrated practical tests for gauging soil composition, compaction, structure, and life.
Practical tests for soil structure
Remember that the soil can be compared to that at the field margins in hedgerows or adjacent woodland as a “reference sample.”
Soil profile test. If you are buying or renting new land this might be your first test, digging down through the soil profile, taking care at least on one side, not to smear the surface. The act of digging will tell you a lot about your soil. It will also give you a clear idea of your earthworm population – the type of earthworm, their numbers and age range, burrowing depth and potential aeration. Rooting depth can also be seen, with roots gathering above any compacted layers. A pen knife or trowel should help you identify compacted areas and the smell of any anaerobic layers should be obvious. Be aware that soil type can change at depth and may affect the performance of the top soil. Sometimes the test pit will fill with water, indicating a semi-permeable layer of soil, this indicates a good response to any drainage work at this depth, allowing installed drains to draw water from the surrounding area. This is something to celebrate rather than fear.
Take a large handful of soil and add water until it can be rolled into a ball. Press down on this ball. If it crumbles easily, you have a sandy soil.
Next, form the soil into a ribbon, squeezing it between the thumb and forefinger in an attempt to push it into a flat ribbon that dangles off the side of your forefinger. See how long you can get this ribbon to be before it breaks off. A true clay soil will stay together for several centimetres before breaking.
Spread the soil on to the palm with your thumb or rub it between your fingers. If it feels soapy, this indicates a silt soil.
Jar test. Dry your soil sample, process it with a mortar and pestle. Place it a jar add water, non-foaming detergent and stir thoroughly. Leave it for a day, after which the sand, silt, and clay will have separated out, allowing you to see your soil composition. If the layers cannot be distinguished by colour and texture, repeat the test marking the level of the soil after five minutes, two hours and then one day, this should correspond to sand, silt and then clay levels. Measuring these in mm will give the proportion of each.
Compaction test. Paul used a penetrometer to measure the amount and depth of compaction. This can be done at much less cost, using a steel rod, approximately 10-15mm diameter, ideally with a T-bar welded across the top, inserting the bar in to the ground to get an immediate sense of compaction.
Cigar test. This was Paul’s favourite test, attempting to roll a “cigar” of soil from a good handful of soil. If this is possible, the soil may be too wet to work with, where it breaks down to a friable tilth without a cigar forming the soil can be worked with less risk of compaction.
pH and nutrient balances
Next up, we looked at soil pH and nutrient levels. The key issue here is that pH affects the ability of plants to absorb various nutrients, and that different crops prefer different pH levels.
The pH can normally be adjusted within a year or two, and here Paul proposed the use of lime, especially for those renting land, due to its cheapness and easiness to apply.
With regards to nutrient levels, rock phosphates and compost were recommended and, again, green manures were brought up - this time as a means of resting and re-fertilising fields.
To test pH, Paul recommended splashing out for a proper laboratory soil test, which he said was well worth the small investment to have certainty about what’s going on in your soil.
Despite us all hailing from fairly diverse farming and growing backgrounds, there was a sense that we all took a lot from the day, whether in theoretical knowledge or practical techniques. There was also a sense of respectful awe for the enormity of the topic at hand. The message, though, was simple and humbling: look after your soil and it will look after you.
Thanks to Max Johnson for writing this piece for us. Max attended this event as part of our Less Toil Better Soil Field lab, which is exploring ways to increase crop yields, improve soil health and reduce the need for weeding in small scale vegetable production, working with social enterprise Locavore.
