Editor Mike Donovan recalls the talk on soil at the 2015 Oxford Real Farming Conference by Bruce Bell of the Scottish Agricultural College. Delegates were asked to bring soil samples so Dr Bruce might analyse them, and the event proved so popular the room in the Town Hall was packed. Unfortunately many farmers thought it a competition to find the best soil and so he was confronted with some examples of the best in the country. A few, including your editor, decided to take samples of poor soil, and the advice was very helpful.
Visual evaluation of soil
Bruce Ball specialises in soil physics and soil management at Scotland’s Rural College, and concentrates on compaction, structure, and porosity. Bruce says “Visual evaluation is something any farmer can do, without involving expense.” Evaluation can suggest remedial action. Bruce told his audience that soil is made up of 45% minerals; 25% air; 25% water; 10% organic material; 10% roots and around 8% humus. Ratios vary through poor structure and soil type. Evaluation involves extracting and inspecting a block of soil. He said Tom Batey, a farmer who has been an inspiration to him, tells people to “observe and let the soil tell its story”.
Comparing soil from the the centre of the field with a block taken from under the fence line, corner, or other area of the field not rolled over with a tractor and/or heavily treaded by cattle is a useful exercise to see the damage which has been created over the years. Soil block extraction Dig at least a spade spit deep, and more if the field has had some deep cultivating and heavy traffic on it. The block needs to be about 6ins wide and be lifted out carefully and placed on a plastic sack. If the soil is hard it might be necessary to get the block from the side of a previous hole.
What to look for If the block has a uniform structure the signs are good, but you need to take more than a passing glance. Soil that has been affected by traffic and other use will have horizontal layers of different structure. The depth of these layers needs measuring and noting down. Breaking the soil in the block reveals more information. The first stage it to move the block on the bag gently and see how and where it fractures. This can show layers which may not be first visible, and you can then pick out lumps from different depths of the soil and take a look at their shape. Breaking lumps into 1/2in – 1.5cm – fragments will show whether their porosity, root patterns.
Shape of soil pieces It’s obvious that you want to see the soil as it is present in the field, so parts that have been squashed by the spade and boots are not representative. Similarly when handling the need is to do it gently. Angular patterns, smooth surfaces rather than pitted indicate problems. Plant roots in the block indicate the quality of the soil. Roots that are clustered, which turn at sharp angles, that are thick and short all indicate soil problems. Soil colour is very important, as that which has been starved of air goes grey, and can smell of sulphur and quite frequently there are ferrous ions present as well. Colour varies with the mineral type. Soil smell is another useful measure of structure. He described three different smells – like leaf mulch, a kind of woodland scent; like old compost which is perhaps slightly more acidic; sulphurous with a hint of rotten eggs in there, and it is the last which indicates problems.
Soil scoring
Scoring the sample starts with the ease of block extraction, the aggregate shape, roots, colour and the ease of fragmentation. Soils have a wide variation of types and qualities, and there’s no point in expecting yours to compare with the best – unless it is the best! All are susceptible to damage, and the damage will show itself in the same manner in each type of soil, which is why visual evaluation is viable.
Improving damaged soils, remedial methods
Bruce had less to say on the remedial work – but then his talk was on evaluation. Yet putting soils right was perhaps the reason for many of the audience being there. Sward lifter warnings He had some words of warning about sward lifters – the Sumo is being well advertised and we featured a home designed one from Gwyn Scourfield, in Whitland. Good grass swards have prime roots which go down a long way. If a lifter is pulled through it the roots get cut and this obviously damages the plant and so reduces grass yield. Lifted swards are also more susceptible to poaching, as the soil can be quite aggressively moved upwards, leaving it less capable of carrying weight.
Aeration with a spiker Bruce explained the value of getting air and moisture into the soil. He said that aeration was valuable, and put up a picture of the aerator your editor made in 1988! Spiking can’t harm soil. Spiking is less drastic than subsoiling and sward lifting. If the soil is damp and smears, the slits are going to be less effective than if the soil breaks like scooping cold ice cream. But the soil which is dry takes a lot more weight to get the spikes in. We used the spiker once or twice a year over the whole farm, but were far too ignorant to understand the value of digging holes and looking at soil strata. I would probably have been horrified at what I was looking at. Aerating worked well absorbing rainwater after thick slurry spreading. Heavy rain all day was going to wash the slurry into the stream, which was monitored less than a mile away for pollution. The spiked holes absorbed had the rainwater and slurry – result: a saturated field which took 10 days to get dry enough to plough, but no officials called!
Getting the camera out Taking pictures over a period of years will show the changes that happen over time and Bruce says it can be a simple way to keep a check on progress.
Broader issues
The trend has been for farmers to rely entirely on soil sampling and analysis, generally done free of charge by their fertiliser supplier. The pH, NPK and S results are a long way from what can be provided to guide the farmer to the most effective ways to manage their soil, not simply for the crop they are planting, but for crops in the future.
This report first appeared in Practical Farm Ideas Vol 23-4 Winter 2014-15
