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Arable

Counting on worms for a healthy soil status The humble earthworm could be the key for growers to properly assess and understand the health status of soils on their farms. Dominic Kilburn attends a ‘Soil Health’ briefing to find out why. Agrii’s Dr Syed Shah (left) and Andrew Richards check out worm populations.

Soil health – it’s a complicated business with a lot of unknowns. But as we continue to experience an increased number of extreme weather events each season – sometimes too wet, sometimes too dry – there is no doubt that the soil’s resilience, structure and productivity in this country is being tested. While the general concept of soil health is nothing new, or the understanding that repeated cultivations and growing of crops can eventually result in soil degradation, Agrii senior agronomist Andrew Richards acknowledges that growers in the UK need to develop more resilient soils containing increased levels of organic matter, if soil health, stability and structure is to be maintained or improved. “Research in the past has highlighted that organic matter levels in England and Wales are mostly in decline, however it plays a huge part in soil structure and its functionality,” said Mr Richards, speaking at a ‘Soil Health’ briefing held at the company’s AgriiFocus site near Marlborough in Wiltshire recently. “The problem for growers is that they need to know what to improve and how to do it,” he added. According to Mr Richards, a Defra-funded initiative at Reading University, which started in 2016, is aiming to develop a clearer definition of soil health and a partner set of indicators that can be used to

Key items for soil focus – Dr Syed Shah • Drainage and compaction • Organic matter (affects • • 8

compaction and soil biology) Soil pH (affects soil biology and nutrient availability) Worm counts

“develop policy, derive research and influence land managers”. “Many farmers are now asking about the health status of their soils while at the same time there are many different soil tests on offer including sampling and visual assessments. “But while there is currently no fundamental baseline measurement, everyone is going off in a range of directions and this is not proving helpful to farmers,” he stressed.

Worm life Agrii’s Dr Syed Shah has been involved in research looking at the impact of organic manures on soil health, and he pointed out that indicators of soil health needed to be sensitive to variations in land management, as well as reliable, easy and inexpensive to measure. “Healthy soil overcomes stresses, it is high in biological diversity and capable of maintaining nutrient availability and supporting the crop. But, while soil biology is an indicator of soil health, you can get different results from testing four different parts of a field giving huge variability, and so I have my doubts about the reliability of soil biology tests. “However, worm populations are a good indicator of a healthy soil and in my opinion more reliable than other soil indicators,” he added. Dr Shah suggested that in November and the early spring, when the ground is most likely to be soft, are the ideal timings for a simple test dig to assess worm counts. He reckoned that if worm counts are in the region of 150m2, it is an indicator of good soil health although he said that worm numbers will vary according to soil type (clay soil containing the highest numbers and sandy soil the least). In addition, he pointed out that worm numbers are usually lower in compacted and waterlogged soils, those which are acidic and where soils are low in organic matter.

Organic matter boost Agrii trials in February 2012 set out to assess the potential to improve worm populations with the addition of organic manure on silty clay land

near Swindon. Five treatments were applied to 150 x 12m plots and they included chicken manure, biosolids, farmyard manure and two rates of green compost (low 25t/ha; high 50t/ha). All applications were made in combination with the farmer’s standard fertiliser treatment. According to Dr Shah, the plots were incorporated three or four days after application of the organic manure and spring barley was drilled later in March. The same treatments were applied again in August that year, ahead of oilseed rape, and again in August 2013 ahead of winter wheat. Although all materials added the same amount of nitrogen, compared with the untreated plot (which only received the farmer’s standard fertiliser treatment) farmyard manure delivered an additional 1.5t/ha yield to winter wheat variety Gallant at the 160kg N/ha rate. “Farmyard manure was better than the other treatments as it was adding high levels of organic matter and supplying food for micro-organisms. It improved yields and soil health. “But different inputs have different affects – green compost for example supplied a good level of organic matter and more carbon, but not other things that micro-organisms in the soil like to eat,” he explained.

Effect on worms In April 2016, four samples per plot were taken to measure the weight of earthworms per m2 and all treatments, except chicken manure, had a beneficial effect on worms. “It shows that you must choose the right product according to soil type,” said Dr Shah. “Because chicken manure is high in acidity, it’s OK to use on high pH soils for balance. It won’t improve worm counts but will help with organic matter levels,” he said. “All treatments improved soil infiltration rates too but, again, farmyard manure was best. Overall it was the most reliable product on a variety of soils for infiltration rate,

Worm population is a good indicator of a healthy soil.

soil biology/earthworms and organic matter levels. “It’s better for the long-term because it improves the biology and soil structure, however it’s a case of horses for courses. Other products will be better in different situations – taking sandy soil as an example, compost is better suited for improving soil resilience and also where soils are deficient in manganese and trace elements. “It’s the quantity of product that’s important for sand whereas with clay it’s more about improving the biology of the soil, and that’s where farmyard manure is better,” he added. Dr Shah pointed out that biosolids were a good option for improving phosphate indices.

Cultivation comparisons AgriiFocus trials work also compared the effects of one- and two-pass autumn cultivation/drilling systems on earthworm populations. When measured the following spring (April 2017), it found that almost double the number of earthworms were in evidence following minimal disturbance, one-pass strip-till type systems compared with the two pass cultivator/drill alternative. “Cultivations can affect soil health and biology and it seems that less cultivations are better in terms of worm numbers,” Dr Shah said, adding that he had observed the same trend when comparing ploughed, min-tilled and zero-tilled land on farm. ■

Characteristics of a healthy soil • Good soil tilth • Sufficient depth • Sufficient but not excessive supply of nutrients • Small population of plant pathogens and insect pests • Good soil drainage • Large population of beneficial organisms • Low weed pressure • Resistant to degradation • Resilient when unfavourable conditions occur Source: Agrii

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