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AHDB: What happens when we plough a direct drilled field?
WHAT HAPPENS WHEN WE PLOUGH A DIRECT DRILLED FIELD?
Using the soil health scorecard to monitor changes in soil health. Dr Jenny Bussell from The Allerton Project at GWCT shares the findings from research aiming to answer this question over the last three years. Growing food and fibre crops requires soils to be maintained in a suitable state that provides optimal soil structure, water retention and nutrient availability. The physical, chemical and biological properties of soil interact to deliver these functions. Measuring soil health therefore requires an integrated approach that combines the assessment of all three of these factors. There is a good understanding of the soil chemical and physical constraints to crop and grassland productivity, however, the role of soil biology is less clear. A key aim of the AHDBBBRO funded Soil Biology and Health Partnership is to improve our understanding of soil biology and to explore ways that farmers can measure and manage soil health. The Partnership has developed a soil health scorecard which aims to provide information on key indicators of soil chemical, physical and biological condition, to help guide soil and crop management decisions.
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There are many recognised benefits of minimising cultivation including changing soil organic matter dynamics, improving soil structure and increasing the soil microbial and invertebrate population along with the cost saving benefits of machinery, fuel and time. However, issues such as a high weed burden or compaction can arise leading to the decision to cultivate a previously direct drilled field. There is currently limited information for growers on how much this will impact on their overall soil health. As part of the partnership the Allerton Project has been running a field trial on their research and demonstration farm at Loddington for three years, looking at the impact on soil health of ploughing a field that has previously been a direct drilled field for seven years. The experiment started in 2017, when three strips were ploughed into the previously direct drilled field, creating three plough plots, and three direct drill plots across the field. These plots were monitored in autumn 2018 after one year of treatment, then again in autumn 2020, after three years of annual ploughing, and ten years of continuous direct drill.
The Soil Health Scorecard approach brings together information about the chemical, physical and biological properties. The integrated report uses ‘traffic light’ coding to identify the properties where further investigation is needed to determine the management steps required to minimise any potential risks to crop productivity. Here we present the scorecard (Figure 1) for those soil properties where there is already an established evaluation framework (e.g. soil nutrients, visual soil evaluation of soil structure score – VESS). This framework is still under review in the final year of the Partnership and the agreed benchmarks will be released in the autumn.
Earthworms are often called indicator species due to their prevalence and visibility in the soil; they are easy to find and count and can be the first to respond to changes in the soil, such as cultivation disturbance. Earthworms are also important for soil health due to the role they play in the breakdown and mixing of soil organic matter, and the improvements they can make to soil structure and water infiltration by moving through the soil and the formation of burrows. However, earthworm numbers are seasonally variable and so should be combined with other measures to assess soil health. In 2020 earthworm numbers were low across all plots probably due to the drier soil conditions at the time of sampling and no differences were seen between the treatments. In the first year (2018) we found smaller numbers of earthworms in the ploughed plots. This was mostly due to a decline in topsoil dwelling (20 cms) earthworms, which are most at risk from direct plough damage.
Topsoil structure (25cm depth) was monitored using Visual Evaluation of Soil Structure (VESS) (Figure 2). The soil at this site is a clay loam and is naturally increasingly compact at depth, however it is also vulnerable to cultivation damage where timeliness of cultivation is compromised. After three consecutive years of ploughing, we found a higher VESS score (poorer structure). In the direct drilled
Treatment Plough Direct Drill
Year (post-harvest, pre-cultivation)
SOM (%)
2018 2020 2018 2020
7.1 7.1 7.2 7.9
pH 6.8 7.4 7.2 7.4
Ext P (mg/l) [index] 22 [3] 21 [3] 26 [3] 28 [3]
Ext K (mg/l) [index] 162 [2-] 137 [2-] 140 [2-] 173 [2-]
Ext Mg (mg/l) [index] 126 [3] 119 [3] 104 [3] 98 [2]
VESS score 2.0 2.9 2.0 2.2
*PMN (mg/kg) 71 54 74 95
Earthworms – total/pit 6 5 10 5
CO2 burst (mg/kg C) 143 101 139 116
No action needed Monitor Investigate
Figure 1. Soil health scorecard for the ploughed and direct drilled treatments in 2018 (one year of plough; eight years of direct drill) and 2020 (three years of plough; ten years of direct drill). Numbers in square brackets indicate soil nutrient index.
plots, the structure in the top layer has a better ‘crumb’ structure, in contrast to the ploughed soil where the soil aggregates are larger and there is less continuous pore space. Good soil structure is key to soil health, allowing water and air to infiltrate, as well as reducing the risk of soil erosion, surface run-off and flooding, making soils more resilient to adverse weather.
During the development of the Scorecard, the Partnership also assessed the value of some simple indicators of microbial activity; the scorecard shows soil respiration
Figure 2: Example VESS profiles from a direct drilled treatment (left) and ploughed treatment (right). Numbers represent the soil layers.
Figure 3: Metabolic activity of microbial community measured using the MicroRespTM system in April 2020. under controlled conditions, (CO2-burst), and an incubation-measure of potentially mineralisable N (PMN). There is no clear pattern of response to the treatments shown here. Therefore, we carried out a further study to investigate the impact of cultivation on the microbial fraction of the soil biology to further understand the impact of this on these important organisms.
Using the MicroRespTM system, the metabolic fingerprint of the soil microbial community could be measured in more detail. MicroRespTM measures the ability of soil microbes to break down a range of different substrates, measured by the amount of CO2 released. MicroRespTM measurements using fresh soil samples from the field showed that the direct drilled treatment had more than double the metabolic activity of the ploughed treatment (Figure 3), indicating a larger, more active microbial population. The microbial population is not only important for improving soil structure but is also responsible for constantly cycling soil nutrients into plant available forms, which is vital for crop growth and health.
A strength of the soil health scorecard is the ability to break down the challenge of improving soil health into individual areas, to help make decisions on where to focus to best manage and improve soils for future soil health. At first glance the differences between the direct drilled and ploughed treatments seem small, but indicators such as earthworm number and the soil structure decline can tip off a grower to changes that might be going on in their soil. The further assessment of microbial activity highlighted the growing differences between the two cultivation treatments after three years of ploughing. This work supports the review of Conant and others (2007; 10.1016/j.still.2006.12.006) showing that whilst there are negative impacts of ploughing, a soil that has an occasional tillage event e.g. to control weeds, will not necessarily suffer long-term impacts, as long as the tillage intensity is not increased permanently.
For more information on the soil health scorecard or wider research projects, please visit:
ahdb.org.uk/greatsoils.
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