Jake studied various cover crops over a four year period working closely with the BBRO. His PhD titled 'The effect of cover crops on soil structure and the subsequent yield of sugar beet'. Please find below abstract and conclusions.
Pressure is mounting on policy makers and farmers to improve the sustainability of UK Agriculture. One area of improvement surrounds possible changes within crop rotations to improve soil health in tandem with increasing crop yield.
The success of a cash crop, is widely determined by the structure of its growing media. In field crops this is considered as the structure of the soil which is responsible for allowing water and nutrient uptake as well as, particularly for root crops, providing a profile for unimpeded root growth.
The effects of cover crops on the soil physical properties and the subsequent crop growth are considered in this thesis. By conducting glasshouse experiments using different cover crop species and soil volumes, the relationship between cover crop root growth, soil moisture and soil aggregation has been tested. This informed the development of a number of field experiments that have investigated the relationship between cover crops, soil structure and subsequent crop growth with typical UK climatic and soil conditions. It was hypothesised that cover crops improve the soil structure, prior to a cash crop, resulting in higher crop yield.
Our findings have established that cover crops do influence the soil structure, demonstrated by aggregation in controlled environment experiments and soil porosity as seen in the field. However, this was greatly influenced by factors including soil texture, soil volume, cover crop growth and weather conditions. We fund that the growth of cover crops was most beneficial on soils with a low clay content where sugar beet yield was 10% greater following a cover crop than following stubble. This was as a result of lower water stress in response to greater soil porosity.
Results showed that soil with a high clay content is susceptible to changes in soil aggregation. There is a link between soil conductivity and plant growth showing it is a useful proxy for water uptake. There is also a positive effect of cover crops on earthworm population. We found that overall cover crop root growth was directly related to above ground biomass and there was no benefit to combining cover crop species in favour of single species cover crop.
It is concluded that the effect of cover crops, is likely to be positive but their efficacy on soil structure and the subsequent crop growth is highly determined by environmental factors.
Implications for farmers
We have seen that cover crops can improve both soil structure and sugar beet growth and yield. This effect, however, is dependent on environmental factors such as soil texture and weather conditions. It is likely that inclusion of cover crops on sandy soils will be easiest to manage due to their free draining nature. We have seen that in these conditions cover crops can lead to increased soil porosity and improvements to sugar beet growth. Introducing cover crops to soils with higher clay content is likely to be more difficult to manage subsequent cultivation, due to the water holding capacity of the soils but as seen in controlled and field experiments, there is the potential for cover crops to increase the proportion of small aggregates in the soil profile which may allow better seedling establishment and removal of soil constraints. These findings are likely to be of particular use to growers who use cover crops as a means of providing environmental focus areas (EFAs) for subsidy requirements. Cover crop root growth is directly related to above ground biomass production and therefore decisions on species, combinations and destruction timing should be easier for farmers.
The effects of cover crops on water availability to the crop suggests that cover crops can improve growth of the sugar beet canopy but in years of extreme heat this can be detrimental to overall sugar beet yield. However, extreme weather conditions such as 2018 are difficult to predict and previous studies have suggested that the long term inclusion of reduced tillage, cover crops and conservation agriculture may improve water and nutrient availability to crops. This may be particularly pertinent in future years as extreme weather events are predicted to occur more frequently which may result in overall sugar beet yield potential decreasing (Collins et al., 2014, Hoffmann and Kenter, 2018). Furthermore, if farmers intend to use cover cropping as a method of reducing nitrate leaching or for prevention of soil erosion, this is possible without having negative impacts on sugar beet yield.
It is important to point out the findings of Allison et al. (1998b) who showed that the lack of positive effects on yield suggests that cover crops are not a financially viable option for sugar beet farmers as costs of establishment are not recouped by an increase in yield. However, if agricultural policy continues in the direction of using techniques to improve soil health through subsidising their use, the financial cost to the farmer may be outweighed by the financial reward of environmental subsidy.
Implications for policy makers
We have found that, excluding periods of extreme weather and unreasonable soil constraints, cover crops can have a positive or neutral effect on sugar beet yield. Furthermore, the effects on earthworm abundance and management of autumn rainfall suggest that inclusion of cover crops are beneficial to the soil environment. While we have not explored the long term effects of cover crops on the environment, our findings in combination with reports such as Cooper et al. (2017) suggest that including cover crops in the crop rotation is a viable option for improving soil structure and nutrient content in the short term.
Our findings show that a combination of cover crop species has no benefit to a single species cover crop and hence do not support the current EFA regulations. It is likely that the short growing period of the autumn sown cover crops does not allow sufficient time for species richness to outweigh the benefits of producing a large food source for soil fauna.
Conclusions
Cover crops were able to alter soil aggregation. This was governed by root density, soil moisture and soil texture. Through the growth of roots, a greater proportion of aggregates was produced which is a likely result of both direct root growth and wetting and drying cycles caused by plant water uptake. This effect was more pronounced when clay content in field soil was relatively high, reflecting the high plasticity of clay compared to sand.
Cover crops had a significant effect on soil moisture during their growth. Larger cover crops were able to have a greater influence on soil moisture as a result of both greater root exploration and higher water demand.
The influence of cover crops on soil moisture did not directly affect the water availability to the sugar beet crop. This is a result of destruction date of the cover crop allowing sufficient time for soil to reach field capacity.
The presence of cover crops increased earthworm populations in some conditions. This was determined by the presence of a food supply for earthworms and the soil moisture being favourable to earthworm activity.
There was no benefit in terms of growth or influence on the soil structure, root growth or earthworm population of combining multiple cover crop species.
Cover crops were able to increase soil porosity on soils with a high sand content which led to better water uptake by the sugar beet and higher yields. This effect was not seen on soil with a higher clay content as a result of soil constraints during drought conditions.
Measurements of soil conductivity were a useful proxy for water uptake by both cover crops and sugar beet and indicated areas of the experiments that took up more water resulted in higher sugar beet yield.
The direct effect of cover crops on sugar beet yield were mixed and tended to be determined by water supply suggesting that water is still the most important environmental factor limiting sugar beet yield.
In reflection of the hypotheses set out at the start of the project it is reasonable to accept that the influence of cover crops on the soil structure is determined by a combination of environmental factors. The project has consistently shown that the volume of soil and the soil type has a considerable impact on the influence that cover crops roots have on the soil structure. Further to this, the variable results of the field experiments confirm that air temperature, soil moisture and water uptake will have a huge influence on the ability of cover crops to change the soil structure.
It is unlikely that the growth of cover crops will have had a detrimental effect on the soil structure, their variable results in years where autumn soil moisture is high and temperatures are low, the effect on cover crop growth will likely result in farmers questioning their investment of money and time at least in the short term.
When comparing results between all chapters and the published literature, we can accept that the success of cover crops will depend on soil texture. When experiments were conducted on soils with relatively high clay content, the effect of cover crop roots on the soil was more permanent. However, it was on soils with relatively low clay content where the effect of cover crops on sugar.
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