FWAG South East Projects
Improving Nitrate Retention using Cover Crops
Since 2015, cover crop trials have been running on two Kent farms, in collaboration with Affinity Water and Kings Seeds. This project aims to evaluate the uptake to nutrients by cover crops compared to stubble fields. In 2020, the trials expanded to include some different grassland types: improved, semi-improved and species-rich pastures.
Nitrate leaching into groundwater drinking sources is highest during the autumn-winter period when there is a lack of root mass to absorb it. This can result in costly water treatment processes for water companies to get to safe drinking levels (50mg/l of nitrate). Cover crops can be used to address the nitrate issue at the source and can have the additional benefits of improving soil structure, carbon storage and resilience.
Sampling methods
Nutrient uptake
All the above ground vegetation within a metre square is collected from the cover crop plots in spring. This is weighed and analysed in a laboratory for nitrogen %, total N, carbon:nitrogen ratio, dry matter and fresh weight.
The results will estimate the nutrients that would have been lost from bare soil and helps plan nitrogen application on the next crop from the different cover crop mixes. It also gives an estimate of root storage as typically these are about 20% of above ground material.
Soil-water nitrate
Every two weeks from autumn to spring, soil water samples are taken from a metre under the cover crops using porous pots. The analysed samples give an accurate measure of nitrogen (N) within the water entering the aquifer.
Results
From this project we have so far identified that cover crops will typically capture 70 kg N/ha but this can be higher depending on what nutrients are left by the previous crop. This also typically relates to the nitrogen found in the soil water samples.
Larger crops with more leaves are able to pull up more nitrogen and brassicas (especially radish) will retain the highest amount of nitrogen. However, non-brassica species will enhance mycorrhizal fungi and improve soil health and so also have a part to play.
In regard to establishment, sowing as early as possible will lower seed rates and cost as there is more sunlight for the plant to use to extract nitrogen.
Based on this project, when choosing cover crops species, we would recommend you consider:
- Deciding what you want to achieve? Such as: capturing nutrients, improve soil structure/health, facilitate less cultivations, graze or pest reduction.
- Whether to join a government or water company scheme to gain a subsidy for sowing cover crops (and any rules to follow)
- Agronomic implications including, how to establish, destroy, sow following crop, pest/weed/disease carry over (all of this will depend on soil type, rotation and number of species within mix). Cover crops should initially be destroyed early as they can take a while to breakdown if soil health is low.
Future opportunities
Affinity Water are working on a research proposal to provide more accurate details about the amount of nutrients returning to the next crop. It will take into account factors that will impact on nutrient availability such as maturity, how crop is destroyed, grazing (which will make nutrients more available), soil temperature and carbon:nitrogen ratio.
FWAG South East LLP
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