A trial at a Cheshire farm has shown how growing cover crops and carrying out soil analysis can reduce artificial fertiliser use to protect raw water sources.
The project, led by United Utilities and Compass Agronomy, took place over two years to establish how much captured nitrogen in a cover crop’s biomass could be released for the following spring’s crop.
Having this information means that a more tailored approach to fertiliser use can be used, ultimately protecting raw groundwater sources by reducing nitrate leaching risks.
The trial at Lower House Farm utilised the Soil Mineral Nitrogen (SMN) Plus system by Compass Agronomy.
This estimates how much nitrogen is available in the soil and the amount likely to be mineralised from soil organic matter and incorporated crop residues.
This data can then be used to calculate how much additional nutrients are required from fertiliser, so farmers do not have the expense of over-using products which would only go unused by the crops and leach out of the soil.
In year one a mixed species cover crop was established on sandy loam soil, with three different sections receiving the farm’s usual practice of nitrogen fertiliser, the SMN Plus recommended treatment, or no fertiliser.
Monthly soil and crop nitrogen measurements were taken during the spring and summer. As soil nitrogen decreased, crop nitrogen content increased for both farm practice and SMN Plus treatments.
At harvest, both treatments yielded 6.7 t/ha, but the farm practice treatment recovered 208 kg N/ha compared to the SMN Plus which recovered less (166 kg N/ha) due to the slower breakdown of nitrogen from the residues.
The second year of the trail established a different mixed cover crop, with the same processes for sampling and fertiliser application.
In this season, hot and dry weather conditions were a factor, likely impacting some residues not being fully incorporated into the soil.
The farm practice treatment out-yielded the SMN Plus by 1 t/ha. This increase was due to the additional supply of nitrogen (53 kg N/ha), maintaining a higher tiller and ear number. SMN Plus did produce the target yield the optimum nitrogen rate predicted.
SMN Plus recovered nearly all of the applied nitrogen (93%) compared to farm practice which recovered half (50%). This resulted in a lower surplus of 56 kg N/ha for SMN Plus, compared to the farm practice treatment of 141 kg N/ha.
The farm practice treatment resulted in 2.5 times more nitrogen being left in the system and at risk of overwinter leaching compared to SMN Plus.
Alli Grundy of Compass Agronomy said the trial demonstrated that reducing fertiliser inputs to account for cover crop residue resulted in improved nitrogen use efficiency overall.
“Including cover crops in arable rotations and utilising SMN Plus can improve nitrogen recovery and uptake," he explained.
"It can also achieve required crop yields with reduced fertiliser inputs, and result in a lower nitrogen surplus after harvest, which all contribute to improving nutrient use efficiency.
"These practices can help farmers and growers protect the environment and raw water quality, by reducing nitrate losses to groundwater.”
Farmer Rob Briscoe, of Lower House Farm, added: “The grey area has always been how much nitrogen has been locked up by the cover crop and how much is available to the preceding crop as the cover crop breaks down over the growing season.
“After several years of growing cover crops we have seen a marked improvement in the condition of our soil, with the added bonus of fields staying drier over winter than when left as bare stubble and of course less leaching of essential nutrients."