Green manures to increase N availability

This field lab aims to compare how different green manures affect the availability of nitrogen and key nutrients to a following spring green crop. While the trial crop is spring greens, this work will be highly relevant to other brassica crops.

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Field Lab Timeline

    4/13/2017 11:00:00 PM
  • Field lab design

    Field lab design
  • 4/29/2017 11:00:00 PM
  • Sow green manures

    Sow green manures
  • 7/17/2017 11:00:00 PM
  • First public meeting

    First public meeting
  • 7/17/2017 11:00:00 PM
  • Soil and tissue sampling

    Soil and tissue sampling
  • 7/20/2017 11:00:00 PM
  • Incorporate green manures

    Incorporate green manures
  • 8/15/2017 11:00:00 PM
  • Plant spring green crop

    Plant spring green crop
  • 8/17/2017 11:00:00 PM
  • Soil sampling

    Soil sampling
  • 9/28/2017 11:00:00 PM
  • Soil and tissue sampling

    Soil and tissue sampling
  • 10/31/2017 12:00:00 AM
  • Soil sampling

    Soil sampling
  • 2/28/2018 12:00:00 AM
  • Preliminary results

    Preliminary results
  • 3/1/2018 12:00:00 AM
  • Soil and tissue sampling

    Soil and tissue sampling
  • 3/16/2018 12:00:00 AM
  • Second public meeting

    Second public meeting
  • 3/26/2018 11:00:00 PM
  • Third public meeting

    Third public meeting
  • 4/8/2018 11:00:00 PM
  • Harvest spring green crop

    Harvest spring green crop
For further information hover over the above milestone marks
  • Discussion

    To see the latest activity please log-in (group members only).

  • Achievements

    April 2018

    Final Results

    The results from the field lab running can be found both in summary on the findings tab, and in full report under the 'Field Lab Documents' section. Please log in and download this for further information.

    Milestone: Harvest spring green crop

    March 2018

    Open group meeting

    At the field lab's third public meeting, coordinator Tony Little introduced the field lab idea to new interested parties and explained the Innovative Farmers process.

    The triallist also reflected on his reasons for initially getting involved "There's all this research out there but I want to know there's something for my problems in my situation." and that the trial provided this opportunity. As with other trials over the last year in the UK, the dry spring of 2017 meant that cover crop establishment struggled. Despite this, the triallist felt that the trial and researcher were flexible; "We restructured as we went along - there's been a development between us [triallist and researcher] and that's what's important."

    Alongside the preliminary results (please log-in for free to download from the (Field Lab Documents section), the triallists observations have been important and shaped his willingness to continue using cover crops. He noted that:

    - Buckwheat was able to germinate on very dry ground, and established in these conditions better than the other cover crops
    - The good leaf colour and size of the cover crops used, and the amount it did establish and grow despite the lack of rainfall
    - The mix of cover crops used flowered at different times which meant a diversity of insects and pollinators and was "absolutely alive" throughout the growing period.

    The triallist would like to continue using the mixed cover crops which included buckwheat, phacelia, westerwold ryegrass, berseem clover, crimson clover, persian clover and yellow trefoil. The initial results show that chemical nitrogen use could be cut by up to 50% if using this cover crop mix.

    Some other questions came up during the discussion including:

    - If there are only a couple of months available to plant cover crops, can they supply enough / a good amount of nitrogen to the soil and following crop?

    The triallist would like to trial this over the coming spring.

    Milestone: Third public meeting

    March 2018

    First year final sampling

    Soils were analysed in the field trial plots for earthworm counts, these included a variety of species of both horizontal shallow living and vertical burrowing deeper living species.

    Final samples for this year's trial were also taken. These have been sent off for analysis into the nitrogen that has been available for the crop (rather than total N as previously measured). Total N measurements were not informative enough to understand the effects of additional N provided by the cover crops once incorporated into the soil.

    The above measurements will be available soon.

    Milestone: Soil and tissue sampling

    February 2018

    Preliminary results cont.

    (...)
    Returning to the question of N availability, the contribution of green manures may have in fact been underestimated in this study because:

    - The variability of the data from the control and the ‘Nitrogen’ plots were, in many instances, more variable than in the green manure plots, although this was not the case in every instance.
    - 2017 was unusually dry between April and July. In more typical years (i.e those with higher rainfall in this period), there may be higher losses of N through leaching/anaerobic loss from the control plots with no green manures.

    Next steps:

    One further data collection period, which will be around harvest in spring. More analysis also needs to be done on the financial implications of using green manure in this context.

    Milestone: Preliminary results

    February 2018

    Preliminary results - February 2018

    Preliminary results, ahead of another data collection period this spring, have been collated by Tony Little in the attached February 2018 report under the 'Field Lab Documents' section of this page. Please log in for free to download this.

    In brief, the results so far are that:

    While the soil analyses did not pick up and differences between any of the treatment, if we look at the whole picture (i.e. N uptake by the plant and N content of the tissues), we can see that green manures make a very important contribution to N availability compared to fallow ground, which shows that relying on soil data alone can be unreliable.

    The data does not show that Green Manures are more effective than synthetic fertiliser at supplying mineral Nitrogen, however, they can reduce the application of mineral fertilisers by 40 – 50%, which is an important contribution by any measure.

    Although this study focused on Nitrogen availability, we should not overlook the importance of green manures from other perspectives. Anecdotal evidence from this study suggests that green manures can lower soil moisture content (this may or may not be desirable). Studies from elsewhere cite a wide range of other benefits including improving soil biology, organic matter levels, soil structure and drainage, enhancing pollinator populations and other insects.

    (...)

    Milestone: Preliminary results

    October 2017

    Interim assessment

    Four weeks after incorporation available nitrogen is higher for the control than for the non-legume green manure. Available nitrogen in the legume and the mixture are comparable with the control but cost of seed and work to chop and incorporate may make you choose bare ground over the green manures from a financial standpoint. Of course we need to look at crop yield/quality.

    We need to look at nitrogen uptake by the crop (10 weeks after incorporation) before drawing firm conclusions. It is also important not to forget the other benefits gained from green manures e.g. in terms of building carbon in the soil, pollinators, and avoiding erosion.

    For more detail see Green manures to increase N availability - interim report - October 2017.pdf [free login required].

    Milestone: Soil and tissue sampling

    August 2017

    Soil samples collected

    2nd set of soil samples taken. See image of samples: https://innovativefarmers.org/media/1802/soil-sampling-molyneux.jpg.

    Milestone: Soil sampling

    August 2017

    Main crop planted

    Spring green crop planted.

    Milestone: Plant spring green crop

    July 2017

    Green manures incorporated

    Green manures have been incorporated prior to sowing the crop. The crop will be sown in the coming days based on a suitable weather window.

    Milestone: Incorporate green manures

    July 2017

    Changes agreed to project design

    Based on discussion and feedback at the public meeting on 18 July 2017 the group recommended changes to the project design as follows:

    i) Change the crop from Kale to Spring Greens
    Spring Greens will grow vigorously between planting and the onset of winter. Switching to this crop will give a much better indication of the uptake of nitrogen derived from the green manures and significantly reduces the risk of losses over winter, eliminating a significant source of variability and therefore improving the quality of the data.

    ii) Inclusion of ‘with N’ control plot
    Each of the control plots will be split roughly in half lengthways. Half the control plots will receive no Nitrogen input until the Spring, acting as a “scientific control”. The other half of the control plots will receive Nitrogen fertiliser at the start of the growing season, to provide a “farm standard practice” control.

    iii) Application of supplementary Nitrogen to the growing crop
    Changing the crop provides opportunities to enable the grower to apply Nitrogen to the crop without compromising the trial results. This will address growers concern that without the legacy of a fertility building rotation/ phase, the yield from the plots (2.4ha) will be very low and the financial cost of that will be unacceptably high.

    Milestone: First public meeting

    July 2017

    Public field day held

    8 people came to an open day introducing the project. The day included a talk on using Green Manures by Growing Nature's Alan Schofield, and a farm tour.

    Discussions brought up issues with the project design and suggested changes.

    See the 'Investigating Nitrogen Availability following Green Manure Mixes update - 18-07-17.pdf' presentation from the day in the sidebar above - (free login required)

    Milestone: First public meeting

    July 2017

    Samples taken

    Soil and tissue samples were taken for analysis on the day of the public meeting.

    Milestone: Soil and tissue sampling

    June 2017

    Public meeting planned for 18/07/17

    Find out more and register here: http://greatsoils-field-day-nitrogen-availability-18-july.eventbrite.co.uk/

    Milestone: First public meeting

    April 2017

    Trial methods agreed

    Three combinations of green manures, with varying proportions of leguminous: non-leguminous species, will be trialled.

    Plots of these three mixtures and an area of bare soil will be planted in a randomised block experimental design with 3 replicates including control strips. See https://innovativefarmers.org/media/1675/using-green-manures-to-optimise-nitrogen-availability-to-kale-crops-trial-design.pdf (free login required).

    The green manures will be incorporated after an appropriate growing period and a following Kale crop planted in each of the plots.

    Measuring nutrient availability:
    The availability of Nitrogen in each plot will be assessed to determine both soil mineral nitrogen, total soil nitrogen and plant tissue nitrogen between July 2017 and April 2018. A minimum of three soil assessments are required.

    The trial presents an excellent opportunity to validate the Laqua twin Nitrate meter, a tool to measure the N content of plant tissue on-farm. Readings of tissue samples taken with the meter can be directly compared to the lab results, enabling robust conclusions to be drawn about the accuracy of the former. While not essential to the outcome of this trial, if the meter proves sufficiently accurate, its use will enable significant savings to be made in sampling time and testing costs in future field labs and experiments of this type.

    Milestone: Field lab design

    April 2017

    Analysis methods agreed

    Samples will be taken at key points during the growing season and the results will be analysed to determine whether significant differences in nitrogen availability and uptake, yield and quality exist as a result of the different green manure combinations.

    • Soil Sampling method: 20 soil samples per plot will be taken and consolidated into one bulked sample per plot. Samples should be taken using a soil auger from a W pattern in the plot to a depth of 15cm. 12 samples will be stored in soil analysis boxes provided and sent to NRM laboratories for Nmin and Total N analysis. Soil sampling will take place on up to 5 separate occasions as outlined in the field lab timeline. Ideally, 2 soil samplings will be taken at the start of the most rapid growth stage of the crop, 1 at green manure incorporation, 1 prior to winter to assess the potential for n leaching and one at harvest.

    • Green manure/Crop Sampling method; 4 quadrat (0.25m²) cuts of above ground biomass should be taken from each of the 12 plots, weighed fresh, and then dried in an oven at Elm Farm to determine dry mass. Samples will then be bulked and sent to NRM laboratories for determination of N content. Biomass samples of the green manure will be taken prior to incorporation to determine the amount of nitrogen contained and therefore incorporated into the soil.

    • Biomass samples of the crop will be taken on 2 occasions to coincide with early growth and development in the Autumn and at harvest of the crop to determine nitrogen uptake.

    Milestone: Field lab design

  • Findings

    April 2018

    Summary of final results

    While the soil analyses did not pick up many differences between treatments, when crop tissue and biomass data (i.e. N uptake by the plant and N content of the tissues) were analysed, it was revealed that the green manures could make an important contribution to N availability compared to fallow ground. This does demonstrate that relying on soil data alone can be unreliable, particularly, as we found in spring, when a crop was actively growing.

    The data did not show that green manures are more effective than synthetic fertiliser at supplying mineral N, but they can reduce the application of mineral fertilisers by 40 – 50%, an important contribution to crop nitrogen requirements.

    Although this study focused on N availability, we should not overlook the importance of green manures from other perspectives. Anecdotal evidence from this study suggests that green manures can lower soil moisture content (this may or may not be desirable). There is strong evidence suggesting a wide range of other benefits for agriculture including improving soil biology, organic matter levels, soil structure and drainage, enhancing pollinator populations and other beneficials.

    Returning to the question of N availability, the contribution of green manures may have in fact been underestimated by this study because:

    • The variability of the data from the control and the ‘Nitrogen’ plots were, in many instances, more variable than in the green manure plots, although this was not the case in every instance.
    • 2017 was unusually dry between April and July. In more typical years (i.e. those with higher rainfall in this period at this trial location), there may be higher losses of N through leaching/anaerobic loss from the control plots with no green manures. The very dry spring also contributed to a poor green manure establishment, that in a “typical year” could be expected to produce a greater biomass.
    • Carrying out tissue analyses at the same time as the Spring soil analyses would have provided more robust data to draw stronger conclusions about the contribution of the green manures in this trial.

    A cost – benefit analysis from data collected in December showed that the cost per kg taken up was significantly lower in the Nitrogen treatment compared to the green manure treatments, but the cost per kg of product was very similar. In respect of the latter, for the N treatment it was estimated at £0.12 £/kg, the same as the ‘Mix’, a penny higher than the ‘legume’ and a penny lower than the ‘non-legume’. This may not sound like much, but with large volumes of product coming off the field, a penny a kg either way can add up to significant sum of money.

    If the trial were to be repeated, tissue analyses would be taken in parallel with the spring soil sampling. While this has budgetary implications, it would unquestionably be a worthwhile investment in terms of increasing the quality of the data.

    Although this is only data from one farm over one year the results are encouraging, and the team are keen to pursue this further.

    An open day was held on 27 March 2018, and a number of suggestions were raised for future work, centred around short term green manures including:

    • Using spring beans as a short-term summer cover crop between harvesting of a brassica cash crop and the subsequent sowing of another.
    • Monitoring the accumulation of N during the growth of short-term green manure crops to establish the optimal timing of destruction and incorporation to deliver maximum N to a following brassica crop.

    These proposals will be put forward to Innovative Farmers for consideration in spring/ early summer 2018.

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