Project number:                    91110003

Lead contractor:                   RSK ADAS Ltd

Start & end date:                   01 January 2018 – 31 March 2018

 

The Problem:

To review available data on offtake values for phosphate and potash for all crops, including grass, forage crops and straw, listed in Nutrient Management Guide (RB209).

 

Aims and Objectives:

  1. Identification of novel sources of data. Preliminary assessment suggests that little data is currently available in the public domain, hence, it is deemed imperative to build a consortium containing organisations such as members of the Forage Analysis Assurance Group; to obtain forage (fresh and ensiled) analyses; fertiliser or seed companies, for trials data; plant tissue analysis labs, anaerobic digestion industry or their trade associations (http://adbioresources.org or http://www.biogas.org.uk) etc.
  2. Review new data for its robustness and reliability, for example can enough background information on the crop be obtained, e.g. maturity, nutrient applications, potential availability and eventual yield.
  3. Make recommendations for revisions to guidance provided in the Nutrient Management Guide (RB209). The review should be written so that the key findings could directly be incorporated in the next revision of RB209.

 

Approach:

On completion of the project, the main outcome will be to verify and/or update RB209 values for phosphate and potash in crop materials for grassland, forage and arable crops; and associated fertiliser recommendations.
Any changes to the recommendations will ensure that farmers, growers and advisers using RB209 will be applying sufficient nutrients to meet crop requirements and to maintain soil indices at target levels, thereby improving nutrient use efficiency, productivity and profitability. These changes can be disseminated via the AHDB website https://ahdb.org.uk/projects/RB209.aspx and directly to those registered to receive any updates.
To ensure that outcomes translate into changes to practice the results will also be promoted through:
  • Dissemination at events, including Cereals, Grassland UK 2018 etc.
  • Dissemination at conferences, including AHDB Agronomists Conference 2018, United Oilseeds/AHDB Joint Seminar, British Grassland Society annual conference etc.
  • Coverage of the project in the farming press (target publications include Farmers Guardian, Farmers
    Weekly, Grassland Farmer and Crop Production Magazine).
  • Publication of the results in ADAS Crop Action and ADAS Technical updates, and in crop management literature produced by project partners.
  • Presentations at ADAS Farm Associations, AHDB Agronomy Series of regional technical seminars and Monitor farms.

Due to the short-time scale of the project, all KE activities will be undertaken following the completion of the project.

 

Results:

Grass

  • Values derived from the experimental database gave mean grass phosphate offtakes of 1.2 kg/t fw (at 24% dry matter), which was lower than the current value in RB209 (1.7 kg/t fw at 25% dry matter). For example, the P2O5 offtake figure for a 40 t/ha silage crop calculated using the database offtake value was 48 kg/ha compared with 68 kg/ha using the current RB209 value.
  • Values derived from the experimental database gave a mean potash content in grass of 5.2 kg/t fw (at 24% dry matter), which was lower than the current value in RB209 (6.0 kg/t). For example, the K2O offtake figure for a 40 t/ha silage crop derived using the database offtake value was 210 kg/ha compared with 240 kg/ha using the current RB209 value.
  • Data from the commercial datasets reported higher grass P (range: 0.29-0.36% dm) and K concentrations (range: 2.3-2.89% dm), than the experimental dataset (P 0.23% dm; K 2.1% dm) and RB209 (P 0.3-0.4% dm; K 2% dm).
  • Without background information on the commercial datasets (e.g. fertiliser application rates, management practices, soil nutrient status, crop yield etc.) it is not possible to be certain why the reported offtakes are higher than from experimental dataset. K concentration in grass is likely to be highest in early cut, leafy crops particularly those that have received potash applications from fertiliser or organic materials. Application strategies that favoured luxury K uptake could also result in atypical grass K concentrations.

Overall, due to the range and variability of phosphate and potash concentrations in grass dry matter from the different data sources sets no changes to the grass offtake values in RB209 are recommended.

 

Cereals

Winter wheat

  • Values derived from the experimental database gave a mean grain P2O5 offtake of 5.7 ± 0.1 kg/t fw, which was lower than the current value in RB209 (7.8 kg/t fw). For example, the P2O5 offtake for an 8 t/ha wheat crop derived using the database offtake value was equivalent to 46 kg/ha, compared with 62 kg/ha using the current RB209 offtake figure.
  • Phosphate offtakes were higher from sites where the soil P Index was ≥2 (6-6.1 kg/t fw) compared with sites at P Index 0 (4.5 kg/t fw) or P Index 1 (5.2 kg/t fw).
  • Values derived from the experimental database gave a mean grain K2O offtake of 4.9 ± 0.1 kg/t fw, which was lower than the current value in RB209 (5.6 kg/t fw). For example, the K2O offtake for an 8 t/ha wheat crop derived using the database value was equivalent to 38 kg/ha compared with 43 kg/ha using the current RB209 figure.
  • Evidence from the experimental database suggested that there was no effect of soil type on the relationship between predicted and measured offtakes for either phosphate or potash.

 

Winter and spring barley

  • Values derived from the experimental database gave an average grain P2O5 offtake of 7.2 ± 0.1 kg/t fw) which was similar to the current RB209 value (7.8 kg/t fw).
  • As for winter wheat, phosphate offtakes were higher from barley sites where the soil P Index was ≥2 (7.5-7.7 kg/t fw) compared with sites at P Index 0/1 (6.70 kg/t fw).
  • Values derived from the experimental database gave average grain K2O offtakes of 5.4 ± 0.2 kg/t fw, which was similar to the value in RB209 (5.6 kg/t fw).

 

Cereal summary

  • There was some evidence that grain P2O5 content was influenced by soil P index with greater offtakes at soil Index 2 and above than at soil index 0 and 1.
  • RB209 reports a single value for cereal grain phosphate (7.8 kg/t fw) and potash (5.6 kg/t fw) content regardless of cereal type (e.g. wheat, barley etc.) or sowing date (winter or spring). For both phosphate and potash there was no evidence to suggest that the relationship between predicted and measured offtake was different for winter wheat, winter barley or spring barley.
  • For an 8 t/ha winter wheat crop, the reference value in RB209 predicted greater phosphate (+16 kg/ha) and potash (+6 kg/ha) offtakes than the values derived from the dataset; equivalent to c.£9.50/ha in fertiliser P2O5 and c.£2.40/ha fertiliser K2O.
  • For barley the reference value in RB209 and the experimental database values gave similar offtakes.
  • Given that the over prediction of cereal (grain) potash values is small (2-6 kg/ha) in practice it will have little economic or environmental significance and it is not recommended that the potash values in RB209 for cereals (grain) are updated.
  • However, as there are consistent differences in measured and RB209 predicted grain P2O5 concentrations there may be justification in reducing the P2O5 offtake values in RB209 to 6.0 kg/t fw

 

Winter oilseed rape

  • Mean oilseed P2O5 derived from the experimental database was 12.1 ± 0.2 kg/t fw, which was lower than the current reference value in RB209 (14 kg/t fw). The P2O5 offtake (based on a 3.5 t/ha seed yield) derived from the data base value was 42 kg/ha compared with 49 kg/ha using the current RB209 figure.
  • As for the cereal crops, phosphate offtakes were higher from oilseed rape sites where the soil P Index was higher (P Index ≥3: 13.6 kg/t fw) compared with sites at P Index 0/1 (10.8 kg/t fw).
  • Mean oilseed K2O derived from the experimental dataset was 8.5 ± 0.1 kg/t fw, which was lower than the current reference value in (11 kg/t fw).
  • Although the P2O5 and K2O offtake values in the experimental dataset were lower than those reported in RB209 it was considered that a larger dataset would be required before any recommendations for changes to RB209 could be made.

 

Forage maize

  • Mean P2O5 concentration in forage maize derived from the experimental database was 1.7 ± 0.04 kg/t fw, similar to the current figure in RB209 (1.4 kg/t).
  • Similarly mean K2O concentration in forage maize derived from the experimental database 4.7 ± 0.1 kg/t fw was close to the current figure in RB209 (4.4 kg/t fw).
  • Mean % P and % K in dry matter derived from the NRM maize dataset were 0.32% and 2.96, for P and K respectively, and higher than the RB209 values (0.2% P in dm; 1.22% K in dm).
  • Overall, due to the limited size of the dataset, and variability within the data, no changes to RB209 values are recommended.