Optimisation of phosphorus and potassium management within organic farming systems (OF0114)

Summary

This is the final report from Defra project OF0114.
The scientific objectives of the project were to
1 - Assess the balance between inputs and offtakes of P and K within a range of UK organic farming systems
2 - Evaluate chemical and bioavailability indices used to assess P and K status of soils
3 - Assess the availability of a range of P and K fertilisers to grass-clover leys and tillage crops
4 – Develop a process-based simulation model and integrate with whole farm nutrient budgeting to allow P and K management decisions to be made for the whole farm.
5 - Provide guidelines for farmers on the use of phosphorus and potassium fertilisers for organic systems.
Farm gate budgets for P and K were collated using farm records, measurements and standard tables of nutrient contents for a number of organic farms and rotations on organic farms. An integrated series of incubation, greenhouse and field experiments was carried out with a range of fertilising materials selected to give a range of likely availabilities for plant uptake and to give a mix of mineral and organic sources of P and K on soils representing the range of soil types under organic management.
Organic farming systems can show both P and K surpluses and deficits depending on management. In mixed systems, manures represent a significant resource of both P and K, which are supplemented through imports of feed and bedding. However, manure handling also therefore gives significant opportunity for losses, particularly of K from the system. K deficits are common in organic rotations, but not necessarily at the whole farm level. These deficits are close to the level which can be sustained from the weathering of mineral reserves in many UK soils. However, more consideration needs to be taken of the potential of soils to supply K when rotations and management plans are designed. There is no reason why organic farming systems, operating within the current UKROFS standards, cannot achieve a nutrient budget in line with long-term sustainability of soil P and K. However, continued monitoring of soil P and K levels in long-term organic trials is necessary to demonstrate these conclusions.
Results of the simulation modelling are further summarised in the executive summary at the start of the main report.
The complex interactions between nutrient cycles in organic farming systems means that the process based simulation model of P and K turnover was necessarily very simplified and the data collected in this project was not long-term enough to allow a full evaluation of all the factors, which influence P and K offtake, particularly the impact of crop establishment and management practices influencing spatial and temporal P nd K management. However, practical guidelines were drawn up for farmers and their advisors.
The project has produced 4 scientific papers and contributed to others. 10 presentations were made at scientific conferences and 5 presentations to farmers’ meetings. The project has also been responsible for simulating debate and encouraging the use of good scientific data in the development of future UK organic farming systems.