Phosphorus in organic farming systems:
Improving supply and use efficiency

Christine A Watson, Robin L Walker, Robert M Rees and Elizabeth A Stockdale

Abstract - Phosphorus (P) is a key plant nutrient but it needs to be managed it carefully in an organic system, in order to obtain sustainable crop yields and quality without contributing to potential damage to the environment. Minimally processed rock phosphate (PR) can be used, but it has very low solubility at optimum soil acidity for crop growth (pH 6.5). This paper describes a project (PLINK), which is investigating methods of improving P efficiency on organic farms. Approaches include fermentation of crop waste material with PR in order to make P more available to the crop, as well as incorporation of crops or varieties with high P uptake efficiency.

Introduction
UK Organic Action Plans (OAPs) from both Defra (Department of Environment, Food and Rural Affairs) and SEERAD (Scottish Executive Environment and Rural Affairs Department) aim to increase the proportion of organic food consumed in the UK so that 70% of it is produced by UK farmers and growers. This would require a considerable increase in the area of organic cropping. Phosphorus (P) is a key plant nutrient, and if this resource is not managed extremely carefully in an organic system, crop yield and quality will be detrimentally affected, with the added risk of potential damage to the environment. The use of soluble P fertilisers such as triple super phosphate are prohibited under organic rules, and although minimally processed phosphate rock (PR) can be used, it has extremely low solubility at optimum soil acidity for crop growth (pH 6.5). Organic systems that incorporate crops or varieties with high P uptake efficiency into the rotation, is one way to help optimise P use within the system. Additionally, the fermentation of crop residues and crop waste material with PR also has potential to increase the availability of P to the crop plants. Neither of these approaches have been investigated in depth in the field, and as yet, never in the UK. In order for improvements to be made in the best way to manage P use and its efficiency on organic farms, more detailed studies of the factors involved need to be made. This paper describes a project (PLINK) which is attempting to do just this.

Christine A Watson is with the Scottish Agricultural College, Organic Systems Team, Aberdeen, UK (christine.watson@sac.ac.uk).
	Robin L Walker is with the Scottish Agricultural College, Organic Systems Team, Aberdeen, UK (robin.walker@sac.ac.uk)
	Robert M Rees is with the Scottish Agricultural College, Organic Systems Team, Edinburgh, UK (bob.rees@sac.ac.uk)
	Elizabeth A Stockdale is with the Newcastle University, School of Agriculture, Newcastle, UK (e.a.stockdale@newcastle.ac.uk)

Biotechnological solubilisation of PR
Rodriguez et al. (1999) successfully demonstrated that PR fermented with sugar beet waste before application to soils improved plant P supply and provided other plant growth and nutritional benefits. There is also evidence that composting rock phosphate with organic wastes can increase P availability to plants (Singh and Amberger, 1991). Vassilev and Vassileva (2003) reviewed studies on biotechnological solubilisation of PR and concluded that the use of agro-industrial wastes is an important route towards realisation of a cost-effective implementation of this approach. The mechanism of fungal solubilisation in the presence of available C and N is thought to be due to the production of organic acids which both directly acidify the medium and chelate the cations (in particular Ca) bound to phosphate. P released through these processes is incorporated by mycelial growth; any excess remains in soluble inorganic forms. Despite the success of biotechnological solubilisation of PR at a small-scale, a usable technology at field or farm scale has yet to be developed. Further understanding is required, in terms of (a) the mechanisms contributing to increased P availability during fermentation of agro-wastes and PR and after application of the product to land and (b) how this approach can be practically and cost-effectively applied within agricultural systems.
We have identified 3 main points in the waste cycle of organic farming systems, where agro-wastes might be combined with PR and biotechnological solubilisation processes potentially applied (a) on-farm; (b) at vegetable processing facilities; and (c) at waste management / composting sites (see Figure 1). Site managers will be consulted in order to seasonally map the availability of the agro-wastes and their quality, particularly in relation to C and N contents and their ratios. Practical/management constraints at each site will also be discussed, elucidated and recorded, so that any fermentation protocols developed at small-scale can be grounded in an understanding of the systems within which they will need to operate. The first part of the project will focus on the manipulation of the fermentation process, with small-scale pilot studies being used to investigate how fermentation of PR with a typical range of agro-wastes can be controlled to create products with a different partitioning of P between soluble P, microbial P and P remaining in the rock phosphate. A separate series of experiments will consider factors relating to the storage and handling. 

Figure 1. Diagram showing potential PR use and links with waste streams for improved P use efficiency on farm.

of fermentation products on the fractionation of P and its potential availability.

P uptake efficiency by crop rotations
Crop rotation design can have an indirect impact on P availability through modification of soil properties – particularly soil structure and microbial activity. It has been suggested that P deficiencies in organic farming systems might critically limit rotational yields because of the impact P deficiency might have in reducing N fixation capacity of legumes. As such it is critical to consider additional agronomic approaches that might be taken to improve the effectiveness of P use within the farming system. The design of crop rotations for increased efficiency is likely to mean the inclusion of crops or cultivars with high P uptake efficiency, particularly as intercrops or as cover crops (Kamh et al. 1999). We will quantify a range of crops and their ability to utilise the PR fermentation products in comparison with composted PR produced from the same feedstock and PR alone. During these experiments inclusion of P-free fermentation products (prepared using Ca in place of PR at the same Ca addition rate) will allow us to separate the indirect effect of adding highly reactive organic acids on ‘native’ soil P, which would stimulate a P response to the added materials. This approach will initially utilise both small-scale pot and plot studies, and later in the project, farm scale ones. The on-farm field trials will compare standard farm rotations to rotations modified to increase P use efficiency. Crops are likely to include white and red clover, kale, oilseed rape and lupins, based on their ability to mobilise P. Once practical methods have been clarified for the preparation of fermented products, a split-plot approach will be used in these trials to study crop response and P use efficiency after the substitution of PR with that pre-treated through the fermentation and composting approaches. As the project seeks to assess the rotational benefits of P management, field trials must continue for at least 3 years. The field trials will allow the practicality and economics of the biotechnical solubilisation process to be assessed from cradle to grave.

Targeted use of radioisotopes
Radioisotopes will be used in targeted laboratory incubation and/or greenhouse experiments within this project where they can give information of significant additional value. They will be used to follow the fate of solubilised P into P fractions, including microbial biomass and to determine dynamic pool sizes and rates of turnover of pools of P in the products before application to the soil, as well as to quantify the P dynamics in the soil in the presence / absence of the residues / PR / products. P from crop residues will be traced into soil P fractions, including microbial biomass and into crop uptake. 

Modelling approach
A defined data handling and modelling structure will be used in the research to integrate data obtained from the field experiments with different types of modelling. Each type of modelling within the hierachy has both type-specific objectives and general function to perform within the overall scheme for understanding the behaviour of P in organic rotations. Thus for example, modelling of biophysical processes, which uses a mechanistic approach, has a specific role in identifying knowledge gaps in understanding the behaviour of these processes in detail. In a wider context, detailed output from the mechanistic models, and data from the field experiments will be used to develop budgeting models for P use in organic rotations and will form the basis of knowledge transfer outputs to the industry via specialist advisors.

Acknowledgements
This project has been sponsored through the Defra LINK programme.

References
Kamh, M., Horst, W.J., Amewr, F., Mostafa, H. and Maier, P. (1999) Plant and Soil 211:19-27.

Rodriguez, R., Vassilev, N. and Azcon, R. (1999). Applied Soil Ecology 11:9-15.

Singh, C.P. and Amberger, A. (1991) Biological Agriculture and Horticulture 7:261-269.

Vassilev, N. and Vassileva, M. (2003) Applied Microbiology and Biotechnology 61:435-440.