The PSALM model

Background:

The overall surplus of P in the European Union, and concerns over eutrophication of fresh waters has lead to an increased interest in P losses from agricultural lands. Until recently P leaching from agricultural soils was considered to be negligible under most conditions so, consequently, it was little studied. However, measurements of P concentrations in drainage water from the famous, 156 year old Broadbalk experiment at Rothamsted have now disproved this. Unexpectedly, above about 60 mg P/ kg soil there was considerable P leaching, which was linearly related to soil P concentration (Heckrath et al. 1995). Other work has now confirmed this. Algal blooms are a common symptom of eutropication of fresh waters

An indicator for the risk of P leaching from agricultural lands has been developed using 0.01 M CaCl2-extractable P and 0.5 M NaHCO3-extractable P (Olsen P) measurements on soil (Hesketh & Brookes, 2000). The indicator proved to be useful in predicting the concentrations of Olsen P above which loss to drainage water occurs, but there were wide variations between soil types. In order to be able to assess the impact of different management regimes and the relative importance of the various loss mechanisms of P in soils, a dynamic modelling approach is required. We are currently developing a dynamic model for the transport and sorption of phosphate in agricultural lands, PSALM (Addiscott & Falloon, 2002).

PSALM, the phosphate solute and leaching model, has been developed at Rothamsted Research by Tom Addiscott, Pete Falloon and Gill Tuck, to simulate losses of phosphorus at plot and field scales from agricultural soils.

Using the PSALM Model to Interpret the Phosphate Change Point in its Relation to Iron within Soil

DEFRA code:	PE 0109
End/status:	30th June 2002
WPT Classification:	KA-FPS (modeling at the soil scale)
IFT Classification:	2,3
Contact:	Professor Tom Addiscott
Telephone:	00 44 1582 762133  Ext 2228
Email:	tom.addiscott@bbsrc.ac.uk

Objectives:

1. Identify appropriate ranges of values for the instantaneous (Freundlich) and time-dependant sorption parameters to represent the variety of soil types with which we are concerned, together with likely values for the Olsen and total P in the soil. Make runs of the PSALM model using the parameter values identified. Use the results to interpret the characteristic shape of the change-point curve and its relation with the % iron in the soil.

2. Develop from the preceding work, a system which will predict the phosphate concentration in drainage from the % total or oxalate-extractable iron, the Olsen P and weather data.

3. Subject this system to ‘error and non-linearity analysis’ to check whether the system is susceptible to problems arising from interactions between error (in the statistical sense) and non-linearity in the model and to provide an indication of how readily the system can be upscaled.

4. Test the system against data from the earlier change-point work on Broadbalk, the Brimstone Experiment and the EU-funded Woburn lysimeters. Use these results to interpret the phosphate results from those sites and extend them to other sites.

Approach:

Experimental and Scientific Circumstances:Modelling with some iterations on lab and field work.

Relevance to DEFRA policy and extension:

The main objective is to use the PSALM model to interpret the relation between the phosphate change-point for a soil and the percentage iron and to incorporate the results in a system for predicting the change-point relations for various soils from the iron in the soil and possibly other properties. This will be developed further to estimate the concentration of phosphate in water draining from the soil on the basis of the iron and the Olsen phosphate. The system will be tested for possible malfunctions and then checked against field data from Broadbalk and Brimstone and ultimately could be used as part of a soil and catchment management tool.

Phosphorus Leaching Losses from Arable Land

DEFRA code:	NT 1046
End/status:	31st March 2003
WPT Classification:	KA-FPS
IFT Classification:	2
Contact:	Professor Phil Brookes
Telephone:	00 44 1582 762133
Email:	philip.brookes@bbsrc.ac.uk

 

Objectives:

1. To predict soil P concentrations in the plough layer below and above which P is at risk of being leached down the soil profile.

2. To estimate field losses of this P (on a kg P ha^-1 basis) by measurements of P concentrations in drainage water from plots and fields of different soil types.

3. Combine the above findings to produce a predictive mathematical model of P leaching losses from fields and catchments.

Approach:

Experimental and Scientific Circumstances:The work will extend findings on the relationship between 'Change Points' and soil Fe concentrations by measuring the relationships in other sites and measuring different soil Fe fractions (e.g. dithionate-extractable Fe, free iron oxides, exchangeable Fe). Various 'underdrainage systems' will be sampled at other sites around the country. This data will be used to examine the relationship between rainfall pattern and P leaching losses above and below the 'Change-Point'. Finally, soil P concentrations will be related to P concentrations in drainage waters to develop and test a model to predict P losses by leaching on a field and catchment basis.

Relevance to DEFRA policy and extension:

A general 'indicator' of the Change-Point' will be used to help to prevent soils becoming so loaded with P that leaching losses become significant. This will be used in a model of P loss from soil to water and therefore provide a scientific basis for providing information that helps make farmers control soil P and water quality

References:

Heckrath, G., Brookes, P. C., Poulton, P. R. & Goulding, K. W. T. (1995). Phosphorus leaching from soils containing different phosphorus concentrations in the Broadbalk Experiment. Journal of Environmental Quality 24, 904-910.

Hesketh, N. & Brookes, P. C. (2000). Towards development of an indicator of risk of phosphorus loss from soil to water. Journal of Environmental Quality 29, 105-110.

Addiscott TM & Falloon PD (2002) PSALM - the phosphate solute and leaching model. European Journal of Soil Science (in preparation)

Publications:

Addiscott TM, Falloon P (2000) PSALM - a phosphate sorption and leaching model. BSSS Eurosoil 2000 Conference, University of Reading, 4-6 September 2000. Abstracts.

Falloon P, Addiscott TM (2000) Factors that control P sorption in soils: a review of literature data. BSSS Eurosoil 2000 Conference, University of Reading, 4-6 September 2000. Abstracts.

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