Long-term changes in extractable soil phosphorus (P) in organic dairy farming systems
On five farms that have been managed organically for several years, all cultivated soils were sampled on two occasions. The time span between the first and second soil sampling varied from 6 to 12 years.
At the first sampling the farms had been managed organically for 3, 4, 6, 11 or 53 years. The average phosphorus (P) concentrations in topsoil (0-20 cm) extracted by ammonium-acetate lactate solution (P-AL) decreased from the first to the second sampling on all farms.
At the second soil sampling, the average topsoil P-AL concentrations on the five farms were 50, 64, 65, 75 and 119 mg P kg-1, which is characterised as medium (26-65 mg P kg-1) or high (66-150 mg P kg-1).
The decrease occurred mostly in soils with high and very high (> 150 mg P kg-1) P-AL concentrations at the first sampling. In these samples, the average value decreased from 100 to 87 and from 188 to 151 mg P kg-1, respectively.
In subsoil (20-40 cm), an increase from 15 to 27 mg P kg-1 (P < 0.01) in P-AL concentration was found in subsoil samples with low P-AL concentrations (0-25 mg P kg-1) at the first sampling. This indicates P transfer from topsoil to subsoil.
The pattern of decrease in topsoil was fairly well explained by farm level P balances. The average topsoil concentrations of P-AL were well below values for comparable conventional farms, but still at a level acceptable for crop production.
Crop yields were acceptable, but the general pattern of decrease shows that in the future, some P should be supplied from external sources to avoid a further decrease, especially on the fields with lowest P-AL concentrations.