A considerable proportion of the dietary nutrients consumed by poultry are excreted in the manure. This becomes an important issue on free-range farms, if manure and/or nutrients are not removed periodically from the range areas.
The nutrients and trace elements in manure can accumulate in the soil and become toxic to vegetation, while also causing pollution of ground and surface water through leaching. Soil samples were collected from fourteen free range layer farms both on the range and control areas (with no exposure to poultry) to investigate comparative soil nutrient concentrations.
Nutrient concentrations were also compared between fixed and rotational ranges and between farms having different bird densities. At each site, soil was collected from 10 sampling points, arranged diagonally in a grid across both the range and control areas. A sampling probe was used to collect soil from the top 10 cm depth. These were submitted for a standardised lab analysis (Apal Agricultural Laboratory, SA, Australia).
Data was subjected to analysis of variance and means considered significant at P < 0.05. The soil nutrient concentrations in range areas were well above the levels required for normal crop/pasture nutrition. Concentrations of N, Ca and S were significantly high (P < 0.05) in the range as compared to control areas.
Soil samples from fixed ranges showed significantly higher concentrations of Ca (P < 0.01) and were significantly more alkaline (P < 0.01) than in rotational ranges, probably due to constant deposition of manure and lack of any rotational cropping program to remove nutrients from the soil. Bird density was not found to be a significant factor for soil nutrient concentrations in range areas. The percentage of birds using the range and the percentage of the range being used by birds may be more important variables than density, for nutrient loading.
The highly elevated concentrations indicate increased risk of off-site export of nutrients, in runoff, or by leaching into groundwater, potentially contaminating surface and groundwater resources. The potential impacts of nutrient export should be evaluated across a wide range of climatic conditions and soil types, to assist in developing design and management practices.
Failure to mitigate adverse environmental impacts may threaten the long- term sustainability and social acceptance of free-range production systems.