Establishing soil nutrient distribution zones across free range egg farms to guide practical nutrient management strategies

Little is known regarding manure nutrient deposition in free range egg layer facilities. Consequently, a significant knowledge gap exists regarding how to best manage soil nutrient loading on these farms. Here, we report on soil nutrient properties across 14 Australian free range farms. Electromagnetic-induction (EM) mapping was performed at each farm to select soil sample locations which were collected at an average depth of 30 cm. EM data exhibited promising relationships with key soil properties. Measured soil properties were highly variable between and within farms. Soil nitrate-N (NO3-N) and Colwell-P (Col-P) concentrations ranged from 1 to 529 mg N kg−1 and 11 to 1856 mg P kg−1. Average NO3-N and Col-P concentrations across farms were 100 mg N kg−1 and 250 mg P kg−1 which exceed typical background nutrient levels and exceed requirements for dryland crop or pasture production. Farms with trees exhibited 2.6 × and 2.1 × greater NO3-N and Col-P concentrations than farms with no trees (P < 0.05), indicating trees attract birds to range areas resulting in higher manure deposition rates. Generalised nonlinear models were derived to describe soil nutrient concentrations with respect to shed location. These models revealed sharp nitrate and Col–P concentration decreases with increasing distance from sheds, with 50% of the NO3-N and Col-P concentration gradients restricted to a radius of <6 m (equating to an area of 0.05 ha) from the nearest shed (P < 0.05). Encouragingly for farms that pose a nutrient accumulation risk, these relatively small impacted areas can be managed with several options which we discuss in this paper.