How do stony soils treat dairy shed effluent?
Soil cores being removed in the Mackenzie Basin
Landcare Research has shed light on the topical issue of how stony soils treat dairy shed effluent (DSE).
With irrigation, dairy farming is now possible on stony soils that previously wouldn’t have supported adequate grass growth because of lack of water.
But these soils are thought to be ‘leaky’. Compared to soils traditionally used for dairying they do not bind microbes and nutrients well. Instead these contaminants can potentially bypass the soil’s filtering and cleansing ability, and flow into the groundwater below.
In 2008 we used research into the microbial bypass flow of 12 different soils to produce a nationwide map of potential for microbial bypass flow through soils. In 2012 we took that a step further by studying how stony soils coped with DSE.
We created soil lysimeters made from cores of intact soil 460mm in diameter and up to 750mm deep. Some very stony soil cores were taken from near Christchurch Airport where the Waimakariri River had previously flowed. Others were taken from the Mackenzie Basin, some with surface stones contained within in a silty matrix, while others had layers of silty material 300mm to 600mm deep above stones.
We applied DSE and irrigated the soil cores – both at standard rates – and captured and measured the Escherichia coli (E. coli) that flowed out the bottom of the lysimeters.
We found high potential for E. coli to leach through the soil cores taken from near Christchurch Airport but low for the Mackenzie soils with 600 mm silty material over gravels.
We also studied the impact of pugging – the breakdown of soil structure that occurs when cattle trample wet soil. Using a post-hole rammer with the head modified like a cow’s hoof to simulate pugging, we found we could induce microbial bypass flow through the Mackenzie soils that had surface stones within silt. In our experiment the microbial movement through Mackenzie soils with 600mm of silty material over gravels did not change with topsoil pugging.
Overall, the results showed that in very stony soils, shallow groundwater may be vulnerable to microbial contamination under irrigation. When pugged, soils with surface stones and silt may also leak microbes. So when soils with stones to the surface are wet, particularly from DSE application, care needs to be taken with stock management to avoid topsoil pugging.
The results of this research have been used to update the DSE Pond Storage Calculator, developed by Massey University, Horizons Regional Council and AgResearch. We have also updated the bypass flow rating information for stony soils in S-map, our digital soil map for New Zealand.