In New Zealand, regulatory authorities have selected controlled application of dairy farm effluent onto pasture as the best practical treatment option. Nitrogen leaching studies have found undissolved or particulate (>0.2 μm in size) as well as dissolved organic material in drainage waters as a result of the application of this effluent onto poorly drained soils. This paper describes a detailed investigation into the leaching behavior of the particulate fraction of dairy farm effluent applied onto a poorly drained soil. The study entailed dividing the particulate fraction of dairy farm effluent into a number of arbitrary size-classes and determining the amounts of organic carbon present in each of these classes by physical filtration and carbon measurement. This effluent was then applied to a number of undisturbed soil cores from soil horizons of a poorly drained soil. The filtration characteristics of the soil horizons were determined by differences between the carbon in the applied effluent and the leachate for each size-class. Results show that when the upper horizons of the soil are at water contents greater than field capacity, between 27 and 34% of the carbon applied in dairy farm effluent can be leached through the top horizons. As expected, most of the material that was filtered out came from the larger size-classes; 84% of the carbon applied was greater than 2.7 μm in size, and more than 83% of this was retained in the top soil horizons. In the smaller size-classes (<2.7 μm in size), nearly all of the carbon applied was shown to leach. However, some of this material may have originated from larger size-classes, which broke down into these smaller size-classes in the soil.
This paper also presents and parameterizes a simulation model that can be used to describe the transport of particulate organic material associated with dairy farm effluent. The model has two pools, a "trapped" pool in which the effluent material is considered to be permanently trapped within a soil layer and a mobile "wash" pool of organic material able to move with water flux events. The simulated response of the carbon leached over the three irrigation events was in very close agreement with the observed values.