Management practices and technologies for reducing nitrogen and phosphorus losses from soils receiving farm dairy effluent : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Soil Science, Massey University, Palmerston North, New Zealand
The loss of nutrients to the aquatic environment caused by the irrigation of farm dairy
effluent (FDE) is a prominent and contentious feature of dairy farming in New Zealand.
This thesis investigates management practices and technologies with potential to reduce
nitrogen (N) and phosphorus (P) losses in drainage water from mole and pipe drained
dairy pasture soils irrigated with FDE.
Farm dairy effluent management was both monitored, using remote sensing
technologies, and modelled on a case study farm. During the winter and spring of 2008,
an estimated 7,890 m3 of FDE was applied in excess of the soil water deficit (SWD).
System constraints were the cause of about two-thirds (5,070 m3) of this over-applied
FDE volume. It was estimated that as much as 502 kg TN and 83 kg TP could have
been lost to surface waters due to inadequate infrastructure. The two main system
constraints were the farm’s insufficient FDE storage capacity (2,000 m3) and the
inability of the farm’s irrigator to apply small application depths (<8 mm).
Furthermore, this study highlighted the significant loss of nutrients that can occur under
FDE irrigation and reinforced the need for tools to assist farmers with FDE
A number of tools were developed to help farmers manage FDE irrigation. The use of a
soil water balance, incorporating actual farm daily rainfall, is an effective method for
informing the scheduling of FDE irrigations. Also, the risk of over-application of FDE
to soils caused by travelling irrigator breakdowns or stoppages was substantially
reduced by the use of a breakdown alert and automatic shut-off system developed and
evaluated in this study.
Given the elevated risk of P losses from soils treated with FDE, a method of capturing P
from drainage waters was investigated. A field experiment was conducted to quantify
the ability of Papakai tephra, installed into mole and pipe drainage systems, to remove P
from drainage waters. This drainage system reduced TP losses in drainage by about
50% (c. 0.14 kg P/ha) over a drainage season, which equated to a 2.8 kg P reduction for
a 20-hectare effluent block.
As farmers frequently crop effluent block soils, the effect of summer forage cropping on
nutrient losses was quantified. The practice of spring cultivating long-term dairy
pasture, summer forage cropping and autumn regrassing increased the quantity of TN
measured in drainage water, over three drainage seasons (2006 to 2008), by 84% (21 kg
N/ha), compared to long-term pasture. If this study had commenced in spring with a
more typical pattern of rainfall and drainage, this increase is estimated to have only
been about 23.7% (5.9 kg N/ha). Based on these results, summer forage cropping is
estimated to increase whole-farm drainage water N losses by about 5%, when 10% of a
farm’s area is cultivated each year.
Of the management practices and technologies studied, the greatest opportunity to
reduce the losses of N and P to surface water from the case study farm’s effluent block,
is through investment in FDE system infrastructure, particularly adequate storage
capacity, and the use of decision support and fail-safe tools to assist the implementation
of deficit irrigation of FDE.