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Chemical amendment of dairy cattle slurry to reduce P losses from grasslands
R.B. Brennan 1 , O. Fenton 2 , M.G. Healy 1
1 Civil Engineering, National University of Ireland.
2 Teagasc, Johnstown Castle, Environmental Research Centre, Co Wexford.
E-mail: r.brennan1@nuigalway.ie
Abstract
In intensive dairy farming systems, phosphorus (P)
inputs can exceed P outputs. Over time, soil test P
(STP) levels build up after repeated manure
applications, and this can increase the risk of P loss to
surface and subsurface waters. There is also the risk of
incidental losses when landspreading slurry.
Amendments could potentially mitigate P losses in
strategic areas. The aim of this study was to identify
amendments with the potential to reduce dissolved
reactive P (DRP) loss from agricultural grassland in
Ireland arising from the land application of dairy cattle
slurry.
1. Introduction
In intensive dairy farming systems, phosphorus (P)
inputs can exceed P outputs. Over time, soil test P
(STP) levels build up after repeated manure
applications, and this can increase the risk of P loss to
surface and subsurface waters. Amendments could
potentially mitigate P losses in strategic areas, while
allowing farmers to utilise nitrogen (N) and other
nutrients in the slurry. The aim of this study was to
identify amendments with the potential to reduce
dissolved reactive P (DRP) loss from agricultural
grassland in Ireland arising from the land application of
dairy cattle slurry.
2. Materials and Methods
An agitator test (Mulqueen et al., 2004), was used to
identify the optimal rates of amendment addition to
slurry to reduce soluble P and to estimate associated
costs. The most effective amendments were as follows:
aluminum sulphate (alum) applied at a metal to total P
ratio of 1.1:1, poly aluminium chloride (PAC) (0.93:1),
ferric chloride (FeCl2) (2:1), and lime (10:1).
Two runoff boxes 200 cm long by 22.5 cm wide by
5 cm deep with side walls 2.5 cm higher than the soil
surface, containing intact soil from a dairy farm treated
with dairy slurry and amended slurry were subjected to
simulated rainfall with an intensity of 11.5 mm hr-1
(n=3) with each runoff box receiving three successive 1
h rainfall events at time zero (RS1), 1 h (RS2) and 24 h
(RS3) with first event being 48 h after land application
of slurry. All experiments were conducted in triplicate.
All runoff samples were tested for suspended sediment
(SS) and DRP.
Proc Mixed of SAS (2004) was used to analyze the
data with a covariance structure to account for
correlations between the repeated measures.
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3. Results
The results of this experiment are shown in Fig. 1.
Alum reduced mean flow-weighted concentration of
DRP by an average of 83% (p