Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 4 Western Australia, Series 4
Volume 25
Number 3 1984 Article 4
1-1-1984
Soil types and drainage Soil types and drainage
Eric Bettenay N. J. Schofield
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Recommended Citation Recommended Citation
Bettenay, Eric and Schofield, N. J. (1984) "Soil types and drainage," Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 25: No. 3, Article 4.
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SOIL TYPES DRAINAGE
By E. B e t t e n a y , CSIRO Division of Groundwater Research, Perth a n d
N. J. S c h o f i e l d , Water Resources Branch, Public Works Department.
At least 90 per cent of the phosphorus entering the Peel-Harvey estuarine system comes from land cleared for agriculture, most of it from the coastal plain. These soils are naturally deficient in phosphorus and sulphur and this is supplied in superphosphate, which contains about 10 per cent phosphorus and 11 per cent sulphur.
However, rain leaches some of this applied phosphorus from the land into drains and rivers which flow into the estuary. In 1981, farmers in the Harvey River-Mayfields Drain catchment lost the equivalent of 1,300 tonnes of
superphosphate into the estuary. Between them they have in effect spent $120,000 to fertilise a crop of blue-green algae in the estuary.
The amount of phosphorus lost from the land depends on a number of factors. They include the nature of the soils and the type of farming;
the amount of land cleared and the effectiveness of drainage from it; and the amount and kind of fertiliser used and whether it is applied before, during or following winter rain.
Drainage 5%
exPott Drainage 25% 35%
Deep Grey Sands
.ural Drainage
"export 20% - 25%
Soil store 5 5 %
Sands Over Clays
..cultural i export
25%
Soil store 70%
Loams and Clays
Figure 1. The approximate fate of the applied phosphorus on different soils.
84 Journal of Agriculture, Vol 25, No. 3, 1984
Soils
Different types of soil vary in their ability to 'bind' or hold phosphorus.
The laterite and clay soils of the Darling Range adsorb most of the phosphorus applied as fertiliser near the soil surface. Even though phosphatic fertilisers are used extensively, only about 10 per cent of the phosphorus entering the estuary comes from east of the Darling Scarp. The heavy soils—the loams and clays—
on the coastal plain also retain the applied phosphorus. At most they release only about 5 per cent of that annually applied to drainage.
In contrast some of the sandy soils of the coastal plain (see map) hold phosphorus poorly. These soils lose a high proportion of applied phosphorus to the estuary.
The main research effort towards reducing agricultural phosphorus losses therefore has concentrated on farming properties to the west of the foot of the Darling Scarp.
Here, the brown and yellow sands and soils which contain ironstone gravels leach relatively small amounts of phosphorus. On the other hand the deep grey sands of the Bassendean Association, particularly the Joel soils, and the sandy surfaces of the Coolup-type soils in the Guildford Association, lose a lot of applied phosphorus.
D r a i n a g e
The leaching of phosphorus from the coastal plain is largely determined by soil characteristics and the network of drains constructed to reduce waterlogging. Phosphorus enters the estuarine system mainly through overland flow and shallow groundwater flow. Deep, regional groundwater movement is barely significant.
The Joel soils have permanent groundwater lying close to the surface (typically at one metre or less depth), which intersects the surface in a number of permanent swamps. After heavy winter rains the watertable rises to the surface and contributes to surface run-off.
However, about 60 per cent of the flow to drains comes from the sub-surface flow. The shallow groundwater contains high phosphate
concentrations of up to 26 milligrams per litre which come from recently applied fertilisers and from the phosphorus store built up in the soil from previous years of application. The mean phosphorus concentration of water in drains is about 1.5mg/L.
In the duplex Coolup-type soils, perched groundwater forms in the sandy surface soil above the clay horizon during winter. Following rainfall, most of the run-off (about 80 per cent) is overland flow, with phosphorus concentrations of about 0.5 mg/L.
During the past 40 years more land has been cleared and cultivated and the drainage system
has been greatly improved. It was only in the 1970s, however, that big areas of the deep grey sands were cleared and drained.
Much of the Harvey River catchment and its associated drains consists of these deep grey sands and duplex soils, and this is the largest source of phosphorus entering the estuary. In 1978, when the Harvey River contributed only 37 per cent of the total flow into the estuary, it carried in 60 per cent of the phosphorus. The following low rainfall year, when the Murray River flowed poorly, the Harvey River accounted for 76 per cent of the phosphorus.
Although research has focused on the Harvey River catchment, areas of deep sand in the Serpentine catchment are also important and will be assessed more critically in the future.
M e a s u r e m e n t s
Extensive sampling of both topsoils and subsoils, measurement of drainage flows and concentrations and estimates of phosphorus export in agricultural products has allowed a phosphorus budget to be drawn up for the major soil classes of the coastal plain (Figure 1).
On sandy soils, almost half the phosphorus is retained in the soil store and only about one- quarter is removed in milk, meat, wool and hay.
The remainder—up to 35 per cent—is lost to drainage, and is a direct loss to farmers.
C o m p u t e r modelling
Because of the complex nature of phosphorus movement in the soil and the cost of field experiments, computer modelling is
supplementing research. The models should give researchers an increased understanding of phosphorus cycling. It should help them to interpret field results better, to estimate long term trends in phosphorus soil storage and leaching, and enable them to apply the results to different districts.
Harvey
• • •
D e e p S a n d s B r a w i a n d Grey Over Yellow S a n d s Clay S a n d s Bassendean* Gurfdtord KaoahaBa Southern Canrangton C o w * *
Loams, Q a y s a n d Peats
Seipentine Riwi Herdsman
Much L i f e Phosphorus released to drainage
Figure 2. Soil types of the Peel-Harvey catchment area and the effects on phosphorus released to drainage.
• Tube wells allow samples to be taken to measure phosphate concentrations in groundwater at various depths.
S5 Journal of Agriculture, Vol 25, No. 3, 1984
