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DAFWA Collections

Natural resources


The current status and trends of soil analyses in the coastal catchment of the Peel Harvey estuary were developed from historical soil data (1982 to 1991) combined with a renewed sampling to greater depth (1 m). This report encompasses the data collected from the first year of the project. Soil phosphorus content was found to be high but has dropped slightly since 1991. Soil pH is very low and is likely to be limiting production but have risen slightly since 1991. Soil potassium content is generally so low that it may be limiting production especially to the west of the south west highway. Correcting these deficiencies would reduce the rate of increase in soil phosphorus and reduce the hazard of phosphorus loss. These findings show some improvement but much greater effort is required to develop better nutrient management for both production and the environment. The sandy soils have not retained phosphorus and the heavier, clay soils have accumulated very large stores of phosphorus. Much of the soil has become saturated with phosphorus with the lighter textured soils being completely saturated and the heavier textured soils also showing signs of phosphorus saturation in the topsoil. The soils with the least stores of phosphorus appear to retain little of the applied phosphorus and are unfortunately very close to the estuary and major drainage lines. The sandy soils do not build up phosphorus stores and the phosphorus that is applied leaches, the heavy textured soils have built up phosphorus to the point that up to 77% do not need more phosphorus to be applied. Total Phosphorus The soils of the Peel-Harvey have become greatly enriched by the application of phosphorus since clearing. The farmed soils have 1020 kg/ha/m stored in them compared with 180 kg/ha/m in the uncleared sites. This is equivalent to about 10 tonnes of superphosphate added to the top metre of soil profile over each hectare or about 50 years of 200 kg of superphosphate per hectare. The greatest absolute enrichment has occurred in the top 5 cm as would be expected for surface applied fertiliser, but enrichment continues deep into the soil profile indicating leaching and incorporation of phosphorus in all soil types. Phosphorus status The phosphorus status of the soil is a measure of how well a pasture will respond to applications of phosphorus. The phosphorus status of the soils, as a group, have dropped shown by the low phosphorus status (those likely to benefit from additions of phosphorus) having risen from 28 to 33% and those in a high phosphorus status (those unlikely to benefit from additions of phosphorus) having dropped from 58 to 42%. When the soils are grouped into their different categories of phosphorus retention the sandier soils have the greatest proportion of low phosphorus status samples (70 to 80% of samples) while the heavier, higher phosphorus retentive soils, have a greater proportion of high phosphorus status (45 to 77%). Phosphorus retention The phosphorus retention mapping has been included into the DoW Modelling project which is renewing the modelling of the nutrient flows in the catchment and the chemical analyses are being included into reviewed soil information held by the Department of Agriculture and Food, WA. The mapping of phosphorus accurate to the high levels achieved here enables a clear understanding of the location and mechanism of phosphorus movement which can be used when targeting extension or planning works as it allows the mechanism of leaching or sediment bound nutrient runoff to be identified. pH The pH of the soils of the catchment are mostly acidic (below 4.5) and most require lime to minimise productivity loss. There was a small increase in pH over the soil sampling period. EC The was no overall trend in the electrical conductivity of the soils of the Peel-Harvey. As would be expected the soils with the higher iron content (heavier clay soils) had higher electrical conductivity and localised drainage conditions are likely to be having the greatest influence on salinity levels. Potassium status Overall, the potassium status of the soils are low and the lowest levels correspond to the phosphorus retention. Although there is a similar distribution of potassium and phosphorus the magnitude is much lower with few sites having built up high enough potassium to effectively utilise other nutrients. The potassium levels have not changed with time. Management implications There are a number of relationships in the soil analyses that have been described that are useful in modelling and for conveying risk as an adjunct to agronomic recommendations when considering the results of soil testing.

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Peel Inlet, Harvey Estuary, phosphorus, nutrient status, pH, potassium

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