Soil analysis by means of a 1:2 volume extract

Abstract

SUMMARYThe use of the 1:5 weight extract in soil analysis presents some difficulties. Conductivity determinations as a means of assessing the salt status are unreliable for those soils containing appreciable amounts of gypsum, and it is also desirable that the soil organic matter content be known when interpreting the analytical data.These objections are largely overcome by using saturation extracts. The principal drawback with this method is, however, its laborions procedure.In an investigation at the Experiment Station at Naaldwijk very good results were obtained using an extract made by adding sufficient field-moist soil to two parts of water until the total volume had increased by one part (1:2 volume extract). The analytical data for conductivity, chloride, nitrogen, phosphate, potassium and magnesium in this extract were very closely related to data obtained by the saturation extract. The correlation coefficientes being between 0.943 and 0.982.The weight ratio water: soil of the 1:2 volume extract was highly correlated with the organic matter content. In soils low in organic matter the ratio was about 1 : 1, and with very peaty soils (40 per cent organic matter) about 3½:1.For precise preparation of the 1:2 volume extract it is necessary to use soil samples at field capacity. A visual check on this condition appeared to be possible. INTRODUCTIONSoil analysis plays an important role in glasshouse management. Because of the rapid changes which occur in both the salt and nutrient status it is desirable that frequent determinations are made, and this results in a considerable number of soil samples for testing. At the Experiment Station at Naaldwijk, for example, more than 40,000 soil samples are tested each year. Testing for salt and nutrient status is usually carried out by means of a water extract of the soil, the extraction ratio normally used being one part by weight of air-dry soil and 5 parts of water (1:5 weight extract). Electrical conductivity and chlorides are used to assess the salt content, and the nutrient status is assessed from the concentration of nitrogen, phosphate, potassium and magnesium.The 1:5 weight extract does, however, have some disadvantages. In interpreting~the analytical data it is desirable to know the moisture content of the soil when at field capacity, and this can be derived from the organic matter content 2. Thus the interpretation of the analytical data for conductivity, chloride, nitrogen, potassium and magnesium is made in relation to the organic matter content. Phosphate is usually evaluated independently of the organic matter. With increasing water: soil ratio the concentration of phosphate insolution tends to be more or less constant as a result of the large reserves in the soil and their low solubility. Only if the water: soil ratio is raised to a high value can a significant decrease ill concentration be expected 4. The concentration of phosphate in the 1:5 weight extract generally equates with the phosphate concentration in the soil solution.Another disadvantage of th...

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