Range in Soil Phosphorus Critical Levels with Time F. R. Cox* ABSTRACT Soil test interpretations differ according to the mathematical func- tion used to determine the critical level, and they also differ because the critical level may not be the same from year to year. Figure 1.5 The solubility of a range of phosphorus minerals occurring in soils (and of the fertiliser material mono calcium phosphate ) (after Lindsay 1979). Feagley. University of Missouri, Columbia, MO. In Victorian pasture soils, plant available phosphorus is usually tested using the Olsen P test and results are presented in milligrams per kilogram (mg/kg) or parts per million (ppm). The Mehlich-3 soil test method extracts more P, K, Mg, and Ca from soil than the Mehlich-1 method. Soils low in organic matter may contain only 3% of their total phosphorus in the organic form, but high-organic-matter soils may contain 50% or more of their total phosphorus content in the organic form. With a graduated cylinder, measure 20 mL of Mehlich 2 soil extractant and transfer to the beaker. Phosphorus fertilizers are the main input of inorganic phosphorus in agricultural soils and approximately 70%–80% of phosphorus in cultivated soils is inorganic (Foth, 1990). It is important to understand that soil test values (lbs/ acre) extracted by a particular soil test should be inter-preted as an index value and are not equivalent to actual pounds of available nutrient per acre. Phosphorus and potassium soil test values are correlated with crop yield response and fertilizer needs (i.e., as the soil test value increases, the need for supplemental fertilizer decreases). Inorganic Phosphorus. To begin extraction of phosphorus and potassium from the soils, use a spatula to weigh out 2 g of dried, sieved soil sample, and transfer to a labeled 100-mL beaker. Phosphorus is an essential element for plant and animal growth, but too much of it can accelerate the natural aging of lakes and streams. 2. Vegetative conversion from grass to forest may influence soil nutrient dynamics and availability. This publication covers essential scientific information about phosphorus and how it behaves in soil, current concerns about phosphorus runoff from agriculture, and how farms can manage this nutrient. Soil Nutrient Analysis: Nitrogen, Phosphorus, and Potassium.

Phosphorus is essential for plant growth and is vital for early root formation. In graphs of this type, the lowest line represents the mineral of the lowest solubility. A test range of 14 to 22 ppm is adequate when using the DTPA extraction. Manganese .

2. Soil minerals can react strongly with applied phosphorus and only a small proportion may be available for plant uptake. Phosphorus in fertilizers is converted to water-soluble inorganic phosphate as orthophosphate ions H 2 PO 4 − and HPO 4 2 − within hours after application to soil ( Schulte and Kelling, 1996 ). However, the presence of mycorrhizal fungi, which develop a symbiotic relationship with plant roots and extend threadlike hyphae into the soil, can enhance the uptake of phosphorus, as well especially in acidic soils that are low in phosphorus.

Soil Test Phosphorus Threshold Levels . Principles. Soil phosphorus is classified into two broad groups, organic and inorganic. Purpose of This Publication . A test range of 20 to 30 ppm of extractable manganese is usually adequate when the 0.1 NHCl extraction is performed.

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