Phosphorus soluble

A commercial fertilizer is a material that contains at least one of the plant nutrients in chemical form that, when applied to the soil, is soluble in the soil solution phase and assimilable or available by plant roots. Most often, this implies chemical forms that are water soluble. However, in the case of phosphorus, solubility in special reagent solutions (citric acid, neutral ammonium citrate, or alkaline ammonium citrate) often is used as a guide for availability to plants. In the case of nitrogen, slow solubility in water may be more desirable from an environmental and efficiency standpoint than easy solubility. 

If we enclose in a flask a mass of white or red phosphorus, and if we keep this flask at a fixed high temperature T long enough for equilibrium to be established, we shall have, at the end of the experiment, the flask filled wilb phosphorus vapor, whose tension will be equal to the tension. of saturated vapor from red phosphorus at the temperature of tiie experiment, and the mass of solid in excess will be in the form of red phosphorus. If, for instance, the temperature is 440 C., we shall have a mass of phosphorus vapor which will fill the flask at a pressure of 1.75 at. this mass will be equal to as many times 3.6 gr. as there are litres in the volume of the flask the rest of the phosphorus will be in the red form. By suddenly cooling the flask we shall find a mass of phosphorus, soluble in carbon bisulphide, equal to 3.6 gr. per litre the rest will be insoluble in carbon bisulphide. 

Moore P. A., Jr. and Miller D. M. (1994) Decreasing phosphorus solubility in poultry litter with aluminum, calcium, and iron amendments. J. Environ. Qual. 23, 325-330. 

The raised solubility of phosphate in some types of soils at both low and high pH has led to the observation that phosphorus solubility is lowest near neutral pH, contrary to the model of solubility based on simultaneous A1 hydroxide, A1 phosphate, and Ca phosphate precipitation. The latter model predicts phosphorus to be most soluble between pH 6 and 7, although there is little evidence from soils to confirm this prediction. While precipitation in very acid and in Ca-rich mineral soils is undoubtedly a key process in determining the solubility of phosphate, oversimplified precipitation models for this and other ions can lead to misleading generalizations about solubility in a material as complex as soil. 

ISO 11263 1994 Soil quality - Determination of phosphorus - Spectrometric determination of phosphorus soluble in sodium hydrogen carbonate solution. 

Sodium is an indispensable element for some crops (notably sugar beet), can partially substitute for potassium in several crops, contributes to neutralizing soil and subsoil acidity, and has a positive effect on soil phosphorus solubility. Sodium is an essential nutrient for cattle, and sodium application to soil increases its content in pastures. Sodium nitrate is particularly effective as a nitrogen source for sugar beet, vegetable crops, tobacco, and cotton (qv), and for any crop in acid soils. 

Huffman, S.A. (1995) Phosphorus solubility and availability in soils formed under different weathering regimes. PhD thesis, Colorado State University, Fort Collins, Colorado. 

Phosphorus solubility is also directly affected by the changes in redox potential (Patrick, 1964). In well-drained mineral soils, some of the inorganic P is bound to oxidized forms of iron such as iron oxyhydroxides. Ferric phosphate is a common phosphate compound in these systems. Similarly, in oxidized portions of mineral wetland soils, some of the inorganic phosphorus is also present as ferric phosphate. The mineral form of FeP04 is called strengite. 

Phosphorus availability in soils is regulated by pH. The relationship between phosphorus solubility and pH is shown in Figure 9.20. In the hgure, the y-axis represents the concentration of phosphate... 

In mineral wetland soils, A1 and Fe are major constituents, which regulate phosphorus solubility by forming insoluble phosphate minerals. These minerals are sensitive to changes in soil pH, whereas phosphorus minerals associated with Fe are redox sensitive. Phosphate minerals associated with A1 and Fe are stable under acidic soil conditions, and their solubility increases with an increase in pH. Solubility of calcium phosphates decreases with an increase in pH. In soils dominated by CaCOj (calcareous soils), when Ca + activity is depressed, phosphorus solubility increases. 

FIGURE 9.35 Influence of Eh and pH on phosphorus solubility in mineral wetland soil. (Adapted from Patrick et ah, 1973.)... 

Solubility of phosphorus is also influenced by soil pH and Eh (Figure 9.35). In a pH range of 5-8, phosphorus solubility is low at 300 mV, resulting in low phosphorus concentration in soil solution (Patrick et al., 1973, Patrick and Khalid, 1974). However, as Eh decreases from 300 to 250 mV, phosphorus solubility increases at all pH levels, reflecting high concentration in soil pore water. Phosphorus solubility is highest under low-pH and low-Eh environments. 

Some of the key concepts on mechanisms regulating phosphorus solubility in wetlands are as follows ... 

Eh In mineral soils dominated by iron, minerals associated with phosphorus are stable under aerobic or drained soil conditions. As the soil undergoes reduction, a decrease in Eh increases phosphorus solubility through reduction of FeP04. Accumulation of organic acids and dissolved carbon dioxide can also potentially increase the solubility calcium phosphates. 

In addition to mineralization of organic phosphorus, microbial decomposition of detrital matter and soil organic matter can have indirect effect on phosphorus solubility, as described below ... 

The reduction and dissolution of iron and its reprecipitation to form ferrous minerals are thought to be the dominant processes controlling phosphorus solubility in anaerobic systems. Soil pore water profiles shown in Figure 9.56 show that the concentrations of soluble phosphorus and dissolved iron are low in the water column and increase with depth. Similar patterns in these profiles suggest that the solubilities of phosphorus and iron are strongly coupled. In the water and in the aerobic soil layer, iron is in the immobile ferric form, which reacts with phosphate and precipitates. Thus, under aerobic conditions, oxidation and precipitation of iron control phosphorus solubility and limit... 

In sulfate-dominated wetlands, production of sulfide (through biological reduction of sulfate) and formation of ferrous sulfides may preclude phosphorus retention by ferrous iron in regulating phosphorus bioavailability (Caraco et al., 1991). In iron- and calcium-dominated systems, Moore and Reddy (1994) observed that iron oxides likely control the behavior of inorganic phosphorus under aerobic conditions, whereas calcium phosphate mineral precipitation governs the solubility under anaerobic conditions. This difference is in part due to a decrease in pH under aerobic conditions as a result of oxidation of ferrous iron compounds, whereas an increase in pH occurs under anaerobic conditions as a result of reduction of ferric iron compounds. The juxtaposition of aerobic and anaerobic interfaces promotes oxidation-reduction of iron and its regulation of phosphorus solubility. 

FIGURE 9.66 Chemical reactions regulating phosphorus solubility in aerobic and anaerobic layers of wetland soils. (Adapted from Moore and Reddy, 1994.)... 

The two preceding examples demonstrate the role of inorganic amendments in regulating phosphorus solubility in wetland soils. Possible sequential reactions after application of chemical amendments are follows ... 

Numerous published articles evaluated the potential of wetlands to retain phosphorus. Most of this information is related to inflow and outflow characteristics of the water, with very limited information on internal processes and mass balances of phosphorus in wetlands. However, a wealth of quantitative information on phosphorus biogeochemistry of upland and of other aquatic ecosystems provides extensive process-level information. These data on chemical and biological reactions regulating phosphorus availability and mobility in soils and sediments form a basis for much of the phosphorus biogeochemistry research related to phosphorus retention in wetlands. For example, early research on flooded soils showed the regulation of phosphorus solubility and retention by oxidation and reduction of iron. Similarly, iron regulation of phosphorus solubility was also shown in lake sediments. More recent research in wetlands also confirmed that these principles are applicable to wetland ecosystems. 

Thrombokinase, a substance lipoidal in nature, lidi in phosphorus, soluble in alcohol, petroleum ether and chloroform, stable toward heat, contained in abundance in the plates, the leucocytes and the tissue extracts. 

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