Phosphate availability

There is a wide variety of phosphates available and they are commonly produced in glassy, crystalline, amorphous powder and liquid forms. Figure 10.1 shows two common phosphate types. 

Mg(H2P04)2.4H2O as was the case in the reaction between MgO and simple phosphoric acid solutions. Inspection of the diagrams of Belopolsky, Shpunt Shulgina (1950) shows that newberyite is the stable phase at lower concentrations of phosphate. Presiunably, in the present case, aluminium locks up some phosphate and so reduces the phosphate available for the magnesium phosphate phase. 

Bieleski, R.L. (1973). Phosphate pools, phosphate transport, and phosphate availability. Annual Review of Plant Physiology 24, 225-51. 

Complexing properties of humic substances can also have a great importance in P nutrition (Stevenson, 1991). Indeed Fe3+ and Al3+ (acidic soils) and Ca2+ (calcareous soils) complexed by humic molecules can bound phosphates to humic substances thus making P in a plant-available form (Gerke and Hermann, 1992). Complexation of cations by carboxylates such as those released by plant roots (e.g., citrate) can increase phosphate availability (Gerke, 1993). 

The acidification of phosphate rock to produce superphosphate is an important method of making phosphate available for fertilizer purposes. 

Some of the triose phosphate available for product synthesis will be transported into the cytosol and converted into sucrose (Fig. 2). In both photosynthetic and nonphotosynthetic cells, sucrose is synthesized via sucrose phosphate synthase, which catalyses the reaction... 

Katunuma and coworkers (1971) described a protease in the rat that hydrolyzes the apoenzymes of a number of pyridoxal phosphate-dependent enzymes it has no effect on other proteins or the holoenzymes. Presumably, it attacks the conserved amino acid sequence around the active lysine residue to which the internal Schiff base is formed. The activity ofthe enzyme is increased some 10- to 20-fold in vitamin Be deficiency, suggesting that its function is to degrade those enzymes that lose their coenzyme more readily, and so make more pyridoxal phosphate available for use by other enzymes. There is also evidence that some pyridoxal phosphate-dependent apoenzymes are modified to become incapable of activation by pyridoxal phosphate, although retaining immunological cross-reactivity with the normal form of the enzyme in vitamin Be deficiency (Nagata and Okada, 1985). 

D ll) Vitamin B, deficiency. By reducing pyri-doxal phosphate availability at the entrance to Porphy s Hemeland, an anemia may result through decreased ability to form heme. 

Grades of anhydrous dibasic calcium phosphate available for direct compression include A-TAB (Rhodia), Di-Cafos AN (Chemische Fabrik Budenheim), Emcompress Anhydrous (JRS Pharma LP), and Fujicalin (Fuji Chemical Industry Co. Ltd.). The EINECS number for calcium phosphate is 231-837-1. 

The need for calcium stimulates parathyroid hormone secretion. Parathyroid hormone, in turn,, suppresscs the 24-hydroxylase and stimulates the la-hydroxylase sy.stem. When phosphate availability is below normal, the la-hydroxylase is stimulated and the 24-hydroxylasc is. suppressed. 

Humic substances might sequester organic phosphorus-containing molecules and render phosphate available only through enzymatic hydrolysis. If so, production and release of organophosphorus compounds by the microflora would gradually result in decreased phosphate availability. Biotic equilibrium would be established after increases in alkaline... 

Maximum phosphate availability in soils tends to occur around pH 6 to 6.5. Explain why in terms of Fig. 9.5. 

The availability of soil phosphate is highly pH dependent and, as with nitrogen, Is only partially understood. The main mechanism for phosphate fixation (decreased availability) under acid conditions appears to be the precipitation of highly insoluble iron and aluminium phosphates. Phosphate availability also tends to decrease at high... 

It was found that acetic acid and formic acid are better solvents of phosphates than carbonic acid. Lipman thinks that sulphur makes phosphorus in phosphates available. 

Barshad, I. (1951). Factors affecting the molybdenum content of pasture plants. II. Effect of soluble phosphates, available nitrogen and soluble sulfates. Soil Sci. 71 387-98. 

Silicon is present in comparatively large amounts in many grasses, rushes, rice straw and hulls, etc. It is a structural component of diatoms, and most workers have considered that it is essential for these organisms. Its function in plants other than for structural purposes is unknown. In this capacity it does increase resistance to disease. In soils it may increase phosphate availability by displacing it. 

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