Physico- chemical characterization of humic- metal- phosphate complexes and their potential application to the manufacture of new types of phosphate- based fertilizers

The aim of this review is to describe the main physicochemical characteristics of diverse types of humic-metal-phosphate acid complexes. The effects of these complexes on phosphorus (P) fixation in soils with different pH values and physicochemical features and on plant phosphorus uptake are also discussed. Humic-metal-phosphate complexes have apparent stability constants in the same range as those of metal-humic complexes, in solutions with diverse pH and ionic-strength values. Likewise, the molecular-size distribution of humic-metal-phosphate complexes as a function of pH is similar to that of potassium or sodium humates and metal-humic complexes. Humic-metal-phosphate complexes are able to decrease phosphate fixation in soils and increase plant growth and phosphate uptake. Phosphorus fertilizers containing humic-metal-phosphate complexes proved to be efficient to improve plant growth and P uptake with respect to conventional fertilizers such as single superphosphate. The values of parameters related to plant phosphorus-utilization efficiency (PUt E) suggest that the regulation of root acquisition of phosphate from these complexes could involve the interregulation of a system for the optimization of metabolic P utilization in the shoot and another system involving stress responses of roots under phosphorus deficiency.