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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.

The design, synthesis, and biological evaluation of phosphoramide derivatives as urease inhibitors to reduce the loss of ammonia has been carried out. Forty phosphorus derivatives were synthesized and their inhibitory activities evaluated against that of jack bean urease. In addition, in vivo assays have been carried out. All of the compounds were characterized by IR, (1)H NMR, MS, and elemental microanalysis. In some cases, detailed molecular modeling studies were carried out, and these highlighted the interaction between the enzyme active center and the compounds and also the characteristics related to their activity as urease inhibitors. According to the IC(50) values for in vitro inhibitory activity, 12 compounds showed values below 1 microM and 8 of them represent improvements of activity in comparison to the commercial urease inhibitor N-n-butylthiophosphorictriamide (NBPT) (100 nM) (AGROTAIN). On the basis of the activity results and the conclusions of the molecular modeling study, a structural model for new potential inhibitors has been defined.

To optimize the economical cost of each unit of fertilizer applied and to reduce the environmental contamination caused by nutrient losses, the development of highly efficient granulated fertilizers is of great importance. This study proposes a strategy that consists of developing specific fertilizers having nutrient release patterns that are dependent on plant activity in the rhizosphere. This type of fertilizer is named "rhizosphere-controlled fertilizer" (RCF fertilizer). This fertilizer is based on the introduction of an organomineral matrix composed of metal [Mg (Ca is also possible), Zn (Fe and other metals are also possible)]-humic phosphates. The presence of this matrix modifies the nutrient release pattern of the fertilizer. In this way there are two main nutrient fractions: (i) a water-soluble fraction or "starter" fraction and (ii) a "rhizosphere-controlled" fraction insoluble in water but soluble by the action of the rhizospheric acids released by plants and microorganisms. This study shows the chemical and structural characterization of the organomineral matrix, as well as its efficiency in slowing the nutrient release rate of the RCF fertilizer, principally with respect to P and N. It is demonstrated how these properties of the matrix were also reflected in the significant reduction in both ammonia volatilization and N leaching in a pot system consisting of wheat plants cultivated in a calcareous soil and fertilized with a RCF fertilizer.

The presence of organic residues on soil surface amended with urea increased ammonia volatilization, and was particularly high in sandy compared with clay soils. Application of NBPT reduced ammonia volatilization although its efficiency is reduced in acid soils. Concerning AN fertilization, there were no differences in ammonia volatilization with or without DCD in no-tillage soils.

Size distribution, maximum complexing ability, and stability constants for phosphate-metal-humic (PO43--M-HA) complexes involving two trivalent (Fe and Al) and five divalent metal (M) bridges (Zn, Cu, Mn, Ca, and Mg) were investigated at the pH values 4, 6, and 8. Results highlighted the existing competition between metal-humic acid (M-HA) aggregation and the formation of PO43--M-HA complexes. However, the fact that only a very low fraction of complexed metal is involved in PO43- fixation seems to be related to the existence of specific electronic and/or steric requirements in the binding site in the metal-humic complex. The importance of the ionic form of phosphate (H2PO4- or HPO42-) and the involvement of phenolic and especially carboxylic groups in the phosphate binding are discussed. Finally, the order of stability obtained for PO43--M-HA complexes was similar to that of M-HA complexes. This result suggests that PO43--M-HA might play a significant role in the dynamics of phosphorus in certain soil types.

The results indicated the potential efficiency of PMHA-based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability.

The aim of this work is to study the suitability of the complementary use of ultrafiltration (UF) and the interaction with an anion-exchange resin (AR) to characterize of phosphate-metal-humic complexes in solution. The results indicate that a methodological approach consisting of the validation and calibration of the AR method by the UF method and the further use of the AR method is suitable for characterizing phosphate-metal complexes. Such an approach has proven to be useful for calculating the phosphate maximum binding capacity of iron-humic complexes and stability constants. It might also be used to obtain valuable purified phosphate-metal-humic complexes for further structural characterization.