Phosphate-metal-humic complexes are very relevant in nature due to their crucial role in phosphate availability for plants and microorganisms. Synthetic phosphate-calcium-humic acid (HA) complexes have proven to be efficient sources of available phosphorus for crops. However, the current knowledge about their structure and molecular features is very poor. The structural implications of phosphate interaction with humic binding sites through calcium bridges, in both monocalcium phosphate and dicalcium phosphate is investigated by using molecular modeling, P-31-NMR, H-1-NMR and X-ray diffractometry. The conformational changes in the molecular configuration of the humic acid involved in the interaction resulting from the synthetic process is also studied by using HPSEC and synchronous fluorescence. The results obtained allow us to identify the phosphate type in the crystalline phase that is involved in the interaction of humic acid binding sites and the different forms of calcium phosphate. Synchronous fluorescence also shows that whereas the conformational configuration of the HA binding site is only partially affected in the monocalcium phosphate interaction, it changes in the case of dicalcium phosphate showing simpler molecular arrangements. These changes in the molecular conformation of the binding site in HA in solution may influence the biological activity of the humic acid.

Phosphate fertilizers play an important role in plant nutrition. Different P fertilizer sources such as high-solubility (simple superphosphate, SSP), low-solubility (rock phosphate, RP) and complex superphosphate (CSP) are available for plant supplementation. The objective of this study was to investigate the short- and long-term redistribution of soil P after application of different P sources at establishment of an Eucalyptus forest stand. We carried out two experiments to identify the short- and long- term changes in a Brazilian Oxisol. To property identify the P pools in different times, Hadleýs fractionation methodology was applied to a long-term studies and citrate and oxalate to short-term. From zero to 180 days, the soluble P fractions were not altered in the non-fertilized treatment. Under SSP, a slight increase in this P fraction was found until 30 days, followed by a decrease in later evaluations. During the same period, a slight reduction in Pi extracted by citrate and oxalate was found under the control and a large reduction (approximately 50%) under the SSP treatment. Intermediate behavior was observed under the CSP and RP treatments, whereas there was an increase in P-citrate and P-oxalate until 30 days followed by a reduction afterwards. These results suggest that this pool comprises a potential bioavailability of P to plants.

The ability of rhizospheric humic substances to improve plant growth has been well established by many studies carried out using diverse plant species cultivated under many different conditions. These beneficial effects of humic substances on plant development are expressed in both root and shoot. However, the mechanisms responsible for this action of humic substances are only partially known and poorly integrated. In fact, although the studies focused on plant root development are numerous, those dealing with plant shoot development are scarce. Likewise, studies integrating humic effects on root and shoot are also few. In this context, the main goal of this work is to summarize some of the results regarding the effects of humic substances on plant development within a hypothetical holistic framework that will allow us to interconnect these findings and disclose some features of the functional crosstalk between the effects on soil, root and shoot. Furthermore, the significance of all these mechanisms in plants growing in the field is also discussed.

This study describes the efficiency of a new coating material for preparing granulated potassium-fertilizers with a potassium release to the soil solution sensitive to rainfall intensity. The composite is prepared by reaction of an alkyd-resin with cement in the absence of water. The complementary use of diverse analytical techniques showed that the presence of the cement fraction induced alkyd resin reticulation and gradual cement-resin hardening. Scanning electron microscopy revealed the formation of micro and nanopores within cement-clusters, whose water permeability is affected by the resin reticulation and amphiphilic character. Potassium release was evaluated in water, soil-columns, and in soil-plant trials in pots and open-field. Agronomic results were consistent with potassium release rates obtained in water solution and soil columns. The composite-coated potassium fertilizer was more efficient than the noncoated one in providing plant available potassium, with this effect being dependent on water presence in soil.

The aim of this review is to describe the main physicochemical characteristics of diverse typesof 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 phosphorusuptake are also discussed. Humic-metal-phosphate complexes have apparent stability con-stants in the same range as those of metal-humic complexes, in solutions with diverse pH andionic-strength values. Likewise, the molecular-size distribution of humic-metal-phosphate com-plexes as a function of pH is similar to that of potassium or sodium humates and metal-humiccomplexes. Humic-metal-phosphate complexes are able to decrease phosphate fixation in soilsand increase plant growth and phosphate uptake. Phosphorus fertilizers containing humic-metal-phosphate complexes proved to be efficient to improve plant growth and P uptake withrespect to conventional fertilizers such as single superphosphate. The values of parametersrelated to plant phosphorus-utilization efficiency (PUt E) suggest that the regulation of rootacquisition of phosphate from these complexes could involve the interregulation of a system forthe optimization of metabolic P utilization in the shoot and another system involving stressresponses of roots under phosphorus deficiency.

Fertilizers based on phosphate-metal-humate complexes are a new family of compounds that represents a more sustainable and bioavailable phosphorus source. The characterization of this type of complex by using solid (31)P NMR in several fertilizers, based on single superphosphate (SSP) and triple superphosphate (TSP) matrices, yielded surprising and unexpected trends in the intensity and fine structure of the (31)P NMR peaks. Computational chemistry methods allowed the characterization of phosphate-calcium-humate complexes in both SSP and TSP matrices, but also predicted the formation of a stable sulfate-calcium-humate complex in the SSP fertilizers, which has not been described previously. The stability of this complex has been confirmed by using ultrafiltration techniques. Preference towards the humic substance for the sulfate-metal phase in SSP allowed the explanation of the opposing trends that were observed in the experimental (31)P NMR spectra of SSP and TSP samples. Additionally, computational chemistry has provided an assignment of the (31)P NMR signals to different phosphate ligands as well as valuable information about the relative strength of the phosphate-calcium interactions within the crystals.

BACKGROUND: Previous studies showed that phosphate can be complexed by humic acids (HA) through stable metal (M) bridges (PMHA). We studied the thermodynamic properties of PMHA and their relationships with the ability of PMHA to both decrease soil P fixation and increase P availability for plants. With this aim, we studied the theoretical stability of PFeHA, PAlHA and PCaHA by molecular modelling methods in relation to the degree and intensity of P absorption in soils and the ability of plants to take up complexed P. RESULTS: A density functional theory (DFT) quantum chemical study enabled us to obtain stable structures for the three PMHA complexes in water solution. The theoretical stabilities (¿G¿) were consistent with that for apparent stability obtained by Scatchard method, PFeHA ¿ PAlHA > PCaHA, though the differences were clearer by the DFT method. Also the reduction of soil P fixation and the release of P from PMHA in the presence of an anionic resin confirmed the stability order of the different PMHA. Plant studies confirmed the ability of diverse plant species to take up both P and metal complexed in PMHA. CONCLUSION: The results indicated the potential efficiency of PMHA-based fertilizers to optimize P fertilization for crops cultivated in soils with high P fixation ability.

A number of studies have shown the ability of natural organic matter (NOM) in general and humic substances (HS) in particular, to affect the development of plants and microorganisms in many different natural ecosystems and agroecosystems. Regarding plants, these NOM and HS effects were expressed in both root growth and architecture, and shoot growth. However, these effects were different in intensity and quality depending on several intrinsic and extrinsic factors associated with HS structure and concentration, plant species and soil properties. Two main mechanisms have been proposed to explain the beneficial action of NOM and HS on plant growth. An indirect effect expressed through the improvement of plant nutrition by increasing soil nutrient availability, principally some micronutrients (mostly P and Fe); and a possible direct action affecting the transcriptional and post-transcriptional regulation of several enzymes and molecular transporters in the root. These biological effects within the plant seem to be associated with both nutrient root uptake ability and the efficient use of the nutrient in plant leaves. In this communication, the relationships between the effects of HS on root development, shoot development, plant nutrition, and soil properties; are discussed. This study is developed in the context of the links existing between the signal role of some nutrients and the hormonal balance in both root and shoot.

A new type of superphosphate (organic complexed superphosphate (CSP)) has been developed by the introduction of organic chelating agents, preferably a humic acid (HA), into the chemical reaction of single superphosphate (SSP) production. This modification yielded a product containing monocalcium phosphate complexed by the chelating organic agent through Ca bridges. Theoretically, the presence of these monocalcium-phosphate-humic complexes (MPHC) inhibits phosphate fixation in soil, thus increasing P fertilizer efficiency. This study investigateed the structural and functional features of CSP fertilizers produced employing diverse HA with different structural features. To this end were used complementary analytical techniques: solid-phase 31P NMR, 13C NMR, laser-confocal microscopy, X-ray diffraction, and molecular modeling. Finally, the agronomical efficiency of four CSP have been compared with that of SSP as P sources for wheat plants grown in both alkaline and acidic soils in greenhouse pot trials under controlled conditions. The results obtained from the diverse analytical studies showed the formation of MPHC in CSP. Plant¿soil studies showed that CSP products were more efficient than SSP in providing available phosphate for wheat plants cultivated in various soils with different physicochemical features. This fact is probably associated with the ability of CSP complexes to inhibit phosphate fixation in soil.

Hydroponic plant experiments demonstrated the efficiency of a type of humic acid-based water-insoluble phosphate fertilizers, named rhizosphere controlled fertilizers (RCF), to supply available phosphorus (P) to different plant species. This effect was well correlated to the root release of specific organic acids. In this context, the aims of this study are (i) to study the chemical nature of RCF using solid-state 31P NMR and (ii) to evaluate the real efficiency of RCF matrix as a source of P for wheat plants cultivated in an alkaline and acid soil in comparison with traditional water-soluble (simple superphosphate, SSP) and water-insoluble (dicalcium phosphate, DCP) P fertilizers. The 31P NMR study revealed the formation of multimetal (double and triple, MgZn and/or MgZnCa) phosphates associated with chelating groups of the humic acid through the formation of metal bridges. With regard to P fertilizer efficiency, the results obtained show that the RCF matrix produced higher plant yields than SSP in both types of soil, with DCP and the water-insoluble fraction from the RCF matrix (WI) exhibiting the best results in the alkaline soil. By contrast, in the acid soil, DCP showed very low efficiency, WI performed on a par with SSP, and RCF exhibited the highest efficiency, thus suggesting a protector effect of humic acid from soil fixation.

The role of soil humus in soil fertility and crop production has been well established by many studies and practical farming experience. This role is related to the presence of a family of organic substances, known as humic substances, with the capacity to increase the pool of plant-available nutrients in soil for root uptake. The improvement in plant growth and mineral nutrition is directly linked to the physicochemical features of humic substances, and more specifically, to their capacity to form stable chemical complexes with metals. This chapter reviews the main mechanisms behind the action of humic substances in improving plant mineral nutrition. It also discusses the main signalling pathways that might be involved in the regulation of these humic substances.