Review Article: The effect of humic substances on phosphate and iron acquisition by higher plants: Qualitative and quantitative aspects

Abstract

Humic substances affect iron (Fe) and phosphate (P) uptake by higher plants by two different ways. First, humic substances bind Fe(III) by forming humic‐Fe complexes of medium to high stability. In soils with pH > 5.0 these complexes account for most of the Fe in the soil solution, while dissolved Fe from Fe‐oxides exhibit an extreme low solubility (<< 0.1 µM). Humic‐Fe and humic‐Al complexes may bind P and form humic‐metal‐P complexes in the soil solution. Also, humic substances may desorb P bound to Fe(Al)‐oxides by binding to the P sorption sites of the oxides. These mechanisms may increase soil Fe and P availability. Second, humic substances affect several physiological processes of higher plants which are related to the acquisition of mineral nutrients from soil such as Fe and P. Soil humic substances exhibit plant hormone features, the quality and extend depending on the humic sources, the apparent molecular weight and the association status of the humic substances. They may affect the rooting pattern, the excretion of protons and carboxylic anions, the plasma membrane Fe‐reductase activity and they may activate Fe(II) and P transporters. Organic acid anions released by the roots can decrease the apparent molecular weight of the humic substances which may in turn increase the plant hormone activity of the humics. These mutual interactions can be described as soil‐root cross‐talking or humic acid cross‐interactions with roots.

Recent Advances in the Molecular Effects of Biostimulants in Plants: An Overview

“…Several roles have been attributed to these substances in regards to soil and plant functions [20]. When it comes to plants, the influence of HS in growth can be indirect, ranging from an increase in microbial populations, improved cation exchange capacity and pH buffering properties of the soil, increased availability and mobilization of soil nutrients and the improvement of soil structure [21][22][23][24]; or direct, by positively influencing several molecular mechanisms such as photosynthetic activity, protein synthesis and enzymatic activity, whilst also being able to effect phytohormones [25][26][27]. Even though HS are promising biostimulants, there is still a need for further research regarding their effects and mode(s) of action [22,28].…”

A contribution of molecular modeling to supramolecular structures in soil organic matter#

“…Despite the definition of HS, which seems to be the only practical definition of this heterogenous group of compounds for the time being (Hayes & Swift, 2020), many important findings could be obtained by assuming that HS represents SOM. For example, important findings concerned the influence of soil management on SOM (de A. Machado & Gerzabek, 1993; Novotny et al., 1999), the transport of contaminants in soils (Haberhauer et al., 2002), possible stabilization mechanisms of SOM (Spaccini et al., 2002), or the uptake of plant nutrients (Gerke, 2021). Analyses of the alkaline extracts by exclusion chromatography on controlled pore glass suggested HS to be mainly macromolecules (Danneberg & Schmidt, 1978).…”

Humic Acid Alleviates Fe Chlorosis in Graminaceous Plants Through Coordinated Fe-Dependent and Fe-Independent Mechanisms

“…Hence, Fe deficiency is one of the major nutritional concerns lowering crop yield and nutritional quality, mainly in alkaline-calcareous soils, making up 30% of the world's agricultural soils (Barton and Abadia, 2006;Abadía et al, 2011). Nonetheless, the levels of Fe in soil solution can increase due to complexation of the micronutrient with soluble organic ligands like organic acids, phenols, phytosiderophores released by roots of graminaceous plants, microbial siderophores, and humified organic matter (Garcia-Mina, 2006;Zanin et al, 2019;Gerke, 2021).…”