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

Abstract

AbstractHumic 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.