Soil humic substances (HS) are known to be beneficial for soils and plants, and most
published studies of HS and humates, usually conducted under controlled conditions, show benefits.
However, the value of commercial humate application in the field is less certain. This review
attempts to answer the question: How effective are commercial humates in the field? Commercial humates, especially K humate, are used widely in agriculture today as “soil conditioners”.
A wide range of benefits is claimed, including growth of beneficial soil microbes; deactivation of toxic metals; improvements in soil structure including water retention capacity, enhanced nutrient and micronutrient uptake and photosynthesis; resistance to abiotic stress, including salinity; and increased growth, yield and product quality. Despite this, there is a surprising lack of solid evidence for their on-farm effectiveness and findings are often inconsistent.
The industry relies largely on anecdotal case studies to promote humates, which are often applied at unrealistically low levels. It is recommended that products should be well characterised, physically and chemically, and that careful field studies be conducted on foliar humate application and pelletised humates at realistic rates, targeted to the seedling rhizosphere, for a variety of crops in a range of soils, including low C sandy and saline soils.
Using wheat treated with foliar fulvic acid, Xudan [57] obtained spectacular results, including
normalising of yield under drought conditions to that of the irrigated control, by reducing stomatal
conductance, increasing chlorophyll and increasing P uptake by roots. Other positive results from the
foliar application of HS include a 22% increase in potato tuber yield associated with application of
just 0.45 kg Na humate/ha [58] and increased potato yield from foliar K humate [59].
These findings,coupled with the relatively high cost of humates, suggest that foliar application may be more economical (due to the small quantities needed) and more effective than soil application [52].
Conversely, in another study, HS applied to soil increased wheat biomass, while foliar HS did not
providing another example of the inconsistent actions of humates on plants.
