Abstract
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.
Highlights
Introduction“Few agricultural products have been subject to more confusion and conflicting information than humic-type additives” [1]
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
Little et al [44] found mostly positive shoot and root growth responses with humic acid addition to two different soils for lucerne but not for ryegrass; microbial biomass increased for ryegrass, but not for lucerne on the high pH
Summary
“Few agricultural products have been subject to more confusion and conflicting information than humic-type additives” [1] With this unpromising but valid observation, we introduce our brief review in the hope that, if not providing notable clarification, we will at least not muddy the waters further. Humates are salts of humic acids, the most active component of HS, which are more soluble and more reactive in soil [5] It has been over 50 years since the publication of Burdick’s article on commercial humates for agriculture. $1500–2200/t are applied in Australia per year at 5–10 kg/ha and 25 kg/ha for saline soils, mostly “down the tube” with fertiliser [10] and in liquid form in irrigation systems [11] Given their widespread use, it is timely to review the current status and future opportunities for the research and application of commercial humates in agriculture. This review will attempt to answer the question: How effective are commercial humates in the field?
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