Agricultural uses of biostimulants: Humic substances

 

With increasing pressure to efficiently produce more food, fiber and fuel to meet world demands, a number of soil and plant amendment products have emerged. These amendments are under the umbrella of "biostimulants." The definition and concept of a plant biostimulant is still evolving, which is partially a reflection of the diversity of products that can be considered biostimulants. In general, biostimulants are defined as substances and/or microorganisms whose function when applied to plants or the soil is to stimulate natural processes to enhance nutrient uptake, nutrient efficiency, tolerance to abiotic stress and crop quality. As soil amendments, they improve soil structure, function or performance and thus enhance plant response.

 

Biostimulants differ from plant protection products (pesticides) because they act only on plant strength without protection against pests, and from fertilizers because they have no principal nutritional activity.

 

The biostimulant global market is projected to grow 10.4 percent and reach a value of $2.91 billion by 2021. Biostimulants are increasingly being integrated into production systems with the goal of modifying physiological processes in plants to optimize productivity. Unlike fertilizers and pesticides, which have a long history of documented research and university recommendations, biostimulants are widely sold and used without much detailed research. In fact, relatively little research has been done to document the effects (or non-effects) of many biostimulants on crop production, or to provide evidence of their potential effects on soil processes.

 

On the basis of biochemical and physiological function and mode of action and origin, biostimulants are grouped into five categories: humic substances, inorganic salts (including phosphite), seaweed extracts, microorganism and hydrolyzed proteins and other N-containing substances. The precise mechanisms activated by biostimulants are difficult to identify and describe because of the complexity and wide range of molecules contained in these products. Frequently, biostimulatant products have a multicomponent composition that includes hormones or hormone-like substances, amino acids, proteins, vitamins, sugars, lipids, sterols, humic substance, etc. Therefore, the mode of action of biostimulants is equally diverse and may include nitrogen metabolism, phosphorous release from soil particles, stimulation of microbial activity or stimulation of root growth and enhanced plant establishment. Various biostimulants are reported to increase plant growth by increasing plant metabolism, stimulating germination, enhancing photosynthesis and increasing the absorption of nutrients from the soil and ultimately increasing plant productivity. One of the most cited benefits of biostimulants is their ability to alleviate abiotic stress such as drought, heat, salinity and frost.

 

Humic substances (humic and fulvic acids) are the most used and researched biostimulants. Humic substances are naturally occurring organic compounds, resulting from the decomposition and transformation of plant, animal and microbial residues. They play important roles in maintaining key soil functions and plant productivity. Agricultural practices such as tillage have resulted in the loss of humic material and soil organic matter. Consequently, there is an increased interest in the application of humic substances to agricultural systems to reverse this trend.

 

There are numerous reports describing the benefits of humic substances. Some of the documented benefits include improved soil aggregation and structure, increased pH buffering and cation exchange capacity, increased water holding capacity, increased bioavailability of phosphorous, iron and zinc, and decreased toxicity of aluminum and heavy metals.

 

Despite all these benefits, there is skepticism about their effectiveness. Part of the reason is that humic substances are mixed with plant nutrients; hence the cause of any benefit cannot easily be attributed to the humic substances. Also, the amount of humic substances applied is very small compared to the natural level in the soil. For example, a soil containing just 1 percent organic matter has 20,000 pounds per acre of organic matter. Therefore, considerable amounts of organic matter additions are required to make significant changes in this soil. Nevertheless, it may be feasible to make important changes in localized soil zones, such as the seedbed or in a fertilizer band.

 

Another touted benefit of humic substances is that they can provide a carbon source for soil microorganisms. This benefit seems to be unlikely. A typical application of 5 to 20 gallons per acre of humic acid will supply only 3 to 15 pounds of carbon per acre. In contrast, a typical corn crop could return about 4,000 pounds of carbon per acre in the residue alone.

 

Wheat responses to humic substances have been studied since the mid 1980's. Most of these studies are under controlled greenhouse and hydroponic conditions for less than 3 months during early vegetative growth stages. Hence, it is uncertain if the trends observed in early stages of growth will be maintained for the duration of the growing season and translates into yield gains at harvest. Under both field and greenhouse conditions results are inconsistent. For example, a foliar application of fulvic acid to a water stressed wheat crop at the time of flag leaf emergence resulted in either a no response or an 18% increase in yield. In a short greenhouse pot study, an application of low concentration of humic acid enhanced wheat growth and nitrogen uptake. At a higher rate, humic acid failed to enhance wheat growth and nutrient uptake.

 

Due to the molecular complexity of humic substances, a particular product may behave completely different under different environmental conditions and when applied to different crops. As with many chemical fertilizers, the source, timing, location, and rate of application could play an important role in determine whether beneficial or harmful effects will occur and whether or not any beneficial effects are economically worthwhile.

 

Most products have received relatively little replicated and rigorous independent validations and have inconsistent responses under field conditions. The prospects of humic substances as plant-growth stimulants in agricultural systems are "theoretically" strong. Continued research is needed to realize their full potential. Nevertheless, crop producers need to be cautious. Biostimulants should not be used instead of good, sound agronomic practices. No additives will compensate for poor management and inadequate crop nutrition.