A meta-analysis of biostimulant yield effectiveness, published last year, shows an average additional yield benefit of about 18 percent, a surprise for many. And yet, not a surprise.

By Janet Kanters

A meta-analysis of biostimulant yield effectiveness, published last year, shows an average additional yield benefit of about 18 percent, a surprise for many. And yet, not a surprise.

For those players in the industry, biostimulants have a place, and many studies have shown their benefits. Indeed, the advantages of plant biostimulants have been reported numerous times. Yet, there has been a general lack of quantitiative assessment of the overall impact of biostimulants on crop production, hence, the doubt in some circles.

The study, ‘A Meta-Analysis of Biostimulant Yield Effectiveness in Field Trials’, was undertaken by Jing Li, doctorate student/research assistant, Thijs Van Gerrewey, post doctoral researcher, and Danny Geelen, all of Ghent University. In it, they summarized over one thousand pairs of open-field datasets to compare the yield gains of different crops (cereals, legumes, vegetables, fruits, root/tuber crops, and others) upon certain biostimulant application. Yield gains in open-field cultivation upon biostimulant application were compared across different parameters, including biostimulant category, application method, crop species, climate condition and soil property.

The overall results showed that the add-on yield benefit among all biostimulant categories is on average 17.9 percent and reached the highest potential via soil treatment; biostimulant applied in arid climates and vegetable cultivation had the highest impact on crop yield; and, biostimulants were more efficient in low soil organic matter content, non-neutral, saline, nutrient-insufficient and sandy soils.

“I’m (pleasantly) surprised that the benefit is so high on average given their broad range in scientific sophistication,” noted Matthew Wallenstein, chief soil scientist at Syngenta Group, on his LinkedIn feed. “No wonder the industry and growers are still enthusiastic about biologicals. If the average yield improvement is 18 percent, just imagine the potential of biostimulants driven by strong and sophisticated science, applied with data-driven precision.”

Biostimulant effectiveness

Crop yield enhancement is a popular claim listed in the product description of many biostimulants. As various environmental factors and management practices influence yield performance, empirical knowledge that depends on different experimental conditions is of critical value for the farmer. Because of the variability in agronomic management and environmental conditions, studies with similar or identical biostimulants have resulted in different effectiveness data, noted the study authors.

“As crop yield is a multi-trait property, meta-analysis has been conducted to gain insight into the impact of soil property (Oldfield et al., 2019), climate change (Challinor et al., 2014) and microbial biostimulants application (Schütz et al., 2018),” they stated. “Hence, effectiveness remains poorly understood to what extent these variables affect non-microbial biostimulants.”

Variation in biostimulant effectiveness is expected as different crops respond differently to biostimulants, and the environmental conditions are likely also influencing the effects. “In this study, we looked at the bigger picture and queried the literature for data published on the effectiveness of biostimulants. The study focused on biostimulants derived from natural resources categorized according to their origin and chemical properties, and restricted to crop yield experiments in open fields that are closer to an application and commercialization target,” stated the authors. “The main result from the meta-analysis was that it revealed correlations between biostimulant effectiveness and impactors that have, insofar as we are aware, not previously been completely recognized.”

For the study, six subcategories of non-mirobial biostimulants were categorized: chitosan (Chi), humic and fulvic acids (HFA), animal and vegetal protein hydrolysates (PHs), phosphites (Phi), seaweed extracts (SWE) and silicon (Si). The authors also reference plant extract-based biostimulants (PE) (excluding SWE). Products with a single active compound were not included in the review. Moringa leaf extract (MLE) was separated as a subgroup of interest, and the rest were other PE under the PE group.

With the high yield gain found, one would expect widespread use of biostimulants in many crop production systems. But this isn’t the case. The authors suggest the average yield increase they reported is an overestimation of what can be expected in a commercial context.

“Noteworthy here is that the efficiency of non-commercial products was seven percent higher than commercial ones (see Figure 1). Indeed, a more conservative estimation of the yield increase is warranted, and there is a need for a more systematic collection of yield data to conclude the effectiveness of commercial crop production systems.”

Previously, a meta-analysis focusing on humic substances under controlled environment and field studies reported an estimated just above 20 percent of the increase in dry weight of shoot and root (Rose et al., 2014), which is close to the yield gain the study authors found (+17.9 percent) (Figure 1). In the case of microbial biostimulants, Schütz et al. (2018) found that the yield benefit in field trials was between +8.5 and +20.0 percent.

“Taken together, the published data show some level of consistency across the different biostimulants analyzed. Chemical fertilizers also contribute to a yield gain, and here the average contribution was estimated to be around 40-60 percent (Stewart et al., 2005),” noted the authors. “Considering that biostimulants are commonly applied as supplements under conventional fertilization schemes and in many cases usually contain NPK fertilizers, the net positive effect of the bioactive ingredients is expected to be a considerable fraction of the total yield gain. Nevertheless, biostimulants sustainably improve the yield and provide a solution to reducing the dependency on synthetic fertilizer.”

The meta-analysis illustrated that the extent of yield improvement varied across categories, with PE biostimulants and MLE as the best performing biostimulants (Figure 1). “MLE has been historically tested on many different crops displaying beneficial effects on seed germination, plant growth and yield, nutrient use efficiency, quality traits, and tolerance to abiotic stresses,” noted the authors. “The significant profitability and variability in MLE and other PE efficacy might be due to their complex composition of plant metabolites, containing many macro and mineral nutrients, osmoprotectants and antioxidants. PE also likely contains plant hormones, which, in small quantities, are known to harbour the capacity to stimulate crop production (Harms and Oplinger, 1988).”

The scientists estimated the effect of SWE products with more confidence, they said, compared to the other biostimulant categories, adding the consistency in yield benefits linked with SWE application is likely a result of the standardization of SWE extraction and formulation methods. The use of SWE as a plant growth regulator can be traced back as far as the Roman Empire (Henderson, 2004), with the first commercial product marketed in 1952 (Milton, 1952).

The meta-analysis also found that biostimulants consisting of complex mixtures were more effective, although it remains to be demonstrated whether the complex biostimulants exert stronger bioactivity because of synergistic interactions between bioactive ingredients.

Impact of application

The meta-analysis showed that biostimulants applied via soil resulted in about 10 percent higher yield benefits than foliar and seed applications “This outcome is surprising as foliar and seed applications deliver the biostimulants directly to the plant, allowing faster uptake of the bioactive ingredients,” noted the study. “For instance, surface spraying acts more directly and results in rapid responses to ripen fruits. Soil application of biostimulants likely has a different mode of action related to nutrient uptake efficiency or enhancing microbial activity on and around the crop.”

Yet foliar application is the favoured method – the authors posit that is because it can be merged with conventional spraying practices. “Remarkably, single biostimulant sprays were nearly as effective as multiple applications. This suggests that the yield benefit is likely due to nutrient supply and other rapid-growth stimulation induced upon spraying the crop once or twice. Also noteworthy is that applications above four times resulted in a negative trend with lower efficiency. The diminishing returns of higher biostimulant application frequencies may be caused by changes in the uptake and assimilation rate of effective agents throughout the germination, vegetative and reproductive plant developmental stages. In general, the efficiency of biostimulants depends on the plant’s nutrient uptake rate, which is highest prior to maximum growth rates depending on the crop type.”

Crop response

Vegetable and legume crops showed the highest gain in yield upon biostimulant application (see Figure 2). A previous meta-analysis study on the crop yield improvement via biofertilization with microbial biostimulants argued that vegetables require higher fertilizer concentrations for optimal growth, and legumes engage in symbiotic nitrogen fixation, which is stimulated upon the addition of microbial biostimulants (Schütz et al., 2018).

“As our analysis included only non-microbial biostimulants, the stronger legume response is not likely attributed to the stimulation of symbiotic interactions with nitrogen-fixing bacteria. It is currently unclear why vegetable and legume crops are more responsive to biostimulant application.”

Growing conditions

Overall, the meta-analysis illustrated that biostimulants showed the strongest crop yield effects in soils of low quality (acid and alkaline soils, saline soils, barren soils with low SOM, and P- or K-deficient soils. “Therefore, we suggest combining biostimulant applications with ‘integrated fertility management’ to maximize yield potential and reduce crop loss risk under climate change scenarios,” the authors advised.

The effectiveness of biostimulant application was the highest under suboptimal growing conditions of arid climates with low precipitation conditions. A similar conclusion was made from a meta-analysis on the yield improvement using microbial biostimulants (Schütz et al., 2018), noted the study authors. In addition, exogenously applied compounds may elicit a stress response that prepares the plant for subsequent stresses caused by limitations in water, soil fertility, or unfavourable temperature conditions.

“Phytohormones are also commonly present in biostimulants, and their interactions with plants are known to enhance osmolyte accumulation and tolerance to stress (Sharma et al., 2019),” stated the study. “In addition, plant-derived biostimulants contain antioxidants and improve the adaptation to unfavourable growing conditions by eliminating reactive oxygen species (ROS) (Drobek et al., 2019). Therefore, the bioactive compounds in biostimulants may evoke either stress alleviation (e.g., suppression of ROS) or induce stress response factors that trigger the immunity against abiotic stresses (Brown and Saa, 2015), validating the hypothesis that biostimulants are more effective under suboptimal growth conditions.”

Conclusions

The authors state the meta-analysis underscores the importance of evaluating biostimulant application methodology and the crop cultivation conditions, and stressed that the impact of biostimulant application on crop yield depends on the type of products and application management.

“Our results also provide various environment-specific assessments of biostimulant performance in open-field conditions, which can be used to set up more effective farming practices for future biostimulant application strategies. In conclusion, biostimulants improve crop yield by reducing yield reductions under stress conditions. This approach can help improve food security for the growing world population under increasing climate change threats.”

https://informa.turtl.co/story/new-ag-intl-eng-marchapril-2023/page/5/1