From Lab to Field: Role of Humic Substances Under Open-Field and Greenhouse Conditions as Biostimulant and Biocontrol Agent

Abstract

The demand for biostimulants has been growing at an annual rate of 10 and 12.4% in Europe and Northern America, respectively. The beneficial effects of humic substances (HS) as biostimulants of plant growth have been well-known since the 1980s, and they can be supportive to a circular economy if they are extracted from different renewable resources of organic matter including harvest residues, wastewater, sewage sludge, and manure. This paper presents an overview of the scientific outputs on application methods of HS in different conditions. Firstly, the functionality of HS in the primary and secondary metabolism under stressed and non-stressed cropping conditions is discussed along with crop protection against pathogens. Secondly, the advantages and limitations of five different types of HS application under open-fields and greenhouse conditions are described. Key factors, such as the chemical structure of HS, application method, optimal rate, and field circumstances, play a crucial role in enhancing plant growth by HS treatment as a biostimulant. If we can get a better grip on these factors, HS has the potential to become a part of circular agriculture.

Linking abiotic stress, plant metabolites, biostimulants and functional food

“…Humic substances are not only considered an essential component of soil organic carbon, but also well known as a biostimulant, which can perform phytohormone‐like activities to induce nutrient uptake, whereas plant response differs depending on the origin of humic acids, concentration, method of application and stage of plant development (Jindo, Canellas, et al, 2020). As stated by Jindo, Olivares, et al (2020), anatomical and biochemical changes in the root system stimulated by humic substances are the main factors responsible for increased nutrient uptake, along with the increase in the nutrient availability through chelation (Jindo, Olivares, et al, 2020). In the case of phosphorus starvation, more research is needed regarding how humic substances relates to crosstalk between plant hormones and other secondary metabolism mechanisms at multiple levels.…”

A Beginner’s Guide to Osmoprotection by Biostimulants

“…In addition, published data show that humic and fulvic treatments can modify the plants’ primary and secondary metabolism against abiotic stress, enhancing water uptake and antioxidant behaviour under stress [ 193 ]. The application of both types of acids is widely reported in the literature as an external treatment to increase tolerance against osmotic stress (see Ali et al [ 128 , 194 ] for an extensive review): Humic acids are able to enhance maize plants’ salt response [ 195 ], increasing proline accumulation and strengthening the enzymatic antioxidant system. In Lima bean [ 196 ] it can ameliorate negative effects exerted by drought, by increasing photosynthetic activity and accumulating sugars and proline in leaves and thus a higher relative water content in these organs.…”

“…Use of humic substances as a biostimulant for plant growth is a promising eco-friendly approach, in accordance with the concept of circular economy. It focuses on a progressive conversion to resources whose consumption can itself alleviate anthropic impacts and pressure, and the impending consequences of climate change [ 194 ]. Finally, there is interesting research where the authors try to isolate humic substance structures and associate them with their activity in plants [ 199 , 200 ], laying the basis for a better understanding of this structurally complex type of biostimulants.…”

Which Traits of Humic Substances Are Investigated to Improve Their Agronomical Value?

“…It is also necessary to confirm whether HS experimental conditions to enhance crop marketability at a lab scale are applicable and reproducible in a field test, where farmers are engaged. In fact, optimal HS treatment concentrations between laboratory and greenhouse plant stimulation assays were found to be different [ 41 ].…”