Biostimulants are chemicals and microorganisms that are widely used in agricultural practice to enhance the growth of plant organisms. Biostimulants are not pure nutrients. On the contrary, they facilitate nutrient uptake or contribute favourably to promoting plant growth or reducing abiotic stress. They are, also, available in a variety of formulations and with a variety of ingredients (Calvo et. al., 2014). With their total gross revenue growing at a steady rate of 12% annually, a new and promising market is starting to develo. EBIC (2012) reports that by 2022, the total gross revenue of biostimulants will have reached $3,000,000 and that over 8 million hectares will be treated with biostimulants in Europe (European Biostimulants Industry Council 2012).

The factors driving this continued growth are numerous:

1. The biostimulants sector has developed new innovative products that target specific agronomic needs, thus attracting new consumers.
2. Biostimulants were initially used mainly for the organic farming system and in high-value fruit and vegetable crops. Nowadays, they are increasingly being introduced in the conventional farming system and in extensive crops.
3. The recent high and volatile prices for fertiliser inputs have created incentives for farmers to optimise the efficiency of their inputs, rather than increase them. It is estimated that the efficiency of fertiliser use increases by at least 5% (and can be as high as 25% or more) when biofertilisers are applied.
4. Growers, responding to consumer demands for safe agricultural products of high nutritional value, are looking for ways to use their inputs more effectively and efficiently. As a result, biostimulants are increasingly seen as a way of improving the return on their investment in other inputs, as well as a way of responding to consumer demands for “softer” agricultural practices. Farmers are, thus, able to secure higher prices for their products when the quality of their crop products is higher. Improved quality also has a positive impact on storage and preservation, giving producers more time to choose the best time to sell their products at the best possible moments.
5. The relatively high investment of companies in research (between 3% and 10% of the annual turnover in the EBIC members that were surveyed), which contributes to a growing list of new products.

After the first international conference on biostimulants (2012), several researchers together with other stakeholders (Calvo et al., 2014; du Jardin, 2012; Halpern et al., 2015; Przybysz et al., 2014) expanded the classification of biostimulants to additional categories, identifying new biostimulants and microorganisms:

• Humic substances (humins, humic and fulvic acids).
• Complex organic materials.
• Salts.
• Proteins and amino acid hydrolysis products.
• Seaweed extracts.
• Chitosan and other biopolymers.
• N-containing substances.
• Yeast extracts.
• Microbial inoculants (fungi, bacteria).
• Glycines / Betaines.
• Synthetic phenolic compounds.
• Elements (Al, Co, Na, Se and Si).

The application of biostimulants in agricultural practice influences a number of physiological processes, such as a more efficient nutrient uptake, the enhancement of beneficial microbial activity, the improvement of plant response to abiotic stresses, root system development and the quantitative and qualitative improvement of the products produced (Du Jarbin, 2015; Halpern et al., 2015).