Plants can absorb water and nutrients through their leaves, which is believed to be a heritage of their evolutionary origin as water plants [1]. Although soil application is still the most common plant nutrition strategy in organic farming [2], plant sprays based on compost, excrements and diverse plant materials have been used for foliar plant nutrition for over a hundred years [3].

Fermented liquid fertilizers (FLF) describe anaerobically fermented animal or plant residues (or a mixture of both) used as liquid organic fertilizers. Other liquid organic fertilizers include compost teas, humic acids, algae extracts, and manure tea [4]. FLF can be distinguished from the (partially overlapping) concepts of biofertilizers and biostimulants, which are produced similarly but do not directly supply nutrients. Biostimulants promote plant growth without providing nutrients. Biofertilizers, a subcategory of biostimulants, increase the soil fertility by enhancing the soil activity through living microorganisms, for example, rhizobacteria [5, 6]. FLF are also labelled as fermented foliar fertilizers, liquid organic fertilizers [7], fermented organic fertilizers [8], or biol (due to their importance in Latin America)[9]. Some of the latter terms can also refer to biofertilizers. Normally, FLF are used for foliar fertilization, where they have shown superior immediate nutrient supply compared to common solid organic fertilizers [10]. Their micronutrient content is reported to meet the requirements of numerous horticultural crops [11].

Regularly, they serve as corrective measures (additional to basal fertilization) when nutrient deficiencies are observed [12]. There are indications that these products could also be used to prevent plant stress in periods of intensive vegetative growth activity and when a plant initiates its reproductive stage [13]. Occasionally, FLF are applied to the soil (e.g. through drip irrigation) or mixed with solid fertilizers to improve their quality. They are used for a variety of annual and perennial crops [14] but most frequently for nourishing vegetables, fruits and ornamentals.

High-quality FLF are characterized by balanced nutrient content and satisfactory plant assimilation of these nutrients [12]. Since FLF can be produced on-farm using accrued residues [8], they increase both the environmental and economic sustainability in agricultural production. Further advantages involve an easily accomplishable preparation process; early harvesting and extended harvest periods; increased post-harvest stress-resistance; a positive impact on harvest taste and quality; the principal absence of toxic residues on harvested food; and a low health risk for producers [11]. Some of the problems of using FLF is that they may excessively contain Na [10]. There is also a risk of contamination with pathogenic microorganisms (e.g. Escherichia coli, Salmonella sp., or Shigella sp.) and with heavy metals (As, Cd, Cr, Hg, Ni, Pb), which is why in the European Union and the USA, the use of FLF is legally restricted [8]. A correctly implemented fermentation process using lactobacilli decreases the risk of contamination with pathogenic microorganisms [15]. As for the risk of heavy metal contamination, explicit studies are necessary.

All plant nutrients were evidenced in FLF solutions. In addition, FLF may contain Na, Si, I, Se, and diverse metals [11]. The most common FLF formulations, all based on cow manure (Table 1), have content of 2101-4800 mg N kg - 1 , 27-32 mg P kg - 1 , 1651-4493 mg K kg - 1 , 931-3716 mg Ca kg - 1 , and 348-1499 mg Mg kg - 1 (Table 2).

During fermentation, nitrate is converted into ammonium and gaseous forms of N [15]. Consequently, the N content of FLF declines once most components are fermented, especially when it is heated as it can happen under direct exposure to sunlight [18]. Ito [8] found the highest total N content of an FLF at 30 days after starting the fermentation process, while the highest ammonium content was observed after 45 days.

Concerning P, the organic acids originating from the fermentation process facilitate its solubilization. The use of molasses and whey additionally stimulates the solubilization of P. The amount of plant-available P in FLF increases during the first ten days of fermentation and then decreases due to its consumption by microorganisms [14, 15, 18].

The K and the Mg content is directly related to the use of molasse as FLF ingredient. In contrast, the presence of Ca is linked to the use of both molasses and manure [8]. From the first day of the fermentation process, the content of ionic Ca decreases slightly, while the ionic K content increases. Furthermore, the methanogenesis reduces the content of ionic Fe and Mn [14, 15].

As for vitamins, provitamins and enzymes, FLF usually contain thiamine, pyridoxine, niacin, pantothenic acid, riboflavin, cobalamin, ascorbic acid, folic acid, beta-carotene, ergosterol, and alpha-amylase [11].

Most FLF show a slightly acidic pH of approximately 6.5, which is a consequence of the formation of acid compounds during the fermentation process. There are also reports of strongly acid FLF with a pH of 4.5 [18] or even of 3.4 [27]. The electric conductivity of FLF is around 5 dS m - 1 . Commonly, the salinity decreases slightly during the fermentation process [15, 27].

A FLF must be diluted with water before it is applied. Undiluted FLF increase the risk of low nutrient absorption, injury to the leaf surface, or general phytotoxicity [8]. Ideal concentrations of foliar-applied nutrient solutions depend on the plant species, plant age, nutritional status and weather conditions [29]. In the case of FLF, common concentrations range between 3% and 7%. If minerals are added, a dilution to reach a concentration of 2% is adequate. Used in short intervals, the dilution should be higher. Ash (150 g L - 1 ) and raw soap (2 g L - 1 ) can be added to the diluted FLF to enhance its adhesiveness on the crops. Cactus juice, aloe, and molasses have a similar adhesive effect [11].

Foliar-applied nutrients are ions that enter the plant metabolism following an aqueous pathway through a leaf’s cuticular wax or the stomata. The penetration of foliar-applied substances is a passive process driven by concentration gradients and stimulated by light and soil moisture [1, 2].

Micronutrient deficiencies can be corrected with one single foliar application (in the case of a crop like maize with a high micronutrient demand, it may be up to three applications). A micronutrient supply only using foliar fertilizers is accomplishable but only would make sense in entirely disturbed soils.

Macronutrient requirements are not entirely met with foliar applications, but (for example in very acid soils) foliar application are effective methods to correct macronutrient disorders. Furthermore, complementary foliar macronutrient supply during determined growth stages can significantly improve crop quality [2].

For an efficient absorption of foliar fertilizers, leaf stomata should be open; the treated plants should be cool and turgid; the temperature should not be above 20 ∘ C to avoid burning of the plant foliage, and the weather should be windy but not stormy. The ambient air humidity should not be too low since dryness may cause the evaporation of the applied solution, immobilizing its compounds. It should not rain for at least three hours after the application of the nutrient solution [2, 29]. According to practitioners, these conditions are more likely to be met in the early morning; yet, Fageria et al. [2] also suggest the late afternoon as a prime opportunity for applying foliar fertilizers. FLF should not be mixed with other substances. The only exceptions are the mentioned adhesives such as cactus juice and whey [11].

Recent studies have emphasized the development of fertilizers made of solid and liquid farm residues. Most of this research (commonly from Asia) centres on biofertilizers, not on FLF [30].

Latin America, the region where FLF were developed, is still the place where most of the respective informal on-farm research on FLF is undertaken. Consequently, the proof of the effectivity of FLF frequently comes from countries like Peru and Ecuador. In such studies, a positive effect of the use of FLF on crop yield was evidenced for bean, broccoli, carrot, cucumber, lettuce, maize, and spinach; whereas a beneficial effect on fruit quality was shown for cucumber and lettuce. As for vegetative growth, FLF were successfully tested in maize and papaya fields; and in banana production for disease control Table 3.

Few extensive studies deal with FLF. Hence, it is necessary to deepen the research in this field. Particularly, more studies are needed emphasizing the concerns of farmers and organic farming authorities (especially about the acceptability of using FLF with different worldwide organic standards). Core research issues include the monitoring and reduction of health and quality risks (heavy metal and human pathogen contamination). Additionally, most FLF formulations are based on what is available in Latin American farming communities. Accordingly, it is necessary to implement research about optimum FLF formulations that work with resources available in other parts of the world. Similarly, research covering the FLF requirements of specific crops would be helpful. Today, most FLF are produced on-farm using easily achievable materials such as plastic barrels and bottles. Considering the high costs of commercial organic leaf fertilizers, this circumstance is vastly beneficial for small-scale producers in developing countries. Nevertheless, research regarding a larger-scale production of FLF would help increase their distribution in industrialized nations.

Despite their popularity in Latin America, FLF are still a new plant nutrition strategy at the global level. Yet, the technique has the potential to become a significant component of sustainable plant nutrition: Its many contributions include agricultural residues that are recycled to nourish plants and a considerably high micronutrient supply that may outperform organic basal fertilization. As they can be produced on-farm, FLF considerably may reduce fertilization and transportation costs, improving the sustainability of farming systems. Today, most FLF are used as a complementary measure to established organic fertilization techniques. However, they have the potential to become a universally important fertilization tool.