Effects of Humic Materials on Plant Metabolism and Agricultural Productivity
Andrés Calderín García, Fernando Guridi Izquierdo and Ricardo Luis Louro Berbara
Humic materials, such as compost or vermicompost, have a high potential for increasing plant production, even in unfavorable environmental conditions. The use of these materials involves two types of action: direct (on plant metabolism) and indirect (increased soil fertility). The materials are composed of mineral elements, plant hormones, and amino acids that create beneficial conditions for crop yield and anti-stress adaptation. These products increase soil mineral content, reduce compaction, increase water retention and aggregate stability, and provide an adequate niche in the soil for microbial development. At the same time, humic materials stimulate root growth, which allows for a greater coverage of plant nutrition and greater activity of biotic and abiotic anti-stress enzyme systems. This stimulus translates into improved plant conditions and the optimization of metabolic processes, which leads to greater agricultural yield. The high potential of humic materials is based on their structural richness, which makes them an ecological and sustainable alternative for plant improvement. In this chapter, we will discuss improvement technologies.
Keywords
vermicompost; compost; agricultural yield; plant metabolism; humic substances; bioactivity; spectroscopy; humus
18.1 Introduction
One of the challenges that the planet faces is the need to avoid increased environmental contamination and improve crop production while providing quality food products. For this purpose, recent studies have focused on the reutilization of easily available materials, such as humic materials (compost and vermicompost), that are innocuous to nature (Bhattacharya et al., 2012; Shafawati and Siddiquee, 2013; De Carvalho et al., 2013). These materials have a variety of environmental functions, such as improving soil quality, increasing plant yield, and interacting with soluble cations (Singh et al., 2013; Doan et al., 2013; Yang et al., 2013).
The fact that these materials have natural effects that are both indirect (on plants, improvement of chemical, physical, and biological soil properties and soil decontamination) and direct (metabolism stimulation) is of extreme environmental importance. Regarding the direct actions, compost materials can improve the agricultural yield of plants by increasing plant nutritional status, improving resistance to stress, and stimulating various metabolic pathways (Gutiérrez-Miceli et al., 2007; Tejada et al., 2009; Lazcano et al., 2010; Srivastava et al., 2012; Azizi et al., 2013; Doan et al., 2013; Singh et al., 2013).
These effects are important aspects of agricultural resource management. Although there is no technology for the use of humic materials, the fact that they reduce the use of synthetic chemicals in the soil makes them an environmentally valuable alternative, particularly because they have a low cost and are easily available and harmless to the environment.
With this general view and without the objective of creating booklets on the use of humic materials for improving crop yields, we will discuss the potential of these substances. We first describe the chemical, physical, and spectroscopic characteristics of humic materials and their fractions to provide context for understanding their properties in the environment. We then discuss the relation between the structural properties and function of humic materials.
18.2 General aspects of the characteristics of humic materials and their functions
18.2.1 Vermicomposting and its application on soil and plants
Vermicomposting is a process that involves chemical, physical, and biological transformations of solid organic materials (agricultural residues of plant and animal origin) through the use of worms and microorganisms (Garg and Gupta, 2009). The use of worms in the process aids the fragmentation of fresh organic matter, thereby increasing the surface area available for microbial colonization (Domínguez et al., 2010). Utilization of vermicompost (VC) in agricultural and environmental activities is related to the ability of this material to improve the chemical, physical, and biological soil conditions and to directly and indirectly improve the biological and agricultural plant yield. Therefore, VC has been used to remediate contaminated soils, to promote plant regulation, and to stimulate plant growth (García et al., 2013b; Oo et al., 2013).
With respect to the use of VC in soil remediation, it has been shown that its use, when based on olive cake compost, may promote a rapid decrease in the concentration of atrazine herbicides in the soil. This reduction occurs because VC may stimulate microbial activity related to the degradation of these substances. The same results were not obtained when using only olive cake that was not previously composted, which indicates that atrazine degradation depends on the type of organic matter applied to the soil, and humic organic matter is more favorable to the degradation of this contaminant (Delgado-Moreno and Peña, 2009).
Other studies have also shown that the application of VC to soil stimulates the bacterial community related to the degradation of organic chemical species that are contaminants, such as polycyclic aromatic hydrocarbons (PAH). Di Gennaro et al. (2009) documented the use of olive mill VC for the degradation of naphthalene in soil based on gene expression related to biodegradation in the community of autochthonous soil bacteria that are able to degrade PAH. Similarly, authors such as Zhang et al. (2012) discuss inhibition of in situ PAH remediation due to the low presence of nutrients and organic carbon in the soil. These authors conclude that the application of organic matter to soil stimulates the activity of dehydrogenase and the microorganisms that degrade PAH.
Humic materials also directly affect physical, chemical, and biological soil properties. The use of composted vinasse residues and vermicomposted plant residues decreased soil loss by 31.2% and increased plant cover by 68.7% (Tejada et al., 2009). Beneficial results have also been obtained when VC inoculated with microorganisms was applied to soil. This VC and microorganism consortium can improve physical conditions of the soil, such as the density and water-retention capacity, as well as chemical properties, such as pH and the levels of organic carbons, N, P, and K (Singh et al., 2013).
Therefore, VC can be an ecological alternative as a regular agricultural practice. However, there are still several questions that must be clarified before establishing phytotechnological recommendations because, as previously discussed, the beneficial effects depend on the type of raw material that was used in the VC, the type of soil to which it will be applied, and the species of plant (Figure 18.1).
18.2.2 Some structural characteristics of vermicomposts
The VC quality depends on the source of the raw materials used. Thus, it is necessary to know the chemical, physical, and biological characteristics before making a recommendation. Table 18.1 shows some chemical parameters of five different VC that were created from different raw materials with varying types of worms and maturation times. The table shows that an important characteristic of VC is the amount of carbon in the form of humic carbon. This factor could have important consequences on the effects of the VC when applied to soil.
Table 18.1
Quantities of Some of the Chemical Parameters Present in Five Different VC Samples
| VC | Pep and PPep | C | N | C/N | M.O | C-FHS | C-FAF | C-FAH | HA/FA |
| 1 | 3.45 | 21.41 | 1.85 | 11.57 | 36.9 | 11.71 | 9.81 | 1.9 | 0.19 |
| 2 | 4.37 | 20.36 | 1.76 | 11.57 | 35.1 | 12.28 | 10.58 | 1.7 | 0.16 |
| 3 | 2.99 | 20.00 | 1.73 | 11.56 | 34.5 | 11.37 | 9.57 | 1.8 | 0.18 |
| 4 | 3.33 | 22.30 | 1.92 | 11.61 | 38.4 | 7.16 | 5.73 | 1.43 | 0.24 |
| 5 | 3.22 | 21.40 | 1.85 | 11.57 | 36.9 | 12.51 | 11.16 | 1.35 | 0.12 |
Pep and PPep (peptides and polypeptides): values in g/100 g (VC).
C, N, M.O, C-FAF, C-FAH, and HA/FA: values in %.
(1) VC from cow manure, Red Californian worms and 1 year of maturation, (2) VC from rabbit manure, Red Californian worms and 1 month of maturation, (3) VC from cow manure, Red African worms and 1 year of maturation, (4) VC from goat manure, Red African worms and 3 months of maturation, (5) VC from cow manure, Red African worms and 3 months of maturation.
Data provided by research professor Eduardo Ruiz Vasallo-UNAH-Cuba.
Another important question related to the chemical characteristics of VC is the insoluble solid fraction of VC in aqueous solution. Researchers have documented the ability of this insoluble residual fraction to retain dissolved heavy metals (García et al., 2007, 2013c). This ability increases the continuation of metal contaminants in the soil and is explained by the presence of ionizable functional groups, which can establish interactions of different natures with metal cations.
Figure 18.2 shows a chemical fragmentation technique (atmospheric pressure chemical ionization-mass spectrometry [APCI-MS]) applied to the insoluble solid fraction of cow manure VC. The mass spectrum of the fraction shows the structural variability that is present in one of the VC fractions, for both the positive and negative fractions, but particularly in the latter due to the importance of interactions with positive metallic elements.
The diverse effects that VC have on soil properties can be observed in the ability of these materials to interact with distinct environments. When these VC are applied, they can enrich the soil with different fragments of diverse chemical properties that provide nutrients for both plants and microorganisms. Evidence of this structural diversity is shown in Figure 18.3, which shows how cow manure VC has considerable quantities of soluble fragments in solvents with different polarities. This figure indicates a greater quantity of less hydrophilic substances solubilized in chloroform and more hydrophobic substances solubilized in water. These characteristics are proof of a more stable structural arrangement of humic organic matter, which maintains its structural integrity. These facts may explain some of the action mechanisms of humic substances (HS) in plants and interaction with roots (Nebbioso and Piccolo, 2011; Song et al., 2013).
Regarding the content of the mineral elements, VC have a variable composition, depending on the VC’s source and production process. Some of these elements can be released and become part of the soil when applied in the field, which contributes to increased fertility. Figure 18.4 shows an EDX spectrum of a cow manure VC. A greater quantity of the elements C, K, Ca, O, and Si, possibly in the form of salts such as carbonates and silicates (CO32−, SiO44−), can be observed in this spectrum. Other mineral elements, such as Mg, Fe, and Al, are part of the inorganic pool of this VC.
A study that used spectroscopic techniques (diffuse-reflectance infrared Fourier transform spectroscopy [DRIFT]) to determine the quantities and relative abundance of structural fragments (by pyrolysis-gas chromatography-mass spectrometry) during the VC of five materials (cow manure [M]; cow manure and sugarcane bagasse [SB]; cow manure and sunflower oil cake [SC]; cow manure, sugarcane residues, and sunflower-oil cake [SBSC], and filter cake [FC]) concluded that the thermochemolysis of the VC released compounds derived from lignins, carbohydrates, proteins, acids, fatty alcohols, terpenic compounds, and hydrocarbons, whose relative abundances varied with the maturation of the VC, and indicated that the relative variations and abundance of these composts were characteristics of each VC studied (Balmori et al., 2013). An example of the dynamics of the composts presented in this study can be found in Figure 18.5.
), SBSC (
), and FC (
). 18.2.3 Chemical composition of liquid humus
Liquid humus is obtained through the extraction of an extracting dissolution with different chemical characteristics and a VC. The liquid humus obtained from the VC can be used as liquid fertilizers applied to foliage because they have high levels of mineral nutrients and HS that regulate metabolic processes in plants that contribute to improved development (Gutiérrez-Miceli et al., 2008; Singh et al., 2010).
Logically, the variability of VC as a source of raw materials causes the liquid humus to have a variable composition as well, although in a general manner, with some parameters fluctuating within the same range. Liquid humus has a high amount of elements and substances with different chemical natures that vary even with the type of method used for attaining the humus (Terry et al., 2012; Aşik and Katkat, 2013). Caro (2004) obtained a humic acid/fulvic acid (HA/FA) relationship of 0.98, pH of 8.4, and electrical conductivity of 12.82 mS/cm−1 from a cow manure VC liquid humus (Liplant®), while Hernández (2011) obtained HA/FA values of 1.21, a pH of 8.7, and an electrical conductivity of 5.81 mS/cm−1 from this same VC but with a different extractive solution. Gutiérrez-Miceli et al. (2008) reported that leachates obtained from cow manure VC had AH/AF values of 1.6, a pH of 7.8, and an electrical conductivity of 2.6 mS/cm−1. In contrast, Canellas et al. (2013), in their study with soluble HS in basic aqueous solution obtained from cow manure VC, found AH/AF values of 0.98, a pH of 8.67, and an electrical conductivity of 11.7 mS/ cm−1.
Arteaga et al. (2007) found in the liquid humus Liplant® a microbial community of bacteria (6.55×107 UFC/mL−1), fungi (2.21×105 UFC/mL−1) and actinomycetes (5.4×103 UFC/mL−1), while Hernández (2011) applied thermal treatments in several of the steps to obtain this liquid humus, changed some of the extracting solution, and obtained a quantity of bacteria (3.4×105 UFC/mL−1) and fungi (1.40×104 UFC/mL−1), but no actinomycetes were detected.
This evidence indicates that liquid humus is enriched with various types of organic substances (plant hormones, proteins, amino acids, peptides, and fatty acids), mineral elements, microorganisms, and HS and thus constitutes a viable source of organic fertilizer. The scientific literature provides methodologies for ecological and sustainable use (García et al., 2013a).
18.2.4 Some structural characteristics of humic fractions
Concern for knowing the structural characteristics of HS and its fractions has led to a group of chemical and physical techniques being used to understand the superstructure of HS. If it is certain that VC HS fractions have spectral differences, it is also possible to observe similarities in spectral “signatures,” such as those obtained by the ultraviolet-visible spectroscopy (UV-vis), Fourier-transform infrared spectroscopy (FT-IR), and carbon-13 nuclear magnetic resonance (13C-NMR) techniques (Ke et al., 2013; Lv et al., 2013; Xiao-Song et al., 2013).
A study by Aguiar et al. (2013) demonstrates the spectral characteristics obtained by FT-IR and 13C-CPNMR of humic acids (HA) isolated from four VC of different raw materials. Although there were some differences, the NMR spectra showed the presence of C-alkyl, N-alkyl/methoxyl, O-alkyl, di-O-alkyl, acrylic, O-acrylic, and carboxylic structures in the HA of all of the VC.
Therefore, HA, even when isolated from composts derived from different raw materials, have great similarities regarding the presence of their chemical structures, although they also show differences in spectral peak intensity, which indicate the quantity of structures (Al-Faiyz, 2013).
Humic substances of sewage sludge VC characterized by FT-IR had a low aliphatic character, with structures that came from proteins and polysaccharides, and an elevated presence of functional groups of acids and high aromaticity (Li et al., 2011). In contrast, an extraction of HA from cow manure VC by chromatography showed that larger molecular fragments of HA have predominantly aromatic characteristics, while smaller fragments have a predominance of more oxidized carbons and are less aromatic (Canellas et al., 2010).
Variability in the structure of HA during the composting process has also been studied. Amir et al. (2010) determined the structures of HA in the starting material of a compost at 15, 60, and 135 days of composting. In the first 15 days of composting, the predominant structures in the HA of the VC were O/N-alkyl and C-carboxyl. In the next 60 days, the predominant structures were C-alkyl and C-aliphatic, while at 135 days, the structural predominance was C-aromatic and C-carboxylic, with an aromaticity/aliphaticity ratio of 1.51. Regardless of the composting processes, HA exhibit characteristic structural characteristics based on the source of material. However, if it is certain that the HS and fractions have a high variability depending on the source of origin and production processes, then it is also certain that there are spectral patterns that repeat and homogenize their structural behavior. Thus, the growing use of VC as biofertilizers that promote plant growth is based more on the elevated content of soluble humic substances that these materials possess than on the composition of nutritional elements (Trevisan et al., 2010).
A complement for understanding the similarities and differences of HS and their fractions is the material on this topic on the International Humic Substance Society (IHSS) site (http://www.humicsubstances.org/).
18.2.5 Some basic knowledge about the structure–activity relationship of humic substances
The most important previous knowledge for understanding HS properties and functions is a structural complexity. According to studies in “Humeomics,” introduced by Nebbioso and Piccolo (2011, 2012), HS have a supramolecular structural organization, distributed in large hydrophobic structures and other smaller hydrophilic ones. The hydrophobic fractions are basically composed of humic fractions of linear aliphatic chains and condensed aromatic rings, while the hydrophilic fractions are composed of more irregular humic fractions. Therefore, it is understood that the supramolecular structural arrangement of HS is a result of a non-uniform relationship of heterogeneous humic molecules that interact as a function of their size, form, chemical affinity, and hydrophobicity (Nebbioso and Piccolo, 2012).
This heterogeneity and variability of HS, both organizational and in terms of the diversity of functional groups, allows us to understand the many chemical, physical, and biological processes in which they are involved in the environment. Among the most studied processes are their ability to interact with metal cations and the possibility of interaction with plant root systems, triggering various beneficial events that result in greater plant and agricultural yield. Studies about the role of HS in the environment have created interpretations to understand the modes of action and structure–activity relationship of HS (Nuzzo et al., 2013; Xitao et al., 2013; Wang-Wang et al., 2014).
Regarding interactions between HS and metals, it is known that these interactions are favored, which causes a high presence of ionizable functional groups (carboxyls: COOH, carbonyls: C=O, and phenols: OH) that allows the formation of HS-metal organometallic compounds. It has been documented that these functional groups have a greater participation in interactions with the elements Cu2+, Pb2+, and Ca2+ for humic and fulvic acids, establishing an order of interaction of Cu2+>Pb2+>Ca2+ (Piccolo and Stevenson, 1982). The structural arrangement of the HS also participates in interactions with metals. Nebbioso and Piccolo (2009) reported that in the interaction of HA with the metals Al3+ and Ca2+, carboxylic acids participated favorably, and this participation led to an increase in the conformational structural rigidity and the size of the hydrophobic domains.
One of the most discussed questions in the scientific literature today in the area of humic materials is about the action mechanisms of HS in plants. The discussion involves the physiological stimulation of the root system of plants by HS action (Muscolo et al., 2013; Zandonadi et al., 2013). Some studies demonstrate a mimetic action of HA with auxins, which stimulates the emission of secondary roots through a mechanism that stimulates the activity of H+-ATPases (Zandonadi et al., 2013). Authors such as Canellas et al. (2012) report that HS, regardless of the source of isolation, have a superstructural organization with a superficial hydrophobic domain and a hydrophilic interior, and a break in the superstructure can release hydrophilic components structurally similar to auxins that could cause these mimetic effects.
In turn, Elena et al. (2009) showed that in isolated leonardite HA, there are no substances that are structurally similar to plant hormones, which indicates that the effects of root stimulation could be exerted through mechanisms that are independent of the pathways used by auxins. Humic substances that are soluble in water with a low molecular weight did not show the same type of activity as auxin in Arabidopsis plants (Schmidt et al., 2007).
Other studies reported that HA can interact with the root system and modify root functionality. This type of event leads to a decrease of up to 44% in hydraulic conductivity and has been called colloidal stress (Asli and Neumann, 2010). Similar results that support this type of interaction have been reported by García et al. (2012b, 2014), who found that HA from VC can form clusters in the roots of rice plants, promote the production of radical oxygen species, and stimulate some enzymes related to antioxidative defense mechanisms. Another study also reported a stimulation of the enzyme catalase due to the application of humic substances in maize plants (Cordeiro et al., 2011).
18.3 Use of humic materials for sustainable plant production
As a result of research performed under controlled environments, the favorable effects of HS on plant growth and development have been documented (Alfonso et al., 2011; Ertani et al., 2013). During the last two decades, many studies were developed to reduce the use of inorganic fertilizers. The theoretical assumption is that there is a possibility of release of mineral elements when the fertilizers are completely mineralized. For example, a dosage of 15 kg/plant VC produced indicators that the given biological and agricultural productivity decreased the demand for mineral fertilizer by 50% in oil palm plantations in Colombia (Garcia et al., 2012). However, in Cleome gynandra, the most significant dosages were 11.5 and 15 t ha−1 (Ng’etich et al., 2012).
The progressive introduction of VC in conventional agriculture has been studied under different conditions. There has been evidence of positive effects on the microbial biomass and organic carbon fractions in the surface layer of tropical soils, which would translate to important improvements in agricultural productivity over time. Santos et al. (2012) considers that a period of at least 2 years in tropical soils is necessary to ensure adequate transfer to organic agriculture.
The application of plant residue compost, particularly to soil affected by salinity in coastal areas or those with a predominantly sandy texture with low fertility, has produced favorable effects on both development and biological and agricultural productivity in crops (Ayyappan et al., 2013) with Vigna radiata L. and Adebayo et al. (2013) with Abelmoscus esculentus). In silt clay soils with less than 1% organic matter, the most widely used composts have been those made from manure, sometimes in combination with urban organic residues (the latter only after previous verification that they do not contain persistent organic contaminants or heavy metals). This type of use requires prior application during planting or sowing, mixed with the soil surface layer. Generally, application of VC in agricultural production occurs in three fundamental variations: incorporating the VC into the soil, using the previously extracted soluble fraction in an aqueous solution (neutral or alkaline), or using dissolutions of HA isolated from the soluble fraction.
VC (created from macrophytes mixed with cow manure) incorporated in the first 15 cm of a soil with low organic matter in proportions corresponding to 2, 4, and 6 t ha−1 used for tomato crops (Lycopersicon esculentum Mill) caused significant increases in biological productivity parameters (height, leaf area, above-ground dry mass, and dry root mass) and agricultural productivity (number of fruits per stem and per plant, average fruit mass, and proportion of commercial quality fruits), which proves that this alternative source of organic matter is ecological and economically viable for agricultural production in this type of soil, especially at a dose of 6 t ha−1 (Ahmed and Khan, 2013).
The considerations and examples cited here about the impact of the use of composts on biological and agricultural productivity confirm the viability of this practice in different parts of the world. The accumulated experience and knowledge that have been cited in the scientific literature are summarized in Box 18.1.
The soils employed in intensive agricultural systems, particularly in tropical and subtropical regions, experience a gradual decline in total organic carbon and alterations in the structural characteristics of the humic fraction (Quintero et al., 2012). These structural modifications in the humic fraction, as previously described, lead to changes in the potential interactions of these substances in the soil–plant system. García et al. (2012a) found that when they applied different procedures for extracting and purifying HA, structural differences were reflected in spectropic studies, as is shown in Figure 18.6.
The above-stated issue is a very important question to consider because another way of applying humic organic matter to stimulate plant development is using extracts of soluble humic substances in aqueous solution (or isolating the HA of these extracts to later dissolve into water), which can be obtained from humic materials (natural or derived from induced humification processes, such as composting and vermicomposting).
This alternative has had practical use in hydroponics through fertigation or direct foliar application. One of the first studies on this topic in the scientific literature discusses the application of humates and fulvates extracted from natural humic material in field conditions with barley (Gonet et al., 1996). The application of the humic material preparations achieved positive results for grain production (15 to 20% greater) compared to mineral fertilizer application, and there was also a higher level of total nitrogen in the grains.
Due to the practical fact that HA are relatively easier to isolate than fulvic acids, most studies of soluble humic fractions applied in agricultural production systems have been developed using HA. VC of different materials are one of the most commonly used sources to obtain HA (García et al., 2013c), although it is possible to use composts and other natural humic materials such as leonardites (Mora et al., 2010). Aqueous dissolutions of HA isolated from VC in different concentrations were tested in different crops to evaluate the impact on the biological development of different commercially important plants. Some dissolutions were applied to the soil, while others were sprayed on leaves and seeds before planting. In direct applications to the soil for tomato (Lycopersicon esculentum Mill) and cucumber plants (Cucumis sativus L.), the addition of up to 500 mg kg−1 HA stimulated plant growth, but the intensity of the effect differed according to the source of the VC used in the extraction and isolation of the HA, and there were differences in the optimal concentration value. Very high values caused unfavorable effects, thus confirming the overall behavior equivalent to plant hormones that is attributed to HA (Atiyeh et al., 2002).
Various studies have been performed to associate structural characteristics of HA with their biological effects. Dobbss et al. (2010) induced structural modifications in humic substances contained in VC with reactions of hydrolysis, oxidation, reduction, and methylation. The biological activity in stimulating root growth and the activation of proton pumps were studied in all of the structural derivatives in tomato and corn seedlings. The effects of the derivatives were more intense than those caused by the original humic substances. There was no defined relationship between biological activity and the molecular size of derivatives, but the hydrophobicity index was directly related with proton pump stimulation.
Applications of HA were also used to protect plants from different conditions of abiotic stress, particularly water and salt stress and by chemical contaminants such as heavy metals. In bean plants (Phaseolus vulgaris L.) grown in soil contaminated with heavy metals, three concentrations (20, 40, and 60 mg of HA L−1) of HA dissolutions (from cow manure VC) were sprayed, and the biochemical–physiological effects of the stress from the heavy metals were mitigated (Portuondo, 2011). Similarly, different concentrations of HA were applied to potatoes (var. Spunta) on a sandy entisol soil in Egypt under different levels of water stress. The results obtained indicated that applications of up to 120 kg ha−1 HA increased growth parameters, tuber production, and biochemical indicators such as the content of chlorophyll, ascorbic acid, starch, and total soluble acids, which proves that HA also mitigate the effects of water stress (Selim et al., 2012).
Another tendency in the development of alternative methods for the application of soluble humic substances to stimulate biological and agricultural productivity has been to combine solutions of HA with suspensions of beneficial microorganisms. In this way, a suspension (109 cells mL−1) of diazotrophic endophytic bacteria (Herbaspirillum seropedicae) combined or not with a dissolution of 20 mg of C L−1 HA extracted from VC of cow manure was used in corn plants (Zea mays L.) under field conditions on an ultisol soil in Macaé, Rio de Janeiro (Brazil), with a low total content of carbon (0.6%) and nitrogen (0.04%). The treatments (a suspension of bacteria, an HA dissolution, and a combination of both) were applied via foliar spray one time 45 days after plant germination (hybrid P6875) at a density of 120,000 ha−1. The results obtained in grain production demonstrated that the independent application of the bacterial suspension and HA dissolution increased production by 17 to 20%, while the bacteria–HA combination produced an increase of 65% greater than the control (Canellas et al., 2013).
Over the past few years, different forms of commercial HA have appeared in the agricultural market, and the possible effects of these products on agricultural production are being studied. In the early twenty-first century, the Chemistry Department of the Agrarian University of Havana (Universidad Agraria de la Habana) standardized a procedure for obtaining an extract of cow manure VC (called “liquid humus”) used in an alkaline aqueous solution, characterized by a high percentage of organic carbon (such as HA, FA, free amino acids, and other compounds with biological activity) and a lower amount of a mineral fraction with both macro- and micronutrients. Caro (2004) demonstrated the physicochemical characterization of HA and FA contained in liquid humus, and dilutions of this extract were sprayed on the leaves of corn plants on a small rural property. Two dilutions (1:20 v:v and 1:30 v-v) of this humus were applied two times with an interval of 25 days. The two dilutions significantly increased indicators of growth and productivity. The 1:20 dilution treatment produced the greatest economic benefits.
The same liquid humus was used to evaluate the impact of foliar application of two aqueous dilutions of the extract on tomato plants (Lycopersicon esculentum Mill) for two consecutive years (Arteaga et al., 2006). Two aqueous dilutions (1/30 v:v and 1/40 v-v) were sprayed 7 days after transplant and repeated every 15 days. The biological factors measured were the stem length and diameter, number of leaves, and number of flowers. Productivity indicators included the number and mass of fruits per plant and the yield per unit surface area (t ha−1). The liquid humus dilutions improved the biological development of plants, with indicators that were superior to those of the controls, and thus produced better results for agricultural productivity. The most notable results were obtained with the 1/30 dilution.
Some modifications were recently introduced to the process of obtaining liquid humus, and the effects of the new extract were tested in bean plants (Phaseolus vulgaris L.) and lettuce (Lactuca sativa L.) (Hernández, 2011). Using these modifications, aqueous dilutions of 1:50, 1:60, and 1:70 (v:v) of the extract were sprayed on the bean plant leaves (primary leaves, trifoliate leaves, and trifoliate tertiary leaves) in CC–25–9 plants planted by conventional planting in a red ferralitic soil (Rhodic Eutrustok according to soil taxonomy). These dilution sprays increased the plant height, dry mass, leaf surface, quantity of pods per plant, number of grains per pod, and yield (Hernández et al., 2012). In urban lettuce crops in Cuba, similar aqueous dilutions (1:50, 1:60, and 1:70 v:v, which corresponds to 1.2; 1.3 and 1.6 mmol C L−1, respectively) were sprayed after 10 and 25 days of transplanting at a dose of 300 L ha−1. Both the biochemical–physiological (leaf composition of carbohydrates and proteins, nitrate-reductase activity, and specific leaf area) and biological and agricultural productivity indicators (number of leaves per plant, leaf length and area, and production cycle length) that were evaluated indicated significantly positive effects for the application of all the dilutions. The 1:60 dilution produced the most notable impacts and reduced the production cycle by 21 days, which greatly benefits producers (Hernández et al., 2013).In an attempt to summarize the general questions regarding the application of humic materials in agricultural production, we suggest Figure 18.7.
18.4 Conclusion and future prospects
A general evaluation of the previously cited examples in relation to the use of VC in plant productions allows the formulation of at least the following considerations:
• There are many organic raw materials (separate or combined) that can be subjected to the VC process, the majority of which are subproducts of the agricultural production systems to which they will be applied, which facilitates the practical application of these products.
• The chemical, physical, and biological characteristics of VC depend on the nature of the original materials, temperature, time of maturation, and type of worm.
• VC generally have a higher proportion of organic carbon (particularly soluble humic substances) than composts and a greater variety of organic compounds that are potentially active in the soil–plant system.
• Four ways of using VC (applying whole material directly on the soil, using soluble humic substances previously isolated from the material, spraying aqueous extracts obtained in an alkaline solution, and spraying aqueous leachates) have demonstrated success in increasing the development and biological and agricultural productivity of various commercial crops.
• Conditions for the applicability of different ways of using VC are similar to those of composts, but the dosages are generally lower.
The use of humified materials in agricultural activities has increased. Its acceptance has been due to its advantages regarding environmental safety, improvement of soil properties (chemical, physical, and biological), and growth and yield of agricultural crops. However, further studies are needed on the structural features of these materials targeting its standardization in relation to its effect and purpose. According to the actual studies, one way to ensure greater acceptance is to focus research on finding alternatives phytotechnical which were included the humified materials. These materials, VC or compost, for example, could be used as a substitute for the large consumption of chemicals and fertilizers of synthetic origin. On the other hand, it is necessary for the preparation and training of skilled manpower for the purpose of acquiring knowledge about concepts of organic agriculture and humus carbon.