**3. Green manures**

Green manure can be defined as the practice of plant cultivation until the flowering stage or until the incomplete development of seeds, with subsequent cutting and/or incorporation of its biomass into soil [57, 58]. The basic purpose of this technique is to improve chemical, physical, and biological soil characteristics in order to increase or stabilize the production of one or more crops in an area [57, 58]. Famers usually use legumes as green manure because of the high biomass yield, biological nitrogen fixation (BNF), and cycling of nutrients from deeper soil layers [57, 58].

After Green Revolution, the practice has gained importance, especially in organic production systems [58]. However, the species used as green manure are not restricted to these systems. They are also used, for example, to control soil degradation in minimum tillage, no‐till (NT) and in integrative systems [58–60]. In some situations, these species are called "cover crops," since the main purpose may be to conserve the soil. Species from the Poaceae family are the most common used cover crops [58]. A better soil cover and diversification of species in an area may also be used to suppress the development of weeds and also the emergence of pests and diseases [58, 61]. From an environmental stand point, the use of these plants can be a form of sequestering CO2 from the atmosphere [58]. This practice can also improve the use of mineral fertilizers, especially for those nutrients more susceptible to losses by leaching or sorption [58].

The main species used as green manure, their benefits, limitations, and managements are discussed in the following topics. More information, you can find in the references [58, 62, 63].

#### **3.1. Plants used as green manure**

Tropical environments allow the cultivation of a wide variety of green manure species, with plants from the families Fabaceae, Brassicaceae, Asteraceae, and Poaceae, among others. Some of the main species used in tropical environments, their characteristics of dry matter yield, amount of fixed nitrogen, and mineral composition of macronutrient and micronutrients is presented in **Table 1**.

#### Green Manures and Crop Residues as Source of Nutrients in Tropical Environment http://dx.doi.org/10.5772/62981 59


rate is higher in soils with neutral pH than in more acidic soils such as tropical soils. However,

The construction and maintenance of microbial diversity in the soil favor higher rates of decomposition [49]. Some agronomic practices are recommended for this purpose, such as: (*i*) regular application of organic residues or the use of biochemically complex green manures associated with those biochemically simple and easy decomposing, which supports the greater diversity of microbial communities in the soil; (*ii*) maintenance of soil cover, which promotes energy (via root exudates) for free‐living and symbiotic microorganisms and produces

Green manure can be defined as the practice of plant cultivation until the flowering stage or until the incomplete development of seeds, with subsequent cutting and/or incorporation of its biomass into soil [57, 58]. The basic purpose of this technique is to improve chemical, physical, and biological soil characteristics in order to increase or stabilize the production of one or more crops in an area [57, 58]. Famers usually use legumes as green manure because of the high biomass yield, biological nitrogen fixation (BNF), and cycling of nutrients from deeper

After Green Revolution, the practice has gained importance, especially in organic production systems [58]. However, the species used as green manure are not restricted to these systems. They are also used, for example, to control soil degradation in minimum tillage, no‐till (NT) and in integrative systems [58–60]. In some situations, these species are called "cover crops," since the main purpose may be to conserve the soil. Species from the Poaceae family are the most common used cover crops [58]. A better soil cover and diversification of species in an area may also be used to suppress the development of weeds and also the emergence of pests and diseases [58, 61]. From an environmental stand point, the use of these plants can be a form of sequestering CO2 from the atmosphere [58]. This practice can also improve the use of mineral fertilizers, especially for those nutrients more susceptible to losses by leaching or sorption [58].

The main species used as green manure, their benefits, limitations, and managements are discussed in the following topics. More information, you can find in the references [58, 62, 63].

Tropical environments allow the cultivation of a wide variety of green manure species, with plants from the families Fabaceae, Brassicaceae, Asteraceae, and Poaceae, among others. Some of the main species used in tropical environments, their characteristics of dry matter yield, amount of fixed nitrogen, and mineral composition of macronutrient and micronutrients is

liming acidic soils promotes accelerated decomposition of the residues.

58 Organic Fertilizers - From Basic Concepts to Applied Outcomes

extracellular enzymes in addition to the enzymes released by plant roots [56].

**3. Green manures**

soil layers [57, 58].

**3.1. Plants used as green manure**

presented in **Table 1**.

Source: Wutke E.B., Trani P.E., Ambrosano E.J., Drugowich M.I. Adubação verde no Estado de São Paulo. Campinas: Coordenadoria de Assistência Técnica Integral‐CATI; 2009. 89 p.

**Table 1.** Dry matter yield, amount of fixed nitrogen, and mineral composition of macro‐ and micronutrients in the shoot of species frequently used as green manure and cover crops in Southeastern Brazil.

The first thing to consider in the process of choosing a green manure is the purpose of the farmer in using it. When the farmer objective is nutrient supply, plants with high biomass production and capable of associating with N‐fixing bacteria can be suggested (**Table 1**). Among these plants are species from the genus *Crotalaria*, *Canavalia*, *Cajanus*, *Leucaena*, and *Acacia*. We recommend inoculation of seeds with *Rhizobium* in order to increase the symbiosis efficiency and consequently the biological nitrogen fixation. On the other hand, when the farmer objective is erosion control, species with higher C/N ratio, fasciculate root system, greater biomass production and lower shoot/root ratio can be suggested, such as some species of genus *Pennisetum* and Brachiaria. (**Table 1**).

Another important point to consider for choosing the correct green manures is the phytosa‐ nitary aspect. Some species can control plant pathogens, such as species from the *Crotalaria* genus, which are effective in controlling certain nematode species that cause root‐knot [58]. However, others are hosts of pathogen species and pests of commercial crops. An example of this problem is the cultivation of common beans (*Phaseolus vulgaris*) in succession or rotation with jack beans (*Canavalia ensiformis*). This green manure hosts the whitefly (*Bemisia tabaci*), an insect that transmits the bean golden mosaic virus [57]. Therefore, the use of species from different phylogenetic groups is recommended. Moreover, the use of species from different families is convenient, since they have different patterns of nutrient accumulation and root architecture, enabling the exploitation of different soil layers.

Many green manure species are better adapted to the climatic conditions of spring and summer while others to the autumn and winter (**Table 1**). Sunnhep (*Crotalaria* sp.) and pigeonpea (*Cajanus cajan*), for example, have greater biomass and nutrient accumulation when grown between the spring and summer due to their water requirements, photoperiod, and/or thermoperiod [63]. In general, sunnhep and pigeonpea plants require short days to flowering, so they have low biomass production when sown in late summer [63]. These plants should be selected for pre‐cultivation management of commercial crop. Other species are more versatile, like the millet (*Pennisetum glaucum*), canavalia (*Canavalia brasiliensis*), and jack bean (*Canava‐ lia ensiformis*) [62], which have little or no sensitivity to photoperiod and good drought tolerance [63]. Such species may have satisfactory dry matter yield in semi‐arid conditions; however, they have to be sown, preferably, when there is still moisture in the soil for germi‐ nation, such as the period after the summer harvest [63]. On the other hand, the genres *Avena*, *Lollium*, *Lathyrus*, and *Vicia* have to be cultivated preferably between the autumn and winter, and some of them are better adapted to subtropical conditions.

Therefore, the success in using the green manures technic depends on its adaptability to the region soil, climate conditions, cultivation system, and on their phytosanitary aspects. Experiments determining which species have greater biomass and nutrient accumulation are recommended when some of this information is unavailable.

### **3.2. Management of green manures in tropical conditions**

Green manure plants can be used in minimum tillage, no‐till or in systems with plowing and harrowing. These plants can be managed as soil cover in fallow, intercropping with perennial crops or in rotation or succession plans with annual crops. In general, when the plants reach the reproductive stage, when approximately 50% of the flowers are opened, a cut must be carried out with or without the plant matter incorporation into the soil. Some species might have longer cycles, but have to be cut before the seeds become viable. The black oat (*Avena strigosa*), for example, should be cut when grain maturation reaches the milky grain stage [63]. Cutting these plants before this stage allows a regrowth, and after this stage the grains may become viable [63]. Both cases may result in economic losses due to competition between the black oat plants and the main crop [63].

The toppling of plants in conservation systems can be obtained using mechanical or manual mowing, a roll‐knife or herbicides for desiccation [62]. In conventional systems, set of harrows are commonly used to incorporate the green manure into the soil or a rotary hoe is used in some cases [62].

Green Manures and Crop Residues as Source of Nutrients in Tropical Environment http://dx.doi.org/10.5772/62981 61

genus, which are effective in controlling certain nematode species that cause root‐knot [58]. However, others are hosts of pathogen species and pests of commercial crops. An example of this problem is the cultivation of common beans (*Phaseolus vulgaris*) in succession or rotation with jack beans (*Canavalia ensiformis*). This green manure hosts the whitefly (*Bemisia tabaci*), an insect that transmits the bean golden mosaic virus [57]. Therefore, the use of species from different phylogenetic groups is recommended. Moreover, the use of species from different families is convenient, since they have different patterns of nutrient accumulation and root

Many green manure species are better adapted to the climatic conditions of spring and summer while others to the autumn and winter (**Table 1**). Sunnhep (*Crotalaria* sp.) and pigeonpea (*Cajanus cajan*), for example, have greater biomass and nutrient accumulation when grown between the spring and summer due to their water requirements, photoperiod, and/or thermoperiod [63]. In general, sunnhep and pigeonpea plants require short days to flowering, so they have low biomass production when sown in late summer [63]. These plants should be selected for pre‐cultivation management of commercial crop. Other species are more versatile, like the millet (*Pennisetum glaucum*), canavalia (*Canavalia brasiliensis*), and jack bean (*Canava‐ lia ensiformis*) [62], which have little or no sensitivity to photoperiod and good drought tolerance [63]. Such species may have satisfactory dry matter yield in semi‐arid conditions; however, they have to be sown, preferably, when there is still moisture in the soil for germi‐ nation, such as the period after the summer harvest [63]. On the other hand, the genres *Avena*, *Lollium*, *Lathyrus*, and *Vicia* have to be cultivated preferably between the autumn and winter,

Therefore, the success in using the green manures technic depends on its adaptability to the region soil, climate conditions, cultivation system, and on their phytosanitary aspects. Experiments determining which species have greater biomass and nutrient accumulation are

Green manure plants can be used in minimum tillage, no‐till or in systems with plowing and harrowing. These plants can be managed as soil cover in fallow, intercropping with perennial crops or in rotation or succession plans with annual crops. In general, when the plants reach the reproductive stage, when approximately 50% of the flowers are opened, a cut must be carried out with or without the plant matter incorporation into the soil. Some species might have longer cycles, but have to be cut before the seeds become viable. The black oat (*Avena strigosa*), for example, should be cut when grain maturation reaches the milky grain stage [63]. Cutting these plants before this stage allows a regrowth, and after this stage the grains may become viable [63]. Both cases may result in economic losses due to competition between the

The toppling of plants in conservation systems can be obtained using mechanical or manual mowing, a roll‐knife or herbicides for desiccation [62]. In conventional systems, set of harrows are commonly used to incorporate the green manure into the soil or a rotary hoe is used in

architecture, enabling the exploitation of different soil layers.

60 Organic Fertilizers - From Basic Concepts to Applied Outcomes

and some of them are better adapted to subtropical conditions.

recommended when some of this information is unavailable.

**3.2. Management of green manures in tropical conditions**

black oat plants and the main crop [63].

some cases [62].

**Figure 1.** Coffee seedlings with pigeonpea tipped over between planting lines. Photos by Lucas de Ávila‐Silva.

Plants from the Fabaceae family with C/N ratio around 20 (Table 2) are rapidly decomposed in tropical conditions. Therefore, the commercial crop planting in rotation/succession to this type of plant should be performed few days (less than two weeks) after the green manure tipping over and/or incorporation, since much of the N is mineralized in the first 60 days [58]. Adopting species with higher C/N rates, around or above 40, such as species from the Poaceae family, enable to wait longer to the commercial crop planting. A 2‐week interval between the grass cutting and the commercial crop planting is recommended, because of the low availa‐ bility of N and possible allelopathic effects. Regarding the interim crops (**Figure 1**), the use of green manure species with climber habits should be avoided, since they may use commercial species as tutors. A solution to this problem is the use of more than one green manure species, a technique known as cocktail, using species with climbing and erect growth habit, so that the latter will serve as tutor for the first. It is also important an adequate inter‐line plant density, in order to avoid competition for growth resources between the green manure and the commercial crop.

A study carried out in Brazil evaluated different green manures in rotation with maize under two managements, minimum or conventional tillage [64]. The green manures used were as follows: *Crotalaria juncea*, *Canavalia ensiformes*, *Rafhanus sativus*, mixed species (cocktail) with *Crotalaria juncea*, *Canavalia ensiformes* and *Rafhanus sativus*, and weeds (control) [64]. The green manures treatments had maize yield higher than the control with weeds, with average of 2.8 t/ha of peeled green ears [64]. Another study, in the same region, evaluated the effect of N topdressing absence, pre‐cultivation of *C. juncea* and topdressing with ammonium sulfate or urea on maize yield produced for silage [65]. The use of *C. juncea* incorporated 12 days before the maize planting resulted in a yield increase of 14 t/ha compared with the control treatment without nitrogen topdressing [65]. However, the treatment with green manure did not surpass the treatments with ammonium sulfate or urea topdressing, which increased silage produc‐ tivity by 33 and 27 t/ha compared to the control without topdressing, respectively [65].

Therefore, in order to maintain a competitive agricultural system, the green manure practice must be managed together with chemical fertilizers. Amado et al., for example, proposed to consider the green manure contribution to define the nitrogen fertilizer dose, which is especially important in no‐till and integrative systems. These authors considered the following criteria for setting the dose: the soil organic matter content; the commercial crop expected yield; and the previous cover crop (green manure) residue nature and amount [66].The previous cover crop contribution was considered in three situations: legumes, grasses in monocrop, and consortia (cocktails) [66].They considered that the higher the green manure dry matter yield, higher the N supply to the commercial crop in succession to legumes [66]. The contribution of grasses for the N supply was estimated to be very small or non‐existent compared with uncultivated areas and may even reduce the availability of N due to its short term immobili‐ zation [66]. Regarding the consortia system (cocktails), the contribution is estimated according to the percentage of legume biomass in total biomass of green manure. In some conditions, fertilizer savings can reach up to 80 kg/ha of N [66].

An example of the green manure potential intercropped with perennials plants is a study with soursop (*Annona muricata*) in southeastern Brazil, in which the author evaluated three green manures species (*Gliricidia sepium*, *Crotalaria juncea*, and *Cajanus cajan*) [67]. The greater amount of biologically fixed nitrogen was found with *G. sepium* (80% of the accumulated N) and with *C. juncea* (64.5% of the accumulated N). During the 2 years of experiment, the *C. juncea* added to the soil, in two cuts, about 149.5 kg/ha of N, from which 96.5 kg/ha came from biological nitrogen fixation (BNF) [67]. The *G. sepium*, managed with three annual pruning, added 113– 160 kg/ha of N, with 90–128 kg/ha derived from BNF. Variations in natural 15N amounts indicated that green manure with *C. juncea* and *G. sepium* contributed to N supply for the soursop, transferring about 22.5 and 40% of the fixed N, respectively [67]. They conclude that the use of these two green manure legumes contribute as organic fertilizer, supplying nutrients, mainly N [67].

The use of fresh organic matter from green manures brings more than just nutrients, affecting physical-chemical and biological characteristics of the soil. Pegoraro et al. [68] showed the effect of the use of green manure (*Acacia mangium*) grown after the *Eucalyptus* cut in short rotation system (6–7 years of growth)*.* The authors noted that the use of a legume in succession enabled the increased in stocks of total C and N, C and N in humic substances and in micro‐ organisms compared to crops without the legume succession [68]. Vegetable residues with lower C/N ratio accelerate the recycling of residues from commercial crops (ex.: *Eucalyptus*), which reduces the microbial attack on soil organic matter (SOM) and increases the levels of stabilized organic matter [68].

The effect on the soil organic matter supply makes the green manure a promising practice for recovery degraded areas and help to restore the A horizon of "beheaded" soil profile. Alves et al. studied green manures as a component of strategies for recovery degraded areas [69]. Positive effects were noted 365 days after the experiment implementation, such as soil compaction reduction and water infiltration time reduction [69]. Kitamura et al. reported, in another paper concerning the same research, that the treatments also provided results related to the soil macroorganism population [70].

#### **3.3. Advantages and disadvantages of green manures**

The main advantages and disadvantages of green manures to tropical weathered soils [58, 62, 63] are listed below.

#### *3.3.1. Advantages*

without nitrogen topdressing [65]. However, the treatment with green manure did not surpass the treatments with ammonium sulfate or urea topdressing, which increased silage produc‐ tivity by 33 and 27 t/ha compared to the control without topdressing, respectively [65].

Therefore, in order to maintain a competitive agricultural system, the green manure practice must be managed together with chemical fertilizers. Amado et al., for example, proposed to consider the green manure contribution to define the nitrogen fertilizer dose, which is especially important in no‐till and integrative systems. These authors considered the following criteria for setting the dose: the soil organic matter content; the commercial crop expected yield; and the previous cover crop (green manure) residue nature and amount [66].The previous cover crop contribution was considered in three situations: legumes, grasses in monocrop, and consortia (cocktails) [66].They considered that the higher the green manure dry matter yield, higher the N supply to the commercial crop in succession to legumes [66]. The contribution of grasses for the N supply was estimated to be very small or non‐existent compared with uncultivated areas and may even reduce the availability of N due to its short term immobili‐ zation [66]. Regarding the consortia system (cocktails), the contribution is estimated according to the percentage of legume biomass in total biomass of green manure. In some conditions,

An example of the green manure potential intercropped with perennials plants is a study with soursop (*Annona muricata*) in southeastern Brazil, in which the author evaluated three green manures species (*Gliricidia sepium*, *Crotalaria juncea*, and *Cajanus cajan*) [67]. The greater amount of biologically fixed nitrogen was found with *G. sepium* (80% of the accumulated N) and with *C. juncea* (64.5% of the accumulated N). During the 2 years of experiment, the *C. juncea* added to the soil, in two cuts, about 149.5 kg/ha of N, from which 96.5 kg/ha came from biological nitrogen fixation (BNF) [67]. The *G. sepium*, managed with three annual pruning, added 113– 160 kg/ha of N, with 90–128 kg/ha derived from BNF. Variations in natural 15N amounts indicated that green manure with *C. juncea* and *G. sepium* contributed to N supply for the soursop, transferring about 22.5 and 40% of the fixed N, respectively [67]. They conclude that the use of these two green manure legumes contribute as organic fertilizer, supplying nutrients,

The use of fresh organic matter from green manures brings more than just nutrients, affecting physical-chemical and biological characteristics of the soil. Pegoraro et al. [68] showed the effect of the use of green manure (*Acacia mangium*) grown after the *Eucalyptus* cut in short rotation system (6–7 years of growth)*.* The authors noted that the use of a legume in succession enabled the increased in stocks of total C and N, C and N in humic substances and in micro‐ organisms compared to crops without the legume succession [68]. Vegetable residues with lower C/N ratio accelerate the recycling of residues from commercial crops (ex.: *Eucalyptus*), which reduces the microbial attack on soil organic matter (SOM) and increases the levels of

The effect on the soil organic matter supply makes the green manure a promising practice for recovery degraded areas and help to restore the A horizon of "beheaded" soil profile. Alves et al. studied green manures as a component of strategies for recovery degraded areas [69]. Positive effects were noted 365 days after the experiment implementation, such as soil

fertilizer savings can reach up to 80 kg/ha of N [66].

62 Organic Fertilizers - From Basic Concepts to Applied Outcomes

mainly N [67].

stabilized organic matter [68].

### *3.3.1.1. Chemical aspects*


#### *3.3.1.2. Biological aspects*


#### *3.3.1.3. Physical aspects*


#### *3.3.2. Disadvantages*

