**4. Crop residues**

Crop residues can be defined as any part of the plant without direct economic value produced in the field (derived from the harvest) or after processing on the farm. The crop residues value in agriculture coincided with the success of no‐till system, until then, soil revolving or even fire was commonly used to accelerate residue decomposition. More recently, the crop‐ livestock‐forestry integrated system is also an example of crop residues conservation. The no‐ tillage system implementation is based on three pillars: (*i*) no soil tillage, (*ii*) crop rotation, and (*iii*) permanent soil cover. Although essential, the soil cover maintenance in tropical environ‐ ment is not a simple task, mainly due to the high rate of straw decomposition from leguminous plants, and is difficult to grow plants in succession (second crop) because adversities such as the rainfall patterns.

The maintenance of residues benefits the agricultural system by conserving the soil free from erosion. The homogeneous distribution of residues in the area with great amount of straw generates a slower runoff and less water and soil loss [71], consequently avoiding topsoil losses, which normally has a great amount of nutrients. Another important feature of crop residues is the maintenance of higher water content in the soil, a problematic factor in the tropical agriculture.

The crop residue maintenance also contributes to a decrease in the soil surface temperature, ensuring a better condition for the plants and other soil organisms. Another relevant factor is the ratio between the straw production, weed control, and soil compaction reduction capabil‐ ities, which is a recurring problem in poorly managed conservation systems.

*3.3.2. Disadvantages*

postures;

crop;

ments of crops;

**4. Crop residues**

the rainfall patterns.

tropical agriculture.

weather conditions;

64 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**1.** Inadequacy of some green manure species to the production system or the soil and

**2.** Lack of interest from consultants and farmers in this technology, which adopt immediate

**3.** Sometimes, green manure involves costs with no direct financial return;

inadequate management of the technology in intercropping systems;

**9.** Uneven seed germination of some species of green manure;

**11.** Lack of functional decomposition models to predict nutrient release.

**10.** Difficulty of obtaining seeds for sowing;

**5.** Some green manures can host diseases and pests that attack the commercial crop;

**6.** Possibility of negative allelopathic effect of green manure residues on the commercial

**7.** Possibility of competition between green manure plants and the commercial crop by

**8.** Some green manures have incompatible decomposition rates with the nutrient require‐

Crop residues can be defined as any part of the plant without direct economic value produced in the field (derived from the harvest) or after processing on the farm. The crop residues value in agriculture coincided with the success of no‐till system, until then, soil revolving or even fire was commonly used to accelerate residue decomposition. More recently, the crop‐ livestock‐forestry integrated system is also an example of crop residues conservation. The no‐ tillage system implementation is based on three pillars: (*i*) no soil tillage, (*ii*) crop rotation, and (*iii*) permanent soil cover. Although essential, the soil cover maintenance in tropical environ‐ ment is not a simple task, mainly due to the high rate of straw decomposition from leguminous plants, and is difficult to grow plants in succession (second crop) because adversities such as

The maintenance of residues benefits the agricultural system by conserving the soil free from erosion. The homogeneous distribution of residues in the area with great amount of straw generates a slower runoff and less water and soil loss [71], consequently avoiding topsoil losses, which normally has a great amount of nutrients. Another important feature of crop residues is the maintenance of higher water content in the soil, a problematic factor in the

**4.** Low development of breeding technologies of green manure species;

The longer the time the crop residues are in the soil surface, the greater the positive effects. The residue permanence time in the soil depends on several factors, such as the: (*i*) fragmen‐ tation level, (*ii*) amount, (*iii*) chemical composition, (*iv*) contact level with the soil, (*v*) weather conditions, (*vi*) microbial community, and (*vii*) soil type. In addition to the factors discussed in the first part, it is also known that residues with greater contact surface with the soil accelerate the decomposition rate.

Green manures residues can be choosing from plants with high mass production, chemical composition with high C/N ratio and lignin, favoring the permanence of straw, the use of equipment which allows less fragmentation, such as the roll‐knife.

Weeds and green manures (or cover plants) can be managed between rows of perennial crops; in this case, we can also use the roll‐knife, although normally brush cutters and grinders are used.

Heavy equipment may be required in some situations, such as the scarifier or harrow, for example, in excessive amount of plant residues, rigid materials, or seeder inefficiency.

The residues management options for residues from commercial crops are less flexible, since the fragmentation is performed according to the harvest type, the amount is dependent on the yield and the material chemical composition is usually consequence of the best‐adapted material. The legume residues have high decomposition rates, often cooperating little to the soil cover. The cultivation of legumes is usually performed first to leave a N balance for the sequential culture.

In specific cases of succession after grasses cultivation, an increase in N doses or even an anticipate application of nitrogen fertilizer is suggested before sowing, basically due to the N immobilization because the high C/N ratio of grasses.

After reminding here, the crucial importance of crop residues to the soil cover and the main factors affecting its decomposition, we can discuss a second advantage of residues from commercial crops: the potential supply of nutrients. The management of residues from a preceding crop (annual) or residues from the crop itself (perennial) can complement the nutrient supply.

Regarding the green manure, nutrient mineralization from the crop residues depends on the residue quality, soil moisture and temperature, as well as specific soil factors such as texture, mineralogy and acidity, biological activity, and the presence of other nutrients [72]. Most of the nutrients are exported in the harvest, remaining just a portion in the residues. In this part, we are going to see yield increases by the nutrient supply potential of the residues from the main crops in tropical environments.

### **4.1. Annual crops residues**

The nutrient net mineralization from the annual crop residues that precede other crop must be considered in nutritional management. We focus here on annual crops that associate with N2 fixing bacteria, because they are the major contributor to the nutrient supply (nitrogen) for crops in succession or rotation. Subsequently, we will see the most used crops in tropical environments that leave nutrient to the system.

### *4.1.1. Soybean*

The soybean (*Glycine max*) represents 50% of the global area of leguminous crops and 68% of global production of this family. The annual input of N fixed is 16.4 Tg, which represents 77% of N fixed by legume crops [73]. A crucial fact to these crop success was the priority given to association with *Bradyrhizobium* and BNF in breeding programs [74].

Most of the soybean crop in Brazil is carried out in no‐tillage system, and this management provides more root nodulation by the bacteria in the soil, best nodulation (nodulation deep in the soil profile), higher rates of BNF and yields compared to conventional tillage [75, 76]. These factors, combined with the efficient association with *Bradyrhizobium*, allow the non‐use of nitrogen fertilizers in this crop.

Approximately 80–83 kg of N is required for each 1000 kg of soybeans produced, from which 51–65 kg are allocated in the seeds and 15–32 kg in the roots, stems, and leaves [74, 77]. The N2 fixation potential of the soybean can be as high as 360–450 kg/ha [78, 79].

In southern Brazil, Paraguay, Uruguay, and northern Argentina (subtropical) is recurrent to get higher wheat yields (winter planting) when soybean is the preceding crop in the summer compared to the maize, precisely because of the remainder N [80]. The maize, in rotation with soybeans, may have its nitrogen fertilizer rate reduced by 20% in this environment, considering the effect of rotation after a soybean crop with adequate productivity [81]. In a similar environment, some producers in various states of the Midwestern United States reduce 45 kg/ ha of the nitrogen fertilizer dose, when maize is planted following soybeans compared to sequential crops of maize [82].

The Cerrado biome (Brazilian savanna) has no winter crops due to insufficient rainfall, and thus the soybean is the main summer crop and, after the harvest, in late summer, usually maize or sorghum is planted ("second crop"). A contribution of 20 kg/ha of N is assumed as a practical parameter in the region for these crops after soybean [83].

The first part of this chapter showed that the C/N ratio and biochemical constitution of the residues are important indicators to predict the nutrient mineralization rate. Regarding the soybean grown in tropical environments, different C/N ratios are observed in the roots (31.6), stems (22.5) and leaves (10.7), as well as larger amounts of lignin–suberin. These data explain the great decomposition rates of shoot residues in the first 20 days after harvest, with N mineralization, especially from leaf residues, under controlled conditions [84].

Unfortunately, there are few long‐term field experiments in typically tropical regions, which hinder an accurate estimate of the decomposition rate of soybean residues and mineralization Green Manures and Crop Residues as Source of Nutrients in Tropical Environment http://dx.doi.org/10.5772/62981 67

**Figure 2.** Residue deposition and homogeneous distribution after the harvest. Photos by Lucas de Ávila‐Silva.

of nutrients. A field experiment carried out for 12 years in Brazil (Parana State, Brazil), estimated the residue decomposition percentage according to the time and management, with y = 93.819e-0.0031x (R2  = 0.91) to the no‐till and y = 90.061e-0.0054x (R2  = 0.92) to the conventional tillage [85]. The residue decomposition time in that work was autumn–winter, a less rainy period that the summer. Moreover, that is a climate transition region (Cfa subtropical climate in the Köppen classification) with low temperatures and a distinct pluviometric regime compared to Brazilian Midwest (**Figure 2**).

#### *4.1.2. Beans*

**4.1. Annual crops residues**

nitrogen fertilizers in this crop.

sequential crops of maize [82].

parameter in the region for these crops after soybean [83].

*4.1.1. Soybean*

environments that leave nutrient to the system.

66 Organic Fertilizers - From Basic Concepts to Applied Outcomes

The nutrient net mineralization from the annual crop residues that precede other crop must be considered in nutritional management. We focus here on annual crops that associate with N2 fixing bacteria, because they are the major contributor to the nutrient supply (nitrogen) for crops in succession or rotation. Subsequently, we will see the most used crops in tropical

The soybean (*Glycine max*) represents 50% of the global area of leguminous crops and 68% of global production of this family. The annual input of N fixed is 16.4 Tg, which represents 77% of N fixed by legume crops [73]. A crucial fact to these crop success was the priority given to

Most of the soybean crop in Brazil is carried out in no‐tillage system, and this management provides more root nodulation by the bacteria in the soil, best nodulation (nodulation deep in the soil profile), higher rates of BNF and yields compared to conventional tillage [75, 76]. These factors, combined with the efficient association with *Bradyrhizobium*, allow the non‐use of

Approximately 80–83 kg of N is required for each 1000 kg of soybeans produced, from which 51–65 kg are allocated in the seeds and 15–32 kg in the roots, stems, and leaves [74, 77]. The

In southern Brazil, Paraguay, Uruguay, and northern Argentina (subtropical) is recurrent to get higher wheat yields (winter planting) when soybean is the preceding crop in the summer compared to the maize, precisely because of the remainder N [80]. The maize, in rotation with soybeans, may have its nitrogen fertilizer rate reduced by 20% in this environment, considering the effect of rotation after a soybean crop with adequate productivity [81]. In a similar environment, some producers in various states of the Midwestern United States reduce 45 kg/ ha of the nitrogen fertilizer dose, when maize is planted following soybeans compared to

The Cerrado biome (Brazilian savanna) has no winter crops due to insufficient rainfall, and thus the soybean is the main summer crop and, after the harvest, in late summer, usually maize or sorghum is planted ("second crop"). A contribution of 20 kg/ha of N is assumed as a practical

The first part of this chapter showed that the C/N ratio and biochemical constitution of the residues are important indicators to predict the nutrient mineralization rate. Regarding the soybean grown in tropical environments, different C/N ratios are observed in the roots (31.6), stems (22.5) and leaves (10.7), as well as larger amounts of lignin–suberin. These data explain the great decomposition rates of shoot residues in the first 20 days after harvest, with N

Unfortunately, there are few long‐term field experiments in typically tropical regions, which hinder an accurate estimate of the decomposition rate of soybean residues and mineralization

mineralization, especially from leaf residues, under controlled conditions [84].

association with *Bradyrhizobium* and BNF in breeding programs [74].

N2 fixation potential of the soybean can be as high as 360–450 kg/ha [78, 79].

The genus *Phaseolus* has more than 200 species described [86], but those that have the greatest economic impact are the *Phaseolus vulgaris*, *Phaseolus coccineus*, *Phaseolus lunatus*, *Phaseolus acutifolius*, and *Phaseolus semierectus*, mainly the first. Different from the soybean, common beans have low symbiotic efficiency [87]. The nitrogen fertilizers use is indicated in some situations to achieve good yields, even though they may affect negativity the BNF efficiency. Mean values of 35% and maximum of 70% of N derived from the atmosphere were observed at the plant biomass considering six field experiments in tropical countries and Austria [88]. Some progress is occurring with the best selection of strains, adapted to the conditions of each site [89–92].

Different C/N ratios were observed in stem fractions (79), straw pods (66), and senescent leaves (24) in four varieties of beans [93]. A shorter leaf half‐life was observed, although the straw pods also showed low half‐life value (both of approximately 70 days). Those authors observed N and P release and half‐life following the residue decomposition rate, while the K release was faster (average half‐life of 18 days) and showed low difference regarding the material quality. In this case, N and P could be better used by the subsequent crop, while K can be reused by the same crop. The yield of 1350 kg/ha presented a potential for cycling 31.5 kg/ha N and 2.37 kg/ha P from the bean residues [93].

The species *Vigna mungo*, *Vigna radiata*, and *Vigna unguiculata* stand out in the *Vigna* genus, the latter being the most cultivated. These species have short cycle, low water requirement, and good development in low fertility soils; in addition, the BNF is capable of supplying more than 100 kg/ha of N [94, 95]. An increase in millet productivity of 9–24% was observed in an experiment carried out in three different locations in Niger, when cowpea was previous cultivated compared to successive cultivation of millet [96].

A C/N ratio of 15.8 was found in cowpea residues grown in a field experiment evaluating N mineralization depending on the phosphorus content in tissues [97]. In the same experiment, the authors found an increasing in N mineralization (25, 32, and 34%) with increasing P concentration in tissues (1.0, 1.2, and 2 g/kg, respectively) in 8 weeks. This fact demonstrates the close relationship between nutrients interfering in the mineralization process, emphasizing the importance of fertilization management together with residues management. A potential mineralization from 6.8 to 9.2 g of N per kilogram of dry matter of cowpea residues is consid‐ ered as a practical reference.

#### *4.1.3. Groundnut (peanut)*

India is largest producer of peanuts (*Arachis hypogaea*), which usually uses 10–20 kg/ha of N from ammonium sulfate. A large number of farmers in that country use crop rotation with groundnut to take advantage of its ability to improve soil fertility and increase the subsequent crop yield, due to the BNF [98].

The peanut cultivation in Brazil is also carried out with the same goal, but most in rotation with sugarcane. A C/N ratio of 15 and 24, and an addition of 70 and 38% of N from BNF in plant tissues, was, respectively, observed in the IAC‐Caiapó and IAC‐Tatu varieties, in acid soils [99]. These authors emphasized that the N values from BNF for IAC‐Tatu were low because the sampling, which was performed 120 days after sowing, when the plant was in an advanced stage of pods maturation and much of the N had translocated to the grains. About 90% of N was from the shoot in the average for these cultivars. In this same experiment, there was a yield increase of 12.2 (IAC‐Caiapó) and 15.5 t/ha (IAC‐Tatu) in the sugarcane crop compared with the control. Bagayoko et al. [96] also reported an increase of approximately 39% in average yield of sorghum after groundnut cultivation compared to subsequent sorghum crops; during 3 years of experiment in Kouaré (Burkina Faso), they found N mineral increment available for sorghum.

The groundnut may also be infected by arbuscular mycorrhizal fungi [99], which favors the next culture infection [96] and improves the P cycling.
