**3. On-farm–produced organic amendments**

### **3.1. SOM source and modified C:N ratio of added residues**

Leguminous green manure has long been used as a SOM source as a component of cropping system in Africa, Asia, and Latin American. Cover crops and intercropping increase C sequestration in the soil [21]. Another possible benefit of legumes is the result of N fixation by the root nodules, which the amount of biological fixed N is a somewhat contentious issue in some case studies, but most of the organic N can be available to plants after residues have been composted [22]. Generally green manure can produce a dry weight of 5-9 tons or more biomass ha–1 year–1; about 40% of dry matter is C and 2-4% is N. While the N productivity of several major cultivated legumes has been reported as from 80 kg (berseem clover) to 190 kg (subc‐ lover) ha–1 [23-25]. The ability of biological N fixation ranges from 40 to 200 kg in aboveground tissues ha–1 due to the species of legume, bacterial strain, and agricultural conditions, such as climate and soil (Table 2).

Factors that influence the ability of microorganisms to break down added plant materials includes the C:N ratio of organic matter and components of organic C and N [26]. The C:N ratio of plant tissue reflects the kind and age of a plant from which it was derived. Non-legume plants may have a high C:N ratio, over 60 (cow straw), up to even 250 (wood tissue) [27,28]. Non-leguminous green manure with high a C:N ratio (over 25), will cause microorganisms to tie up available N in the soil. Thus, added materials with C:N ratios above 25:1 can result in N being bound by soil microbes in the breakdown of C-rich crop residues, thus pulling N away from the root zone of crop plants. Legumes usually have a low C:N ratio (about 10-15) and can help to modify the SOM C:N ratio to an adequate level. The optimal C:N ratio for rapid decomposition of organic matter is between 15:1 and 25:1 [29]. The addition of N-rich plant residues, such as legume plants, to aid the decomposition process may be advisable with these high-C residues: the lower the C:N ratio, the more N will be released into the soil for immediate crop use [30].


**Table 2.** Yearly average biomass yields and nitrogen yields of several legumes.

#### **3.2. Intercropping with green manure**

Intercropping refers to multiple crops planted at same time on the same land, growing and interacting with each other during the whole or part of the growing season within a crop system [31,32]. Intercrops or mixture crops influence the farming system in ways including the lower density of each species helping to reduce plant pathogen infection opportunities, raising land productivity as a result of reducing the effects of unsuitable conditions, which may be not so unsuitable to other intercrops, positive effects on weed control, border effects, and enhancing plant nutrient and soil humidity use efficiency. Intercropping can be regarded as a method of weed suppression and offering habitats for beneficial organisms. According to space arrange‐ ments and temporal practices, intercropping is commonly divided into four subcategories [31]:


Crop mixtures may gather more light and plant nutrients than pure standing crops, as a result of differing root depth and stem height. Generally, the land equivalent ratio (LER) is one of the major measures to judge how complementary crops are under intercropping. The mathe‐ matical equation is stated as [33]:

$$\text{LER} = (\text{Yiji} + \text{Yji} \, / \, \text{Yji} + \text{Yii})\_{\prime \prime}$$

where Yij is the grain yield per unit area of species i grown in a mixture with species j, Yii is the grain yield per unit area of species i grown in a pure stand, Yji is the grain yield per unit area of species j grown in mixture with species i, and Yjj is the grain yield per unit area of species j grown in a pure stand.

When the LER is below 1, it indicates that competition exists between different components rather than them being complementary. Generally, under a well-managed intercropping system, the LER is 1.2-1.5 and sometimes even above 1.5. These results are due to the rational use of natural resources for crop growth. As Paolini et al. reported, after 2-year case studies on mixture crops of sunflower and chickpea, total LER figures averaged 1.16 as to aboveground biomass yield and 1.25 as to grain yield [34]. They also pointed out that an unfavorable climate for one species is not so unfavorable to another. The conclusion can be made that when the climate or other agricultural factor becomes the key limiting factor for one crop, the other crop will likely get sufficient plant nutrients and soil moisture water supply; one species will be more tolerant of the unfavorable condition than the other.

Altieri found that in Mexico, 1.73 ha of land had to be planted with maize to produce as much food as 1 ha planted with a mixture of maize, squash, and beans [35]. Additionally, maize + squash + bean polyculture can produce up to 4 tons ha–1 of dry matter for plowing into the soil as compared with 2 tons in a maize monoculture. In Brazil, a maize or sorghum mixture with cowpeas or beans can lead to LER values of 1.25-1.58 [35]. Sometimes under intercropping management, a major crop cannot get the maximum yield obtainable in a monoculture due to competition from the other crop or a lower density versus a pure stand. By interplanting, farmers achieve several production and conservation objectives simultaneously [36]. Polycul‐ tures produce more combined yield in a given area than could be obtained from monocultures of the component species; sometimes, the LER can be above 1.5, although the yield variability of cereal + legume polycultures are much lower than that for monocultures of the components [37]. The intercrop treatments represented the highest LER was 1.52 for baby corn/pea intercropping system [38]. And other maize/bean intercropping achieved LER values were 1.76 and 1.92 [39]. Cover cropping is also considered as a the practice of growing pure or mixed strands of legumes, cereals, or natural vegetation to protect the soil against erosion, ameliorate soil structure issues, enhance soil fertility, and suppress pests, including weeds, insects, and pathogens [31]. The cover crop approach has been used for thousands years ago and is regarded as a sustainable method of agricultural production.

#### **3.3. Living mulch**

decomposition of organic matter is between 15:1 and 25:1 [29]. The addition of N-rich plant residues, such as legume plants, to aid the decomposition process may be advisable with these high-C residues: the lower the C:N ratio, the more N will be released into the soil for immediate

**(tons ha–1)**

Intercropping refers to multiple crops planted at same time on the same land, growing and interacting with each other during the whole or part of the growing season within a crop system [31,32]. Intercrops or mixture crops influence the farming system in ways including the lower density of each species helping to reduce plant pathogen infection opportunities, raising land productivity as a result of reducing the effects of unsuitable conditions, which may be not so unsuitable to other intercrops, positive effects on weed control, border effects, and enhancing plant nutrient and soil humidity use efficiency. Intercropping can be regarded as a method of weed suppression and offering habitats for beneficial organisms. According to space arrange‐ ments and temporal practices, intercropping is commonly divided into four subcategories [31]: **1.** *Row intercropping*: growing two or more crops at the same time with at least one crop

**2.** *Stripe intercropping*: growing more than one crop together in stripes wide enough to permit separate crop production using machines but close enough for the crops to interact with

**4.** *Relay cropping*: planting a second crop in a standing crop at a time when the standing crop is at its reproductive stage but before harvest. According to plant density or land features, the crop with high seeding rates is called the major crop, and the other crop is the

Crop mixtures may gather more light and plant nutrients than pure standing crops, as a result of differing root depth and stem height. Generally, the land equivalent ratio (LER) is one of

**3.** *Mixed intercropping*: growing two or more crops with no distinct row arrangement.

Sweet clover 4.3 130 Berseem clover 2.7 75 Crimson clover 3.5 108 Hair vetch 4.3 118 Subclover 5.4–10.1 184

**Nitrogen (kg ha–1)**

crop use [30].

\*

**Crop Biomass\***

294 Organic Fertilizers - From Basic Concepts to Applied Outcomes

Dry weight of plant aboveground material (sources: [2325])

**3.2. Intercropping with green manure**

planted in rows.

secondary crop.

each other.

**Table 2.** Yearly average biomass yields and nitrogen yields of several legumes.

Living mulch refers to a legume cover-crop, which is undersown with an annual crop. Common living mulches include white clover, hairy vetch, and red clover. A living mulch can improve soil structure and water penetration, prevent soil erosion, modify the microclimate, and reduce weed competition [40]. An ideal crop occupies underused time or space in an existing system. It does not compete with the cash crop for light, water, or nutrients, and attracts beneficial organisms, while keeping harmful pests away. It should be readily estab‐ lished and grow rapidly. It should produce an abundant growth of both shoots and roots in a short time, and its growth habits should encourage ground cover soon after its establishment [41]. Common living mulches can offer soil cover, especially during the seedling period and after harvest of the target crop when the crop plant does not cover most of the soil surface. Subclover planted as a living mulch was able to regenerate and provide the succeeding crop with abundant and N-rich residues [42]. The main benefits of living mulches include enhance‐ ment of soil structure, improvement of soil fertility, and positive effects on pest management and environmental quality [43].

Changing from a monoculture to an intercropping system requires several important man‐ agement practices based on natural laws. Successful management requires investment in experience and research to modify the system into an economically acceptable, ecologically sustainable, and technologically practicable one. In the case of winter wheat, a major crop around the world, legumes, as rotation crops or intercrops, have been tested for the establish‐ ment of the cropping systems. According to Caporali and Campiglia, in search of strategies for increasing sustainability in cropping systems, they have been focusing for 10 years on the use of plant resources, such as self-reseeding winter annual legumes (*Trifolium* and *Medicago* species) native to the Mediterranean environment [44]. Although subclover and annual medics are well-known forage crops in cereal-lay farming systems under the Mediterranean climate around the world, their use is practically unknown in more intensive cash-crop sequences, such as the 2-year rotation between a winter cereal (wheat, barley) and a summer crop (rainfed sunflower, irrigated corn), as is common in central Italy. In this rotation, an annual legume is used as a living mulch in winter cereals, and after its self-reseeding, as either a green manure or living mulch for the succeeding summer crop. This alternative cropping system has proved to have the potential to induce a significant shift toward a less energy-intensive and a more environmentally friendly management type, while maintaining the same cash-crop sequence of the conventional one [44].

The foundation of the system began with the screening of self-reseeding legumes species and cultivars and ended with the implementation and performance assessment of an entire alternative cropping system (winter cereal/summer crop rotation). The yield of winter wheat intercropped with subclover was not significantly different from that of a pure wheat stand in a drier crop year, while in the wetter year the grain yield of the intercropping system was significantly higher than the pure stand. However, in both crop years, grain yields were significantly lower that obtained using 130 kg ha–1 mineral N fertilizer, by 11% and 23%, respectively. Additionally, a positive correlation between the amount of subclover biomass plowed in and the vegetative and productive characteristic of sunflower was found in dry and wet years. Subclover green manure was so effective that sunflower yield in the alternative system was higher than that of the conventional one fertilized with 130 kg ha–1 of inorganic N. Subclover green manure also affected biomass and the composition of the weed community in the sunflower crop. Subclover mulch from a sod strip intercropping system with wheat was also effective in positively influencing the aboveground biomass production of the succeeding crops, the effect being dependent on the amount of dry mulch left by the different subclover species and cultivars [44].

## **3.4. Competition between intercrops and intercropping principles**

and reduce weed competition [40]. An ideal crop occupies underused time or space in an existing system. It does not compete with the cash crop for light, water, or nutrients, and attracts beneficial organisms, while keeping harmful pests away. It should be readily estab‐ lished and grow rapidly. It should produce an abundant growth of both shoots and roots in a short time, and its growth habits should encourage ground cover soon after its establishment [41]. Common living mulches can offer soil cover, especially during the seedling period and after harvest of the target crop when the crop plant does not cover most of the soil surface. Subclover planted as a living mulch was able to regenerate and provide the succeeding crop with abundant and N-rich residues [42]. The main benefits of living mulches include enhance‐ ment of soil structure, improvement of soil fertility, and positive effects on pest management

Changing from a monoculture to an intercropping system requires several important man‐ agement practices based on natural laws. Successful management requires investment in experience and research to modify the system into an economically acceptable, ecologically sustainable, and technologically practicable one. In the case of winter wheat, a major crop around the world, legumes, as rotation crops or intercrops, have been tested for the establish‐ ment of the cropping systems. According to Caporali and Campiglia, in search of strategies for increasing sustainability in cropping systems, they have been focusing for 10 years on the use of plant resources, such as self-reseeding winter annual legumes (*Trifolium* and *Medicago* species) native to the Mediterranean environment [44]. Although subclover and annual medics are well-known forage crops in cereal-lay farming systems under the Mediterranean climate around the world, their use is practically unknown in more intensive cash-crop sequences, such as the 2-year rotation between a winter cereal (wheat, barley) and a summer crop (rainfed sunflower, irrigated corn), as is common in central Italy. In this rotation, an annual legume is used as a living mulch in winter cereals, and after its self-reseeding, as either a green manure or living mulch for the succeeding summer crop. This alternative cropping system has proved to have the potential to induce a significant shift toward a less energy-intensive and a more environmentally friendly management type, while maintaining the same cash-crop sequence

The foundation of the system began with the screening of self-reseeding legumes species and cultivars and ended with the implementation and performance assessment of an entire alternative cropping system (winter cereal/summer crop rotation). The yield of winter wheat intercropped with subclover was not significantly different from that of a pure wheat stand in a drier crop year, while in the wetter year the grain yield of the intercropping system was significantly higher than the pure stand. However, in both crop years, grain yields were significantly lower that obtained using 130 kg ha–1 mineral N fertilizer, by 11% and 23%, respectively. Additionally, a positive correlation between the amount of subclover biomass plowed in and the vegetative and productive characteristic of sunflower was found in dry and wet years. Subclover green manure was so effective that sunflower yield in the alternative system was higher than that of the conventional one fertilized with 130 kg ha–1 of inorganic N. Subclover green manure also affected biomass and the composition of the weed community in the sunflower crop. Subclover mulch from a sod strip intercropping system with wheat was

and environmental quality [43].

296 Organic Fertilizers - From Basic Concepts to Applied Outcomes

of the conventional one [44].

The struggle for nature's resources is always an issue within any crop system because plant growth needs not only space and time, but also light, mineral nutrients, and water. The competition can be considered to have two main aspects: aboveground competition and root system competition. In rain-fed agriculture, under limited water conditions, a major compe‐ tition can occur between the target crop and legumes for water resource. The wheat yield under intercropping conditions with legumes reportedly decreases with less water availability versus a pure stand, although legume intercropping with a major crop can enhance the N content of the soil [45]. Mc Gowan and Williams found that subclover depleted soil moisture more than barley. At 19 weeks after sowing, maximal soil moisture was observed when barley was in a high density and a pure stand, 7.5% at 5-15 cm and 9.9% at 15-30 cm depth, while for subclover in a pure stand, it was 6.2% at 5-15 cm and 8% at 15-30 cm depth [46]. Taking into account negative effects between intercrops, how to best balance between competition and companion effects is a task for experimental research.

As Altieri and Rosset pointed out, a production system must be designed to reduce nutrient losses by effectively containing leaching, runoff, and erosion, and improving nutrient recycling mechanisms [40]. Diversity is a natural design, while monoculture is an anthropogenic creation. So intercropping should be organized according to natural laws. The cultivars within a cropping system must be suitable for the local climate and soil conditions. Under the circumstances of intercropping, cooperation between different species is also required; at least, competition should be eliminated as much as possible. As the main competition between two crops is canopy competition and root system competition, the principles of a well-managed intercropping system can be summarized as follows:

Tall and short crops are growing together to minimize struggle for sunlight and reduce air humidity of the microclimate. Crops with deep and strong root systems intercropped with the species with sallow root systems can reduce underground competition. The density of a major crop should be reduced to adjust the growth of itself and leaving optimum space for another intercrop.

Select different maturity dates to minimize competition as much as possible [47].

When crops are planted together according to these principles, competition between different species will be less than would exist within the same species. The success of intercropping systems at low levels of interspecific competition has also been explained in terms of more balanced and efficient use of soil moisture due to temporal complimentarily in water require‐ ments of the two species [48]. In the case of legume intercropping, high companion effects between the two crops are caused by biological N fixation, producing N that benefits the target crop and offering soil cover. Simultaneously, the negative effects of legumes on the major crop should be reduced by a well-managed system.
