The use of cover crops is important for the physical quality of the soil, both in protecting the surface and in providing phytomass from the aerial part and roots (SOUZA et al. 2014). Furthermore, they accumulate nutrients in the plant material and release them during its decomposition, enabling the maintenance and improvement of soil fertility (SILVA et al. 2014) and stimulating biological activity through the positive interaction between plants and soil microbiota (REIS et al. 2012).
Rossetti et al. (2012) observed improvements in the physical attributes of the surface layer of the soil and an increase in the content of organic matter, which is a source of nutrients for crops, through the use of cover crops compared to fallow land. In a study carried out by Alcântara et al. (2000), the authors found greater accumulations of the elements nitrogen, potassium, calcium and magnesium on the soil surface through the use of legumes as cover crops compared to the use of grasses and attributed this effect to the greater capacity for recycling and mobilization of nutrients in the former.
For the cultivation of cover crops, Pauletti et al. (2009) emphasize that the ability to promote the absorption of nutrients in deep layers of the soil and accumulate them in the aerial part provides benefits for the successor crop after the degradation of the straw on the soil surface, in addition to demonstrating importance in management for weed control (SODRÉ FILHO et al., 2008).
For CHERUBIN (2022), the benefits of using cover crops in agricultural systems go beyond covering the soil surface. Figure 1 demonstrates multiple benefits of using cover crops in improving soil health, that is, improving the physical, chemical, and biological components responsible for the functioning of the soil as a living ecosystem. Above the surface, cover crops promote erosion control by reducing the impact of rainfall, reducing runoff, and increasing soil resistance to disintegration. In addition, these plants help regulate soil temperature, reduce evaporation, and control weeds, whether by competing for light, water, and nutrients, allelopathic effect, or acting as a physical barrier against the emergence of invasive plants.
Figure 1. Benefits to soil functioning provided by the use of cover crops. Source: CHERUBIN, 2022.
The cultivation of cover crops, mainly legumes, contributes to nitrogen fixation through association with microorganisms. In addition to the important role in biological fixation, the vegetative cycle and subsequent decomposition of crop residues regulates nutrient cycling and increases the availability of phosphorus, potassium, nitrogen and other macro and micronutrients for subsequent crops (CHERUBIN, 2022).
Among the summer legumes with potential for use as green manure, the following stand out: crotalaria (Crotalaria juncea), the jack bean (Canavalia ensiformis), the dwarf pigeon pea (Cajanus cajan) and black velvet bean (Mucuna lands) (PEREIRA et al., 2017). According to Foloni et al. (2006), the production of dry mass of the aerial part of crotalaria is not influenced by soil compaction and the species helps to improve the physical characteristics of the soil, in addition to presenting a good supply of macronutrients in the dry mass of the aerial part (LEAL et al., 2013). The jack bean adapts to different soil and climate conditions (TEODORO et al., 2011) and its rusticity qualifies it as an alternative for green manure in regions with acidic, saline, poorly drained and low fertility soils (PADOVAN et al., 2011). The dwarf pigeon pea has a deep root system and allows the recycling of nutrients in deeper layers of the soil (FARIAS et al., 2013). Despite presenting high nitrogen accumulation in the dry mass of the aerial part (BARROS et al., 2013), mucuna-preta has a longer development cycle, which can hinder its use in crop rotation systems (TEODORO et al., 2011).
Among the summer grasses, Sudan grass stands out (Sudanese sorghum) because it presents high phytomass productivity (BORGES et al., 2015). On the other hand, buckwheat (Fagopyrum esculentum), belonging to the family Polygonaceae, is a plant with high hardiness and a short cycle. It stands out for its tolerance to soil acidity and its ability to absorb phosphorus and potassium salts that have low solubility. In addition, it develops well in low fertility soils (KLEIN et al., 2010).
In winter crops, wheat cultivation (Triticum aestivum L.) is the main winter species cultivated in the South region, but recurring problems such as the failure of previous harvests due to climate problems mean that this species does not develop in isolation throughout the cultivation area (ZIECH et al., 2015). Thus, the oat crop (Oatmeal spp.) being a little more rustic, it has the potential to fill areas in the winter period for the purpose of soil cover. In this context, there is still potential to fill areas with barley crops (Hordeum vulgare L.), canola (Brassica napus L.), triticale (X Triticosecale Wittmack) and rye (Dry cereals L.) (ZIECH et al., 2015), among others.
As cited by Silveira et al. (2020), cover crops can be planted in extreme cultivation or in consortia. The use of high-quality winter crops can present a series of benefits to the soil and the subsequent crop (AITA et al., 2001). Among the advantages of consortia compared to isolated cultivation, the greatest production of dry matter, of the aerial part and roots, accumulation and recycling of nutrients and soil protection stand out. Ziech et al. (2015) used winter cycle plants as cover: black oat (Avena strigosa Schreb.); ryegrass (Lolium multiflorum Lam.); rye (Dry cereals L.); white lupine (Lupinus albus L.); common vetch (Vicia sativa L.); forage turnip (Raphanus sativus L.); and consortia between black oats + common vetch (A+E) and black oats + common vetch + forage radish (A+E+N). And as a result they obtained that black oats (Avena strigosa), in single and intercropped cultivation, has a high potential for soil coverage 49–50 days after sowing, with a dry matter (DM) contribution of more than 2,600 kg ha-1 on the soil surface. Still the consortium between black oats, common vetch (Vicia sativa) and forage turnip (Raphanus sativus) (A+E+N) provides plant residues on the soil surface with a balanced C/N ratio compared to single crops. And pure grasses and consortia with grasses have greater potential for soil protection, due to the residues remaining on the surface. In addition, the consortium between A+E+N presents intermediate decomposition to single crops and promotes the maintenance of at least 1,045 kg ha-1 of straw on the soil 120 days after its management.
The advent of direct seeding has brought great advances in reducing water erosion in agricultural areas, however, in many situations there is a lack of soil cover (WOLSCHICK et al., 2016). In general, monoculture or crop succession aggravate this problem, which could be solved by using soil cover species before commercial crops (WOLSCHICK 2014).
The application of the concepts of no-till farming and sustainability with the use of plants for soil protection or for green manure and nutrient cycling in agricultural areas has become an important practice both in winter/summer and in the off-season (PEREIRA et al., 2017). The subsequent cultivation of species without crop rotation considerably reduces the deposition of plant residues on the soil surface and results in reduced levels of organic matter (PERIN et al., 2000) and the degradation of physical, chemical and biological characteristics of the soil, with direct damage to the environment (ERNANI et al., 2001).
Among the cover crops used as green manures, legumes have the ability to biologically fix nitrogen and make it available to the succeeding crop (PEREIRA et al., 2017). However, because they have a low C/N ratio and a high rate of plant biomass decomposition, they have a shorter soil cover time compared to grasses (TEIXEIRA et al., 2009). In turn, grasses have a high degree of rusticity, high accumulation of green matter (BARRADAS, 2010), act as regulators of soil temperature and humidity, and reduce the risk of erosion (BRANCALIÃO & MORAES, 2008) due to the high C/N ratio and lower rate of plant biomass decomposition (ALVARENGA et al., 2001). Thus, green manures are important for agriculture because they promote the rapid cycling of nutrients, favoring their use by the crop in sequence, especially those elements with leaching potential such as nitrogen or those that can be retained in weathered soils, such as phosphorus (RODRIGUES et al., 2012).
The use of the no-till system, when managed following its basic principles of minimum disturbance, maintenance of the soil permanently covered and crop rotation, constitutes a technological complex capable of ensuring soil and water conservation in Brazilian soils (DE MORAES et al., 2016). The layer of straw on the soil surface acts as a physical barrier to water loss through evaporation due to the reduction in maximum soil temperatures.
According to what was described in the text, it is possible to observe the importance of cover crops and the way they impact the quality and increase in soil fertility, making the use of this management practice essential to obtain healthier soils capable of allowing crops to express their maximum productive potential.
Bibliographic references
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Authors
- Agr Eng. Dr. Angélica Schmitz Heinzen
- Agricultural Eng. Msc. Carolina Custodio Pinto
- Agricultural Eng. Msc. Thiago Stella de Freitas
