India has been bestowed with wide range of climate and physic-geographical conditions and as such is most suitable for growing various kinds of vegetables. They are important food and highly beneficial for the maintenance of health and prevention of diseases. Vegetables contain valuable food ingredients which can be successfully utilized to build up and repair the body. Adequate vegetable consumption can be protective for some chronic diseases such as diabetes, cancer, obesity, metabolic syndrome, cardiovascular diseases, as well as improve risk factors related with these diseases. Vegetables may be edible roots, stems, leaves, fruits or seeds. Each group contributes to diet in its own way (
Robinson, 1990). Most of the vegetables are quick growing and ready for harvest within a short time enabling the grower to practice succession cropping and inter cropping and thus provide more profit compared to cereals. Although, the production cost of vegetables is high and needs heavy inputs like fertilizers, insecticides-pesticides, irrigation water, weeding and other cultural operations. Even though, the establishment of seedlings in agricultural filed is a tedious job in water limited regions which is considered as a dwindling natural resource. In hot dry and semi-bone-dry regions, use of mulching in vegetable cultivation has been prompted in order to cut-down the expense of production and acquire quality produce, return boost with increased benefits.
The word mulch has been derived from the German word molsch means “easy to decay” and mulches have widely been used for vegetable production since ancient times (
Lightfoot, 1994). Mulching is referred as spreading various covering materials on the surface of soil to minimize moisture losses and weed population and to enhance crop yield (
Nalayini, 2007;
Kader et al., 2019), i.e., it is the process or practice of covering the soil/ground to make more favourable conditions for plant growth, development and efficient crop production. Organic-based mulches such as plant waste, straw, sawdust and manure have also been used to a great extent for vegetable production. Traditionally, organic mulches have consisted of materials which are locally plentiful (
Coolong, 2012). Organic mulches are cheap materials; therefore, the cost of mulching is also economical
(Ranjan et al., 2017). Continuous use of mulches is helpful in improving the organic matter content of the soil, which in turn improves the water holding capacity of the soil
(Agrawal et al., 2010). It prevents rapid evaporation from the soil surface and reduces quick drying thereby conserving soil moisture. The purpose of this review is to provide a comprehensive analysis of the scientific research on the benefits of mulches used in vegetable crops.
Types of mulches
The type of mulch to be used depends on the type of weed, type of soil/topography, prevailing weather conditions of the area, crop to be cultivated and the availability of mulch. Basically, there are two types of mulches depending upon the material used as mulching. They are as under:
Organic mulches
In organic mulching, suitable animal or plant residue such as bark, wood chips, dry grasses, paddy/wheat straw, dry leaves, saw dust, grass clipping, farm yard manure
etc., is used to cover soil so that it will enhance the growth of crop and also improve the condition of the soil. As these mulches slowly decompose, they provide organic matter which helps keep the soil loose. They serve as food for many microorganisms in the soil. These organisms are necessary for maintaining and promoting soil granulation
(Barche et al., 2015). This organic matter becomes food for the beneficial earthworms and other soil micro-organisms in the soil and create a very good porous soil
(Patil et al., 2013). Expense and logistical problems have generally restricted the use of organic mulch in vegetable crop production with only limited use on a large commercial scale. Natural materials cannot be easily spread on growing crops and require considerable human labour (
Bhardwaj, 2011).
In-organic mulches (Plastic mulches)
Plastic mulches are one of the important components of plasticulture and have been used commercially for the production of vegetables since the early 1960’s, and their usage is still increasing throughout the world. Polyethylene is one of the most commonly used plastic materials for mulching, due to the fact that it is easy to process, has excellent chemical resistance, high durability, flexibility and is odorless as compared to other polymers
(Helaly et al., 2017). As far as color of the plastic film used for mulching is concerned, black color film (black plastic) was found to be the best in most the crops and seasons
(Solia et al., 2018 and
Kumar and Sharma, 2018). Different plastic mulches
i.e., red, black, yellow, green, brown
etc. are used in vegetables (
Orzolek and Lamont, 2015). By proper selection of plastic mulch composition-colour and thickness, it is possible to precisely control the soil environment.
Limitation of mulching
Mulching also shows some limitations beyond many advantages as it may harbor some insects, pests and diseases and also create some weed problems sometimes. Plastic mulches, especially black plastic, do not break down and should never be disked or incorporated into the soil. Many types of organic mulching such as grass and straw contain seeds that may allow to grow weeds and release acid to soil (
Chalker, 2007;
Patil et al., 2013). Juglonic acid is mostly isolated from the bark of black walnut and used in the inhibition of many weeds and non-desirable plants. This acid can also destroy the seedlings and shallow-rooted plants though it has no effect on established plants (
Harris et al., 2004).
Effect of mulching on soil and vegetables
Improve soil health
The practice of applying or retaining crop residues in the field as mulch is imperative to prevent soil erosion, maintain soil quality and improve crop productivity
(Mgolozeli et al., 2020). Mulch is categorized as inorganic or organic with the latter being more commonly favored due to its biodegradable nature
(Lalljee et al., 2013). Organic mulching like crop straw, grasses, sawdust,
etc., improve soil physical, chemical and biological quality by adding organic matter into the soil during the decomposition process
(Yadav et al., 2017). Organic mulches serve as food for many microorganisms in the soil. These organisms are necessary for maintaining and promoting soil granulation
(Barche et al., 2015). Furthermore, it can increase soil organic matter content and plant nutrients. It can also provide an ideal environment for earthworms and useful for soil micro-organisms (
Dickerson, 2000). The practice of applying or retaining crop residues in the field as mulch after harvest is imperative in maintaining soil health and productivity
(Dube et al., 2012). Organic mulch materials are is commonly used in arable systems to improve soil health, but the use of inorganic plastic mulch has gained global importance in recent decades
(Ngosong et al., 2019).
Conserve soil moisture
Water is a major input for agricultural production. In the current situation, it is a scarce resource and there exists a large gap in terms of water available and its requirement for irrigation. It is therefore, necessary to minimize the loss due to evaporation and to conserve the moisture. Evaporation from soil is mainly due to the degree of saturation of soil surface, temperature of air and soil, humidity and wind velocity. Several factors are greatly influenced only by the vegetative cover. Therefore, the only way to conserve the moisture in such condition is to spread mulch over the crop
(Agrawal et al., 2010). Adoption of innovative irrigation techniques can also increase the efficiency of water usage by minimizing deep percolation losses. Mulches could potentially minimize water runoff, improve infiltration capacity of soil, restrain weed population
via shading and perform as obstacle in evapotranspiration
(Rathore et al., 1998). In addition plastic mulch helps in shedding excessive water away from the crop root zone during periods of excessive rain fall. This can reduce irrigation frequency and amount of water used. The mulching techniques were being used widely in irrigated crop production worldwide.
Ramakrishna et al., (2006) noted that evaporation from the soil accounts for 25-50% of the total quantity of water used. The notable advantage of the use of plastic mulch is its impermeability which prevents direct evaporation of moisture from the soil and thus reduces the water losses. Mulching experiment conducted on brinjal crop with black polyethylene revealed that it conserved 29-56 and 22-107% more moisture as compared to straw mulches and control, respectively
(Singh et al., 2006). Likewise,
Agrawal et al., (2010) found that among the mulch covers the water use efficiencies of 0.57, 0.42, 0.37 and 0.21 q/ha-mm was observed in red, black, white mulch and control plots respectively. In that treatments water use efficiency was increased by 63, 50 and 43% in red, black and white mulch respectively as compared to the no mulch plot.
Leaf mulch of local grasses like sevan (
Lasiurus sindicus), kheep (
Leptodenia pyrotechnica) and lasoda (
Cordia myxa) with brinjal under hot arid conditions conserved higher moisture content in the range of 33-100% compared to un-mulched plots
(Awasthi et al., 2006). Manyatsi and Simelane (2017) also observed that significant difference between the treatments in terms of the moisture retention properties. Organic compost mulch recorded the highest mean percentage moisture retention (22.9%), followed by treated sewage mulch (20.9%) and no mulch recorded the lowest percentage moisture retention in the soil (14.4%). Similarly,
Mahadeen (2014) examined that water use efficiency in agriculture is mostly increased by mulching process. Covering of soil with different mulch materials is an efficient method for soil and water conservation along with weed management; it was examined by
Bobby et al., (2017). Ashrafuzzaman et al., (2011) observed that transparent polythene mulch apparently showed highest moisture (21.1%), followed by black (20.4%) and blue (19.2%) polythene mulch. The lowest moisture (14.6%) was recorded in the control plot. Increased moisture retention capacity due to mulching with polythene could be attributed to less evaporation from the soil.
Maintain soil temperature
Agricultural management practices, such as mulching and irrigation can change the characteristics of the soil surface and hence influence the hydrothermal properties of the soil. Straw mulching system can conserve soil water and reduce temperature because they reduce soil disturbance and increase residue accumulation at the soil surface
(Zhang et al., 2009). Mulch is an excellent insulator and prevents drastic fluctuations in soil temperature. It keeps the soil cooler in summer and warm in winter (
Musie et al., 2015). Wheat straw mulch raised the soil temperature by 2-3°C in peak winter season (
Sarolia and Bhardwaj, 2012). At night, condensation on the underside of the mulch absorbs the long wave radiation emitted by the soil thereby slowing cooling of the soil (Lamont, 2005). Similarly,
Maida and Kumar (2020) study indicate that soil temperature under the various coloured mulches was 2 to 4°C warmer compared to bare soil. The highest soil temperature was recorded under black mulch (20.63). Experiment conducted in the hot arid conditions of western Rajasthan, opined that in general, straw mulches applied curtailed soil temperature by 1.1-5.6°C during warmer months, while an increase in soil temperature by 0.6-3.2°C at 20 cm depth was recorded during the winter months of December-January
(Awasthi et al., 2006). Rajablariani et al., (2012) also studied the effect of coloured plastic mulches (black, blue, clear, red, silver on black) on tomato and they reported that that soil temperature increased under the various colored plastic mulches about 3 to 6°C more than it in bare soil. Soil temperature was highest under blue mulch followed by red and clear plastic silver on black had the lowest soil temperature among all the plastic mulches.
Improve plant growth parameters
Increase in soil temperature and moisture content stimulate root growth which leads to greater plant growth. The retention of moisture also leads to higher uptake of nutrients for proper growth and development of plants. Therefore, mulched plants usually grow and mature more consistently than un-mulched plants (
Bhardwaj et ., 2011;
Sarolia and Bhardwaj, 2012). In brinal crop maximum leaves/plant, plant height, no. of shoots/plant, shoot length and plant spread was found in black polyethylene mulch as compared to control
(Kumar et al., 2019). Similarly,
Maida and Kumar (2020) observed best results of chilli grown on silver/black mulch.
Ashrafuzzaman et al., (2011) reported the tallest plant (78.45 cm) in transparent, followed by black (77.58 cm) and blue (77.03 cm) plastic mulch in chilli crop, while, the smallest plant (61.15 cm) was observed in control plot.
Tyagi and Kulmi (2019) obtained maximum plant height and number of structural branches/plant in chilli with the application of silver/black plastic mulch as compared to no mulch.
Agrawal et al., (2010) observed that plant height (90.74 cm), no. of primary branches (7.31), stem thickness (1.87), no. of flowers/cluster (7.02), no. of locules (5.17) were greatest in red plastic mulch followed by black plastic, white mulch, whereas, the same characteristics was obtained lowest in control plot.
Promote early harvest
The greatest benefit from plastic mulch is that the soil temperature in the planting bed is raised; promote faster crop development and earlier harvest. Black plastic mulch can give a harvest earlier by some 7-14 days, while clear plastic may advance the harvest date by 21 days
(Parmar et al., 2013). Warm season vegetables such as cucumbers, muskmelons, watermelons, eggplant, peppers, usually respond to mulching in terms of early maturity and higher yields. An early maturity is probably due to maintenance of favorable temperatures during growing season (
Bhardwaj, 2013). Applications of polyethylene films as mulch have shortened growing season and enhanced earliness and yield in different vegetable crops
(Goreta et al., 2005; McCann et al., 2007). Similarly,
El-Nemr (2006) reported earliness in cucumber with use of clear and black polyethylene mulches, compared to no mulch.
Soleymani et al., (2015) noted earliest maturity with use of polyethylene mulch (black and clear). Beneficial effects of mulches on early harvest were also found by
Singh et al., (2017) and
Agrawal et al., (2010) in tomato;
Kumar and Sharma (2018) in summer squash.
Improve quality and yield
Plastic mulches are adopted on large scale in commercial vegetable production due to their large scale effects on gaining higher yield with better quality which might be due to decreased water evaporation, increased soil temperatures and less weed infestation (
Coolong, 2010 and
Gordon et al., 2008). Black plastic mulch is effective in increasing soil temperature (
Mahadeen, 2014) and thereby improved the yield of summer squash by 74% over control
(Bhatt et al., 2011) and transparent mulch helps in soil solarization by increasing soil temperature that lead to yield gain of 25-28% in melon crop over control
(Patil et al., 2013; Ekinci and Dursun, 2009). Black polythene mulch produced significantly 19.0% and 50.3% more fruit yield of brinjal crop over wheat straw mulch and without mulch, respectively
(Kumar et al., 2019). Similarly,
Tyagi and Kulmi (2019) reported significantly maximum fruit length (16.48 cm), fruit girth (1.29 cm), green chilli weight (8.19 g), number of fruits/plant (212.80), dry chilli weight/plant (267.50 g) and dry chilli yield (53.50 q/ha) in chilli.
Kayum et al., (2008) reported that water hyacinth and straw mulches have potentiality effect on increase the yield of tomato, which involves minimum cost of production. The black polyethylene mulching material was noted with significantly enhanced fruit length and fruit weight, whereas, white plastic mulch significantly improved the number of fruit/plant, fruit diameter and total yield of cucumber at both early and late seasons followed by grass-mulch, whereas control consistently produced the highest number of nonmarketable fruits (
Ajibola and Amujoyegbe, 2019). Similarly,
Maida and Kumar (2020) also recorded maximum fruit length (8.74 cm), fruit diameter (0.88 cm), average weight of fruit (4.49 g), number of fruits/plant (155.25), fruit yield/plant (744.14 g), fruit yield/plot (11.82 kg) and ascorbic acid (269.07 mg/100 g) in chilli grown on silver/black mulch.
Chandra et al., (2002) found that potato yield increased with the use of mulches. Tuber weight was 0.23 kg/plant for no mulch but increased to 0.29 kg/plant and number of tubers/plant increased to 6.5 from 5.4 when pine mulch was used. Total yield was 165.0 q/ha for no mulch and 222.7 q/ha for pine mulching.
Nijamudeen and Dharamasena (2002) also observed 32% increase in yield and 28% lower water consumption in chilli due to the application of mulch.
Vankar and Shinde (2007) reported statistically higher yield of fruit (22.06 t/ha), leaves and stem dry matter, number of fruits/plant, weight of fruits/plant, length of fruit and total biomass production in okra with use of white polythene mulch than black, straw and no mulch treatments. Beneficial effect of polyethylene mulch on higher yield was also found for watermelon
(Parmar et al., 2013) and tomato
(Agrawal et al., 2010).
Reduce weed population
Weeds provide major challenge in the cultivation of any crop, universal. They decrease crop productivity by interfering with crop growth. Apart from reducing yield, weeds contaminate and taint farm product to reduce their market values and change their end use. Mulching is an effective method of manipulating crop growing environment to increase yield and improve product quality by controlling weed growth. Mulches typically function by blocking light or creating environmental conditions which can prevent germination or suppress weed growth shortly after germination (
Coolong, 2012). By providing a physical barrier, mulching reduces the germination and nourishment of many weeds (
Vander Zaag et al., 1986).
Luqman et al., (2013) noted that among different forms of mulch (
Rumex crispus,
Silybum marianum, newspapers and sawdust) and herbicide, stomp 330 EC (pendimethalin) the efficiency of mulching treatments was substantially more successful.
Polyethylene mulches are widely used in vegetable production and contributed significantly for reduction of losses due to weeds (Ngouajio and Ernest, 2004). Black plastic films do not allow sunlight to pass in to the soil. Hence, it arrests weed growth completely because photosynthesis do not completed in absence of sunlight
(Barche et al., 2015). In contrast, transparent and blue plastic mulches allow sunlight to pass through therefore it encouraged weed population.
Ashrafuzzaman et al., (2011) recorded highest number of weeds/m
2 in transparent plastic mulch (186.5) and the lowest in black plastic mulch (54.25). The weed population increased 11, 5 and 3 times in transparent plastic, blue plastic and control, respectively compared to black, indicating black plastic mulch was more effective than the other mulches in suppressing weed growth.
Mahajan et al., (2006) also observed significant effect of black polythene mulch on weed control in red chilli in respect of rice straw mulch.
Effect of mulching on insect and micro-flora
Pest and diseases are the major limitations for the crop because of labor and the high costs needed to manage them
(Davis et al., 2008; Sapkota et al., 2010). Plant mulches can be an effective way to provide shelter for predatory insects
(Johnson et al., 2004). Mulch application reduced the severity of some above ground diseases of plants in crops such as tomatoes (Abbasi
et al.,
2002).
Cookey et al., (2016) observed that plastic mulch was significantly effective to the control of root gall nematode infection on cucumber than the control plots. The ability of clear mulches to produce soil temperatures high enough to control weeds, plant pathogens and nematodes forms the basis for the soil solarization process (
Sciortino, 2001 and
Stapleton et al., 2005). Santos et al., (1995) tested transparent polyethylene mulch for its efficacy in reducing insect populations, virus diseases and increasing soil temperature and yield of cantaloup (
Cucumis melo L.) in a tropical region. They observed that transparent mulch reduced whitefly populations, aphids caught in yellow traps and virus incidence, with respect to bare soil (control). Also, soil temperatures in the morning, midday and afternoon were significantly increased in the transparent mulch.
Improve economics
Whenever we use synthetic chemicals, fertilizers and mulches, they estimate the cost and benefit which comes out from the investment. The mulching can be used to solve the problem of weed infestation, it stimulates the microbial activity in soil through improvement of soil properties, it minimizes the requirement of nitrogen fertilizer, warms the soil and suppresses weed growth and as a result increases yield.
Kumar et al., (2019) observed that black polyethylene mulch and wheat straw mulch earn 106.4% and 70.4% more net return, respectively over without mulch in brinal crop. B:C ratio was also 37.0% and 33.3% more with black polyethylene mulch and wheat straw mulch, respectively over control. Similarly,
Tyagi and Kulmi (2019) stated that treatment receiving 30 micron silver on black plastic mulch resulted in higher gross returns, net returns and cost: benefit ratio as compared to without mulch in chilli.
Narayan et al., (2017) reported the higher B:C ratio with double coated black polythene mulch in chilli.
Parmar et al., (2013) also observed that highest net return with maximum CBR in silver on black mulch treatment followed by black on white mulch, whereas, it was lowest in control (no mulch) in water melon.
Kundu et al., (2019) noted that among all the inputs for tomato production technology, labour alone accounts for more than 70% of the cost of operations and it was observed that maximum return can be fetched from black poly mulch followed by jute felt and straw mulch. Similarly, rice straw mulch recorded significantly higher benefit cost ratio (5.65) followed by water hyacinth (5.56), wastage of rice straw (5.01) and no mulch (4.51) in tomato by
Rahman et al., (2006). Lourduraj (2003) also observed that gross return and net returns favorably influenced by plastic mulching compared to unmulched in chilli.