Mulching has been practiced in India since long time using mainly the crop residues including straw, thrash, stalk leaves, etc., but plastic mulches have come into use due to their inherent advantage of efficient moisture conservation, weed suppression, maintenance of soil structure, raising soil temperature, etc.
Mulching is a practice of covering the soil around plants to improve crop growth and development. Mulch materials may be organic (leaves, straw, grass, etc.) or synthetic (plastic). Plastic mulches have been used commercially for vegetable production since the early 1960s in the United States and in several European countries. Nowadays plastic mulches are commercially used in protected vegetable production.
Vegetable growers often try to extend the growing season for selected crops for the purpose of obtaining a marketing advantage. Although, a variety of vegetables can be grown successfully using plastic mulches, but vegetables like muskmelon, tomato, pepper, cucumber, squash, brinjal, watermelon and okra have shown significant increase in earliness, yield and fruit quality.
The growing season can be prolonged by following early planting during winter season using protective structures and synthetic mulches. Greenhouse farming virtually allows all year-round vegetable production. But whether to go for this system depends upon the crops and their suitability to this production system, increase in the cost of production and probable increase in the sale price of the produce early or late in the season.
Type of Mulches:
(A) Organic Mulches:
Grasses, leaves and straw are some of the materials that can be used as organic mulches. The use of organic mulches does not affect soil temperature to the extent of synthetic mulches. They do not suppress weeds, reduce crusting and preserve moisture. The gradual decomposition of organic mulches can also add organic matter to the soil.
The commonly used organic mulches and their effects on soil are as follows:
(i) Grass:
Grass mulches have a low C: N ratio that may result in later release of nitrogen to the crops.
(ii) Straw:
Straw mulches are time consuming to apply, introduce weeds and create an environment for pests like slugs and rodents.
(iii) Saw Dust:
It has a high C: N ratio and requires nitrogen from the soil for decomposition. It is easy to apply and inexpensive, but should only be used in well-rotten state.
(B) Synthetic Mulches:
There are three primary non-degradable mulch types- black, clear and white or white-on-black, which are commercially used in the production of vegetable crops.
Synthetic (plastic) mulches are expensive which need to be removed and disposed of at the end of the growing season:
(i) Black Plastic Mulch:
It is the most popular mulch as it retards weed growth and warms the soil during winter season encouraging plant growth.
(ii) Clear or Transparent Plastic Mulch:
A clear plastic used as mulch material warms the soil environment often referred to as mini greenhouse effect. However, this mulch requires the use of herbicides to prevent weed growth under it. Additionally it drastically raises the soil temperature affecting, thereby, plant growth adversely. Clear plastic is also used as a means of solarizing soil in the field.
(iii) White Plastic Mulch:
White or white-on-black mulch reflects more light to the plant compared to black mulch and has little or no effect on soil temperature. It keeps the soil temperature down. It is good for establishing crops under hot summer conditions. Besides, white mulch also repels some insects.
(iv) Degradable Mulch:
Degradable plastic mulches have various properties as they combine all the advantages of non-degradable polyethylene mulches with the ability to degrade after having received a critical amount of sun light. It may be bio-degradable or photo- degradable and could offer a solution to the disposal problem associated with plastic mulches.
When the film has received sufficient light, it become brittle and develop cracks and holes. As result small sections of the film may tear off and be blown away. The film eventually break down into small flakes and disappear in the soil. The edges of the mulch covered by soil retain their strength and breakdown more slowly.
Recently, newer photo-degradable products have shown more satisfactory degradation characteristics.
Three major commercial products used in USA are as follows:
(a) Plastigone:
It is an ultraviolet activated and time controlled biodegradable plastic.
(b) Biolan:
It is an agricultural mulch film designed to photo-degrade according to a predetermined schedule into harmless particles, which then biodegrade into CO2.
(c) Agplast:
A photo-degradable material made by lecofilms.
(v) Selectively Permeable or Reflective (Coloured) Plastic Mulch:
Besides black and white, other colours of plastic mulch are also available, including a new generation of selectively permeable and wavelength selective-reflective mulches. Selectively permeable mulches allow certain wavelength of light to pass through the mulch and warm the soil besides discouraging weed growth.
Wavelength selective or colour mulches reflect wavelength of light into the plant canopy to influence plant growth and production or insect population. The reflective properties of aluminum faced plastic interfere with the movement of aphids by reflecting both short and long wavelength light from their surface, disorienting the aphid’s flight.
Similarly, yellow plastic mulch repels the whitefly which spread leaf curl in solanaceous vegetables. Painting the shoulders of black mulch with aluminum paint increases its reflectivity besides being effective in delaying the onset of insect vectors/viral diseases which spread. By using reflective mulches, the growers may be able to harvest marketable produce over longer period of time.
(vi) Photo-Selective Plastic Mulch:
It is recently developed and also known as infra-red transmitting (IRT) mulch. These plastics transmit high proportion of solar infra-red radiation, while blocking most photo-synthetically active radiation. These materials warmer the soil more than black plastic, but less than clear plastics. The IRT mulches retard the growth of weeds including nutsedge. Crops grown on IRT mulch become ready to harvest 7-10 days earlier than crops grown with black plastic mulch.
The potential plastic mulches for the production of high value vegetable crops were recognized during late 1950s.
Advantages of Using Plastic Mulches:
The advantages of using plastic mulches for commercial vegetable production are detailed here under:
(i) Increase in Soil Temperature:
The use of black and clear films increase the soil temperature by about 4-5°F and 8-10°F, respectively up to 2 inch depth of the soil.
(ii) Increase in Plant Growth:
Mulch film is impervious to CO2 which is necessary for the photosynthesis. Research has now shown that high levels of CO2 may build up under the plastic. Because the film does not allow the gas to penetrate, therefore it escapes through the holes punched for the plants and a “chimney effect” is created, resulting, thereby, in abundant availability of CO2 for the actively growing leaves leading to increased plant growth.
(iii) Earlier Harvest:
The greatest benefit from plastic mulches is that these raise the soil temperature of planting beds, especially during the winter season, promoting, thereby, faster crop development and earlier harvest. Clear plastic mulch may advance harvesting in different vegetables by 14-21 days depending upon the outside temperature, whereas black plastic mulch may advance the harvesting by 7-10 days.
(iv) Reduces Evaporation:
Plastic mulches check soil water loss. As a result, more uniform soil moisture is maintained and irrigation frequency can be reduced. The growth of plants can be doubled with mulch compared to that of un-mulched soil in several vegetables, especially in tomato, cucurbits, etc.
(v) Reduces Weed Problems:
Black and white mulches reduce light penetration to the soil, hence weeds cannot survive under such mulches. However, weed growth remains unchecked under clear plastic mulches.
(vi) Reduces Fertilizer Leaching:
Excess water runs off the impervious mulch. Thus, fertilizers beneath the mulch are not lost by leaching and are available for plant use.
(vii) Reduce Soil Compaction:
The soil under plastic mulches remains loose, friable and well aerated. Roots have access to adequate oxygen, and microbial activity is enhanced.
(viii) Root Pruning Eliminated:
Vegetable cultivation is eliminated, except in the area between the mulched strips. Weed growth in these areas can be controlled by cultivation or by use of chemical herbicide.
(ix) Cleaner Vegetable Produce:
The edible product from a mulched crop is cleaner and less subjected to rotting, since no soil is splashed on the plants or fruits. To keep the plants clean, they should be grown on raised beds which should be firm and slightly convex, with the highest point drawn in the centre of the bed and the plastic should be stretched tight to encourage run off of water.
(x) Mulches Aids Fumigation:
Mulches increases the effectiveness of chemicals applied as soil fumigants. Because of the impervious nature of the plastic mulch, it acts as a barrier to gas escape and keeps gaseous fumigants in the soil. For intensive vegetable cultivation, effective soil fumigation is quite necessary for control of several soil borne diseases like Fusarium wilt, etc.
(xi) Helps in Insect Management Strategies:
Use of reflective mulches for reduction of viral diseases in several vegetables has also been found quite effective. The reflective properties of aluminum faced plastics have been found to interfere with the movement of aphids by reflecting both short and long wave-length of light from the surface, disorienting the aphid flight. There are several species of aphids or other vectors for different viruses. Similarly, yellow plastic mulch also reduces the incidence of leaf curl virus in tomato, chilli and pepper crop by repelling white fly.
Disadvantages of Using Plastic Mulches:
(i) Greater Initial Cost:
The use of plastic mulch increases the cost of production which is especially, due to investment in some specialized equipments like a bed press, mulch layer and mulch trans-planter or plug mix seedier. These costs should be offset by increased income due to earlier harvest, better quality and higher yields.
(ii) Removal and Disposal:
A major problem with plastic mulch is removal from the field after the crop is over. Plastic mulch, especially black plastic does not breakdown and should never be disked into the soil. Clear plastic does breakdown over time, but small pieces may remain in the field for several years. To overcome this problem, several new photo-degradable, bio-degradable mulches have been developed, which will solve the problem of removal or disposal of the plastic after completion of the cropping season.
(iii) Increases Crop/Weed Competition:
Weeds can grow out of the holes in close proximity of the crop.
Degradation of Plastic Mulch Film:
In protected cultivation, plastic mulch is widely used at commercial scale. Therefore, before the using of plastic mulch we must know about the process of plastic degradation and its effect on soil, plant and human health with its methods of application for vegetable cultivation on commercial scale.
Absorption of plastic films in the soil must be carefully checked for immediate and long term effects on the biomass of the soil, the ground water and uptake of elements into the plant mass. During first stage of degradation, the film is reduced to a fine powder. This powder is totally inert and physiologically harmless to plants, human being and animals. This is followed by continuous shortening of molecular long chain structure of poly-metric material.
Plastic mulches can be used on large scale for not only the increasing productivity of different vegetables, but also for increasing the quality of the produce without environmental pollution.
Application of Plastic Mulches:
On commercial scale, mulch is typically laid down by machine but it can also be done manually, although it is labour intensive.
There are basically three operations involved in applying the plastic mulches viz.,:
(i) Bedding and Pressing the Soil
(ii) Application of the Mulch, Drip Lines and Fumigation
(iii) Laying of the Mulch
These can be accomplished as separate operations or in combination:
(i) Bedding and Pressing the Soil:
Now-a-days several machines are available to the growers for bed preparation and pressing the soil like single and multiple row models, rotavator, etc. First the soil is raised in one direction and then the beds are compressed according to the soil condition and type.
The beds should be spaced with their centres 1.5 to 1.8 meter apart depending upon the equipment. Beds of 8-12 cm in height and 80-90 cm width with the slopes from the centre to the edge of 3 cm are commonly used for commercial vegetable production. Such a slope will allow excess rainfall to run off the mulch.
(ii) Application of the Mulch, Drip Lines and Fumigation:
Soil must contain adequate moisture for seed germination when the plastic mulch is laid. The soil temperature should be at least 10°C, it should be well worked and free from undecomposed plant debris if a fumigant is used. Drip irrigation is most effective and suitable method of irrigation under mulched crop.
Therefore, the drip lines are laid after preparation of beds with the help of tractor or manually as per need of the vegetable crop. Generally, for tomato, brinjal and cucurbits, a single drip line is laid, whereas two lines are laid for cole crops and three lines for onion and garlic crop on 80-90 cm wide beds.
For single-row crops such as tomato, cucumber, muskmelon, watermelon and pumpkin, a drip irrigation tube should be placed 4- 5 inches from the centre of the bed and 2-3 inches deep with the hole facing upward. In double-row crop of summer squash, okra, brinjal and pepper, the drip tube should be placed directly on the centre of the bed and buried 2-3 inches deep.
Fumigants are primarily used to control nematodes and soil borne diseases, which should be applied done before laying the plastic mulch. The fumigants escape through plastic mulch in 12-14 days during hot weather when the soil is also warm.
(iii) Laying of the Mulch:
After laying of the drip lines on beds, the plastic mulch of our choice is covered or laid on the beds with the help of tractor drawn machine or manually. The plastic mulches are available in rolls of 1.2-1.5 meter wide and 800-1000 meter long. The thickness is, usually, 30-100 micron (0.30 -1.0 mm).
The vegetable growers should take time to adjust the plastic laying machine in such a way that the press wheels hold the plastic against the beds firmly and the covering disc places soil half way up the side of the beds, but not on top of the bed. Also, when starting applications, anchor the plastic and drip tube by covering the ends with soil and a person standing on the drip tube.
The plastic mulch can be covered manually in a way that two person can hold the plastic roll with the help of a pipe and two person can press the plastic by putting soil with spades. If machine is used for laying the mulch then the speed of tractor must be slow and care should be taken that sticks, sharp pebbles or any such items may not puncture the plastic. Rows should be straight and the soil should have even touch with the mulch throughout to avoid air trapping.
After spread of the mulch, the plastic can be punched exactly on drippers at the required planting distance and the crop is transplanted accordingly. The vent should be sufficient only for transplanting of the seedlings or sowing of the seeds as the case may be.
Planting of the Crop in Plastic Mulch:
Vegetable crops can be established in plastic mulch by transplanting of seedlings. Transplanting can be done by hand or with a mechanical trans-planter or semi-automatic trans-planter. For extra earliness of green pepper and tomato, large container cells with their diameter of 5-8 cm should be used. For other vegetable crops, a cell size of 2.5-5.0 cm is recommended.
When plants are being planted/sown by hand, several tools can be used to make the holes in mulches. A long handled bulb setter or a sturdy can or steel cylinder welded onto the end of a handle can be used. The size of hole should be 8-10 cm wide and deep enough to hold the plants. Before planting, dip the roots of seedlings in a ‘starter solution’—a soluble fertilizer high in phosphorus to get good start of plants. A suitable ratio of NPK in the starter solution is 1: 2: 1.
Weed Control:
Only approved herbicides can be used between rows of plastic as this is not fallow area. A cultivator or hoe can be used to control the weeds but the grower must be sure not to rip or tear the mulch.
Irrigation:
Drip irrigation is recommended for use with plastic mulches, although other types can be used successfully. Irrigation frequency will depend on the season, type of soil, crop to be grown and stage of the crop growth. Plastic mulch should not be used without irrigation.
Double Cropping with Plastic Mulch:
Once the first crop has been harvested, it is recommended that a second crop can be grown on the existing mulch. This intensive cropping produces two crops from a single annual expenditure of mulch and drip irrigation line. The herbicides like Glyphosate or Paraquat can be used to destroy the rouble of first crop quickly, or it can be removed manually.
Care should be taken not to damage the drip tubes and during planting of the second crop. The second crop is fertilized through drip irrigation (fertigation) using water soluble fertilizer and a fertilizer injector. Never plant a field to the same crop twice in one year. Disease and insect cycle can best be broken by adopting three or four year’s rotation.