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Advantages of soil mulching in agriculture (part 1)

26/08/2021 - François-Xavier Branthôme
A crucial issue for growers

Mulching is a well-established practice in annual and perennial crops, aiming to form a physical barrier between the soil and the atmosphere, allowing water evaporation reduction, increasing water content in the soil, reducing weed pressure, maintaining high soil temperatures and generally inducing better growing condition for plants and enhanced yield compared to no-mulching. Soil mulching initially used organic substances such as animal manure, plant residues and other farm organic byproducts that were spread over the soil surface. In some cases, paper mulching was used, but the short time of decomposition and the cost of paper and labor made this solution ineffective for agriculture. Plastic mulching, which dominates the mulching methods nowadays, was first used in the early 1950s for vegetable crops and is now used for many other crops that can benefit from its advantages. Plastic film made up about 28.8% of all plastic products for agriculture in the USA in 1994, used for greenhouse cover and mulching. Total plastic consumption for agriculture in the USA has increased from 225 million tonnes in 1995 to about 450 million tonnes in 2010. In Western Europe, the total plastic used for agriculture in 1997 was 700,000 tons, of which plastic films for mulching and silage films were about 25%.

A typical plastic film for mulching is 0.0152 to 0.0509 mm {0.6-2.0 mils (mils = thousandth of an inch, or 25.4 micrometers)) thick and can have different colours or be a transparent film with variable levels of opacity. The choice of the plastic film colour and opacity level need to be considered upon usage. Black plastics are generally used to increase soil temperature early in the season, allowing early planting and crop development. White plastics absorb less radiation and therefore can lower soil temperatures in hot regions and clear plastics can be used for solar soil disinfection. Most plastic mulch can reduce weed emergence and the need for chemical or manual weeding. Plastics in agriculture are not limited to soil mulching, they are also widely used as covering membranes for green houses, nets to keep pests out, pipes and drip-lines for irrigation. All those products and more in the agriculture sector, enhance yield by improving the plants' ecosystem.

The source of energy for plants is sunlight, which for humans seems like a simple white light that enables us to see things during the day. In fact, our eyes are very limited and allow us to see only a portion of the entire light beamed from the sun. Sunlight consists of a wide range of electromagnetic wavelengths called the electromagnetic spectrum. This spectrum starts from the very short wavelengths of Gama rays (1 Pico meter) which are very destructive and dangerous since they carry a lot of energy, and at the other end of the spectrum are long wavelengths like the radio waves (1-1000 Km) that carry low energy and are not dangerous. In the middle there are many wavelengths, such the X-rays (1 nano meter) that we use in hospitals, or the thermal bends (100 micrometer) that are used for thermal imaging. In the middle of this spectrum, there is a narrow range of white light between 400-700 nanometers that the human eye can see. This range is made up of all the colours from violet (400 nanometers) to red (700 nanometers), but we cannot really separate them. Plants are also a "blind" form of life like us and they use (or see) fewer wavelengths than we do. Plants use blue light (400-600 nm) and red light (600-700 nm) for the photosynthesis process, but those are not the only wavelengths they use. Plants also need heat in order to crop up, develop and produce yield. In cold regions, the growing season can be very short due to short days and low temperatures, while in other regions, high temperatures can be also an obstacle for plant development. The use of mulching on the soil surface can alter the temperatures of both below and above mulching, meaning, the soil temperature and the plant microclimate temperature. In addition, by selecting the right type (colour), mulching can also affect the photosynthetic radiation (PAR) reflected by the mulch and improve plant performance.

Duration and performance
In a potato experiment conducted in China, two types of plastic mulch were examined (black and transparent) and compared to no mulching. The results from this two-years experiment showed that the black mulching had higher soil temperatures, as well as higher radiation reflected from the black mulching which enhanced plant growth (Exh. 1).

Another experiment conducted on potatoes in China tested the time effect of plastic mulching, to examine the duration needed for mulching during the growing season. The researchers used transparent plastic for durations of 65, 85 days after planting (DAP), throughout the season, mulching and no mulching control. The two years experiment showed that the soil surface temperatures of the topsoil layer were up to 3.2°C higher with mulched treatments compared to the control during the early season. Those small differences were significant for the emergence and establishment of the crop during the cold days of the early season and emergence took place earlier by 8-12 days with the mulched treatments. Later in the season, the temperature differences were less notable and even negative with the mulched treatments, which implies that mulching is important during the early stages of the growing season during emergence and crop establishment (Exh. 2). The yield results from this experiment showed a yield increase of 92% for short-term mulching (65 DAP) that was higher than the whole season mulching treatment. The removal of mulching after the early growing stages, managed to maintain adequate soil moisture and temperatures, allowing longer season and better tubers bulking.
Despite the above, not all mulching methods can increase soil temperatures as plastic mulching does (black or transparent). Plant residue or wood chips are also used for soil mulching as alternatives to plastics. The effect on soil temperature with organic mulching, however, can even lower soil temperatures in comparison to plastic mulching and even bare soil. Organic mulching prevents heat flux to penetrate the soil and no thermal conductance occurs in the topsoil layers. That type of mulching can be used for hot regions where soil temperatures are relatively high and can damage crop performance, similar to white mulching.

Soil moisture
The water balance in cropping systems can be described with the equation below: 
ETc = l + R + ΔS + RO +D, 
ETc: crop water requirement (loss by transpiration and evaporation of water from the soil
I: irrigation
R: rain
ΔS: changes in soil water content
RO: surface runoff
D: deep water drainage below root zone

The second part of this special feature will be made available shortly on the Tomato News' website.

Source: Plasticulture 2021 n°140
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