- François-Xavier Branthôme
A crucial issue for growers
In most cases, RO and D can be negligible, when drip irrigation is used. The ETc, then, is the primary factor to extract water from the soil by plant transpiration which is beneficial, and soil water evaporation which is pure loss of water. During early stages of crop development the soil evaporation portion can be significant and can represent more than 50% of the ETc. When the crop is well established and the plant cover protects most of the soil, the proportion of water from the soil lost to evaporation becomes minor. In order to reduce the amount of evaporative water from the soil, a barrier must be placed between the soil and the atmosphere, where soil mulching can perform well, Many experiments reported higher yield and higher soil water content when using soil mulching, where the main impact of the mulch was to reduce water evaporation from the soil and maintain more water in the soil. Lower water evaporation allows reduced irrigation, mainly in the early growing stages when most of the soil is exposed.
The results presented in the previous graph (Exh. 3) shows higher water content in the topsoil layer in a potato experiment conducted with different periods of mulching compared to no mulching control. However, not all types of soil mulching have the same efficiency in reducing water evaporation from the soil surface. In an experiment conducted on baby corn, two types of soil mulching were tested: rice straw and plastic mulching, compared to no mulching. In addition to mulching types, different water quantities were tested: 120%, 100% and 80% from reference ET (ET0). The results showed that soil mulching had higher yield than no mulching in all irrigation rates. In addition, plastic mulching was found to have higher yield than rice straw mulching, indicating higher water use efficiency.
The yield results presented in Exh. 4, show that corn yield from the plastic mulched treatment and irrigation of only 80% of ET0, was higher than the 120% irrigation rate with the un-mulched treatment, saving 30% in water and increasing yield by 28.6%
Weed and pest control
One of the key impacts of soil mulching is weed control in the field. Infestation of weeds in fields are challenging for the grower, since they compete with the crop for water, light and fertilizer resources. Weeds increase soil water depletion since they consume water, and since they are opportunist plants, they develop faster than the crop itself and, delay its development Weed control is one of the farmers’ main challenges during the growing season. Weed control methods are chemical spray application or occasional mechanical or manual weeding. Those methods are costly, labor intensive and not sustainable due to chemical usage. The most sustainable weed control method is the prevention of weed germination to begin with. Like other plants, weeds also need soil, water and sunlight to grow, so any curtailment of those resources can reduce their population. Soil mulching will not prevent weeds from extracting water, but it does form a barrier between the soil and the atmosphere, thus preventing sunlight to penetrate, and it physically prevents weed growth. Soil mulching can drastically reduce weed pressure in the field, up to 100% if ail the soil surface is covered by mulch. In most cases, only the elevated beds where the crop is planted is covered with mulch, with the furrows between the beds remaining exposed. Those furrows can be easily treated by mechanical weeding.
In a corn trial comparing two mulching methods, plastic and rice straw, plastic mulching was found to reduce the weed bio-mass by 37.4 and 63.8% compared to rice straw mulching and no mulching control, respectively. The results from this experiment shows that mulching was effective in reducing weed pressure, but plastic mulch was more effective due to better prevention of light penetration in the soil and thus reducing weed germination.
The experiment shown in Exh. 5 compared two organic mulching methods (wheat straw and pine needles) and black plastic mulch to no mulch control. All the mulching methods reduced weed pressure, but plastic mulching had the lowest weed biomass of all.
Natural materials, however, are often not available in adequate amounts, their quality is inconsistent, and they require more labor for spreading. Natural mulches do not always provide adequate weed control; on the contrary, they may carry weed seeds and supply enhanced growing conditions for those weeds.
Plastic recycling and degradable plastic from mulching
The advantages of plastic mulching are well recognized and help increase yield. However, plastic has a downside: plastic is made of petroleum, usually as polyethylene, which persistence in the environment many years after the crop was harvested presents an environmental issue. One option for plastic disposal is landfill, however, that solution only removes the problem from view since plastic will not degrade for years. Another option for plastic disposal is recycling. Since plastic is a polymer, it can be reused entirely after treatment in a recycling facility. However, this solution is not as easy as it sounds. The collection of plastics from the agricultural sector (mulching, driplines, netting, greenhouses) is time and labor intensive and the price for recycled plastics not always worth the efforts. In addition, not all plastics can be recycled since they often contain contaminants such as plants and root debris, or UV and other additives that makes plastic unsuitable for recycling. Europe is a world leader in terms of plastic awareness and sustainability, and thus, has many programmes and directives for plastics recycling. One of them is the National Collection Scheme by the Committee for Plastics in Agriculture (CPA), whereby member countries adopting this scheme collect 75-95% or their agricultural plastics for reuse. The challenge in plastics disposal could be resolved if plastics, just like organic compounds, could be degraded in a short period of time. To address the demand for a more sustainable solution for the plastics' persistence in the environment, several biodegradable and photodegradable plastics have been developed. Photodegradable plastic uses sunlight to initiate a chemical reaction in the plastic that transforms it into water (H20) and carbon dioxide (C02). Photodegradable plastics, which are petroleum or starch based, can usually degrade in 46-64days, but their degradability depends on sunlight exposure which can be limited if buried in the soil or covered by the canopy. An alternative to photodegradable plastics is biodegradable plastics, which rely on the soil flora to convert the plastic into nonpolluting products. Petroleum based plastics are usually non-biodegradable since there is no specific enzyme that can break down those polymers. However, starch and cellulose plastics are highly biodegradable and can be composted after use and can replace the petroleum-based plastics for agriculture.
Up to now, some of those products are too expensive and their price cannot justify their use. More and more biopolymers are developed for the plastic industry, in order to meet the growing demand for a sustainable plastic solution for agricultural uses.
Source: Plasticulture 2021 n°140