How was a biodegradable but sufficiently durable paper mulch developed commercially to replace slowly degrading plastic? Senior Scientist Antti Korpela explains the research and development behind the product.
In terms of mulching, Finns are probably most familiar with the black plastic used on strawberry fields. The main purpose of mulching is to prevent weed growth, even out temperature and moisture variations, and prevent erosion. An estimated 90,000 km2 of arable land are covered in plastic mulch each year. Due to the huge benefits, use of this technique is expected to continue spreading.
Plastic film has unpleasant side-effects
But plastic mulch causes a major problem related to its main raw material, polyethylene. Polyethylene is inexpensive and can be used to manufacture a thin and mechanically strong plastic cover. However, it also degrades very slowly on farms. To avoid a deterioration in the quality of arable land and avoid the transfer of plastic waste into the environment, plastic mulch must be carefully removed after use. However, this is laborious, often expensive and not always a complete success. Soil becomes attached to the plastic, making the mulch unsuitable for combustion in incinerators. Recycling, at least on a large scale, has not proven to be economically viable. This means that discarded plastic mulch ends up – expensively – in landfills. Environmental hazards are generated around the world when plastic waste is piled up on the edges of arable land and burned in fields.
Biodegradable plastic mulch, which can be left on the soil after the growing season, has been developed to replace polyethylene. While biodegradable plastics enable farmers to avoid the trouble and expense of collection and disposal after use, they are seldom used due to their high price. They are most commonly used in areas where the costs of collecting and disposing of conventional plastic mulch are very high.
New use for paper machines
VTT and the University of Helsinki’s Department of Agricultural Sciences launched the Agripap project in 2010 , to develop methods of manufacturing paper mulch to replace plastic mulch. Stora Enso, UPM, Walki, Kemira and the agricultural machinery specialist Avagro, also participated in the project, which was part-funded by Tekes.
Paper manufactured from wood fibre is inherently biodegradable and, as estimated at the beginning of the Agipap project, can form a highly competitive alternative to biodegradable plastic mulch in terms of its price per square metre and metre. The global market is very large: around 5–6 million tonnes of paper mulch a year would be needed to cover 90,000 km2.
My colleagues and I familiarised ourselves with plasticulture at the beginning of the Agripap project. Specialists in the physics of paper, the chemistry of papermaking, biodegradation, product safety and environmental accounting were involved in our project. I, at least, made intensive use of a dictionary, as agricultural terms came thick and fast in various texts and presentations. The project steering group also came up with some questions: What is a mulch laying machine and how does it work? What is a raised bed and why is it created? What is a drip hose? Luckily for us, Professor of Agrotechnology Jukka Ahokas was able to explain the terms.
The project included a high number of laboratory studies and field trials. Field trials were conducted on experimental crops in Finland, Turkey and Spain; at the early stages, it became clear that no ordinary paper would serve as mulch.
Only durable and slowly decomposing paper works as mulch
Paper mulch has two critical features: First, the paper has to be durable enough for machine laying onto a field. When being laid, the paper is subject to strong mechanical stress, which it must withstand without tearing. It would be best if paper could be laid using the same laying machines – and just as quickly – as those used for laying plastic.
Picture: Stora Enso
Second, the paper needs to decompose slowly enough to remain intact until the end of the growing season. The edges of the paper, onto which soil is piled to bind the paper cover to the ground, are particularly prone to decomposition. As the edges weaken, the wind can blow the paper mulch off the ground.
The study revealed that making paper which is mechanically strong enough, or that decomposes slowly enough, is easy. But manufacturing paper with both qualities at once is difficult. For example, ordinary sack kraft paper would be suitably strong, but on humid and warm arable land it almost completely decomposes in just 3–4 weeks. Paper made of lignin-rich fibres is much more resistant to decomposition, but producing sufficiently strong paper mulch from such material is difficult.
Stronger, slowly decomposing paper can be made using various chemical additives, but because crops – such as strawberries or lettuce leaves – may be in contact with the material, all raw materials and additives have to be completely safe for humans and the environment. It is recommended that paper mulch meets the strict product safety requirements set for food packaging materials. This rules out many strong additives which would improve the strength of the paper or reduce its rate of decomposition.
Some of the companies involved in the project continued developing the paper after the end of Agripap in 2014. They have explored the practicality of the trial paper mulches on agricultural land at home and abroad. The know-how of institutions such as the MTT (now the Natural Resources Institute Finland – Luke) has been of major assistance to the companies.
For global markets
The development of paper mulch meant learning new things for the companies involved: such as the country and region-specific analysis of farming practices and needs, and sales and distribution planning. The market is global.
A few years ago, I met two product developers from a North American paper manufacturer at the Agricultural Film Conference, where we were gathering information on plasticulture and plastic mulch. They said that paper mulch would be of interest to their company, since there could be big markets for the product, but there was “just too much to learn” about making and selling such paper. This was followed by a deep sigh.
In the spring of 2016, Stora Enso launched a mechanically laid paper mulch for large-scale farming. On my own behalf and that of my colleagues, I would greet this by saying “Well done Stora Enso – what a great achievement!” I hope all that you have learned and your years of investing in paper mulching pay off!
Antti Korpela, Senior Scientist
Picture: Stora Enso