Grantham Director Prof Tony Ryan OBE believes that plastic could be the solution to soil pollution
UN World Soil Day asks us all to be the solution to soil pollution. As a plastics scientist, it might seem there’s little I can offer to help soil, as plastic is widely considered an ecological evil. But a well designed and implemented plastic technology could contribute enormously to soil health.
Soils are a limited natural resource and their role in food security is crucial. Increasing food production without causing yet more damage to soil is a classic ‘wicked’ problem – but one we have to tackle. The best thing we could do for soil is to leave it alone to look after itself – but we don’t have that luxury – we’ve more than 7 billion people to feed.
With the combined effects climate change, soil degradation and erosion, farmers struggle to protect soil health whilst providing enough food. Uniform field sizes combined with ploughing and tillage, means that much of what the farmer introduces to improve productivity ends up in the wrong place.
Large scale surface irrigation has a whole host of associated environmental problems – erosion, compaction, eutrophication or run-off – that come with a lack of control. Despite this, the 16% of agricultural land that is irrigated produces 40% of the world’s crops: irrigation works – but we need it to work better.
Advances in plastic materials and polymer processing techniques enable a type of precision horticulture impossible either at scale or in an economically viable way with metal, glass and ceramics or by using the original methods from antiquity with perforated clay pipes.
For example, drip irrigation – using plastic pipes – can save water and decrease the need for fertiliser compared with conventional flood irrigation. This modular technology has efficient seals to prevent leakage and flow can be controlled at individual plant level if needed. This maximises the effects of water and nutrients by delivering them into the root zone at the optimum rate, keeping nutrient run-off and water evaporation to a minimum.
The combination of drip irrigation and plastic mulches (covering the soil with plastic sheets), brings even more benefits. Weeds are suppressed, meaning herbicide applications are reduced or even eliminated, and water evaporation is further reduced. In higher latitudes black plastic absorbs the sun’s heat more effectively and this allows earlier planting as well as encouraging faster growth; at lower latitudes or in mid-summer, white mulch can reflect the sun’s rays reducing the temperature of the soil and allow cultivation that would not be otherwise possible. The reduction in weed quantity means that there is less need for ploughing, one of the major factors in soil carbon depletion.
Reduction in weeds means less weedkiller use. But some plants killed using weedkiller are planted deliberately. A lot of soil conservation strategies use cover crops in winter or in fallow period to protect soils before planting crops by directly drilling the seeds into soil or with minimum tillage/ploughing. To do this, the cover crop needs to be killed off and at the moment that means glyphosate! Using plastic sheeting instead could provide a major solution allowing soil conservation agriculture to move away from herbicides.
Another benefit of plastic mulches is the reduction in compaction caused by the crusting action of sun and rain. The soil under the mulch stays loose and well aerated, allowing more oxygen and microbial activity.
Polytunnels and hoop houses are another plastic invention that could be the solution to soil pollution. Constructed from polyethylene they are an evolution of the glass greenhouse – but they are cheap, flexible and easy to move. Every factor influencing a crop can be controlled in a polytunnel and every soil benefit of plastic mulch can also be delivered.
Polytunnels can be used to create warmer, cooler, moister or drier environments and can also protect crops from intense heat, bright sunlight, strong winds, heavy rain and cold snaps. This allows fruit and vegetables to be grown in the off-season, or even in regions that would not otherwise allow that type of crop production – reducing the need for fruit and vegetables to travel overseas.
But what about the plastic? It’s production currently only uses 4% of the world’s oil and gas, so what we need to do is stop it, and put these natural resources to better use. We need to make better, more thoughtful use of plastic.
The overall sustainability of all the plastic technologies to benefit soils needs a proper life-cycle and environmental risk assessment, not only in terms of long term soil quality and ecosystem services, but also the fate of the plastic itself. The drip irrigation and polytunnel materials are easy enough to recover and either reuse or recycle. But some of the plastic mulch will inevitably end up in the soil to become microplastic – we need to find ways to mitigate this.
We need to develop a circular plastics economy whereby we recover and reuse all of the material used in intensive agriculture, and good materials selection and design can enable this. Then will we be able to solve one problem, soil pollution, with what is seen to be another problem, plastic.