Groundcovers, a tool for sustainability by caring for soil

"Groundcovers, a tool for sustainability by caring for soil"

By José Alfonso Gómez

Cover crops are a specific case of the broader concept of intercropping, a common practice in traditional agriculture and especially in agroecological systems, which seeks to leverage synergies arising from the simultaneous cultivation of more than one species. A common case is combining woody and herbaceous crops to increase the resilience of the system and improve the use of resources.

In the specific case of cover crops, these are crops whose primary objective is to protect and improve soil health, not to harvest them. In annual crops, such as wheat, cover crops are normally planted during fallow periods, while in woody crops, such as olive groves, they are planted between trees.

Traditionally, ground covers have been used to cover the soil to prevent erosion, suppress weed growth, and improve soil fertility and structure. This has recently been combined with improving biodiversity, as canopies provide habitat and food for numerous species, and the quality of landscape values.

Depending on the objective, the composition and handling of the covers will be different. Thus, if the aim is to improve soil fertility, the choice should be made for leguminous plants, which can produce a lot of biomass that will be incorporated into the soil as green manure. A similar use is bio-fumigation, for example, with Sinapis alba (a cruciferous plant that is a species of jaramago) to reduce the concentration of a fungus that is harmful to olive trees. If the objective is to prevent erosion, species that cover much of the soil with the lowest possible consumption of water and nutrients should be used, for example, short-cycle, low-growing grasses. If, on the other hand, we want to prioritize biodiversity enhancement, we would choose a mix of species that produce flowers over a longer period of the season. All of these ground cover alternatives will have a positive effect on all of the above-mentioned aspects, although with varying intensity for each depending on the species used and how they are managed.

In all these cases, using cover crops is a commitment to improving the provision of ecosystem services of crops, using agroecological principles where, with a more sophisticated approach to agronomy, a balance is sought between providing different ecosystem services (e.g., food production, better regulation of the hydrological cycle, improved biodiversity, etc.). This approach is based on the understanding that a farm will be more sustainable (economically and environmentally) and resilient to extreme events or situations (droughts, torrential rains, market fluctuations, etc.) if it seeks a medium- and long-term balance between productivity and sustainability.

Currently, it is estimated that farmers in Spain use ground covers on 23% of the cultivated area, preferably on woody crops. In the latter, most of the covers used are the vegetation naturally present in the soil seed bank, called "spontaneous vegetation cover", which is the option in 95% of the area. This situation occurs because it is the most economical and many times the species present are already adapted to the growth cycle being pursued. The other 5% of farms do it because there are circumstances where it is necessary to plant a cover crop, such as when you want to do bio-spraying or green manuring; or when the soil is degraded and you cannot generate a spontaneous cover crop that offers the desired benefits. Some barriers to more widespread use of seeded cover crops are the high cost of seed for the most suitable cover crops, and the difficulty of planting on very degraded soils or soils with a high density of herbivores, e.g. rabbits.

In the Mediterranean climate, where rainfall is always limited and unpredictable, properly managed cover crops are essential to avoid severely affecting the productivity of the main crop. This can occur, sometimes due to competition for water, as, although plant covers improve water inflow, in contrast, their water consumption by transpiration during spring and summer is higher than evaporation from bare, dry soil.

Because of its importance for sustainable soil use agriculture, the Community Agricultural Policy (CAP) aims to help support soil use by offering the option of using cover crops as one of the requirements that farmers must choose from to qualify for the full and significant portion of the grant. Today, after decades of research on plant cover, scientific and technical research must fulfill the need for knowledge, mainly applied, to provide farmers with tools to optimize the benefits of cover crops and prevent their potential risks to help them make management decisions specific to farm conditions and the year's weather conditions. This does not exclude other possibilities for basic and applied research, which could lead to new canopy management strategies and other potential uses. These could even lead to the paradox of returning to situations where a woody crop is intercropped, or between rotations, where the aim is to use part of the crop or biomass of the cover crop as a crop, for example, for energy purposes. Provided that this farming is done with the principles of balancing the provision of ecosystem services, the possibility of a higher income to farmers could allow for a higher quality vegetation cover, which if well managed, could allow for a higher biomass return and better soil cover than what has been observed in some situations with spontaneous vegetation cover.

José Alfonso Gómez (Researcher at the Institute of Sustainable Agriculture of CSIC, member of the Interdisciplinary Thematic Platform (ITP) of CSIC HorizonteVerde and coordinator of the ITP SoilBio).