Preparing the ground for nature: Grass-clover effective in removing soil phosphorus
When combined with regular mowing and removing the cuttings, growing grass-clover on phosphorus-rich soils is an effective technique for reducing soil phosphorus levels and accelerating the development of species-rich grassland. This is the conclusion of a recent publication by the Louis Bolk Institute, “Phytoextraction of soil phosphorus by potassium-fertilized grass-clover swards”, published in the March 2016 issue of the Journal of Environmental Quality. Based on long-term field studies in the Dutch province of Noord-Brabant, there are also strong indications that this method minimizes the amount of phosphorus leaching to surface water and groundwater. Phosphorus pollution is a major problem in the Netherlands, leading to reduced plant diversity and surface water algal blooms.
Phosphorus 'mining' with grass-clover: from concept to evidence
The concept of phosphorus 'mining' with grass-clover was developed by the Louis Bolk Institute in 2001 as a method for reducing phosphorus levels in former agricultural soils. The hypothesis was that grass-clover – fertilised with potassium and sulphur where necessary, and combined with regular mowing and removing the cuttings – would reduce soil phosphorus levels and hence create the nutrient-poor conditions required for the development of species-rich grassland . Between 2002 and 2009 this method was tested at various locations in the Dutch province of Noord-Brabant, including the Loonse en Drunense Duinen. The conclusion is clear: grass-clover effectively reduces soil phosphorus levels down to levels found in species-rich grasslands. "Our study demonstrates the feasibility of converting agricultural land to nature," concludes Bart Timmermans, researcher at the Louis Bolk Institute. "Public authorities and NGOs involved in nature management are often struggling with how to realize ambitious nature targets in the most effective manner. In the case of converting agricultural land to nature areas, bulldozing away the nutrient-rich topsoil is a rapid but also relatively expensive method. Phytoextraction is a cost-effective and now scientifically proven alternative, with the added advantage that soil history and soil biota are conserved."
Environmental impact of phosphorus
Soil phosphorus levels in Europe are among the highest in the world, with the largest phosphorus surplus found in the Netherlands: more than 60% of Dutch agricultural sandy soils are saturated with phosphorus. This phosphorus is prone to leaching to surface water and groundwater, resulting in eutrophication and algal blooms. Furthermore, in natural areas, high soil phosphorus levels lead to impoverished plant communities dominated by one or few species. Reducing phosphorus levels in these soils would allow re-establishment of species-rich vegetation.
While the Netherlands is faced with phosphorus surpluses, global mineable phosphorous supplies are running out: phosphorus is a finite resource. "This is another reason why phytoextraction is a better idea than topsoil removal," says Timmermans. By using the grass-clover cuttings as feed for (dairy) livestock, phosphorus is recycled within the agro-chain: a win-win solution for all parties involved.
“Phytoextraction of soil phosphorus by potassium-fertilized grass-clover swards” is published in the March 2016 issue (issue 45: 701-708) of the Journal of Environmental Quality. Previous publications of the Louis Bolk Institute on soil phosphorus removal with grass-clover, providing practical guidelines for farmers, include: 'Reducing soil phosphorus in nature areas: practical guidelines' (http://www.louisbolk.org/downloads/2402.pdf) and 'Nature development on former agricultural land' (http://www.louisbolk.org/downloads/2823.pdf) (both in Dutch).