HOW COMPOST IMPROVES THE AGRONOMIC PROPERTIES OF PLANTING HOLES

Physical properties of urban soils are often limiting to tree development. Urban soils are compacted, resulting in reduced water and air infiltration, and insufficient root growth. For street trees, the planting hole can be filled with different soil materials. The most commonly used material is arable soil, which can be mixed in the upper part of the anthropogenic soil with organic matter just before tree plantation. Green waste compost is the main organic product used. Added volumes vary from 10 to 50% of the soil volume, that is to say, about 30 times more than for organic inputs in agricultural fields. However, only a few studies have focused on the influence of such high levels of organic matter on soil physical properties and tree development. To investigate this, we carried out an experiment in October 2004. Soils for tree plantation were reconstituted in 600-L containers with two layers. The top layer was a sandy loam amended with 20% or 40% of organic product: sewage sludge and woodchip compost (SW), green waste compost (GW) and sphagnum peat (SP). The control container was composed of two layers of sandy loam (SL). We assessed soil properties every year. Carbon contents rapidly decreased in all of the mixtures due to the biological stability of organic matter, but remained higher in 40% volume mixtures. Compost addition increased structural stability, porosity and hydraulic conductivity, especially with a 40% volume of organic product. Tree development in reconstituted soils was greater for compost than for peat because of different nutrient supplies. Soil physical properties were practically not improved in soil reconstituted with 20% of organic product. These results clearly emphasize the interest of incorporating high levels of organic matter in urban soil.