Favorable Growth of Trees in Soils Amended with Compost

Effect of organic amendment for the construction of favourable urban soils for tree growth

L. Vidal-Beaudet^1,4, G. Galopin² and C. Grosbellet^3
1 Agrocampus Ouest, Centre d’Angers, Unité de Recherche EPHor, Angers, France
² Agrocampus Ouest, Centre d’Angers, UMR IRHS, Angers, France
³ Florentaise, Le Grand Pâtis, Saint Mars du Désert, France
⁴ IRSTV-FR CNRS 2488, Ecole Centrale de Nantes, Nantes, France

SUMMARY 

Ornamental tree planting and establishment in cities is a great challenge because urban soil physical properties are unfavourable to the development of root systems. Our objectives were to measure (i) the effects of organic matter on soil physical properties and tree development, and (ii) the effects of ensuing root development on soil physical properties. Using twenty-four 600-L planted or bare soil containers, we monitored physical properties such as dry bulk density, aggregate stability and near-saturated hydraulic conductivity of our reconstituted soils over a 5.5-year period. A 28-cm thick top layer of sandy loam amended with 40% (v/v) sphagnumpeat or organic composts was laid on top of a 28-cm thick layer of sandy loam. Bare-root Ostrya carpinifolia trees were planted in half of the 24 containers, and we monitored shoot development and root biomass and distribution. After 5.5 years, trunk diameter had increased from 59 mm for the control soil to 66 mm for soil mixed with green waste compost, and 74 mm for soil mixed with co-compost of sewage sludge and wood chips. After 4.5 years, trunk diameter was strongly correlated with the total number of axes (r=0.94) and fine root length density (r=0.98), and was confirmed as a good indicator of tree development. Fine root development increased stable aggregate formation in all treatments as compared to bare soil. After 4.5 years after planting, the tree root system induced by a high organic matter input had significantly improved near-saturated hydraulic conductivity and was fit to support fertile urban soils.

Keywords aggregation, artificial soil, compost, Ostrya carpinifolia, tree development, urban horticulture

Significance of this study

What is already known on this subject?

• The quality of the constructed soil used for backfilling the tree pit plays a major role in tree performance. Soil organic matter content also plays a key role in agronomic fertility, so the use of organic amendments and especially composts appears as an ideal solution to boost urban tree growth.

What are the new findings?

• The addition of high levels of organic composts immediately improved the soil physical properties (e.g., dry bulk density, aggregate stability and hydraulic conductivity) and in return fine tree roots modified soil structure and decreased dry bulk density in the top layer. Correlation coefficients confirmed trunk diameter as a very good indicator of tree above and belowground architecture.

What is the expected impact on horticulture?

• Sustainable tree planting and establishment in cities is a great challenge for managers of urban tree resources. In these cases of urban horticulture, to decrease tree replacement frequency and ensuing costs, it is necessary to improve the establishment and maintenance of trees in cities. To develop urban horticulture, it is essential to educate all the city makers (landscape architects, civil engineers, urban planners, municipal officials and urban farmers) about optimal soil specifications expected for fertile urban soils: choice of organic matter nature, sources of organic matter or mineral material, proportions of organic matter in term of soil structuring and soil profile design.