Spatial Characterisation of Hydrodynamic Properties of Organic Soils Used for Vegetable Production.

Lettuce production in southern Quebec, Canada, is mainly executed on organic soils. Recent greenhouse studies showed the beneficial effect of irrigation on productivity during dry seasons, but when it comes to planning irrigation treatment on the field, results are not as clear. A strong heterogeneity of soils characteristics is visually observable within the soil profile, where an average 20 to 30 cm thick, strongly humified and dense peat horizon is found over the original fibric sedge peat layer (0 to 150 cm thick). Since the existence of the Oh horizon is due to agricultural practices themselves, there is a strong spatial variability to expect within the different farms and different fields used for vegetable production since up to 50 years. The objective of this study was to characterize the spatial variability of organic soils hydrodynamic characteristics over a farm area in order to specify adapted parameters for precision irrigation.

A random sampling was conducted in October 2009 on an area of 6 km² in southern Quebec. In total, more than 80 pedons were dug. In addition to the complete description of the soil profile, 6 unperturbed soil samples (300 cm³) were collected in each of them. Effective porosity, bulk density, saturated and unsaturated hydraulic conductivity and retention curves were later determined in laboratory on a selection of the collected soil samples. The constant head method and the multi-step outflow method were used for saturated and unsaturated hydraulic conductivity determination, respectively.

Soil properties showed significant differences within the same soil profile and from one field to another. Generally, the top 30 cm of soil showed much lower hydraulic conductivity than soils found deeper but the contrary was also observed. The strong spatial variability of soil properties justifies the need of a precise monitoring of soil characteristics and adapted irrigation practices.