Modelling soil pore characteristics from measurements of air exchange: the long-term effects of fertilization and crop rotation

Long-term cultivation of soils for arable crops and without application of organic manure is thought to reduce the soil quality for crop production. We studied the porosity of two sandy loam soils from organic dairy farms with a ley cropping system and compared them with a conventionally managed dairy farm growing arable crops only and an arable soil receiving only mineral fertilizers, respectively. Saturated hydraulic conductivity, water retention characteristics, air diffusivity and air permeability were determined in the plough layer of the soil. The soil of one of the organic dairy farms contained many more earthworm burrows and had a significantly larger hydraulic conductivity than the counterpart soil of the conventionally managed dairy farm. In other respects, these two soils were much alike. The soil of the other organically managed dairy farm was less dense and had a larger volume of pores > 30 mum than the arable soil receiving only mineral fertilizers. A tube model was used to combine the air exchange measurements in a description of the soil pores. The pore system of the mineral fertilized soil consisted of continuous arterial pores with only a small volume of blocked and marginal pores embedded in the soil matrix next to the arterial pores. The porosity of the counterpart organic dairy farm soil had a considerable volume of blocked and especially marginal pores. The complexity of the latter was considered beneficial for local aeration in the soil and for derived soil properties such as fragmentation on tillage.