A high level of organic matter in soils is crucial to maintain structural stability but organic matter sources differ in their effectiveness in stabilizing structural units. Objectives of this study were, first, to determine the optimal rate of sludge and fertilizer application to improve soil physical properties, and second, to investigate a possible correlation between hydraulic conductivity and structural stability measurements. A 4‐yr field study (1994–1997) was conducted on three different soil types to evaluate the effect of different amounts of de‐inking secondary paper sludge on the soil physical properties. The soil physical properties we monitored were structural stability, water desorption characteristics, bulk density, and saturated hydraulic conductivity. Structural stability was increased by 17% in silty clay soil (SCS) and 15% in loamy soil (LS), but decreased by 35% in sandy loam soil (SLS). Results suggest that the effect of sludge application (SA) is short‐lived and that an annual application of sludge is necessary to obtain a year‐to‐year effect on structural stability. Measured bulk density dropped significantly in the SCS (4–10%) and in the LS (1–6%). A significant increase in air capacity and available water values revealed that SA increases both transmission and storage pores in the SCS. Field‐saturated hydraulic conductivity (Kfs) was increased in the SCS, but decreased in the SLS and the LS. A good correlation was observed between structural stability and hydraulic conductivity measurements in the SCS and the LS.
Insufficient aeration in artificial mixes used for nursery and greenhouse production is commonly reported to be a problem. To diagnose aeration problems, gas relative diffusivity estimates can be used. These estimates can be obtained indirectly from measurements of air entry value, saturated hydraulic conductivity and the water desorption curve. In here, a rapid tension chamber method for measuring air entry is proposed. To verify applicability of the method, different potted substrates were inserted into a tension chamber apparatus and water was extracted from the saturated substrate by establishing a contact with an unsaturated body located outside the chamber. The potential within the chamber was monitored over a 5‐min period and the point of air entry was calculated thereafter. Statistically equal estimates of air entry were obtained with this technique compared with values derived from the tension table, for a wide range of air entry values in peat‐based substrates.
An adequate understanding of the mechanisms involved in the structural stabilization of soil by different sources of organic matter is needed to help design management strategies aimed at maintaining a stable soil structure. The objective of this study was to identify mechanisms involved in soil structure stabilization by paper sludge application, either by increasing the soil resistance to external stresses (aggregate stability) or by decreasing the magnitude of the external stresses (diminution of the wetting rate). A laboratory study was conducted on three different soil types with application of paper sludge at three rates (8, 16, and 24 dry t ha−1). The mean weight diameter, bulk density, hydraulic conductivity, and water retention properties were measured before and after a wetting event. The results indicate that most of the changes in physical properties resulting from rapid wetting took place at the soil surface (0–50 mm) and the magnitude of these changes gradually decreased down to a depth of 150 mm. Paper sludge application significantly improved the stability of 1‐ to 4‐mm aggregates to the destructive action of wetting in all three soil types. Paper sludge application increased porosity at potential > −2 kPa, which resulted in higher hydraulic conductivity values (up to 88%) and a smaller increase in soil bulk density (down to 67%) relative to a control following rapid wetting. The wetting rates observed during the wetting event were similar regardless of the treatment, because the increase in the water potential at the wetting front was compensated for by an increase in hydraulic conductivity with increasing rates of sludge application.
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