Soil Bulk Density, Stress at Compaction Threshold and Susceptibility to Compaction as Affected by Long Term Incorporation of Three Types of Manure

Soil compaction is considered as a major threat to long-term productivity of agricultural soils. Two important mechanical properties which can be obtained from the log (stress) - strain curves of compression are: pre-compaction stress (?pc) which is assumed as a transition point between soil elastic to plastic behavior, while the other is the slope of Linear Virgin Compression (VCL) curve at higher stress, called compression index (Cc). Pre-compaction stress and compression index of a silty clay loam soil under long-term (seven years) application of organic matters (municipal solid waste compost, fresh air-dried sewage sludge, farmyard manure) at four rates (0, 25, 50 and 100 Mg ha-1) and one inorganic fertilizer combination (250 urea Mg ha-1and 250 ammonium phosphate Mg ha-1). These mechanical properties were determined at two gravimetrical Water Contents (WCs) (17.1 and 20.9% WC) on remolded soil samples. The experimental designs consisted of: 1) factorial design (organic manure type ? organic manure rate) and 2) orthogonal contrasts: i) control with inorganic fertilizer treatment and ii) control and inorganic fertilizer with factorial group in a completely ranomized design of three replications. With an increase in Organic Carbon (OC), the increase in bulk density (BD) after soil compression and at both WCs was lower. The least increase in bulk density was observed in soil amended with 100 SS ha-1 at WC of 17.1%. At the same WC, the apparent ?pc of the soil amended with high rates (50 and 100 Mg ha-1of organic amendments had higher values as compared with control. For high water content (20.9%), a linear and negative relationship was observed between soil OC and Cc, whearas the relationship for 17.1% WC was found as not significant. The decreases in pre-compaction stress and compression index, for the inorganic fertilizer were not significant as compared with control. In general, since with an increase in OC, the stress at compaction threshold is increased and soil susceptibility to compaction decreased at high water contents, therefore, it can be stated that a long-term incorporation of manure into soil at high rates, irrespective of manure type, could be employed as a means to increase soil resistance to compaction in central Iran.