Long-term crop management and formation of plow pan: its consequences on soil physico-chemical properties

Soil compaction and formation of plow pan in Iranian agricultural soils became a serious problem, which threatens regional productivity. The formation of plow pan due to the long-term use of tillage tools at a constant depth, is a common consequence of conventional tillage, which limits crop growth. In order to prove the existence of plow pan and its effect on physical and chemical soil characteristics, 32 soil profiles (eight soil profiles in each region) were analyzed in Qazvin, Golestan, Markazi and Khuzestan provinces. In all four regions and in 70% of the soils, plow pan with an average thickness of 24 cm was observed at 16 to 64 cm below the surface. Total porosity and the amount of bulk density showed respectively about 7-19% decrease and 5-14% increase in the plow pan compared to other horizons. The depth of the formation of this layer and its vertical expansion as well as the intensity of compaction in the soils of the studied areas have been different. The presence of the plow pan (except for Abyek region of Qazvin) had caused the accumulation of 14 to 40% of dissolved salts and the creation of a relatively saline layer under the surface horizon (Ap). The most important reason for the high compaction of the plow pan in the Gonbad agricultural area compared to the other three areas, was the high average percentage of silt (about 60%) in this area. In addition to the effect of soil texture, the predominance of submerged irrigation, despite the aridity of the climate, intensifies the compaction and resistance to root penetration especially in the agricultural areas of Golestan, Khuzestan and Markazi provinces, because annual plowing at constant depths along with heavy irrigation with a large volume of water leads to the eluviation of clay and silt particles from the surface and illuviation in depth.