Saline-sodicity and soil physical impact on root growth

Of the earth’s 840 million hectares are of soil, roughly 683 million hectares are saline, and 157 million hectares are saline-sodic.  The direct impact of an osmotic stress to plant growth in salt affected soils is well known. Plant roots in salt-affected soils often have morphological changes, and ionic imbalance that interfere with nutrient uptake. In saline-sodic soils, decreased physical stability is typical, likely driving greater penetration resistance and decreased soil aeration.  This could reduce root growth, but research is missing that directly links these measurements of physical behaviour to plant growth. The present study explores these effects in repacked cores of sandy loam and clay loam soils in saline-sodic (NaCl,1.76 g kg^-1 soil) or saline (KCl, 2.25 g kg^-1 soil) conditions. Different physical conditions of light  (50 kPa) and high (200 kPa) compaction stresses, and wet (-5 kPa) and drier (-50 kPa) water potentials were imposed under controlled conditions. Physical data of compression characteristics, bulk density, water content, air-filled porosity, and penetration resistance were measured on the soil cores. Wheat (salt intolerant) and barley (salt tolerant) were grown in the cores and the lengths of their seedling roots were measured 48 hours after sowing in a rapid growth screen. This study investigates the comparative impacts of saline-sodic and saline soils on soil physical properties and the subsequent effects on barley and wheat root growth. Saline-sodic soil exhibited significantly greater penetration resistance, ranging from 0.58 to 2.73 MPa, compared to the control range of 0.62 to 1.70 MPa. In contrast, saline soil demonstrated less penetration resistance, with a maximum value of 1.84 MPa. Additionally, air-filled porosity in saline-sodic soil decreased to 19%, indicating reduced oxygen availability, while saline soil retained higher aeration (43%), surpassing the control value (34%).

These alterations in soil properties significantly influenced root growth. Barley root elongation was more strongly linked to physical changes, while wheat root growth was adversely affected by both physical and chemical alterations due to its lower salt tolerance. In saline-sodic soil, barley and wheat root elongation were reduced to 32% and 20% of the control, respectively, primarily due to increased penetration resistance. A reduction in air-filled porosity further restricted root growth to 46.7% for barley and 30.6% for wheat. Conversely, the lower penetration resistance in saline soil supported higher root elongation, reaching 82.8% for barley and 63.6% for wheat in comparison to the control. Our analysis with concepts from the least limiting water range indicate that soil physical constraints exacerbate root growth restrictions in saline-sodic soils.