Soil hydraulic properties are important for water management in salt and sodium (Na) affected soils. The soil saturated hydraulic conductivity (Ksat) has been found to decrease when high soil solution Na adsorption ratio (SAR) is coupled with low solution electrical conductivity (EC). Soil clay type plays an important role in changes to soil Ksat when the soil solution composition has a high SAR and low EC. Little is known about how soil water retention changes from solution EC and SAR. The objective of this study was to determine the impact of a range of solution EC and SAR values on soil Ksat, and water retention for soils containing swelling and nonswelling clay minerals. The soil Ksat and water retention were measured on four smectitic soils, a kaolinitic soil, and an illitic soil. Smectite swelling caused large changes in soil hydraulic properties in smectitic soils at high exchangeable Na percentage (ESP) and low EC levels, whereas there was a reduced effect on soils with little smectite clay. Increasing soil solution ESP and decreasing EC level caused the Ksat to decrease and the water content at a given pressure head to increase on soils that contained smectite. A narrower range of EC and SAR of waters suitable for use on smectitic soils are required to prevent changes in soil water flow rates and soil water retention.
Planting cover crops can improve soil health and help to sustain agricultural crop yields. In northern climates where corn (Zea mays L.) and soybean (Glycine max L.) are grown, cover crop biomass production can be low. This has led to people investigating the potential of interseeding cover crops into the growing main crop. This paper sought to determine biomass production and the benefit to grain yields, weed control and soil properties from interseeding cover crops into corn and soybean. This review included 70 studies published prior to 15 March 2024. Interseeded cover crops that were winter-hardy such as cereal rye (Secale cereale L.) produced more biomass in the spring (1.04 Mg ha−1 average biomass production) than any of the interseeded cover crops did in fall (0.35 Mg ha−1 average biomass production), primarily at crop row spacings of 76 cm. Factors that affected cover crop biomass production were crop stage, planting method, tillage practice, irrigation and row spacing. There was not a consistent widely planted cover crop species that produced the most biomass. Interseeded cover crops reduced weed biomass by 46% compared to weed control and generally did not affect crop grain yields when planted after V4 crop stage. Interseeded cover crops reduced soil nitrate concentration but generally did not affect other soil properties including soil water content. However, most of these studies planted cover crops at the same site for less than three years. Early interseeded cover crops generally did not perform better than interseeded winter-hardy cover crops planted around crop physiological maturity.
Crop residue removal may negatively affect soil mechanical properties, which are key components of soil quality. To evaluate potential long-term effects, we assessed the 10-yr impact of corn (Zea mays L.) residue removal (59 % of non-grain biomass annually) on surface soil mechanical properties (0–20 cm). We also evaluated whether adding carbon (C) amendments, such as using a winter rye (Secale cereale L.) cover crop or surface-applying cattle manure (24 Mg ha−1 biannually) can ameliorate the effects of crop residue removal. This long-term study was under irrigated no-till continuous corn on a silt loam soil in south-central Nebraska, USA. Measurements included soil penetration resistance, field bulk density, aggregate strength, Atterberg limits (liquid limit, plastic limit, and plasticity index), Proctor maximum bulk density, and the water content at which the Proctor maximum bulk density (critical water content) occurs. Reduction in soil organic carbon (SOC) concentration explained most of the changes in soil mechanical properties. Long-term corn residue removal increased penetration resistance (+40 %) for the 0–20 cm depth, and reduced aggregate strength (−44 %), plasticity index (−22 %), and critical water content (−13 %) in the 0–5 cm depth. Residue removal also reduced field bulk density (−5%), liquid limit (−12 %), and plastic limit (−10 %) in the 0–10 cm depth, but increased Proctor maximum bulk density (+8 %) in the 0–5 cm depth. Winter rye cover crop reduced field bulk density (−5%, 0–15 cm depth) and increased penetration resistance (+52 %, 0–20 cm depth). Surface-applied manure amendments increased the near-surface soil liquid limit (+8 %) and plastic limit (+8 %) in the 0–5 cm depth. Given the high rate of residue removal used in this experiment, our findings support that excessive corn residue removal over the long-term (∼10 years) negatively affects near-surface soil mechanical properties, but that use of winter rye cover crop or surface-applied manure can minimally to partially ameliorate these effects.
Hydraulic conductivity is important in the transport of water and salts in salt affected soils. The objective of this study was to determine how saturation of soil columns with saline and sodic solutions changes soil saturated hydraulic conductivity (Ksat) and subsequent salt leaching on a variety of soil textures and smectite clay content. Soil columns were capillary wetted with solutions of varying electrical conductivity (EC) and sodium adsorption ratio (SAR). Ksat was measured by infiltrating deionized water at the soil surface. Ksat as a function of leachate pore volume was fit with an exponential decay equation and a pedotransfer function was created to predict the decay coefficients. Higher salt contents in waters were more successful at increasing soil SAR as compared to low salt content waters. The pedotransfer functions performed similar to the McNeal and Simunek and Suarez models to predict Ksat change due to solution composition without the need of X-ray diffraction of the soil clay fraction. Soils with a greater clay content and specifically smectite clay content have larger changes in saturated water flow rates whether from one time wetting or under long term use of irrigation waters at high EC and Low SAR. High clay smectitic soils require careful management when they are saline-sodic and have a smaller range of safe irrigation waters to prevent reduction in hydraulic conductivity from SAR.
Decreasing water resources available for irrigation will require a thorough reconsideration of how water is allocated and managed for crop production. Electromagnetic (EM) soil water sensing is an important tool that can facilitate spatial and temporal allocation decisions to increase crop water productivity. Accuracy of volumetric water content measurements in the field, however, is problematic with EM sensors, especially in soils with high clay contents and pronounced horizonation. Under many circumstances, measurement uncertainties are large compared with the range of managed allowed depletion. Soil specific calibrations can improve accuracy although the procedures required to achieve this are normally impractical for routine field deployment of sensors. Herein we present our current efforts in improving the accuracy of TDR soil water sensing and their utility in irrigation management, especially under conditions of limited water availability.Earlier work using a quasi-theoretical model to describe the complex permittivity of soil demonstrated that bound water near clay surfaces and high frequency filtering of the broadband signal were major sources of error for TDR water content estimation. The specific surface area of the soil is partly responsible for these effects, which can also vary in the field because of the dependency of volumetric bound water on bulk density. Although theory can describe how soil apparent permittivity changes with respect specific surface area and bulk electrical conductivity; this does not necessarily reflect how these properties influence the measured travel time. Bound water polarization and dc losses result in signal attenuation of the high frequency components thereby increasing travel time greater than that expected from changes in apparent permittivity.To circumvent these difficulties, we are currently using a supervised machine learning approach to develop an empirical soil water content calibration based on measured travel time, measured state properties (temperature and bulk electrical conductivity), and inferred properties based on TDR waveform features (specific surface area). For example, at a given water content, the shape of the waveform reflection for a soil dominated by kaolinite is distinct from the reflection of a soil dominated by 2:1 phyllosilicates. Essentially the bulk density x specific surface area modifies the waveform features which in turn can be used to develop in essence an in-situ soil specific calibration.Introduction of measured soil water contents into crop models provides a way to facilitate real-time yield predictions of alternative water allocation decisions. The Richards Equation will necessarily be incorporated into crop models to permit a mechanistic basis of redistributing soil water in the profile. Soil water sensing can permit the accurate determination of both irrigation application efficiency and infiltration. Incorporation of measured soil water into crop models allows for “course corrections” of simulated profile water and potentially improvements in the estimation of evapotranspiration and yields.
Abstract Crop residues are often removed for livestock and biofuel production, but how such removal impacts soil physical properties has not been widely discussed. The objectives of this paper were to discuss: (a) the impacts of crop residue removal on soil physical properties, (b) factors affecting residue removal effects on soils, (c) threshold level of residue removal, (d) strategies to offset the removal negative effects, and (e) research needs. We compiled 66 studies on crop residue removal and soil physical properties published prior to 17 Aug. 2021. Residue removal may not affect bulk density and water infiltration rate but increases penetration resistance by 55% and soil temperature by 1.8 °C in spring. However, it reduces wet aggregate stability by 31%, dry aggregate stability by 44%, and water retention at –33 kPa by 24% in most studies, indicating residue removal can increase erosion risks and reduce soil water storage. Residue removal rate is the leading factor that explains changes in soil physical properties. Residue removal at rates above 50%, in general, adversely affected soil physical properties, which correlates to retaining about 4 Mg ha –1 of residue. Cover crops and manure application may partially offset adverse effects of residue removal on soil physical properties, but studies are too few to make a strong conclusion. Consistency of soil sampling depths, accurate reporting of residue removal rates, and additional data from long‐term experiments are needed. Overall, high rates of residue removal can increase erosion potential and reduce soil water but have mixed impacts on other physical properties.
Soil-shrinkage characteristics affect fluid transport and soil mechanical properties, with broad implications for environmental flows, crop production, and civil engineering designs. We quantified mild-saline-solutions effects on soil shrinkage curves and developed pedotransfer functions to predict curve parameters. Seven soil and soil mixes were equilibrated with solutions of 0.5-to-8 dS m−1 and 0-to-20 sodium adsorption ratios (SAR). Saturated paste rods were dried; water contents and isotropic shrinkage measured. Texture affected shape-forming factors when clay and smectite contents were >260 and 140 g kg−1, respectively. Solutions ≥2 dS m−1 affected the coefficient of linear extensibility for smectitic soils containing clay ≥300 g kg−1. Solution SAR affected only the highest clay content (530 g kg−1) and mixed mineralogy soils. However, the solution salinity levels were not high enough to affect shape factors of the shrinkage curves. Pedotransfer functions successfully described soil shrinkage with root-mean-squared-errors 1 to 4 magnitudes lower than the highest measured values.
