Low Exchangeable Sodium Percentage Research Articles

Digital soil mapping and modeling in Loess‐derived soils of Iranian Loess Plateau

The exchangeable sodium percentage (ESP) is the parameter that causes poorly developed soil structure and weak drainage over time, subsequently increasing the dispersion of soil particles, especially silt and clay, which cause soil erosion in Loess-derived soils. The number of 474 soil samples was collected within 0 to 30 cm of soil depth. Five non-linear models named Ridge regression (RidgeR), random forest (RF), support vector regression (SVR), cubist, and generalized boosted regression model (GBM), were considered to link several parameters with ESP in the region of approximately 2,730 ha. The mean square error (MSE), root mean square error (RMSE), and coefficient of determination (R2), were considered to assess the models. The RF and RidgeR algorithms were used to prioritize the variables impacting the ESP. The prediction model for mapping soil ESP had very high accuracy from RidgeR (RMSE = 0.039, R 2 = 0.99), RF (RMSE = 0.257, R2 = 0.97), SVR (RMSE = 0.81, R 2 = 0.544), cubist (RMSE = 0.006, R 2 = 0.99), and GBM (RMSE = 0.426, R 2 = 0.87). The sodium adsorption ratio (SAR) and slope steepness were the most important factors affecting ESP, and slope steepness was recognized as the most effective factor. We also identified that the applied models could simply assess ESP map. In addition, an action plan in different soil horizons, considering erosion and sediment processes, is efficient in controlling runoff at low ESP.

Impact of brackishwater shrimp farming at the interface of rice growing areas and the prospects for improvement in coastal India

The coastal plain of Odisha in the northeastern region of India is designated the “rice bowl” of the state and is vulnerable to the impact of brackishwater shrimp farming, a prominent livelihood in southeastern Asia. Shrimp farming is highly profitable. However, owing to plentiful resources, shrimp farming has encroached on several rice-growing areas and a decline in the quality of natural resources in the coastal neighborhood has since been reported. This paper aims to study the effects of the unplanned expansion of brackishwater shrimp farming on natural resources and to provide a pathway to suitable utilization in order to improve the livelihood security of marginal shrimp farming communities in coastal Odisha, India. The practice of brackishwater shrimp farming has been determined to induce salt stress at 341–9387 ppt ha−1 crop−1 with a soil EC ranging from 0.3 to 3.4 and 0.5 to 9.5 dSm−1 under the Scientific Extensive Traditional (SET) method and 0.1 to 1.4 and 0.2 to 3.6 dSm−1 under the Traditional/Improved Traditional (IT) practice during post– and pre–farming periods, respectively. Soil with ≥35% clay content underwent a severe loss of saturated hydraulic conductivity (Ks), and soil with a low exchangeable sodium percentage (6.09–8.03%) showed more susceptibility towards Na saturation than did soil with a high exchangeable sodium percentage (>10%) after brackishwater shrimp farming. Growing salt-tolerant rice in shrimp ponds during non–farming periods was observed to reduce soil Ks by only 1.2–1.3-fold compared to a reduction of 22–40-fold under shrimp farming. The paper concludes that by promoting salt washing and alleviating salinity hazards, the shrimp–rice sequence has shown promise to restore soil quality, reduce vulnerability, enhance resilience in brackishwater shrimp farming where the farms interface with rice-growing coastal areas, and provide support to conserve the coastal environment.

Remediation of saline soils by a two-step process: Washing and amendment with sludge

Soil salinization is a serious land-degradation problem that affects soil, plants and groundwater within ecosystems. In this work, the effect of washing saline–sodic soils with CaCl2 at 2% (w/v) and amendment with composted and dry sewage sludge was evaluated in a remediation assay. The effectiveness of amendments to improve the soil fertility was investigated with seed germination assays of Raphanus sativus L. Two different organic wastes were used: sewage sludge composted with pruning wastes (CP) and thermally dried sewage sludge (T) at rates of 1 and 2% (w/v) on organic matter basis. In general, there was an increase of germination percentage in the amended washed soils. The best indices were obtained at the 1% amendment dose with CP. The low exchangeable sodium percentage (ESP) in washed–amended soil (3 to 6 times lower than that of saline–sodic soil) could partially explain the increase in seed germination percentage of R. sativus L. The incorporation of organic amendments to washed soil also seemed to be beneficial as a step in the restoration process according to the chemical characteristics, especially by increasing the level of nutrients such as carbon, nitrogen, and phosphorus. A significant multiple linear regression of radish germination index in terms of pH values, sodium adsorption ratio of soil (SAR), and different forms of nitrogen (NH4+ and NO2−) was found.Overall, saline–sodic soil washing with CaCl2 followed by addition of organic amendments seems to be a promising strategy for restoration of such soils with simultaneous improvements of cation balance and nutrient/organic matter contents.

Process and Mechanism for the Development of Physical Crusts in Three Typical Chinese Soils

To compare the development of physical crusts in three typical cultivated soils of China, a black soil (Luvic Phaeozem), a loess soil (Haplic Luvisol), and a purple soil (Calcaric Regosol) were packed in splash plates with covered and uncovered treatments, and exposed to simulated rainfall. Meshes covered above the surfaces of half of soil samples to simulate the effects of crop residue on crusting. The results indicated a progressive breakdown of aggregates on the soil surface as rainfall continued. The bulk density and shear strength on the surface of the three soil types increased logarithmically as rainfall duration increased. During the first 30 min of simulated rainfall, the purple soil developed a 7–8 mm thick crust and the loess soil developed a 3–4 mm thick crust. The black soil developed a distinguishable, but still unstable, crust after 80 min of simulated rainfall. Soil organic matter (SOM) content, the mean weight diameter (MWD) of soil aggregates, and soil clay content were negatively correlated with the rate of crust formation, whereas the percentage of aggregate dispersion (PAD), the exchangeable sodium percentage (ESP), and the silt and sand contents were positively correlated with crusting. Mechanical breakdown caused by raindrop impact was the primary mechanism of crust formation in the black soil with more stable aggregates (MWD 25.0 mm, PAD 3.1%) and higher SOM content (42.6 g kg−1). Slaking and mechanical eluviation were the primary mechanisms of crust formation in the purple soil with low clay content (103 g kg−1), cation exchange capacity (CEC, 228 mmol kg−1), ESP (0.60%), and SOM (17.2 g kg−1). Mechanical breakdown and slaking were the most important in the loess soil with low CEC (80.6 mmol kg−1), ESP (1.29%), SOM (9.82 g kg−1), and high PAD (71.7%) and MWD (4.6 mm). Simulated residue cover reduced crust formation in black and loess soils, but increased crust formation in purple soil.

Effect of initial water content on saturated hydraulic conductivity in desalinization with slaking and drying

Soil slaking is an environment-friendly technique that is gaining importance in restoring saline soils. The objective of this article is to evaluate the effect of initial water content (IWC) on saturated hydraulic conductivity (Ks) in desalinization with slaking and drying. Accordingly, a slaking test was carried out during February, 2009 for evaluating the effects of slaking and drying on Ks, sodium adsorption ratio (SAR) and exchangeable sodium percentage (ESP) under various IWC. We prepared natural and air-dried soils of paddy field in Kojima Bay Polder, Japan to give different pre-drying, air-dried, and not dried (natural). The air-dried soils were resaturated. Each soil was well mixed, then dried to different initial moisture contents (60, 50, 40, 30, 20, and 10% by weight). The specimens were immersed into water in the pot for 24 h. The Ks was measured, and cations in slaked and unslaked soils were analyzed. The Ks was high under the water content below 30% in both the natural and the air-dried soils. But the effects were more pronounced in the natural soil. The air-dried soil showed far smaller Ks than the natural soil. In outer solution, the highest SAR was noted at 30% in the natural and 30 and 20% in the air-dried soils. Significant decrease in ESP of the soils (slaked + unslaked) was also observed at the same water content. Lower water content was more effective in decreasing the soil ESP after desalinization from saline soil. The natural soil showed lower ESP and higher porosity, which was considered as a reason for higher Ks of natural soil than that of air-dried soils. The results indicated that lower water content (10–30%) had no hazardous effect on Ks by slaking and drying of soil.

Effects of sodic water on soil sodicity and nutrient leaching in poultry and sheep manure amended soils

The need to use saline and sodic waters for irrigating agricultural lands has increased due to a shortage of fresh water resources, especially in arid and semi-arid regions. One way to decrease undesirable effects of sodic waters on the physical and chemical properties of soils is to apply organic and chemical amendments. In this study, the effects of two organic fertilizers (poultry and sheep manure), and gypsum were evaluated in soil that received sodic water. These materials were added to a sandy loam soil at the rate of 5%, after which the treated soils and a control sample were incubated for one month at field capacity at 25–30 °C. Column leaching experiments using treated soils were then conducted under saturated conditions using two types of NaCl–CaCl 2 solutions and three sodium adsorption ratios (SAR) (0, 10, 40) with a constant ionic strength (50 mmol l − 1 ). The results indicated that the application of sheep and poultry manure to soils caused an increase in cation exchange capacity (CEC) and greater adsorption of cations such as calcium, magnesium and potassium than sodium (Na +). Conversely, the addition of organic amendments led to increased Na + leaching and a lower exchangeable sodium percentage (ESP). The ESP of the control soil after leaching with solutions with SAR of 10 and 40 increased significantly, but the level of sodification in treated soils was lower. The ESP of the poultry and sheep manures treated soils after leaching with solution with SAR 40 was significantly lower than control soil. Specifically, the rate of soil sodification in the treated soils declined in the following order: control soil > sheep manure treated soil > poultry manure treated soil > gypsum treated soil. Finally, the effects of gypsum on the sodification of soil by SAR 10 solution were the same as the effects of poultry manure, whereas the loss of cations and anions from poultry and sheep manure treated soils was greater than the loss from gypsum treated and control soils.

Infiltrability in soils from south-western Africa: effects of texture, electrical conductivity and exchangeable sodium percentage

The effects of particle size, electrical conductivity (EC) and exchangeable sodium percentage (ESP) on infiltra-bility were investigated in 35 soils from Namibia and western South Africa. Samples were grouped as follows: sand (5–9% clay + silt), loamy sand (16–20%) and silty loam (73–80%). Subgroups included: low EC and low ESP (LL); low EC and high ESP (LH) and high EC and ESP (HH). Infiltrability was measured using a rapid laboratory syringe method. To investigate dispersion and flocculation processes the effect of four mobile phases on infiltrability was examined. Mobile phases included distilled water (W), a gypsum solution (G), a 1:5 soil suspension in gypsum solution (GS), and a 1:5 soil suspension in water (WS). Infiltrability was highest in the sand group, and lowest in the silty loam group. Electrical conductivity and ESP effects varied between particle size groups. In the silty loam group, EC and ESP effects were not significant due to swelling of clay minerals and rapid blockage of soil pores with dispersed particles. In the sand group, soils with high ESP (22) had significantly lower (p<0.05) infiltrability than soils with low ESP (4) levels. The application of gypsum in these soils enhanced infiltrability and minimized the negative effect of high ESP. The ameliorating effect of gypsum was apparent in treatment GS, but not treatment G, which highlights the dominant effects of mechanical disturbance and clay dispersion on crust formation and infiltrability.

GEOCHEMISTRY OF SALT-AFFECTED AQUALFS IN NORTHEAST THAILAND

A geochemistry study was carried out on 248 samples from 28 pedons of salt-affected soils (Natraqualfs) formed on alluvial and wash deposits over salt-bearing rocks in five transects in the Khorat basin, Northeast Thailand, under tropical savanna climate to determine if salinization had affected the chemical properties of the soils. This project had the objective of providing the more detailed information needed to improve agricultural practices in the area. The soils were saline sodic, sodic, and acid saline sodic with halite in salt crusts and calcite in the lower part of some soil profiles. Smectite was a minor constituent of these kaolinitic soils and was more abundant in saline soils where it may have crystallized from soil solution or derived from underlying saline rocks. The chemical composition of the soils was highly diverse caused mostly by variation in texture of parent materials rather than the effects of salinity. Plots of geochemical factor scores and textural properties versus depth showed clear evidence of depositional layering in many profiles. Five main chemical affinity groups may be recognized; Group 1 consisted of elements associated with clay minerals and oxides: Al, Ga, Cs, Sr, Fe, U, Pb, Rb, V, Be, and Zn; Group 2 consisted of sand, Si, Mo, and Cr; Group 3 consisted of pH in water and pH in KCl; group 4 reflected biological activity and consisted of S, organic carbon, and N; and group 5, composed of electrical conductivity, Na, Cl, Br, and I, represented the only distinct effect of salinity on soil composition. The higher percentages that could be explained by the two factors of the low electrical conductivity and low exchangeable sodium percentage samples indicated well the difference between the salt-affected soils and the adjacent nonsalt-affected soils in the total set of samples. They also indicated that the geochemical diversity existing in these soils could at least partially be attributed to the effect of salt in the natural soil system.

Effects of wastewater irrigation on soil sodicity and nutrient leaching in calcareous soils

Soil column studies were conducted with two soils to assess the effects of irrigation with wastewater on soil and groundwater quality. Upon the application of wastewater, exchange occurred between solution sodium (Na +) and exchangeable cations (Ca 2+, Mg 2+, K +), whereby these cations were released into solution. The average exchangeable sodium percentage (ESP) of the soils increased during leaching from 9 to 21 and 28.8 to 29.7 after applying 5.0 and 3.5 l (about 7 and 6 pore volumes) of wastewater to the soils columns, respectively. Adverse effect of high Na + concentration in the wastewater on raising ESP was less pronounced in the soil having initial high ESP than in the soil with low initial ESP. Salinity of the soils was also increased with the application of wastewater and Mg 2+ and K + were leached from the soils. These losses would be more severe on soils having a low cation exchange capacity and if, uncorrected could lead eventually to their deficiencies for plant growth. When the soil columns were leached with distilled water the flow rate of one soil decreased to zero after 2.2 pore volume indicating damage to soil structure. Irrigation with wastewater, which is generally more sodic and saline than regional groundwater, increases the rate of soil sodification of shallow groundwater. A relatively simple chromatographic model was used to estimate final ESP profiles in the soils assuming the condition of local equilibrium. This approach had a limited success for one of the soil. Since the final leached concentrations are in good agreement with those of wastewater, we attribute these differences to non-uniform flow through the column. In terms of practical soil and water management, our study reveals that relatively simple means can be useful to predict the water quality in soils, their discharge to ground water, and the hazard of soil structure deterioration.

Amelioration of alkali soil using flue gas desulfurization byproducts: Productivity and environmental quality

In this study, flue gas desulfurization (FGD) byproducts are used to ameliorate alkali soil. The average application rates for soils with low exchangeable sodium percentage (ESP), mid ESP, and high ESP are 20.9, 30.6, and 59.3 Mg ha −1, respectively. The experimental results obtained for 3 consecutive years reveal that the emergence ratios and yields of the crops were 1.1–7.6 times and 1.1–13.9 times those of the untreated control, respectively. The concentrations of Cr, Pb, Cd, As, and Hg in the treated soils are far below the background values stipulated by the Environmental Quality Standard for Soils (GB15618-1995). Their concentrations in the seeds of corn and alfalfa grown in the treated soils are far below the tolerance limits regulated by National Food Standards of China. The results of this research demonstrate that the amelioration of alkali soils using FGD byproducts is promising.

SODICITY AND WATER QUALITY EFFECTS ON SLAKING OF AGGREGATES FROM SEMI-ARID SOILS 1

Aggregate slaking is one of the main mechanisms responsible for the breakdown of aggregates. Exchangeable sodium percentage (ESP) and electrolyte concentration (C) of the soil solution play a significant role in determining soil physical properties and the response of soil clays to dispersion and swelling. However, studies of sodicity and C effects on aggregate stability have yielded inconsistent results. Our objective was to evaluate the effects of sodicity and C on aggregate slaking in soils with different clay contents. Using the high-energy-moisture-characteristics (HEMC) method, we studied aggregate slaking in 56 samples of Israeli topsoils varying in clay content (80-675 g kg -1 ) and ESP levels (0-30). In this method, accurately controlled wetting of the aggregates (i.e., the driving force for slaking) was the only force exerted on the aggregates. Aggregates (0.5-1.0 mm) were placed in a funnel equipped with a fritted disk and, using a peristaltic pump, were wetted either fast (100 mm h -1 ) or slowly (2 mm h -1 ). Two salinity levels were studied: dionized water (DW, C = 0.04 mmol c L-1) and saline water (SW, C = 20 mmol c L -1 ). Thereafter, the aggregates were subjected to stepwise increases in matric potential up to 5.0 J kg -1 to obtain a moisture retention curve that served as the base for calculation of aggregate susceptibility to slaking. The latter was expressed in terms of stability ratio (SR). The SR of low ESP soils increased from 0.298 to 0.751 and from 0.508 to 0.799 for DW and SW, respectively, with the increase in soil clay content from 11.3 to 67.4%. Increase in ESP decreased SR (i.e., enhanced aggregate slaking), whereas use of SW increased SR compared with DW. A triple interaction among ESP, C, and clay content (P = 0.001) in their effect on aggregate slaking suggested that the combined effects of these variables on slaking were complex. Our results suggested that: (i) in soils having inherently low aggregate stability (clay 35%), aggregate slaking decreased upon use of SW only when ESP > 15.

A model to explain high values of pH in an alkali sodic soil

For alkali sodic soils (pH&gt;8.5), the "hydrolysis of exchangeable sodium" has been used as a possible explanation for the alkalinity production and rise in pH of these soils. As an alternative to this hypothesis, a model was developed to simulate and to explain that the alkalinity production and rise in pH is possible in a soil that accumulates alkaline sodium salts and CaCO3. Several simulations were performed by using different combinations of CO2 partial pressures (P), presence or absence of MgCO3, along with experimental values of exchangeable sodium percentage (ESP) and ion concentrations in saturation extracts from an alkali sodic soil (named Pantanal). A hypothetical system with similar conditions to the Pantanal soil but with a Gapon selectivity coefficient (KG) of 0.01475 (mmol L-1)-1/2 was also considered. Good agreement was obtained between experimental and predicted values for pH and ion concentrations in the soil solution when the model (without MgCO3) was applied to the Pantanal soil. However, KG values calculated for the Pantanal soil were generally higher than 0.01475 (mmol L-1)-1/2. Moreover, high pH values and elevated ionic strength were obtained when a KG of 0.01475 (mmol L-1)-1/2 was used at high ESP (similar to those found in the Pantanal soil). KG values obtained for the Pantanal soil and the results obtained in the simulation of the hypothetical system are suggesting that a value higher than 0.01475 (mmol L-1)-1/2 should be used to adequately simulate the behavior of the Pantanal soil at low ionic strength and high ESP values.

WATER QUALITY AND SODICITY EFFECTS ON SOIL BULK DENSITY AND CONDUCTIVITY IN INTERRUPTED FLOW 1

Interruption of flow during furrow irrigation (e.g., use of surge technique) consolidates soil near the furrow surface, causing reorientation and rearrangement of soil particles, and leads to increased surface bulk density (BD) and reduced hydraulic conductivity (HC) of this surface layer. We hypothesized that soil consolidation could be affected by irrigation water quality and soil sodicity. We studied in the laboratory changes in the BD and HC of an alfisol (Calcic Haploxeralf) and two vertisols (Chromic Haploxerert) having different exchangeable sodium percentage (ESP) levels that were subjected to five cycles of leaching and draining under matric potential of up to -5 J kg -1 . Four different water qualities (electrical conductivity (EC) of 0.01, 0.95, 2.0, and 4.0 dS m -1 ) were tested. Final BD was significantly greater than the initial value when matric potential was applied. Conversely, for continuous leaching (i.e., no application of matric potential), differences between final and initial BD were insignificant. Water quality and sodicity did not affect BD, suggesting that for a given soil exposed to a low level of matric potential, soil consolidation was not affected by water quality or ESP. Final HC values were always lower than initial ones, with the decrease in HC after application of matric potential being by far greater than that observed when continuous leaching was used. The decrease in HC relative to initial HC depended on both water quality, and soil sodicity, was greater with the decrease in water EC and an increase in ESP. Adverse effects of low EC and high ESP on HC were less pronounced in matric potential application than in continuous leaching. Our results suggest that the quality of water available for irrigation and soil sodicity should be taken into account in cases where interrupted flow is considered for improving furrow irrigation efficiency via reducing soil infiltration rate.

Prewetting rate and sodicity effects on the hydraulic conductivity of soils

Contribution from the Agricultural Research Organization (ARO), The Volcani Center, Bet Dagan, Israel. No. 604/2000 series. Exchangeable sodium deteriorates the hydraulic conductivity of soils. The susceptibility of soils to sodicity depends on the soils’ inherent properties, and is known to increase with an increase in clay content and the presence of 2:1 clay minerals, and decrease with an increase in sesquioxides content. The objective of this study was to determine the effect of an extrinsic time-dependent property, such as soil prewetting rate (PWR), on the hydraulic conductivity (HC) of smectitic soils varying in their exchangeable sodium percentage (ESP) between 1 and 10 and in clay content. Five soils were studied: a loamy sand, a loam, a sandy clay, and 2 clay soils. Air-dried soil samples with ESP of 2, 6, and 10 were packed in plexiglass columns and were prewetted from below at 3 rates (1.7, 4.25, and 50 mm/h) with deionised water. The columns were then leached at constant hydraulic head with deionised water, and flow rates were measured. The reference hydraulic conductivity (HC0) of the loamy sand was not affected by both PWR and sodicity. In the loam, HC0 was not affected by rate of wetting. However, increasing the ESP from 2.1 to 9.5 decreased HC0 from 15.8 to 5.0 mm/h for the fast wetting. Similar decreases were noted in the other 2 wetting rates. The hydraulic conductivity of the sandy clay and the 2 clay soils were effected by PWR. In the sandy clay with ESP 5.5, increasing PWR from 1.7 to 50 mm/h resulted in a decrease in HC0 from 78.7 to 3.2 mm/h The decrease in HC0 in these soils with the increase in wetting rate was ascribed to aggregate slaking. The effect of PWR on HC0 was more notable, especially at the low ESP, as clay content increased. The hydraulic conductivity of the sodic soils leached with distilled water decreased more steeply and to lower values with increasing PWR. Fast prewetting, which increased breakdown of soil aggregates, increased the susceptibility of the soils to sodicity

Wastewater Effects on Montmorillonite Suspensions and Hydraulic Properties of Sandy Soils

Recycled wastewater effluent is an important source of irrigation water in arid and semiarid regions. In these regions, however, irrigation water quality is one of the main factors limiting plant growth. Wastewater effluents generally contain high concentrations of suspended and dissolved solids, both organic and inorganic. Inorganic dissolved solids are only minimally removed from the effluent during conventional sewage treatment (secondary or tertiary). As a result, most of the salts added during domestic and industrial usage remain in the irrigation water and may eventually reach the soil. A number of researchers have reported reduced hydraulic conductivity for soils to which treated wastewater has been applied. In this research, the influence of dissolved organic matter (DOM) contained in reclaimed wastewater effluents (mainly humic substances) on the flocculation of montmorillonite and on the hydraulic properties of soils was studied. Flocculation values (FVs) for Na-montmorillonite increased with increasing concentrations of DOM at all pH levels analyzed (5.5, 7.0, and 8.5). Maximum FV levels were exhibited for Na-montmorillonite at the highest DOM concentrations. The effect of DOM on FV can be explained by the mechanisms of edge-charge reversal and mutual flocculation. The hydraulic conductivity (HC) of a sandy soil was determined in the laboratory by leaching columns with an electrolyte solution chemically similar to that of the wastewater effluent (but without DOM). In columns treated with wastewater effluent, the HC exhibited a sharp decrease to only 20% of its initial value. The adverse effect of DOM on HC was evident for this soil despite a relatively low exchangeable sodium percentage (ESP). The reduction in HC is likely to be the result of decreased soil pore-size, which reflects two processes: (i) retention of part of the DOM during water percolation; and (ii) a change in pore-size distribution due to swelling and dispersion of clay particles. The latter may result from a higher percentage of adsorbed sodium combined with the presence of humic substances from the wastewater effluent.

Clay mineralogy as affecting disaggregation in some palygorskite containing soils of the Jordan and Bet-She'an Valleys

The influence of clay mineralogy on disaggregation in some palygorskite-containing soils of the Jordan and Bet-She'an Valleys was examined. The disaggregation potential of different minerals in the soil clay fraction was investigated by establishing differences in the mineral suite between the original and disaggregated clay fractions. By shaking the soil with distilled water, calcite, dolomite, feldspar, and palygorskite were disaggregated preferentially. The pattern for quartz was inconsistent. Among phyllosilicates, palygorskite was the most strongly, disaggregated, while smectite was the least disaggregated mineral. The disaggregation potential of kaolinite was of intermediate value. By shaking the soil with 0 ⋅ 01 N NaCl, calcite and dolomite were released preferentially, and were the dominant minerals in the disaggregated clay fraction. Scanning electron microscopy observations indicate that palygorskite fibres do not associate into aggregates in soils and suspensions, even when saturated with calcium ions. The present findings are relevant for soils with low exchangeable sodium percentage. These soils can be expected under rain-fed agriculture or irrigation with high quality water which has a low sodium adsorption ratio.

MICROBIOLOGICAL AND CHEMICAL AMELIORATION OF ALKALINE SOIL BY GROWING KARNAL GRASS AND GYPSUM APPLICATION

In a field experiment, microbiological and chemical amelioration of a highly deteriorated alkaline soil (pH 10.6, exchangeable sodium percentage 95) was undertaken during 1989–92 using two reclamation technologies: namely, growing Karnal grass (Leptochloa fusca) as a first crop in the absence of any amendment (biological reclamation) or applying gypsum as a chemical amendment for different cropping sequences. After ten months average dehydrogenase activity, used as a measure of catabolic activity of micro-organisms under anaerobic conditions, was 49.5 μg triphenylformazan (TPF) per g soil in treatments where the soil was reclaimed by growing Karnal grass, compared with 26.8 μg TPF by applying 50% of the estimated requirement of gypsum. After three years of reclamation, average dehydrogenase activity was 118.7 μg TPF per g soil with Karnal grass and 96.1 μg TPF with gypsum. After three years average microbial biomass carbon was 28% greater in the gypsum treatments (206.5 mg per kg soil) compared with Karnal grass treatments (161.7 mg). However, at this stage the ratio between average dehydrogenase activity and microbial biomass carbon was greater with Karnal grass (0.77) than in gypsum treatments (0.47). The microbiological properties changed more than the chemical properties of alkali soil as the time period advanced. After three years of reclamation, the average pH of the alkali soil dropped from 10.6 to 9.45 and the average exchangeable sodium percentage was reduced from 95 to 47.5. The increase in total organic carbon was 64% and in available nitrogen about 38% compared with the original soil. Higher microbial biomass carbon in gypsum treatments corresponded with lower exchangeable sodium percentage, higher availability of nitrogen and potassium and greater total yield of sensitive winter clovers.

Factors and processes of soil degradation in vertisols of the Purna Valley, Maharashira, India

The Vertisols of the Purna Valley, which cover the districts of Amravati, Akola and Buldhana in the state of Maharashtra, India, lack any perceptible evidence of salt efflorescence on the soil surface which would indicate the presence of salt, but the drainage conditions are poor. The limited data available indicate that the adverse physical condition of the soils is due to their poor hydraulic conductivity (HC), which is impaired by sodium in the exchange complex. However, the factors and processes that are inherently related to the development of sodicity in these shrink-swell soils are not yet understood. In order to establish the cause-effect relationship, eight Vertisol pedons from methodically selected sites were studied morphometrically in the field, and for their sodicity-related physical and chemical properties in the laboratory. The soils are deep, calcareous, clayey and very dark greyish brown to dark brown in colour. Cracks extend up to the slickenside zones in soils of Pedons 1–3 in the northeastern area of the valley, while they cut through the slickenside zones in soils of Pedons 4–8 in the southwest. The slickenside faces were larger in the soils of the southwest than in those of the northeast. All these soils meet the specifications of the Vertisols order of soil taxonomy. Saturation extracts of the soils had very low electrical conductivity (ECe ⩽ 2 ds m−1). They ranged from moderately alkaline in the northeast, Pedons 1–3, to strongly alkaline in the southwest, pedons 4–8. In soils from the northeast the exchangeable sodium percentage (ESP) was less than 5 throughout the depth of the pedons, whereas in other soils it was up to 6 in the surface horizons and between 7 and 26 in the subsoil horizons; four of these soils qualified as sodic according to the criteria of the United States Salinity Laboratory. The inherently low hydraulic conductivity was due to the dispersion of clay particles caused by a high percentage of exchangeable magnesium (EMP) in the highly smectitic soils, and also to a slight increase in ESP (⩾5). The results of this study suggest that ESP 5 should be used as the lower limit for sodic subgroups of Vertisols, rather than ESP 15 as given in Keys to Soil Taxonomy (Soil Survey Staff, 1994). This is because there are severe limitations to the use of such soils owing to the development of adverse physical conditions even at such a low ESP. The authors emphasize the need to keep this fact in mind during future land resource management programmes on the soils of this valley as well as on similar soils occurring elsewhere. The development of sodicity in the soils of the southwestern part of the valley has been attributed to the semi-arid climatic conditions that have induced the pedogenetic process of depletion of calcium ions from the soil solution in the form of calcium carbonate, thereby resulting in an increase of both the sodium adsorption ratio (SAR) and the ESP with pedon depth. This chemical degradation, which affects the sodicity of Vertisols, appears to be a basic process that needs to be recognised in the future along with those already described as natural processes of soil degradation.

Influence of Salinity and Sodicity on Soil Structural and Hydraulic Characteristics

AbstractAn exchangeable sodium percentage (ESP) &gt;15 is traditionally considered to affect soil structural and hydraulic characteristics. Recent investigations show that both this critical value and the concept of critical threshold need reconsideration, because soil degradation often takes place even at lower ESP in dilute solutions, and soil behavior at increasing ESP appears to be a continuum. This study was carried out to analyze the response of two Sicilian Typic Haploxererts to ESP values up to 15, at a low cationic concentration. The investigation was carried out on aggregate stability, rating of soil shrink‐swell potential, and both saturated and unsaturated hydraulic conductivity. The specific purpose was to verify if a critical ESP threshold exists, or if the hypothesis of a continuum behavior is more appropriate. The high‐energy moisture characteristic technique was used for determining aggregate stability, and Brasher's method for the shrinkage characteristic. The constant‐head method was used for determining saturated hydraulic conductivity, and the parameter estimation method based on one‐step outflow experiments for unsaturated hydraulic conductivity. Almost linear relationships were found between the investigated soil properties and ESP, indicating no critical ESP threshold; furthermore, the results obtained indicate that an effective hazard of soil quality degradation can be forecast even in a 2 to 5 ESP range at a low cationic concentration. This is a basic indication for irrigation management aimed to combat and prevent degradation of soil quality.

Polymer effects on runoff and soil erosion from sodic soils

High levels of soil sodicity, resulting from intensive irrigation with saline-sodic waters, lead to an increased soil susceptibility to seal formation and to severe problems of runoff and soil erosion. The objective of this study was to investigate the efficacy of the addition of small amounts of an anionic polyacrylamide (PAM) to the irrigation water in controlling seal formation, runoff and soil erosion. Two predominantly montmorillonitic soils were studied, a grumusol (Typic Haploxerert) and a loess (Calcic Haploxeralf), having naturally occurring exchangeable sodium percentage (ESP)>12. The soils were exposed to 60 mm of simulated irrigation with commonly used tap water (TW, electrical conductivity=0.8 dS m−1; sodium adsorption ratio (SAR)=2), or saline water (SW, electrical conductivity=5.0 dS m−1; SAR>12). PAM effectiveness in controlling runoff and erosion from the sodic soils was compared with runoff and erosion levels obtained from untreated soils having low ESPs (<4). For both soils and for both water qualities and polymer concentrations in the irrigation water, PAM was efficient in controlling runoff at low ESP levels and inefficient at high ESP levels. At moderate ESP levels, PAM's efficacy in controlling runoff was inconsistent and varied with water quality and polymer concentration. Conversely, in general, soil loss originating from rill erosion, was significantly and effectively reduced in moderate and high ESP soils by addition of PAM to the irrigation water, irrespective of water quality and polymer concentration. PAM was more effective in reducing rill erosion than in reducing runoff in the moderate and high ESP samples, because the energy involved in generating runoff is much higher than that involved in rill erosion. PAM treated surface aggregates were not stable against the distructive forces leading to seal formation and runoff production; but they were stable enough to resist the hydraulic shear exerted by the runoff flow.