Mobility of the herbicide pyrithiobac through intact soil columns

The relative mobility of pyrithiobac [sodium 2-chloro-6-(4, 6-dimethoxypyrimidin-2-ylthio)benzoate], a new herbicide used for postemergence control of broadleaf weeds in cotton (Gossypium hirsutum), was evaluated and compared against that of bromide (Br(-)) tracer on four soils representative of cotton-growing regions using intact soil columns under saturated flow conditions. Pyrithiobac breakthrough curves were asymmetrical in shape with significant tailing and displaced to the left of 1 pore volume in the Houston Black clay (fine, montmorillonitic, thermic Udic Pellustert), Orelia fine sandy clay loam (fine-loamy, mixed, hyperthermic Typic Ochraqualfs), and Ships silty clay (very-fine, mixed, thermic Udic Chromustert) soils. Breakthrough of pyrithiobac in the Hidalgo sandy loam soil (fine-loamy, mixed, hyperthermic Typic Calciustoll) was delayed and more symmetrical, with peak pyrithiobac concentration reached after 1.2 pore volumes. The immobile pore water (IPW) fractions estimated from the Br(-) breakthrough curves ranged from 20 to 87% of total pore water. The IPW values demonstrated that soils with the greatest amount of IPW (Ships with IPW = 87.3%) exhibited the most rapid movement of pyrithiobac (peak concentration after 0.04 pore volume). The experimentally determined pyrithiobac breakthrough curves confirmed the high mobility of this herbicide in these alkaline and predominantly smectitic soils. These results indicate that pyrithiobac mobility was influenced by soil type and preferential flow processes when leached through intact soil columns.     

Comparing Nutrient Availability in Low-Fertility Soils using Ion Exchange Resin Capsules

Assessing the nutrient status of low-input, low-fertility desert soils poses some unique challenges. Commonly used soil analysis procedures and resin capsules generally assess nutrient status of fertile agricultural soils. Ion-exchange resin capsules (Unibest Company, Bozeman, Mont.) provide a viable alternative. A study was conducted to determine effectiveness of resin capsules to extract low levels of nutrients applied to native soils. Loamy sand and sandy clay loam desert soils from Utah were treated with combinations of four rates of nitrogen (N) as ammonium nitrate (34–0–0), three rates of phosphorus (P) as phosphoric acid (0–72–0), and two rates of iron sulfate (FeSO₄·7H₂O) and zinc sulfate (ZnSO₄·7H₂O) (include an untreated control). Each soil treatment was implanted with a resin capsule placed into either 250 or 1000 cm³ of soil after addition of water equivalent to 50% field capacity and incubated for either 60 or 120 days at 25 °C. After the appropriate incubation time, capsules were washed and extracted using 2 M hydrochloric acid (HCl), and the extract was used to measure iron (Fe), ammonium (NH₄)-N, nitrate (NO₃)-N, sulfur (S), and zinc (Zn). Conventional soil tests were completed on incubated soils (60 or 120 days). Resin capsules reflected NH₄-N and P fertilizer applied at low rates in the loamy sand but not in the sandy clay loam. Neither Fe nor Zn application was reflected in resin capsules, but the accompanying S was clearly quantified. In comparison to conventional soil test procedures, resin capsule NH₄-N was clearly a better indicator than KCl-extractable NH₄-N; resin capsule NO₃-N was effective, but not as good an indicator as water extraction; and resin capsule P was reflective of soil applied P in loamy sand but not in sandy clay loam, whereas sodium bicarbonate was effective in both soils. Resin capsules show promise for use in low-input conditions, but additional understanding of interactions in variable soils is needed.     

Correlation of solution and extractable phosphorus with vegetative growth of teff

Abstract

Three soils, Hiwassee loam (clayey, kaolinitic, thermic, Typic Rhodudults), Vaiden clay (very fine, montmorillonitic, thermic, Vertic Hapludalfs) and Marvyn sand (fine, loamy, siliceous, thermic, Typic Hapludults), were used in this study. Phosphorus sorption and desorption isotherms of the soils were determined in the laboratory. Average P sorption maximum (b) was calculated using the Langmuir isotherm equation. Based on the P sorption capacity, each soil was fertilized with different P rates and teff [Eragrostis tef (Zucc)] was grown in the greenhouse. The amount of P sorbed and desorbed increased as the concentration of equilibrating solution P increased. Phosphorus sorption maximum from sorption and desorption isotherms was 278, 251, and 37 mg P/kg for Hiwassee, Vaiden, and Marvyn soils, respectively, and for maximum dry matter yield of teff the soils needed a minimum of 0.029, 0.048, and 0.065 mg/L soil solution P, respectively, which were all below the soil solution P at P adsorption maximum.

Soil P was extracted by the Mehlich I, Mehlich III, modified Mississippi, Bray P‐l and Olsen methods. Plant‐available P extracted by each of the five methods was significantly correlated with teff dry matter yield, and statistically any of the five methods can be used as the basis for predicting teff yield responses.     

Potassium fertilization effects on yield and longevity of established alfalfa

Abstract

Limited information is available which describes the response of established alfalfa (Medicago sativa L.) to topdressing applications of K fertilizer in the Southeastern United States. Field experiments were conducted for three years to determine alfalfa response to rates and time of K application. The experiments were established in two‐year old stands of alfalfa on a Decatur silty clay loam (clayey, kaolinitic, thermic Rhodic Paleudults) and a Hartsells fine sandy loam (fine loamy, siliceous, thermic Typic Hapludults) located in northern Alabama. Potassium as KCl was broadcast in the spring prior to regrowth. For split application treatments, the K was applied in early spring and after the second cutting. Annual total K rates ranged from 56 to 596 kg/ha. Potassium fertilization maintained alfalfa stand density on both soils, but the experiment on the Decatur soil was discontinued after two years due to severe stand loss when the lowest rate of added K was used. Alfalfa yields were increased by the application of K and maximum yields occurred when K was applied according to soil test recommendations made by the Auburn University soil testing laboratory. Potassium applications increased the concentration of K, decreased the concentration of Ca and Mg and had little effect on the concentration of N in tissue from the two cuttings sampled. The split application of K did not consistently increase forage production. There was little movement of K below 25 cm in either soil when K rates of 56 to 596 kg/ha/yr were repeated yearly for up to three years.     

A potential method to incorporate quantity/intensity into routine soil test interpretations

Abstract

Exchangeable potassium (K), extracted with 1M ammonium acetate, widely used as the measure for plant available K does not estimate the K supplying capacity of the soil. This research was undertaken to quantity the K supplying capacities of some Missouri soils and evaluate a modified K Quantity‐Intensity (Q/I) approach for adaptation into routine soil analysis. The K supplying capacities as analyzed by the modified Q/I and plant removal were measured on the 0–20 cm and 20–40 cm depths of Broseley loamy fine sand (loamy, mixed, thermic Arenic Hapludalf), Mexico silt loam, (fine, montmorillonitic, mesic Udollic Ochraqualf), Waldron clay loam [fine, montmorillonitic (calcareous) mesic Arenic Fluvaquemt] and the 0–20 cm of Sharkey clay (very fine, montmorillonitic, nonacid, thermic Vertic Haplaquet). The dominant clays in the clay fractions were identified by X‐ray diffraction to be composed of montmorillonite and illite. The high linear coefficient of correlation (R₂ = 0.92**) between the potassium buffer capacity index (KBC Index) and cation exchange capacity [CEC, sum of exchangeable Ca, Mg, K, Na, and Neutralizable Acidity (NA)] suggests that KBC Index of soils with similar clay mineral compositions, but different clay quantities, could be estimated from the measured CEC. Plant K uptake was highly linearly correlated with K quantity (Kq, R₂ =0.98**) as measured by the modified Kq/i method. It appears to be possible, using the KBC Index derived from the CEC, to calculate a Kq/i value with a single measurement of the K in equilibrium with 0.01 M CaCl₂. With the modified Kq/i method, and using this approach, a total plant available K model is presented. This modified Kq/i approach offers the opportunity for soil testing laboratories to complete a calculation for total plant available K with only one additional measurement of the equilibrium K in 0.01 M CaCl2. This would facilitate the fine tuning of soil test interpretation by basing K fertilizer recommendations on the quantity of plant‐available K in the soil.     

Effects of soil compaction and water‐filled pore space on soil microbial activity and N losses

Abstract

Soil compaction is a significant production problem for agriculture because of its negative impact on plant growth, which in many cases has been attributed to changes in soil N transformations. A laboratory experiment was conducted to study the effect of soil compaction and water‐filled pore space on soil microbial activity and N losses. A hydraulic soil compaction device was used to evenly compress a Norfolk loamy sand (fine‐loamy, siliceous, thermic Typic Kandiudults) soil into 50 mm diameter by 127 mm long cores. A factorial arrangement of three bulk density levels (1.4, 1.6, and 1.8 Mg/m³) and four water‐filled pore space levels (60, 65, 70, 75%) was used. Fertilizer application of 168 kg N/ha was made as 1.0 atom % ¹⁵N as NH₄NO₃. Soil cores were incubated at 25°C for 21 d. Microbial activity decreased with both increasing water‐filled pore space and soil bulk density as measured by CO₂‐C entrapment. Nitrogen loss increased with increasing bulk density from 92.8 to 334.4 g N/m³ soil at 60% water‐filled pore space, for 1.4 and 1.8 Mg/m³, respectively. These data indicate that N loss and soil microbial activity depends not only on the pore space occupied by water, but also on structure and size of soil pores which are altered by compaction.     

Soil texture effect on nitrate leaching in soil percolates

Abstract

Nitrate nitrogen (NO₃‐N), which is an essential source of nitrogen (N) for plant growth, is now also considered a potential pollutant by the Environmental Protection Agency (EPA). This is because excess applied amounts of NO₃‐N can move into streams by run‐off and into ground water by leaching, thereby becoming an environmental hazard. Soils have varied retentive properties depending on their texture, organic matter content, and cation exchange capacity (CEC). The purpose of this study was to determine the effect of soil texture on NO₃‐N retention to reduce NO₃‐N contamination in the environment. A sand, 85:15 sand:peat Greensmix, a loamy sand, and sandy clay loam soils were placed in 2×3 inch metal cylinders and soaked in a 240 ppm solution of NO₃‐N for seven days to saturate the soil with NO₃ ions. The columns were leached with water to collect 10 soil percolate samples of 50 mL each until a total volume of 500 mL was collected. Nitrate‐N was measured in each 50‐mL aliquot by automated colorimetry. The results showed that soil texture affected the retention of N03‐N in the sand, which adsorbed the least amount of NO₃‐N at 119 ppm, followed by the Greensmix at 125 ppm, loamy sand at 149 ppm, and sandy clay loam at 173 ppm. More NO₃‐N was released in the first 50 mL of the sand percolate at 63% followed by the Greensmix, loamy sand, and sandy clay loam at 58,46, and 37% NO₃‐N released, respectively. Soils with more silt, clay, and organic matter retained more NO₃‐N than the straight sand. Therefore, a straight sand would be the poorest of soil types since NO₃‐N retention was low.     

Beryllium phytotoxicity in soybeans

A greenhouse study was conducted to assess the effects of soil-applied beryllium (Be) on the growth and Be content of soybeans [Glycine max (L.) Merr.], grown on acid southeastern soils under limed and unlimed conditions. This study was conducted using a factorial design, with two soil types varying in clay content (Blanton sand, a loamy, siliceous, thermic Grossarenic Paleudult; and Orangeburg loamy sand, a loamy, siliceous, thermic Typic Paleudult), two soil treatments (limed and unlimed) and five Be concentrations (0, 25, 50, 100, and 150 mg Be kg^?1 t soil). Addition of Be to unlimed Blanton soil had the most toxic effects of all treatment combinations; at the 150 mg Be kg^?1 treatment plant biomass was reduced as much as 90% and plant Be concentration was as high as 226 mg Be kg^?1. Beryllium concentrations were greater in plants grown in a soil low in clay (Blanton soil). Liming of soils treated with Be resulted in lowered tissue Be concentrations in plants grown on either soil type.     

Long‐term effects of copper rich swine manure application on continuous corn production

Abstract

Three long‐term field experiments were established in the spring of 1978 and continued through 1988 to evaluate corn (Zea mays L.) response to high Cu levels from Cu rich swine manure and CuSO₄ applications. The field research was conducted on soils with diverse properties, i.e., on a Bertie fine sandy loam (fine‐loamy, mixed, thermic, Aquic Hapludults), a Guernsey silt loam (fine, mixed, mesic, Aquic Hapludalfs), and a Starr‐Dyke clay loam (fine‐loamy, mixed, thermic Fluventic Dystrochrepts‐clayey, mixed, mesic, Typic Rhodudults). Three treatments in the long‐term field experiments on the three soils were i) a control, ii) annual applications of Cu as Cu rich swine manure, and iii) annual applications of Cu as CuSO₄ equivalent to that in the manure. After the 11 years, 1109 mt ha^‐1 of wet Cu rich swine manure were applied for the manure treatment. The Cu rich manure, which contained an average of 1316 mg Cu kg^‐1, was produced by swine fed diets supplemented with an average of 251 mg Cu kg^‐1 as CuSO₄. An average of 325 kg Cu ha^‐1 added to the soils from the manure application over the 11 years exceeded U.S.E.P.A. guidelines for safe copper loading levels for cropland by 45 kg ha^‐1. Copper concentrations in corn ear leaves were within the normal range of 3 to 15 mg kg^‐1 where the 325 kg Cu ha^‐1 were applied to the soils as either Cu rich manure or CuSO₄ over the 11 years. Concentrations of Cu in the grain also were in the normal range of 1 to 5 mg kg^‐1 where the high level of the two Cu sources were applied to the soils. There was no decrease in corn yield on the three soils from application of either Cu source. The lack of yield decrease from the Cu rich manure and CuSO₄ applications may reflect decreased Cu availability at the near neutral pH in the soils under study.     

Impact of addition of different rates of rice-residue biochar on C and N dynamics in texturally diverse soils

Impacts of crop residue biochar on soil C and N dynamics have been found to be subtly inconsistent in diverse soils. In the present study, three soils differing in texture (loamy sand, sandy clay loam and clay) were amended with different rates (0%, 0.5%, 1%, 2% and 4%) of rice-residue biochar and incubated at 25°C for 60 days. Soil respiration was measured throughout the incubation period whereas, microbial biomass C (MBC), dissolved organic C (DOC), NH₄⁺-N and NO₃^–N were analysed after 2, 7, 14, 28 and 60 days of incubation. Carbon mineralization differed significantly between the soils with loamy sand evolving the greatest CO₂ followed by sandy clay loam and clay. Likewise, irrespective of the sampling period, MBC, DOC, NH₄⁺-N and NO₃^–N increased significantly with increasing rate of biochar addition, with consistently higher values in loamy sand than the other two soils. Furthermore, regardless of the biochar rates, NO₃^–-N concentration increased significantly with increasing period of incubation, but in contrast, NH₄⁺-N temporarily increased and thereafter, decreased until day 60 in all soils. It is concluded that C and N mineralization in the biochar amended soils varied with the texture and native organic C status of the soils.     

Toxicity of brass powder to corn and the relationship to soil fractionation of copper and zinc

Abstract

The relationship of Cu and Zn fractionation in soils to plant growth was investigated on a Sassafras sandy loam (fine‐loamy, siliceous, mesic Typic Hapludult) and a Joppa gravelly sandy loam (loamy‐skeletal, siliceous, mesic Typic Hapludult). Pot studies were conducted in the greenhouse exposing corn (Zea mays cv. Silver Queen) to concentrations of 0, 100, 200, and 400 mg brass powder/kg soil. The corn was grown over a two week period, during which time individual plant heights were taken to note differences in plant growth. Data showed that plant height was reduced when the concentration of brass powder in the soil was increased. Total and fractionated Cu and Zn levels in the soils were analyzed and compared to height. Regression equations showed a more explicit relationship between height and exchangeable Cu levels rather than other Cu and Zn levels in the two soils. Additionally, a more pronounced effect was seen in plants grown on the Sassafras sandy loam, which was attributed to differences in soil physicochemical properties.     

Changes in Soil Test Phosphorus from Broiler Litter Additions

Abstract

Nutrient surpluses on the Delmarva Peninsula have led to a continual accumulation of soil test phosphorus (STP), a potential source for transport of phosphorus (P) to surface waters. This article examines the effects of initial soil test P concentrations and broiler litter additions on STP accumulation. Broiler litter (BL) was applied at rates of 0, 2.5, 5, 7.5, and 10 g kg^?1 (dry weight) to three soils: an Evesboro sandy loam (Mesic, coated Typic Quartzipsamments), a Pocomoke sandy loam (coarse‐loamy, siliceous, thermic typic Umbraquults), and a Matapeake silt loam (fine‐silty, mixed, semiactive, mesic Typic Hapludults). Soils and BL were incubated for 16 weeks with subsamples analyzed after 4 and 16 weeks. There was a linear increase in STP (Mehlich‐3), water‐soluble P (WS‐P), iron‐oxide strip‐extractable P (FeO‐P), and Mehlich‐3 phosphorus saturation ratio (M3‐PSR) with broiler litter additions. Regression analysis indicated few significant differences in STP response to added BL between soils within the same soil group having different initial STP levels. Correlation analysis and stepwise regression indicated that increases in WS‐P and FeO‐P from added BL were more closely related to the degree of P saturation of the soil rather than traditional STP measurements. Therefore, decisions regarding manure placement within a watershed should be based on the potential P sorption capacity of the soil as well as potential P transport pathways when the goal is the reduction of P transfer to waterbodies.     

Nitrogen transformations in cold and frozen agricultural soils following organic amendments

Though microbial activity is known to occur in frozen soils, little is known about the fate of animal manure N applied in the fall to agricultural soils located in areas with prolonged winter periods. Our objective was to examine transformations of soil and pig slurry N at low temperatures. Loamy and clay soils were either unamended (Control), amended with ¹⁵NH₄-labeled pig slurry, or amended with the pig slurry and wheat straw. Soils were incubated at −6, −2, 2, 6, and 10 °C. The amounts of NH₄, NO₃ and microbial biomass N (MBN), and the presence of ¹⁵N in these pools were monitored. Total mineral N, NO₃ and ¹⁵NO₃ increased at temperature down to −2 °C in the loam soil and −6 °C in the clay soil, indicating that nitrification and mineralization proceeded in frozen soils. Nitrification and mineralization rates were 1.8-4.9 times higher in the clay than in the loamy soil, especially below freezing point (3.2-4.9), possibly because more unfrozen water remained in the clay than in the loamy soil. Slurry addition increased nitrification rates by 3-14 times at all temperatures, indicating that this process was N-limited even in frozen soils. Straw incorporation caused significant net N immobilization only at temperatures ≥2 °C in both soils; the rates were 1.4-3.4 higher in the loam than in the clay soil. Nevertheless, up to 30% of the applied ¹⁵N was present in MBN at all temperatures. These findings indicate that microbial N immobilization occurred in frozen soils, but was not strong enough to induce net immobilization below the freezing point, even in the presence of straw. The Q₁₀ values for estimated mineralization and nitrification rates were one to two orders-of-magnitude larger below 2 °C than above this temperature (13-208 versus 1.5-6.9, respectively), indicating that these processes are highly sensitive to a small increase in soil temperature around the freezing point of water. This study confirms that net mineralization and nitrification can occur at potentially significant rates in frozen agricultural soils, especially in the presence of organic amendments. In contrast, net N immobilization could be detected essentially above the freezing point. Our results imply that fall-applied N could be at risk of overwinter losses, particularly in fine-textured soils.     

Soil nitrogen and carbon status following clover production in louisiana

Abstract

Field studies were conducted for four to seven years on two soils, Tangi silt loam (Typic Fragiudalf, fine‐silty, mixed, thermic) and Dexter loam (Ultic Hapludalf, fine‐silty, mixed, thermic), to determine the effects of phosphorus (P) applications on growth and nitrogen (N) content of white clover (Trifolium repens L.) and subterranean clover (Trifolium subterranum L.) and on ammonium (NH₄ ⁺)‐ and nitrate (NO₃ ^‐)‐N, total N, and organic carbon (C) levels in the soils at the end of the study. Phosphorus applications consistently and significantly increased forage yields and led to significantly higher N yields by the clovers. Increases in plant yields and N₂‐fixation, however, were not reflected in higher soil N and C levels. On Tangi soil, NH₄ ⁺‐ and NO₃ ^‐‐N levels were lowest where no P was applied but no statistically significant differences (P < 0.05) were found among P rates above 20 kg/ha. On the Dexter soils, no significant differences were found at any P application level. Significant differences due to higher clover yields at increasing P rates were not found in total N or organic C . levels in either soil. Greenhouse evaluations showed no differences in bermuda‐grass yield, N concentration, or total N recovery despite increasing subclover yields in the field during the previous seven years. Harvesting nearly all above ground clover growth caused plant roots to be the major N and C contributor to the soil. It is possible that root production was not increased in proportion to forage production as P applications increased. Perhaps increased microbial activities and some leaching losses also minimized accumulations of N and C released by clover roots.     

In‐row soil aggregate characteristics for two Iowa soils

Abstract

Chemical and physical characteristics of soil aggregates from Clarion loam (fine‐loamy, mixed, mesic Typical Hapludoll) and Edina silt loam (fine, montmorillonitic, mesic Typic Argialboll) soil were evaluated to compare two alternate farming practices. Aggregate size distribution was measured for the 0‐to‐3 mm and 0‐to‐76 mm fractions collected from within existing ridges to a soil depth of 0.15 m prior to planting corn (Zea mays L.), after the first cultivation, at anthesis, and after harvest in 1990. Selected physical and chemical properties were determined for air‐dry aggregates in six size classes (0‐to‐0.5, 0.5‐to‐1.0, 1.0‐to‐2.0, 2.0‐to‐3.0, 3.0‐to‐4.0, and 4.0‐to‐5.0 mm) collected from the two Iowa soils prior to planting. Temporal changes in aggregate size distribution were observed for both soils, presumably because of combined rainfall and crop management effects. Physical and chemical properties of soil aggregates showed large differences between locations and within size fractions when compared to the bulk soil. Clarion loam had relatively more consistent exchangeable cation concentrations compared to Edina silt loam, presumably because of sustained incorporation of manure and municipal sludge at that site. Soil aggregation and aggregate properties can be used as indicators to evaluate the effects of alternate fanning practices.     

Tillage effects on shear strength and bulk density of soil aggregates

An experiment was conducted to determine the effect of four tillage systems (moldboard plow, chisel plow, Paraplow and no-till) on soil aggregate shear strength and bulk density. Two soils, a Canisteo clay loam (fine-loamy, mixed (calcareous), mesic, Typic Haplaquoll) and a Haig silt loam (fine, montmorillonitic, mesic, Typic Argiaquoll) were used in this study. Soil samples were collected from the 0.075–0.15-m-depth increment in 1983 and the 0.075–0.15- and 0.225–0.30-m-depth increments in 1985. Shear strength of soil aggregates 0.02–0.03 m in diameter was measured by a fall-cone penetrometer and bulk density of the same aggregates was measured by gamma-ray attenuation. Aggregates were tested at soil water matric potentials (ψ_m) of −0.2, −1.1 and −4.0 kPa in 1983 and at ψ_m of −0.2, −1.1, −4.0 and −7.9 kPa in 1985. Tillage for the 1983 growing season was conducted under very wet conditions, whereas tillage for the 1985 growing season was conducted under much drier conditions. Samples collected in 1983 showed little tillage effect on shear strength or bulk density. In 1985, tillage had an effect on shear strength and bulk density for the Haig soil, but not for the Canisteo soil. Much of the tillage effect on soil aggregate shear strength could be explained by tillage-induced changes in the aggregate bulk density. As bulk density decreased, soil aggregate shear strength decreased.Sampling depth had no effect on soil aggregate shear strength or bulk density. Matric potential had an effect on soil aggregate shear strength and bulk density. As matric potential decreased, both shear strength and bulk density increased.     

Salinity effects on carbon mineralization in soils of varying texture

In salt-affected soils, soil organic carbon (SOC) levels are usually low as a result of poor plant growth; additionally, decomposition of soil organic matter (SOM) may be negatively affected. Soil organic carbon models, such as the Rothamsted Carbon Model (RothC), that are used to estimate carbon dioxide (CO₂) emission and SOC stocks at various spatial scales, do not consider the effect of salinity on CO₂ emissions and may therefore over-estimate CO₂ release from saline soils. Two laboratory incubation experiments were conducted to assess the effect of soil texture on the response of CO₂ release to salinity, and to calculate a rate modifier for salinity to be introduced into the RothC model. The soils used were a sandy loam (18.7% clay) and a sandy clay loam (22.5% clay) in one experiment and a loamy sand (6.3% clay) and a clay (42% clay) in another experiment. The water content was adjusted to 75%, 55%, 50% and 45% water holding capacity (WHC) for the loamy sand, sandy loam, sandy clay loam and the clay, respectively to ensure optimal soil moisture for decomposition. Sodium chloride (NaCl) was used to develop a range of salinities: electrical conductivity of the 1:5 soil: water extract (EC_1:5) 1, 2, 3, 4 and 5 dS m⁻¹. The soils were amended with 2% (w/w) wheat residues and CO₂ emission was measured over 4 months. Carbon dioxide release was also measured from five salt-affected soils from the field for model evaluation. In all soils, cumulative CO₂-C g⁻¹ soil significantly decreased with increasing EC_1:5 developed by addition of NaCl, but the relative decrease differed among the soils. In the salt-amended soils, the reduction in normalised cumulative respiration (in percentage for the control) at EC_1:5 > 1.0 dS m⁻¹ was most pronounced in the loamy sand. This is due to the differential water content of the soils, at the same EC_1:5; the salt concentration in the soil solution is higher in the coarser textured soils than in fine textured soils because in the former soils, the water content for optimal decomposition is lower. When salinity was expressed as osmotic potential, the decrease in normalised cumulative respiration with increasing salinity was less than with EC_1:5. The osmotic potential of the soil solution is a more appropriate parameter for estimating the salinity effect on microbial activity than the electrical conductivity (EC) because osmotic potential, unlike EC, takes account into salt concentration in the soil solution as a function of the water content. The decrease in particulate organic carbon (POC) was smaller in soils with low osmotic potential whereas total organic carbon, humus-C and charcoal-C did not change over time, and were not significantly affected by salinity. The modelling of cumulative respiration data using a two compartment model showed that the decomposition of labile carbon (C) pool is more sensitive to salinity than that of the slow C pool. The evaluation of RothC, modified to include the decomposition rate modifier for salinity developed from the salt-amended soils, against saline soils from the field, suggested that salinity had a greater effect on cumulative respiration in the salt-amended soils. The results of this study show (i) salinity needs to be taken into account when modelling CO₂ release and SOC turnover in salt-affected soils, and (ii) a decomposition rate modifier developed from salt-amended soils may overestimate the effect of salinity on CO₂ release.     

A study of some tillage practices for sustainable crop production in India

Soil tilth has been defined in terms of a ‘Physical Index’ based on the product of the ratings of eight physical properties — soil depth, bulk density, available water storage capacity, cumulative infiltration or apparent hydraulic conductivity, aggregation or organic matter, non-capillary pore space, water table depth and slope. The Physical Index and a tillage guide were used to identify the tillage requirements of different soils varying in texture from loamy sand to clay in the semi-arid tropics. The physical index was 0.389 for a loamy sand, 0.518 for a black clay loam and 0.540 for a red sandy loam soil and the cumulative rating indices in summer and winter seasons were 45 and 44 for loamy sand, 52 and 51 for red sandy loam and 54 and 52 for black clay loam soils, respectively. The compaction of the loamy sand by eight passes of a 490 kg tractor-driven roller (0.75 m diameter and 1.00 m length) increased the physical index to 0.658 and chiselling of the red sandy loam and black clay loam increased the physical indices to 0.686 and 0.729, respectively. The grain yields of rainfed pearl millet and guar and irrigated pearl millet, wheat and barley increased significantly over the control (no compaction) yields by compaction.

The chiselling of the soils varying in texture from loamy sand to clay at 50 to 120-cm intervals up to 30–40 cm depth, depending upon the row spacing of seedlines and depth of the high mechanical impedance layer, increased the grain yields of rainfed and irrigated maize on alluvial loamy sand, rainfed maize on alluvial sandy loam and red sandy loam, rainfed sorghum on red sandy loam and black clay loam, irrigated sorghum on black clay loam and rainfed black gram on red sandy loam, pod yield of rainfed groundnut, tuber yield of irrigated tapioca and fresh fruit yield of rainfed tomato on red sandy loam and sugarcane yield on black clay soil, significantly over the yields of no-chiselling systems of tillage such as disc harrow and country plough.     

Zeolite Effects on Immobile Water Content and Mass Exchange Coefficient at Different Soil Textures

The concern for groundwater pollution by agrichemicals through solute movement within the soil is widespread. Zeolite is a type of soil amendment that is utilized to improve physical properties of soil and ameliorate polluted soil. The high negative charge of the zeolite and its open space structure allows adsorption and access of heavy metals and other cations and anions. The objectives of this research were (i) to determine the effects of different application rates of zeolite (0, 2, 4, and 8 g kg^?1) on the immobile water content and mass exchange coefficient in a loam soil and then (ii) to determine the effects of optimum application rate of zeolite on the immobile water content and mass exchange coefficient of sandy loam and clay loam soils in saturated conditions by a mobile and immobile (MIM) model. In a disturbed soil column, a method was proposed for determination of MIM model parameters, that is, immobile water content (θ_im), mass exchange coefficient (α), and hydrodynamic dispersion coefficient (D_h). Breakthrough curves were obtained for different soil textures with different zeolite applications in three replicates, by miscible displacement of chloride (Cl^?1) in disturbed soil column. Cl^?1 breakthrough curves were evaluated in terms of the MIM model. The results showed that the pore water velocity calculated based on the total soil volumetric water content (θ_im+ θ_m) and real pore water velocity calculated based on the mobile water content (θ_m) increased in the loam soil with an increase in zeolite application rate, so that, between these different rates of zeolite application, the maximum value of pore water velocity and real pore water velocity occurred at zeolite application rates of 8.6 and 11.5 g kg^?1, which are indicated as the optimum application rates. However, the comparison between different soils showed that the zeolite application rate of 8 g kg^?1 could increase pore water velocity of sandy loam and loam soils by 31% more than that of clay loam soil. The immobile water content and mass exchange coefficient of loam soil were correlated with the zeolite application rate and reduced with an increase in the rate of applied zeolite. In a comparison between different soils at zeolite application rate of 8 g kg^?1, the immobile water contents of the zeolite-treated soil decreased by 57%, 60%, and 39% on sandy loam, loam, and clay loam soils, respectively, compared with the untreated soil. Furthermore, zeolite application could reduce mass exchange coefficient by 9%, 43%, and 21% on sandy loam, loam, and clay loam soils, respectively. A positive linear relationship was found between θ_im and α. Zeolite application increased real pore water velocity of sandy loam soil by 39% and 46% compared with loam and clay loam soils, respectively. In other studies there was a decrease in ammonium and nitrate leaching due to the zeolite application, and therefore, an increase in real pore water velocity due to zeolite application in sandy loam soil, as compared with the loam and clay loam soils, may not show more rapid movement of solute and agrichemicals to the groundwater.     

Influence of tool angles and speed on the soil reactions of a bent leg plough in two soils

The soil reactions of bent leg ploughs, as influenced by tool angles, need to be studied in detail in order to optimise their performance under different soil and operational conditions. In a field study conducted at Kumulur, south India, three bent leg tool models, with 30°, 37.5° and 45° bend angles, were tested for their soil reactions in a Typic Ustocrept (loamy sand) and a Typic Chromustert (clay loam) soil, under different speeds of operation and rake angles. A simple and reliable instrumentation system, capable of measuring the soil reactions of the tools, was developed and used in this study. Mathematical response models were built on these soil reactions to optimise the parametric levels yielding maximum performance. It was found that the tool, while working in the Ustocrept (loamy sand) and Chromustert (clay loam) soils, should have a rake angle between 9° and 15° for minimum horizontal and lateral soil reactions, and maximum downward suction, aiding penetration.