Influence of soil properties on the aggregate stability of cultivated sandy clay loams

Purpose

Frequent cultivation and overhead irrigation have led to severe surface crusting, erosion and poor irrigation performance on sandy clay loam soils in the Coal River Valley, Tasmania, Australia. This study was established to identify the key soil properties related to aggregate breakdown determined by different methods, and explore options for reducing soil crusting.

Materials and methods

Soil aggregates were collected from 0 to 5 cm depth from 20 sites managed for packet salad and lettuce production. The stability of air-dried 2.00–4.75 mm aggregates was determined by rainfall simulation, wet sieving and clay dispersion. Soil aggregates were analysed for particle size, mineralogy, soluble and exchangeable cations, pH, EC, labile carbon and total carbon. The association between aggregate stability and the measured soil properties was explored using Spearman correlation, linear regression and regression tree analysis.

Result and discussion

Aggregate stability determined by rainfall simulation was closely associated with soil properties that promote aggregation, including effective cation exchangeable capacity (ECEC) and the proportion of polyvalent cations (Ca²⁺, Al³⁺). In contrast, aggregate stability determined by wet sieving was associated with soil properties that promote disaggregation, including quartz and sand content, and to lesser extent, the proportion of monovalent cations (especially K⁺). Clay dispersion was closely associated with pH, quartz content, soil texture and the sodium adsorption ratio. Soil carbon appeared to have only moderate influence on aggregate stability, but not clay dispersion, while labile carbon was not significantly related to any measure of aggregate stability or clay dispersion. Similarly, the proportion of Na⁺ ions was not related to either measure of aggregate stability and was only moderately related to clay dispersion.

Conclusions

Options for improving aggregate stability appear limited as aggregate stability was strongly related to the content of inherent soil properties such as sand/quartz and smectite contents. However, high correlation between exchangeable Ca²⁺ and aggregate stability determined by rainfall simulation indicates that soil crusting may be reduced through application of products that rich in Ca²⁺ such as gypsum.

     

Studies on aggregate stability. 11. The effect of humic substances on the stability of re-formed soil aggregates

The effects of both physical addition and adsorption of humic substances on the aggregation of soils were studied. To investigate the effect of adsorbed humic materials onto soils it was necessary to develop an effective adsorption procedure. Mono-ionic soils (either sodium- or calcium-saturated) were used to ensure that adsorption had occurred and to enable the results to be properly interpreted. Physical addition of humic acid followed by incubation with glucose produced aggregates with low stability values. When humic acid was adsorbed onto the soil minerals and incubated, significant increases in aggregate stability were observed which persisted with time. The stability of the re-formed aggregates was even greater when the samples were incubated with glucose. This work has shown that humic substances were capable of stabilization of aggregates under difficult conditions where extracellular polysaccharides were ineffective, and that the stabilization had long term persistance. It is suggested that humic substances are particularly involved in the formation of micro-aggregates.     

Variation of intra‐aggregate organic carbon affects aggregate formation and stability during organic manure fertilization in a fluvo‐aquic soil

The aggregate formation and stability are controlled by the dynamics of soil organic matters (SOM), but how it is related to SOM chemical composition within different‐sized aggregates is largely unknown during manure fertilization. In this study, the variations of intra‐aggregate organic carbon (OC), including intra‐particulate organic matter (iPOM) and mineral‐associated organic matter, were quantitatively and qualitatively analysed, and then, their effects on aggregate formation and stability were assessed under four treatments: control (CK), mineral fertilizer (NPK), reduced manure (30%M) and manure fertilizers (M). Manure application (M) significantly increased macroaggregate proportion, mean weight diameter (MWD), and OC contents within different‐sized aggregates compared to CK, NPK, and 30%M. The OC accumulation of macroaggregate in M was attributed to OC content increase in silt plus clay subfraction rather than iPOM with more labile organic groups; oppositely, in microaggregate it was located in the relatively stable fine iPOM. The macroaggregate formation and stability were controlled by the fine iPOM within macroaggregates, whose abundant polysaccharide‐C and aliphatic‐C after manure fertilization advanced the microbial growth except for Gram‐positive bacteria, which further promoted macroaggregate formation and stability. The free silt plus clay fraction also affected macroaggregate formation and stability, and its polysaccharide‐C derived from microorganisms or decomposing SOM was positively associated with MWD and macroaggregate proportion. Because polysaccharide‐C can be easily associated with mineral particles, further improving micro‐ or macroaggregation. We conclude that continuous manure fertilization could increase labile SOM accumulation within aggregates and then facilitate microbial growth, which collectively are responsible for aggregate formation and stabilization.     

Cation treatment and drying‐temperature effects on nonylphenol and phenanthrene sorption to a sandy soil

The objective of this study was to investigate the effects of mono‐ and polyvalent cations on sorption of the two hydrophobic compounds nonylphenol (NP) and phenanthrene (Phe). To this end, exchange sites of a sandy soil were saturated with either Na⁺, Ca²⁺, or Al³⁺ and excess salts were removed by washing. The samples were then sterilized and either stored moist, dried at room temperature, or at 20°C, 60°C, or 105°C in a vented oven. Saturation with Na⁺ led to an increase of dissolved organic C (DOC) concentration in the soil water extracts, whereas the polyvalent cations Ca²⁺ and Al³⁺ decreased it. The ¹H‐NMR relaxometry analyses showed that Al³⁺ restricted the mobility of water molecules that are confined within the SOM structure to a higher extent than Ca²⁺ or Na⁺. According to contact‐angle (CA) analyses, cation treatment did not significantly change the wetting properties of the samples. Batch sorption–desorption experiments showed no clear salt‐treatment effects on the sorption and desorption equilibria or kinetics of NP and Phe. Instead, the sorption coefficients and sorption hysteresis of NP and Phe increased in dry soil. With increasing drying temperature the CA of the soils and the sorption of both xenobiotics increased significantly. We conclude that structural modifications of SOM due to incorporation of polyvalent cations into the interphase structure do not modify the sorption characteristics of the soil for hydrophobic compounds. Instead, increasing hydrophobization of organic soil constituents due to heat treatment significantly increased the accessible sorption sites for nonpolar organic compounds in this soil.     

Effect of ammonium fertilizer on NH3 loss and Ca,Mg, ammonium and nitrate content in a calcareous soil solution

Summary This study examined the effects of NH _inf4 ⁺ fertilizers [(NH₄)₂SO₄, (NH₄)₂HPO₄, CO(NH₂)₂, NH₄OH, and NH₄NO₃] on NH₃ loss and the quantity of Ca + Mg, NH _inf4 ⁺ and NO _inf3 ^sup– in the solution of a calcareous soil (Harkey sicl, Typic Torrifluvent). Various NH₄ fertilizers applied at a depth of 5 cm in the soil produced differing NH₃ loss characteristics. Applying (NH₄)₂SO₄ (AS) resulted in high volatile NH₃ losses as compared with NH₄OH (AH) and (NH₄)₂CO₃ (AC). The AS treatment formed an equal molar amount of CaSO₄, which increased the mobility of ammonium, while AH and AC treatments caused Ca precipitation and decreased ammonium mobility. Leaching the AS system before NH₃ loss could occur resulted in the most rapid nitrification rate. Lower nitrification rates were found with AH and AC than AS under the same conditions. Surface placement of NH₄ fertilizers resulted in variable leachate contents of Ca + Mg. Ammonium sulfate reacted with CaCO₃ either to solubilize some Ca + Mg or simply to replace exchangeable Ca + Mg with NH₄, while AH, AC, and (NH₄)₂HPO₄ (DAP) precipitated essentially an equivalent molar amount of soluble and adsorbed Ca + Mg. Use of NH₄NO₃, which does not form an insoluble calcium precipitate, resulted in the leaching of an equivalent molar amount of exchangeable Ca + Mg from the Harkey soil.The authors are Professor and former laboratory technician, respectively, at Texas A&M Research Center at El Paso, 1380 A&M Circle, El Paso, TX 79927, USA     

四种土壤调理剂对镉、铅的吸附效果研究

采用等温吸附试验研究了主要原料为麦饭石(M)、蒙脱石(T)、牡蛎壳(O)和硅钙矿(S)4种土壤调理剂对溶液中镉和铅的吸附效果。结果表明,4种土壤调理剂对镉和铅的吸附均可用Langmuir方程进行描述;相比而言,4种土壤调理剂对镉吸附量的大小顺序为O调理剂>S调理剂>M调理剂>T调理剂,对铅吸附量的大小顺序为M调理剂>S调理剂>O调理剂>T调理剂;随溶液pH值的增大各调理剂的吸附量逐渐减小,尤其当Cd溶液pH值>8,Pb溶液pH值>6,各调理剂的吸附量迅速降低。     

The influence of the soil conditioner ‘Agri-SC' on splash detachment and aggregate stability

The influence of the soil conditioner ‘Agri-SC' on splash detachment and water-stable aggregation of an erodible clay Vertisol from Oahu, HI, was assessed. Laboratory rainfall simulation was used to assess splash detachment from soil treated with 0 (untreated control), 0.3, 3.0, 30, and 300 l ha⁻¹ of Agri-SC. Results indicated that the quantity of sediment splashed was significantly lower for Agri-SC application rates of 0.3 and 3.0 l ha⁻¹ (rates are equivalent to 1 and 10 times the manufacturer's recommended rates, respectively), than for the control, or for Agri-SC applied at 30 and 300 l ha⁻¹ (100 and 1000×, respectively). A second experiment was designed to test the influence of Agri-SC on water-stable aggregation of the Vertisol. Aggregates were subjected to rapid immersion in solution, shaken and washed through a series of sieves. Data indicated that there were no statistically significant differences in geometric mean aggregate diameter between the untreated and treated aggregates. The effect of the active ingredient, ammonium laureth sulfate (an anionic surface active agent) on splash and erodibility are discussed. These preliminary results indicate that further testing of Agri-SC is warranted on a variety of soils with different textures and mineralogies.     

Validation of water sorption‐based clay prediction models for calcareous soils

Soil particle size distribution (PSD), particularly the active clay fraction, mediates soil engineering, agronomic and environmental functions. The tedious and costly nature of traditional methods of determining PSD prompted the development of water sorption‐based models for determining the clay fraction. The applicability of such models to semi‐arid soils with significant amounts of calcium carbonate and/or gypsum is unknown. The objective of this study was to validate three water sorption‐based clay prediction models for 30 calcareous soils from Iran and identify the effect of CaCO₃ on prediction accuracy. The soils had clay content ranging from 9 to 61% and CaCO₃ from 24 to 97%. The three water sorption models considered showed a reasonably fair prediction of the clay content from water sorption at 28% relative humidity (RMSE and ME values ranging from 10.6 to 12.1 and −8.1 to −4.2, respectively). The model that considers hysteresis had better prediction accuracy than the other two that do not. Moreover, the prediction errors of all three models arose from under‐prediction of the clay content. The amount of hygroscopic water scaled by clay content decreased with increasing CaCO₃ content. The low organic carbon content of the soils and the low fraction of low‐activity clay minerals like kaolinite suggested that the clay content under‐predictions were due to large CaCO₃ contents. Thus, for such water‐sorption based models to work accurately for calcareous soils, a correction factor that considers the reduction of water content due to large CaCO₃ content should be included.     

In‐season calcium‐spray formulations improve calcium balance and fruit quality traits of peach

Experiments to evaluate the effect of in‐season calcium (Ca) sprays on late‐season peach (Prunus persica L. Batsch cv. Calrico) were carried out for a 2‐year period. Calcium formulations (0.5% and 1.0% in 2008 and only 0.5% tested in 2009) supplied either as CaCl₂ or Ca propionate in combination with two or three adjuvants (0.05% of the nonionic surfactants Tween 20 and Break Thru, and 0.5% carboxymethylcellulose, CMC) were sprayed four to five times over the growing season. Peach mesocarp and endocarp Ca concentrations were determined on a 15‐day basis from the beginning of May until the end of June. Further tissue analyses were performed at harvest. A decreasing trend in fruit Ca concentrations over the growing season was always observed regardless of the Ca treatments. Both in 2008 and 2009, significant tissue Ca increments associated with the application of Ca‐containing sprays in combination with adjuvants were only observed in June, which may be coincident with the period of pit hardening. In 2008, both at harvest and after cold storage, the total soluble‐solids concentration (° Brix) of fruits supplied with Ca propionate (0.5% and 1.0% Ca) was always lower as compared to the rest of treatments. The application of multiple Ca‐containing sprays increased firmness at harvest and after cold storage, especially when CaCl₂ was the active ingredient used. Supplying the adjuvants Tween 20 and CMC increased fruit acidity both at harvest and after cold storage. Evaluation of the development of physiological disorders after cold storage (2 weeks at 0°C) indicated a lower susceptibility of Ca‐treated fruits to internal browning. Fruits treated with multiple CaCl₂‐, CMC‐, and Break Thru®‐containing sprays during the growing season were significantly less prone to the development of chilling injuries as compared to untreated peaches.     

Distillery effluents effect on soil organic carbon and aggregate stability of a Vertisol in India

Distillery effluent, a waste by-product of distillery industries, is usually applied to arable land near the distilleries as irrigation water or as a soil amendment. To evaluate the effect of distillery effluent, both spent wash (SW) and post-methanated effluent (PME), on soil organic carbon and aggregate stability, a field experiment on a soybean (Glysine max L.)–wheat (Triticum aestivum L.) system was conducted for five years on a Vertisol of central India. The treatments were control (no fertilizer or manure or SW or PME, T₁), 100% NPK + farmyard manure (FYM) @ 4 Mg ha⁻¹ to soybean (T₂), four graded levels of SW, viz., 2.5 cm SW to soybean and none to wheat (T₃), 2.5 cm SW to soybean and 1.25 cm to wheat (T₄), 5 cm SW to soybean and none to wheat (T₅), 5 cm SW to soybean and 2.5 cm to wheat (T₆), and four graded levels of PME, viz., 2.5 cm PME to soybean and none to wheat (T₇), 2.5 cm PME to soybean and 1.25 cm to wheat (T₈), 5 cm PME to soybean and none to wheat (T₉), 5 cm PME to soybean and 2.5 cm to wheat (T₁₀). The organic carbon of the surface (0–15 cm) soil that received either PME or SW (treatments T₃–T₁₀), was significantly (P < 0.05) higher than in treatments T₁ and T₂. The mean weight diameter (MWD) of water stable aggregates in this soil layer was also significantly higher in treatments T₃–T₁₀, compared with T₁ and T₂. The MWD showed a positive linear relationship with the organic carbon content of the soil (R² = 0.54**). The proportion of macro-aggregates was higher in SW treated plots than PME, no distillery effluents and NPK + FYM treatments. However, the micro-aggregates showed the reverse trend. The macro-aggregate-associated carbon was higher in SW treated plots. It was highest in T₆ and lowest in T₁. The plots receiving the PME and SW showed increased soil organic carbon, MWD, percentage macro- and micro-aggregate-associated carbon than T₁ and T₂. Application of distillery effluents increased the aggregate stability of the Vertisol through enhanced soil organic carbon as well as the aggregate-associated carbon. So application of SW or PME could be a viable option for soil aggregate stability and enhanced productivity.     

Biochars improve aggregate stability,water retention,and pore‐space properties of clayey soil

The influence of biochar amendments on the physical quality of a clayey soil (Vertisol) was evaluated by aggregate‐size distribution and stability, water retention, and pore‐space structure of biochar‐amended soils. Clayey soil was treated with three kinds of biochars (straw biochar, woodchips biochar, and wastewater‐sludge biochar) at the rate of 0, 20, 40, and 60 g biochar (kg soil)^–1 and incubated for 180 d in glasshouse. The application of straw biochar (SB) and wastewater‐sludge biochar (WSB) significantly enhanced the formation of 5–2 and 0.25–0.5 mm macroaggregates in the clayey soil relative to the control treatment, while the < 0.25‐cm microaggregate decreased with biochar additions. However, woodchips biochar (WCB) had no obvious effect on the formation of macroaggregate. The application of SB and WSB increased the mean weight diameter (MWD) and geometric mean diameter (GMD) of clayey soil, implying that biochar increased the aggregate stability. They improved the aggregate stability through an enhanced resistance to slaking and increased interparticular cohesion. The SB‐amended soils exhibited significant increases in the available water contents of soils. The application of SB significantly increased pore volume in the macropore (> 75 μm) and mesopore (30–75 μm) ranges, which may be the result of the reorganization of pore‐size distribution and aggregation processes induced by the addition of biochar. Results indicated that biochar had the potential to improve the physical quality and pore‐space status of clayey soil. It is suggested that biochar may be considered as a soil amendment for improving poor physical characteristics of clayey soil.     

耕作对土壤有机物和土壤团聚体稳定性的影响

Agricultural sustainability relates directly to maintaining or enhancing soil quality. Soil quality studies in Canada during the 1980‘s showed that loss of soil organic matter (SOM) and soil aggregate stability was standard features of non-sustainable land management in agroecosystems. In this study total soil organic carbon (SOC), particulate organic matter (POM), POM-C as a percentage of total SOC, and aggregate stability were determined for three cultivated fields and three adjacent grassland fields to assess the impact of conventional agricultural management on soil quality. POM was investigated using solid-state ^13C nuclear magnetic resonance (NMR) to determine any qualitative differences that may be attributed to cultivation. Results show a highly significant loss in total SOC, POM and aggregate stability in the cultivated fields as compared to the grassland fields and a significant loss of POM-C as a percentage of total SOC.Integrated results of the NMR spectra of the POM show a loss in carbohydrate-C and an increase in aromatic-C in the cultivated fields, which translates to a loss of biological lability in the organic matter. Conventional cultivation decreased the quantity and quality of SOM and caused a loss in aggregate stability resulting in an overall decline in soil quality.     

Effects of two contrasting agricultural land‐use practices on nitrogen leaching in a sandy soil of Middle Germany

The objective of this study is to evaluate different agricultural land‐use practices in terms of N leaching and to give recommendations for a sustainable agriculture on sandy soils in Middle Germany. Soil mineral N (N_min) and leachate N were quantified at a sandy soil in N Saxony during 3 years. Two treatments were applied: intensive (I)—using inorganic and organic fertilizer and pesticides, and organic (O)—exclusively using organic fertilizer, legume‐based crop rotation, and no pesticides. Split application of mineral fertilizers did not result in substantial N losses at treatment I. Legumes induced a considerable increase of soil mineral N and particularly of leachate mineral N (N_{min_perc}) at treatment O. High N_{min_perc} concentrations (up to 78 mg N L^–1) were observed during as well as after the cultivation of legumes. These high N_{min_perc} concentrations are the reason why clearly higher N_{min_perc} losses were determined at treatment O (62 kg N ha^–1 y^–1) compared to treatment I (23 kg N ha^–1 y^–1). At both treatments, the quantity of N losses was strongly affected by the precipitation rates. Concentrations and losses of dissolved organic N (DON_perc) were assessed as above average at both treatments. The results suggest that the DON_perc concentration is influenced by precipitation, soil coverage, and organic fertilizers. Higher values were determined in the percolation water of treatment O. The average annual DON_perc losses amounted to 15 kg N ha^–1 at I and to 32 kg N ha^–1 at O. The average monthly percentage of DON_perc losses on the loss of the dissolved total N of percolation water (DTN_perc) ranged between <1% and 55% at O and between 2% and 56% at I. For the whole measuring period of 29 months, the relative amounts of DON_perc of DTN_perc (21% at O and 25% at I) were more or less the same for both treatments. The results show that DON_perc can contribute significantly to the total N loss, confirming the importance to consider this N fraction in N‐leaching studies. It was concluded that at sandy sites, a split application of mineral fertilizers, as applied at treatment I, seems to be more expedient for limiting the N leaching losses than legume‐based crop rotations.     

Effects of HC1 acid and lime amendments on soil ph and extractable Ca and Mg in a sandy soil

Abstract

High vater table sandy soils present special problems when establishing soil pH variables under field conditions. In order to examine the response of a coarse‐textured soil to lime and HC1 acid treatments, data are reported for soil pH and extractable Ca and Mg for a field experiment where Mn treatments on soybeans was the primary objective. Three treatments included HC1 acid, control, and lime. Acid (742 liters/ha 3N HC1) was added only at the beginning of the experiment but dolomitic lime treatments were added each year (2240, 2740, and 2900 kg/ha). The lime and acid were applied to the soil surface and incorporated to a depth of 10 to 13 cm. Soil samples were taken every 2 to 3 months at 3 depths (0 to 15, 15 to 30, and 30 to 45 cm) and analyzed for pH and extractable Ca and Mg. Acid treatment decreased the pH by 0.2 units below the untreated soil at the 0 to 30 cm depth and the effect lasted the entire 3 years of the study. Calcium values were lowered only slightly by the acid treatment. Lime additions caused steady increases in soil Ca. Magnesium values increased several months after each of the first and second lime applications. Lime raised the subsoil (30 to 45 cm) pH after 4 to 6 months. Seasonal variations in pH were very wide with the untreated soil pH varying from 6.1 to 6.8. The high pH level of 7.0 was not maintained for an entire season until the third year of the experiment. Soil pH as well as extractable Ca and Mg showed fluctuations that were the result of seasonal variations and soil moisture content at the time of sampling. Soil pH variables on a sandy soil should be established at least a year in advance of starting an experiment and must be closely monitored in order to maintain the desired pH levels.     

Potential of sawdust and corn cobs derived biochar to improve soil aggregate stability,water retention,and crop yield of degraded sandy loam soil

Abstract

Poor soil structure is the main cause of soil degradation; however, biochar the solid carbon-rich production of pyrolysis biomass could improve the soil structure. Biochar from the feed stock sawdust (SD) and corn cobs (CC) was pyrolyzed at 450?°C. Wheat was grown as a test crop and treatments were control, NPK, SDB1% (sawdust biochar), CCB1% (corn cobs biochar), SDB0.5%+CCB0.5%, SDB1%?+?½ NPK, CCB1%?+?½ NPK. The higher growth, higher grain and dry matter yield were displayed by biochar?+?NPK. The lowest pH, the higher organic matter, available P and available K were observed in SDB0.5%+CCB0.5%. However, the highest total N (1.43?g kg^?1) was by NPK treatment. The biochar increased plant available water contents, water contents at field capacity and permanent wilting point, soil porosity and decreased bulk density. The highest stable aggregates were in SDB0.5%+CCB0.5%. Biochar application was found as a useful practice for soil sustainability     

The influence of freeze–thaw cycles and soil moisture on aggregate stability of three soils in Norway

Winter conditions with seasonally frozen soils may have profound effects on soil structure and erodibility, and consequently for runoff and erosion. Such effects on aggregate stability are poorly documented for Nordic winter conditions. The purpose of this study was to quantify the effect of variable freeze–thaw cycles and soil moisture conditions on aggregate stability of three soils (silt, structured clay loam—clay A and levelled silty clay loam—clay B), which are representative of two erosion prone areas in southeastern Norway. A second purpose was to compare aggregate stabilities measured by the Norwegian standard procedure (rainfall simulator) and the more widely used wet-sieving procedure. Surface soil was sampled in autumn. Field moist soil was sieved into the fraction 1–4 mm and packed into cylinders. The water content of the soil was adjusted, corresponding to matric potentials of − 0.75, − 2 and − 10 kPa. The soil cores were insulated and covered, and subjected to 0, 1, 3 or 6 freeze–thaw cycles: freezing at − 15 °C for 24 h and thawing at 9 °C for 48 h. Aggregate stability was measured in a rainfall simulator (all soils) and a wet-sieving apparatus (silt and clay B). The rainfall stability of silt was found to be significantly lower than of clay A and clay B. Clay A and clay B had similar rainfall stabilities, even though it was expected that the artificially levelled clay B would have lower stability. Freezing and thawing decreased the rainfall stability of all soils, but the effect was more severe on the silt soil. There was no evident effect of water content on the stability, probably due to experimental limitations. The same effects were observed for wet-sieved soil, but the wet-sieving resulted in less aggregate breakdown than the rainfall simulator. Rainfall impact seemed to be more detrimental than wet-sieving on more unstable soil, that is, on silt soil and soil subjected to many freeze–thaw cycles. Such conditions are expected to occur frequently during field conditions in unstable winters.     

The effect of Gypsum amendment on transport of phosphorus in a sandy soil

The effect of gypsum amendment on transport of phosphorus (P) in a Pineda sand (loamy, silicious, hyperthermic Arenic Glossaqualf) was investigated in a leaching column experiment. Phosphorus was either placed on the surface of the soil or mixed with the surface 2 cm depth of soil in the column. Gypsum amendment decreased the transport of P in soil. Compared to the unamended soil, transport of P decreased by 35 and 54% in soils amended with 4.5 and 9.0 M.T./ha gypsum, respectively. The transport of P was further decreased by 74% when P was premixed with the surface 2 cm of soil. The transport of P was not influenced by the SO₄ ion from gypsum amendment. Instead, formation of Ca-P precipitate appeared to be responsible for the decreased transport of P in the gypsum-amended soil.     

The effects of organic inputs over time on soil aggregate stability - A literature analysis

Since the beginning of the last century, many studies have reported evidence describing the effects of organic inputs on soil aggregate stability. In 1965, Monnier proposed a conceptual model that considers different patterns of temporal effects on aggregate stability depending on the nature of the organic inputs: easily decomposable products have an intense and transient effect on aggregate stability while more recalcitrant products have a lower but longer term effect. We confronted this conceptual model with a literature review of experimental data from laboratory and field experiments. This literature analysis validated the conceptual model proposed by Monnier and pointed out gaps in our current knowledge concerning the relationships between aggregate stability and organic inputs. Noticeably, the experimental dataset confirmed the biological and temporal effects of organic inputs on aggregate stability as proposed in the model. Monnier's model also related the evolution of aggregate stability to different microbial decomposing agents, but this relationship was not made clear in this literature analysis. No direct or universal relationship was found between the aggregative factors induced by organic input decomposition (binding molecules or decomposers of biomass) and temporal aggregate stability dynamics. This suggests the existence of even more complex relationships. The model can be improved by considering (i) the direct abiotic effect of some organic products immediately after the inputs, (ii) the initial biochemical characteristics of the organic products and (iii) the effects of organic products on the various mechanisms of aggregate breakdown. For now, no trend is evident in the effect of the rate of organic inputs or the effect of the soil characteristics (essentially carbon and clay contents) on aggregate stability.     

Analyzing the efficiency of soil amendments and irrigation for plant production on heterogeneous sandy soils under greenhouse conditions

Variability in soil properties is a complication for fertilization, irrigation, and amendment application. However, only limited progress has been made in managing soil variability for uniform productivity and increased water‐use efficiency. This study was designed to ameliorate the poor‐productivity areas of the variable sandy soils in Florida citrus groves by using frequent small irrigations and applying organic and inorganic soil amendments. Two greenhouse experiments were set up with sorghum and radish as bioassay crops in a randomized complete block design (RCBD). The factors studied were two soil‐productivity classes (very poor and very good), two water contents (50% and 100% of field capacity), two amendments (phosphatic clay and Fe humate), and two amendment rates (10 and 25 g kg^–1 for sorghum and 50 and 100 g kg^–1 for radish). Amendments applied at 50 and 100 g kg^–1 increased the water‐holding capacity (WHC) of poor soil by 2‐ to 6‐fold, respectively. The lower rates (10 and 25 g kg^–1) of amendments were not effective in enhancing sorghum growth. The higher rates (50 and 100 g kg^–1) doubled the radish growth as compared to the control. The results indicate that rates greater than 50 g kg^–1 of both amendments were effective in improving water retention and increasing productivity. Irrigation treatment of 100% of field capacity (FC) increased the sorghum and radish growth by about 2‐fold as compared with the 50%–water content treatment. The results suggest that the root‐zone water content should be maintained near FC by frequent small irrigations to enhance water availability in excessively drained sandy soils. In addition, application of soil amendments in the root zone can enhance the water retention of these soils. Furthermore, managing variable sandy soils with WHC‐based irrigation can increase water uptake and crop production in the poor areas of the grove.