جستجوی مقالات مرتبط با کلیدواژه « wetting » در نشریات گروه « کشاورزی »

Monitoring the changes in physical and hydraulic properties and stability of growth media due to root growth effects and wetting and drying cycles is important. Wetting and drying cycles can probably change physical characteristics, availability of water, air and nutrients for the plant and, as a result, might affect the growth and yield of the greenhouse plants. The growth period greatly affects the physical characteristics of the growth substrates; therefore, the watering of growth substrates should be managed according to these changes to avoid improper irrigation.

In this study, 14 growth media were prepared from individual substrates with different volumetric ratios. In order to evaluate the changes of growth media over the time (i.e., during consecutive irrigation events) in the greenhouse, 10 wetting and drying cycles were applied on the growth media in the lab. Several physical indicators including easily available water (EAW), air after irrigation (AIR), water buffering capacity (WBC) and water holding capacity (WHC) of the growth media were determined before and after the wetting and drying cycles. Besides, the subsidence, decrease of mass and decomposition of the growth media were determined over the time. Total porosity (TP), bulk density (BD), particle density (PD), pH and electrical conductivity of the mixtures were measured as well.

The pH values in the growth media varied from 5.72 to 6.94. The maximum pH value was related to sawdust- sugarcane bagasse biochar produced at 300◦C vermiculite-zeolite, and wheat straw-vermiculite substrates, and the minimum value was related to the cocopeat-perlite substrate. The values of EC in the growth media varied from 0.21 to 1.43 dS m-1. The highest and lowest EC values among the growth substrates were related to date palm bunches-vermiculite-rockwool and rockwool (0.2)-perlite substrates, respectively. The bulk density (BD) values of the growth media varied in the range of 0.163-0.401 Mg m-3. The values ​​of total porosity (TP) of the growth media varied in the range of 64.8-82.8%v/v. The highest TP was related to the cocopeat-perlite substrate. The TP values ​​of most of the substrates were greater than 70%v/v. The average values of EAW in the growth substrates ranged from 0.123 to 0.272 cm3 cm-3. The highest EAW was related to the sawdust-sawdust biochar produced at 500 ◦C vermiculite-zeolite substrate. The application of wetting and drying cycles increased EAW in most of the growth media. Therefore, it can be stated that the time had a positive effect on the EAW in most of the growth media. The average values of AIR before and after the application of wetting and drying cycles for the growth media varied in the range of 0.063-0.240 cm-3 cm3. The highest value of this indicator was observed in the sawdust-date palm bunches biochar produced at 300◦C vermiculite substrate. In all substrates (with the exception of the sawdust-sawdust biochar produced at 500◦C vermiculite-zeolite), the AIR increased after wetting and drying cycles. The range of WHC values before and after applying wetting and drying cycles was 0.453-0.699 cm3 cm-3. The highest WHC belonged to the wheat straw-vermiculite substrate. The WHC values of five growth media, including cocopeat-perlite, decreased due to the application of wetting and drying cycles, and the WHC values of nine growth media decreased. The most stable substrate after the wetting and drying cycles was rockwool-sawdust-vermiculite. The effect of time on the quantity of WBC was positive, so that with the application of wetting and drying cycles, the WBC values of most of the substrates increased. In all substrates, subsidence and dry weight reduction were observed after the wetting and drying cycles. These changes were low for the substrates with a high volumetric ratio of inorganic materials. The least change among the growth substrates in terms of decomposition (dry weight reduction) was related to the completely inorganic substrate rockwool (0.1)-perlite (%0.17). The most stable substrate in terms of subsidence after wetting and drying cycles was the rockwool-sawdust-vermiculite, which has a large volumetric ratio of individual inorganic substrates. The highest subsidence was observed in the substrates containing wheat straw (wheat straw-vermiculite and date palm bunches biochar produced at 300◦C wheat straw-vermiculite). The organic matter content in all the growth substrates decreased over time (after wetting and drying cycles). The decrease of organic matter in the substrates can be related to the decomposition of organic materials as a result of wetting and drying cycles.

The BD, TP, EAW and WHC of the majority of growth media were in the optimal ranges and for some mixtures even better than cocopeat-perlite. Wetting and drying cycles could affect the growth media through several processes such as decomposition of organic compounds, displacement and rearrangement of particles, fragmentation of particles, shrinkage, hardening and subsidence. The growth media with a high percent of organic substrates were unstable as compared with those containing a high proportion of inorganic substrates. In general, the wetting and drying cycles increased the frequency of micropores in the growth media. The wetting and drying cycles positively affected EAW, WHC, AIR and WBC of most growth media. These findings imply that wetting and drying cycles may improve the growth media according to the studied extensive variables. However, it is necessary to study the intensive variables such as hydraulic conductivity, oxygen diffusion and pore tortuosity in the growth media for better evaluation of the impact of wetting and drying cycles as well.

Shear strength is one of the critical properties of soil that affects the interaction of machine and soil. Wetting-drying cycles change the structure the soil (due to frequent surface irrigation) and affects the shear strength parameters of the soil. Therefore, the effect of this process on the shear strength parameters of coarse-textured soil (sandy loam) is studied. In this study, large soil samples were prepared and some of them were subjected to 5 wettingdrying cycle. After that, the large soil samples with/without wetting and drying cycles were compressed under three loads (0, 100 or 200 kPa) at two water contents (0.9 plastic limit; PL and 1.1PL). Then three samples were prepared for direct shear tests. The results showed that the increase in water contents will increase bulk density and cohesion, and reduce the internal friction angle. Compressing the soil after wetting and drying process resulted in an increase in bulk density and cohesion, and reduction in internal friction angle. These changes were due to clay particles mobility and sedimentation among larger particles as well as developing water films among aggregates during wetting.

Potassium fixation is one of the most important factors influencing the availability of this ion for plants. This research was carried out to evaluate the relationship between potassium (K) fixation with some physical and chemical characteristics of soils and clay minerals and to investigate the effect of the dry and wet cycle on potassium fixation in Kakan Plain, in Kohgilouye & Boyerahmad Province. To measure the amount of Potassium fixation, four levels of K were added to the samples and the samples were shaken for 24 h and then dried in the oven at 50°C for 24 h. The drying and wetting cycle was repeated three times. Another set of soil samples was similarly incubated for a period similar to the previous treatment, but drying was performed at room temperature in an equilibrium state. The results showed that potassium fixation was increased with the potassium concentration increment, whereas K fixation percentage was reduced. Also, potassium fixation showed a positive significant relationship with cation exchange capacity (CEC) as well as clay content, in both normal and dry and wet treatments, and a negative significant relationship with organic carbon. Moreover, potassium fixation was enhanced with the increase of smectite content in both normal and dry and wet treatments. Besides, due to more organic carbon and less smectite, surface horizons fixed K less than the subsurface horizons.

IntroductionThe compaction of soil by agricultural equipment has become a matter of increasing concern because compaction of arable lands may reduce crop growth and yield, and it also has environmental impacts. In nature, soils could be compacted due to its own weights, external loads and internal forces as a result of wetting and drying processes. Soil compaction in sugarcane fields usually occurs due to mechanized harvesting operations by using heavy machinery in wet soils. Adding plant residues to the soil can improve soil structure. To improve soil physical quality of sugarcane fields, it might be suggested to add the bagasse and filter cake, which are the by-products of the sugar industry, to the soils.
When a soil has been compacted by field traffic or has settled owing to natural forces, a threshold stress is believed to exist such that loadings inducing lower than the threshold cause little additional compaction, whilst loadings inducing greater stresses than the threshold cause much additional compaction. This threshold is called pre-compaction stress (σpc). The σpc is considered as an index of soil compactibility, the maximum pressure a soil has experienced in the past (i.e. soil management history), and the maximum major principal stress a soil can resist without major plastic deformation and compaction. Therefore, the main objective of this study was to investigate the effects of wetting and drying cycles, soil water content, residues type and percent on stress at compaction threshold (σpc).
Materials and MethodsIn this research, the effect of adding sugarcane residues (i.e., bagasse and filter cake) with two different rates (1 and 2%) on pre-compaction stress (σpc) in a silty clay loam soil which was prepared at two relative water contents of 0.9PL (PL= plastic limit, moist) and 1.1PL (wet) with or without wetting and drying cycles. This study was conducted using a factorial experiment in a completely randomized design with three replications. A composite disturbed sample of topsoil (0–200 mm deep) of a silty clay loam soil was collected from Isfahan province (32 31.530 N; 51 49.40E) in center of Iran. The mean annual precipitation and temperature of the region are about 160 mm and 16 C, respectively. Sugarcane residues (bagasse and filter cake) were obtained from the sugarcane fields in Ahvaz, Khuzestan province (Iran). The samples were air-dried and passed through a 2-mm sieve. Soil treated by bagasse and filter cake in different rates was poured and knocked lightly into cylinders with diameter and height of 25 and 8 cm, respectively. Large air-dry disturbed soil samples were prepared and some of them were exposed to five wetting and drying cycles. Finally, the soil surface was covered by a plastic sheet and was left overnight in the laboratory (for 24 hours) to enable the moisture to equilibrate. The loading tests were performed the next day. The pre-compaction stress was determined by plate sinkage test (PST). The loading test for PST was performed using CBR apparatus. The compression for PST was continuous at the same constant displacement rate of the CBR (i.e. 1 mm min-1). Determination of the σpc was done using Casagrande’s graphical estimation procedure (Casagrande, 1936) in a program written in MatLab software.
Results and DiscussionThe results showed that σpc was significantly decreased by adding residues to the soil at both water contents, and with/without wetting and drying process. For untreated treatments (control), the σpc decreased with increasing water content. Although σpc decreased with adding the residues to the soil, however, the effect of residue types and percentages and soil water content on σpc was not significant for the soil samples treated with residues.
ConclusionsIn order to prevent re-compaction of the soil and improve its structure, it is suggested that traffic control system with permanent routes for the movement of machinery to be used in sugar cane plantations and the residues (after desalination) to be added into strips that are placed under cultivation.

Due to the large surface area and high cation exchange capacity zeolite as an amendment of physicochemical soil characteristics can be secondary effects on the ability to use nutrients such as potassium and changing different forms of the element and thus the cycle. However, the addition of this compound to Vertisols mainly with high amounts in clay mineral with high capacity for k fixation such as smectite can affect the distribution of potassium. To investigate this, the effects of adding different amounts of the zeolite and wetting and drying treatment were studied on 6 Vertisols that were collected from different regions of Fars province.
Based on previous soil maps, aerial photos and topographic maps of Fars province`s Vertisols were selected and some soil profiles were dug in different climatic conditions. After description, six pedons were chosen as representative that almost all Vertisols studied in xeric moisture regimes in the northern and central parts of the province and all of them are in great groups Haploxererts and Calcixererts. Soils were located in the piedmont plain, plateau and lowland physiography. Experiment was a completely randomized 3×2×4×6 factorial arrangement on surface horizons after treatment of soils with zeolite (0, 1, 2.5 and 5%) and wetting and drying treatment. After 8 days, different forms of potassium consisting of soluble, exchangeable and non-exchangeable forms) were determined in the samples. Statistical analysis was done by using the software SPSS, Excel, SAS and comparison of means was carried out using the Duncan’s test.
The studied soils comprise ustic and xeric moisture regimes and mesic, thermic and hyperthemic temperature regimes. Although the mineralogical composition of the various soils were more or less the same, due to the different amounts of various clay minerals in different soils the amounts of different kinds of K was very different in soils. The results showed that zeolite application can increase the amounts of soluble and exchangeable potassium while it has no effect on non exchangeable potassium, which could be due to the high amount of potassium in zeolite and great tendency of zeolite to absorb potassium. Zeolite application with and without wetting and drying treatment has increased the amounts of soluble potassium in different soils. Maximum increase was observed in soil which has the lower amounts of smectites. Wetting and drying treatment caused significant reduction of soluble potassium in all samples. Exchangeable K has significantly increased by the addition of zeolite. Highest concentrations of exchangeable potassium were observed in soils with less amounts of smectite. Zeolite application has reduced non-exchangeable potassium in soils but had different effect on soil no. 4 and has increased the non-exchangeable potassium. This indicates potassium fixation in the soils which could be due to the high amounts of minerals capable to fix potassium. Wetting and drying treatment caused the reduction of soluble and non-exchangeable K and increase of exchangeable K.
The increase of soluble K as a result of zeolite application can enhance possibility of potassium leaching but on the other hand it is also able to prevent the leaching and fixation of K by retention of potassium in exchangeable sites. Considering the very low amount of available potassium in highly calcareous soils of Fars, which is the results of intensive agriculture and little or no K fertilization, zeolite application, may be able to improve the status of potassium in these soils.

Flood irrigation after planting induces wetting and drying cycles in arable soils. For this reason, the effect of this process on load-bearing capacity (pre-compaction stress; pc) of a fine textured soil (silty clay) was studied. In this research, large air-dry disturbed soil specimens were prepared and some of them were exposed to five wetting and drying cycles. Next, the large soil specimens with/without wetting and drying cycles were compressed under three preloads (0, 100 or 200 kPa) and then the centre section of the preloaded soil specimen was firstly submitted to a plate sinkage test (PST). Then immediately one cylindrical sample was cored for confined compression test (CCT). The results showed that for reconstructed soil samples without wetting-drying cycles, the predicted σ pc using PST didnt significantly differ from the applied preload. Therefore, the PST can be used to determine the load-support capacity of the tilled soils. In PST, with an increase in soil water content from 0.9PL to 1.1PL, the amount of over-prediction in σ pc decreased. However, wetting-drying process significantly increased over-prediction in σ pc at the same water content. Hence, soil compressibility does not simply depend on the actual soil water content but also on the previous history of water content changes (i.e., wetting-drying cycles).

Expansive soils are of the kind the volume of which will be changed to extremes by being exposed to varying conditions of moisture. This volume change is a serious menace to structures built on these types of soils. Throughout the present research the potential of volume change in these soils was investigated when exposed to the phenomenon of wetting and drying. For the purpose، samples of a statically compacted soil were positioned in a consolidation test set، and tests conducted under two surcharges of 10 and 20 KPa and along with two kinds of water quality (pure and acid). Results showed equal swell and shrinkage occurring، following a repetition of about five test cycles. Furthermore، changing water quality (to acid) was found as an effective element in decreasing of the swelling potential.

For evaluating effects of different soil moisture conditions, zinc (Zn) and phosphorus (P) on extractability of P in a calcareous soil, two experiments were conducted as factorial on the basis of a completely randomized design with two replications under laboratory conditions in 2012. The first experiment was arranged with four factors, including Zn at two levels (0 and 20 mg Zn per kg of soil as ZnSO4.7H2O), P at two levels (0 and 60 mg P per kg of soil as Ca(H2PO4)2.H2O), soil moisture at two levels (0.6FC and FC) and incubation time at four levels (1, 10, 30, and 60 days) and the second experiment was accomplished with three factors of Zn and phosphorus at the above mentioned levels, and wetting-drying cycles at three levels (1, 10 and 20 cycles). The extractable soil- P values were measured at the end of each incubation period or wetting-drying cycle by Soltanpour and Schwab method. The results showed that application of P increased the extractable P in both soil moisture conditions. The extractable P significantly decreased with increasing duration of incubation, especially in the first 10 days and then remained constant for the rest of the period. The Zn supply decreased extractable P and this reduction was more pronounced with increasing incubation time. Increase in soil water content from 0.6FC to FC increased the extractable-P. The effects of wetting-drying cycles on the extractable-P were different with and without application of 60 mg P per kg of soil. Under no P supplying condition, wetting-drying cycles exhibited higher amounts of extractable P compared with incubation at constant moisture condition; while under P applied condition, imposing wetting-drying cycles led to a decrease in amounts of extractable P compared to incubation under constant moisture condition.

Agricultural soils are continuously under wetting and drying cycles during flood irrigation. Large air-dry disturbed soil specimens with dry bulk density (BD) of 1. 2 g cm-3 were prepared and the effect of wetting and drying cycles on compactibility of fine and coarse textured soils was studied. The large soil specimens with/without wetting and drying cycles were compressed under three preloads (0، 100 or 200 kPa) and then the small soil samples for water content and bulk density measurements were removed. The results showed that the wetting and drying process resulted in increase in an BD for both soils without applying any compacting load. This process resulted in soil susceptibility to compaction for coarse textured soils، whereas it increased the resistance to soil compaction for fine textured soils. Therefore، in fields with coarse-textured soils and under flood irrigation، are more sensitive to soil compaction at harvest traffic at both soil conditions as compared with the fine-textured soils. Therefore، with reducing contact pressure and axal loads the soil compaction could be prevented.

In most civil projects، such as irrigation and drainage networks constructions، soil stabilization has an important role. Achieving maximum durability against wet-dry cycles is one of the soil stabilization objects. Therefore، depend upon project type and its importance; various hydraulic binders with different amounts are tested to obtain desirable results from technical and economical view. In this research، the effect of dry-wet cycles on engineering properties of clayey soils (low plasticity) was studied and lime، cement and lime-cement binders، with (2-6) percent of soil weight، were used. The results showed that with changing type and percent of binders، various sample durability against dry-wet cycles is different so that without binder samples did not have durability and the samples stabilized by 4% cement and 4% lime tolerance 12 wet-dry cycles. The results also showed that after dry-wet cycles، the mass and volume of sample were changed and its unconfined compressive strength was decreased so that the decrease of the unconfined compressive strength was between 40% and 60%. The present study showed that dry-wet cycles significantly cause to change the soil geotechnical properties.

Wetting and drying induce stresses in soil which changes the physical structure of the soil. For this reason، the effect of this process on evaluating the methods for estimating the threshold of compactibility (load-bearing capacity، pre-compaction stress) of a coarse textured soil (sandy loam) collected from the fields of the Sugarecane and By-product Development Co. in Ahvaz province was studied. In this research، the plate sinkage test (PST) and confined compression test (CCT) methods were evaluated. Large air-dry disturbed soil samples were prepared and some of them were exposed to five wetting and drying cycles. After that، the large soil samples with/without wetting and drying cycles were compressed under three loads (0، 100 or 200 kPa) and then the centre section of the preloaded soil samples was firstly submitted to PST. Then immediately one cylindrical sample was cored for CCT. The results showed that for the soil samples without wetting and drying process، there was no significant difference between the applied load and the estimated values using PST method. However، for the soil samples under wetting and drying process، the obtained values were significantly greater than the applied loads. Therefore، for tilled soil، PST can be used as a quick method to determine the compression parameters of the soil in laboratory or in the field.

Expansive soils swell and shrink periodically when subjected to seasonal water content changes. As a result, they are a constant source of problem in the design and construction of foundations. In this study, the behaviour of an expansive soil was studied through a number of experiments involving cycles of wetting and drying using three different water qualities. Laboratory tests were performed on statically-compacted samples of an expansive soil in a modified Oedometer under constant surcharge pressure of 10 kPa. Vertical deformation of the soil sample was recorded continuously, and during the test, void ratio and water content of the sample were determined at different stages. The results indicated that the equilibrium condition was reached after about six wetting-drying cycles. It is shown that the swelling potential changes with changes in water quality; the saline water reduced the swelling potential of the soil compared to the distilled water during wetting and drying cycles. Furthermore, the results showed that the variations of water content-void ratio paths during wetting-drying were the same (no hysteresis) when the equilibrium condition was achieved

Several quantitative stability indices including wet aggregate stability (WAS), mean weight diameter under fast wetting (MWDf) and dispersible clay (DC) have been employed to evaluate soil structure stability. Polyacrylamide (PAM) and organic matter as soil conditioners are vastly used to improve soil structure. This project was carried out to study the effects of PAM and organic matter on aggregate stability of two sandy loam and clay loam soils. It was conducted in pot under greenhouse conditions with applying anionic PAM at three rates (0, 10, 30 mg kg-1), organic matter to form decomposed animal manure < 0.5 mm at two rates (0 and 5 % by weight) and cycles of wetting and drying at five rates (1, 3, 5, 10, 15) with moisture contents ranged from FC to about 1/2 FC. The pots were kept at 22 – 250C under laboratory condition. The experiment was conducted as a factorial based on randomized complete block design with three replications. Indices of WAS, MWDf and DC were measured after 1, 3, 5, 10 and 15 wetting and drying cycles in three replicates. Results showed that with 5 % OM and 30 mg kg-1 PAM, WAS increased about 3.28 times in sandy loam and nearly to 2.4 times in clay loam soil. The climax effect of PAM on WAS in sandy loam occurred after 3 cycles of wetting and drying and thereafter it gradually declined to about half. In the clay loam soil both WAS and MWDf steadily increased up to 15 cycles and never decreased. The effect of PAM on DC at both rates of PAM application was the reverse of WAS and MWDf. As an organic amendment, PAM appeared to have great ability in improving structure stability in both soils.

Expansive soils swell or shrink when subjected to water content changes. These volume changes can give rise to ground uplift which may cause damages to the on site structures. In this research the behavior of an expansive soil was investigated through a number of wetting and drying cycles. Laboratory tests were carried out on statically compacted samples in a modified odometer under surcharge pressures of 10 and 20 kPa. The vertical deformation of the soil sample and EC (electrical conductivity) of the water of the consolidation cell were measured during cycles of wetting and drying. The void ratio and water content of the soil samples were determined at different stages. The results showed that swelling and shrinkage were reversible after the fifth cycle while soil deformation reached the equilibrium condition. The value of EC increased during each cycle and attained to a constant value but it decreased with increasing the number of cycles. Wetting and drying paths were as S shaped curves. These curves consisted of a linear and two curved portions، and in equilibrium condition these paths coincided. The majority of the soil deformations occurred in the middle part of the curves between the degrees of saturation 50% and 90%.

The withdrawal strength of nail and screw fasteners was studied in rice straw fiber-thermoplastic composites. Virgin high-density polyethylene (HDPE) was selected as the composite matrix. Three levels of dry rice straw fiber, 45%, 60% and 75%, based on weight and passed through a 40-mesh size screen were mixed with the polymeric matrix without or with 2% (based on weight) maleic anhydride grafted polypropylene (MAPP) as coupling agent. A dry-blending method was used for compounding the materials. The 12 formulations for the polymer composites were used to prepare samples with dimensions of 25 cm by 15 cm by 1 cm. The final composites were made by pressing the prepared mats between the hot plates of a compression press by employing combinations of temperature and pressure in three stages. After keeping the composites at room temperature for 15 days, the withdrawal strengths of nails and screws were measured according to BS Standard for dry composites. Withdrawal strengths were also measured after immersion of the composites for 24 hours in distilled water (wet condition) (cycle 1). Wetting for 24 hours followed by drying for 24 hours was then repeated for the other 3 cycles. The results showed that the withdrawal strength of screws is more than that of nails. Also, rice straw fiber content may significantly influence the withdrawal strength of fasteners, especially at higher fiber to polymer ratios. Furthermore, it was found that the coupling agent slightly affected the withdrawal strengths of the fasteners. The results also showed that withdrawal strength of fasteners are significantly reduced at more wetting cycles.