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| Ana Sayfaya Dönüþ |
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IN RELATION TO : TIME, WATER TABLE, TEXTURE, AND TREATMENTS A.A. Jafarzadeh Tabriz University, Faculty of Agriculture, Soil Science Department, Tabriz. I.R. Iran. ABSTRACT An important mechanism by which soils become gypseous is the upward movement of ground water with dissolved calcium and sulphat and its subsequent evaporation at the soil surface. High soil evaporation rates, which cause salt accumulation on the surface of these soils, can thwart all efforts to increase crop production. Evaporation in gypseous soils experimentally (over 140 days) was studied by French workers, which they have shown decreases of evaporation for first thirty days and there after irregularly changes. Later this opinion has confirmed by Nafie (1989). But the present study not only evaluates the evaporation in gypseous soils with time, but also examines the relation of evaporation rate to different textures, water table level and treatments. The highest evaporation occurred during the first ten days and then tended to decrease in the second ten days with regularity for 40 to 50 days. From 50-120 days, it became irregular but never returned to the first value (Table 1). Evaporation rate in acid washed sand was slower that the other texture while maximum rate and hence of capillary rise, was with sandy loam. Lowring of water table is not effective unless to one meter or more, just in sandy loam+13% gypsum and 50 cm height results show a lower rate than sandy loam + 13% gypsum with 25cm height. All samples with NaCl show lower evaporation and presence of CaCO3 increase evaporation except in sandy loam with 50 cm and acid washed sand with 25cm heights. Also the high concentration of gypsum in samples of sandy loam + 13% gypsum reduced evaporation rate. It also investigates three quite different effects of capillary movement such as (a) height of rise(b) speed of rise and (c) volume transmitted to the surface at equilibrium. Results grouping in gypseous soils a water table at <1m as a suitable mapping unit in coarse and fine textures. Also speed of rise in these soils not only related to texture but also differs according to different solution, total porosity and different size of pores. INTRODUCTION Many studies on the patterns of water movement in soil provide detailed descriptions of the dynamics of soil moisture, but they do not, as a rule, fully elucidate the problems connected with movement of salt solutions. This reduces the value of these studies, because the liquid phase of the soil is not pure water but solutions of gypsification, one most take into account the quantity of gypsum in the liquid and movement through the soil profile in relation to different factors. Movement per ascensum where gypsum is moved upwards by capillary rise and deposited at the surface following evaporation. This is sometimes from a water table, but may be in moisture from previous rain or dew. Upward movement of water from the water table and subsequent evaporation at soil surface is an important feature of some gypseous soils, especially in fallow land. The process of evaporation from the soil surface can hardly be overemphasised in arid regions of the world with hot climate and low rainfall, and when one considers the importance of evaporation from soils is an important consideration in the program of many farming operations in both irrigated and dry land agriculture. French workers, Mme Christine Plet-Lajoux, Gerard Monnier and Georges Pedro (Glangeaud 1971) have shown that evaporation rates in soil columns in contact with gypsum saturated water for 140 days show a decrease during the first thirty days and thereafter change irregularly but never return to the original value. Nafie (1989) has described evaporation rates of gypsum saturated water in four different soil columns 30 cm in height, over six weeks, and confirms the result of the French workers that for thirty days there is a regular decrease. Therefor no large amount of investigative work concerning evaporation in gypseous soils has been conducted, and with regard to the basic process or the effect of the complexity of the soil or the continuously changing variables associated with atmosphere, much remains to be learned. But the present paper not only evaluates the evaporation with time and other factors but also investigate three quite different effects of capillary movement such as (a) height of rise (b) speed of rise and (c) volume transmitted to the surface at equilibrium. MATERIALS and METHODS The study was carried out with laboratory columns involving five different soils [silt loam (ZL), sandy loam (SL) sandy loam with 13% gypsum, medium fine sand (MFS), acid washed medium sand (AWS)] and all with three different treatments (soil, soil plus 10% CaCo3 and soil plus 2% NaCl). Sandy loam with 13% gypsum/ acid washed sand and all samples with 10% CaCo3 and 2% NaCl were prepared in the laboratory. The natural soils, free of gypsum, were collected from four areas of Kent (UK) as follow: 1) Silt loam : Btc horizon at about 50-100 cm depth from the Hamble series at Withersdane. Wye. Kent. 2) Sandy loam: Bw horizon at about 20-35 cm depth from the Bearsted series at the Hothfield Common, near Ashford, Kent. 3) Medium-fine sand: Creatceous Folkestone Beds sand from Hinxhill. near Ashford, Kent. 4) Acid- washed medium sand : beach sand from Kingsgate. Thanet. Kent. which was washed with hydrochloric acid for one week, then with hot water and finally with distilled water, and dried in the laboratory. The pipete method was used for particle size analysis. After boiling with hydrogen peroxide (H2O2) for the destruction of organic matter, dispersion was performed by shaking in a 10% sodium hexametaphosphate solution for 16 hours. Particle separation followed the procedure proposed by Tinsley (1970). The wet combustion method of Tinsley (1970) was used for organic matter content determination. All selected materials are essentially finer grained than sand (<600µ) (Table 3 and 4). The two loamy materials are mainly finer than medium sand (<212µ), but the silt loam has twice as much silt + clay that the sandy loam. The sandy materials slightly vary in their dominantly medium sand content, and the beach sand has been additionally cleaned by acid- washing. Thus the materials represent four textural conditions sufficiently differentiated to significantly influence the ascending water movement from a constant, gypsum saturated water table at variable depths. All samples were placed in gypsum saturated water (prepared from plaster of paris and distilled water) in beakers. The sun heat in deserts was simulated during the experimental periods (120 days) by men of continuously glowing 100 watt radiant lamps placed 20 cm above the soil surfaces. Evaporation rate in relation to different factors were studied in columns with 25 and 50 cm length and 6 cm internal diameter and measured every two days (later every three days) during the experiments (120 days) and calculated for every ten days (Table 1). But for the study of capillary movement effects, columns 7.5cm high with 6cm id, columns l m in length and 9 cm internal diameter were used. Volumetric water content and water retention of soils were studied by the pressure plate melthod. The porosity of the disturbed samples, representing a condition similar to that of the materials packed in the experimental tubes, was studied and capillary continuity was confirmed using the suction method in a pressure plate at tensions of -5 kPa, -20 kPa,-50 kPa, -100 kPa (Table 2).    RESULTS and DISCUSSION The simulated arid zone environmental conditions induced a high evaporation rate when the water table was close to the soil surface. Experimental information about gypsum dissolution and transport was obtained from both experiments by introducing gypsum saturated water (gsw) to the bottom of soil columns in relation to different factors. The highest evaporation occurred during the first ten days and then tended to decrease in the second ten days for both experiments (five different soils and each with three different treatments [soil - soil 10% CaCo3 and soil + 2% NaCl]) with regularity for forty to fifty days. From 50 to 120 days, the evaporation rate became irregular, but never returned to the first value, which confirms the findings of previous workers about evaporation rate and its relation with time in gypseous soils. The evaporation rate was measured in this study with regard to four different textures (ZL, SL, MFS and AWS). The water rose from the water table to the surface of the soil columns by capillarity, and was evaporated from the surface. The sandy loam in both experiments (25cm, 50cm) with three replications in each experiment shows a lower rate than medium fine sand. Therefore texture of soil affects capillary conductivity by its influence on the size and continuity of the interspaces or pores, which results obtained from experiments can be as follows: SL > MFS > ZL > AWS Moore (1939) has shown that saturated permeability increases with increasing coarseness of texture from shallow water tables. Sand > fine sandy loam > light clay > clay But according to the results observed from both experiments evaporation rate in AWS is slower than the other textures, while the maximum rate of evaporation and hence of capillary rise, is with sandy loam. Thus the results from these experiments do not agree with Moore's (1939). However, they do confirm the result described by Nafic (1989) about medium acid washed sand and also suggest an explanation for the effect of texture on capillary rise and evaporation rate. In order to study the relation between evaporation rate and depth to the water table, columns as described earlier were packed with soil, with a perforated aluminium foil in the bottom of each column. The simulated water tables were 25 cm and 50 cm which in each experiment all previously explained soils and treatments were used. Lowering the water table from 25 cm to 50cm in all different soils except sandy loam almost sandy loam with 13 percent gypsum would decrease the evaporation rate by a factor of height (Table.1).Therefore upward movement and evaporation of water is possible with water table as deep as 50 cm and although the rate will be slow, accumulation of harmful amounts of gypsum is possible if the ground water contains substantial amounts of calcium and sulphate ions and sufficient time is allowed. In such a case the lowering of water table is not effective, unless to 1.0cm or more. Table.1 confirm this effect which the evaporation rate in sandy loam with 13% gypsum and 50cm height shows a lower rate than sandy loam with 13 percent gypsum with 25cm height in comparison with sandy loam samples. The concentrations of different treatments (soil, soil + 10% CaCo3, soil + 2% NaCl and so soil +13% gypsum in sandy loam texture) were determined for all soils with three replications in both experiments. Evaporation of water samples with NaCl differs from that of just soil in that under comparable conditions, the gypseous soils + NaCl has a lower evaporation rate than others. There are three principal reasons for this. In the first place, as water evaporates from the surface of the soil, the salt concentration in the soil surface increases, which lowers the vapour pressure of the solution and increases its osmotic pressure. This reduces its rate of evaporation, and so allows solution to move up from the subsoil for a longer time than if no NaCl were present. Secondly the soil surface becomes dry and covered with salt crust, the rate of evaporation drops just as in a salt-free soil. Thirdly sodium chloride collapses the soil structure, and with cementing and pushing up of the particles causes a low evaporation rate (table.1). The results obtained from the five soils in two different experiments with three replications confirm this idea about gypseous soil with NaCl. But the results of the samples with 10 percent calcium carbonate in both experiments except SL + 10% CaCo3 (50cm height) and AWS + 10% CaCo3, (25cm height) show a high evaporation rate in comparison with Haplogypsid samples. Leenheer (1964) has described how application of calcium carbonate has sometimes given a marked visible improvement in the structure of soils low in organic matter, even if they were neutral, and there is evidence that excess calcium carbonate may reduce the cohesion between clay particles in moist clods (Russel and Basinki 1954) and reduce the size of the water-table crumbs in a soil (Williamson 1959). therefore the results confirm the effect of calcium carbonate in the improvement of soil structure in calcigypsids in comparison with Haplogypsids . Also a high concentration of gypsum has reduced evaporation rate in sandy loam samples with 13 percent gypsum in comparison with sandy loam samples in experiments which refers to gypsum concentration that reduce total porosity and therefore it causes low evaporation rate. CONCLUSION The results obtained from the present column experiments polyethylene tubes of one metre length and 9cm internal diameter, with silt loam and medium fine sand textures confirm the result of szabolcs and Lestaks (1971) experiment for eight months with sulphate, that during a four month experiment, the gypsum saturated water never reached the surface, and indeed there was no accumulation of gypsum in the surface (0-1cm) of silt loam samples with 50 cm height. Therefore with regard to results (during 8 months and 4 months) and the studied profiels with water table at 60-85cm depth (BAH3 and BAH15) (Jafarzadeh 1991) grouping soils with a water table at <1m is suitable as a mapping unit in gypscous soils with coarse and fine textures. This limit however should be related to the nature of soil particles, since results of column experiment with 50cm height show that gypsum saturated water does not reach to the surface and there is even no evidence of gypsum accumulation at 44cm depth from the top in acid washed medium sand samples with clean particles. Also the results of capillary movement speed with time during the first fifteen and thirty days in column experiment of 50cm water table height confirm both ideas of previous workers, ie that gypsum saturated water after fifteen days reaches to soil surface of columns in sandy loam samples with enough connected, medium size pores, but in other textures such as silt loam with clean particles and large pores there is no evidence of gypsum accumulation in the soil surface and even in acid washed medium sand samples with 50cm height gypsum saturated water does not reach to the surface. Therefore the speed of capillary rise is not only related to the texture, but also differs according to different solutions, for all samples show sodium chloride accumulation at the surface after fifteen days, even acid washed medium sand. There was confirmation of previous authors results about texture, in that Premi and Oswal (1984) reported greater capillary rise in sandy loam than in sand or sandy loam/sand mixtures , in agreement with results obtained from the present study (Table 1). Porosity was studied and capillary continuity confirmed in different textures with the pressure plate (Suction method) in different pressures of -5 kPa, -20 kPa, -50 kPa and -100 kPa.  Total porosity of samples was first measured by saturation and without any pressure. The results (Table 2) refer to silt loam with an abundance of well connected water filled pores, but they are of fine calibre and so water movement is slower (Tables 2 and 3). Sandy loam has an abundance of well connected, medium sized water filled pores and very few air filled pores (Tables 2 and 3). Natural medium fine sand with dirty grains carries thicker water films and has fewer obstructing coarse air filled pores (Tables 2 and 3) than acid washed medium sand with clean grains carrying thin water films and having large size of pores give many barrier air spaces (Tables 3 and 4).  REFERENCES Glangeaud L 1971. Authors: C plet-lajoux, G Monnier and G Pedro Pedologie-Etude experimental sur la genese et al place des encroutements gypseux. C R Acad Sc, Paris T 272 (14 Juin 1971). Jafarzadeh, A.A. 1991. Experimental studies of gypsum migrtion and deposition in soils porfiles.PhD. Thesis. Wye College. University of London. Leenheer L de 1964. Preservation of soil structure on mechanised farms in Belgium. Trans 8th Int Congr Soil Sci 2:561-570. Moore R E 1939. Water condition from shallow water tables. Hilgardia 6^383-426. Nafie, F A A 1989. The properties of highly gypsiferous soils and their significance for land management. PhD Theis, Wye College, University of London. Premi S C nd Oswal M C 1984. Comparison of observed and predicted maximum rise of water in soil. J Indian Soc Soil Sci 32: 654-657. Russell E W and Basinski J J 1954. The effect of heavy dressings of calcium salts on the consistency of clay soils and subsoils. Trans 5th Int Congr Soil Science 2: 166-173. Szabolcs I and Lestak V 1971. Capillary movement of sodium salt solutions in soil columns. Soviet Soil Science 483-488. Tinsley, J 1970. A manual of experiments for students of soil science. Soil Scinence Department Aberdeen University. Williamson W T H, Pringle J and Coutts J R H 1959. Short and longly rotations and soil structure in the north east of Scotland. Proc Int Symp Soil Structure 1958, 176-187. |