| Study in Morocco Highlights Efficiency of SAG Technology |
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Study in Morocco Highlights Efficiency of SAG Technology Abstract: A study of the SAG technology in Morocco was conducted by IAV Hassan II and released in May 2002 by the Ministry of Agriculture of Morocco in the Monthly Bulletin of Technology Transfer Connection and Information of the PNTTA, Synopsis 92. The study found that SAG technology, “corrects the alkalinity and reduces the pH of initially alkaline water, strongly diminishes the global salinity in the whole soil profile, reduces the exchangeable sodium percentage (ESP) and the sodium absorption ratio of the soil, and increases the permeability of the soil.” It also reported that “SAG is a practical method to treat irrigation water,” making it an attractive alternative to other methods of soil reclamation utilizing sulfuric acid. Note: The original release of this study was in French and is available via the link above. The English translation of that release is below. TRANSFER OF TECHNOLOGY IN AGRICULTURE The water resources for agriculture in arid and semi-arid zones are limited, especially in comparison to the needs of the local populations. In these regions expansion and development of agriculture exceeds the development of agricultural irrigation. However, if irrigation contributes generally and rapidly to an increase in agricultural production and to the improvement in the quality of life of rural populations in developing countries, than its expansion is often accompanied by serious hazards to preservation of the environment, especially soil quality. In an arid setting, the use of irrigation presents a risk of salinization and/or alkalization to the soil of which manifestations have been identified in numerous irrigated locations around the world. The principal consequence of this is a reduction in fertility seen over both short and medium time periods in numerous soils, thus putting development in danger in these regions. In Morocco, the majority of the post-project studies done in different irrigated plots showed that soils initially non-salinated became salinated after irrigation. In fact, the area of salinated soils in Morocco is estimated to be 350,000 ha, which is about 35% of all irrigated land in the country. Improvement of the physical and chemical properties of soil affected by soluble salts and/or by a significant accumulation of sodium comes by the removal alkaline ions from the soil structure. For calcareous soils, which are most of the soils in Morocco, sulfuric acid is the most effective and fastest way to restore them. However, problems tied to operating and handling this treatment has limited its use in Morocco. To resolve these difficulties, Sweetwater International, Inc (Utah, USA) has developed a new technology. It is the treatment of irrigation water on site through the use of a Sulfuric Acid Generator (SAG). This technology has been tested by the IAV Hassan II on different crops (corn, citrus fruits, peaches, and tomatoes) in the regions of Tadla, Haouz, and Souss-Massa using different types of irrigation (gravity and drip). The objective of this report is to present the results of a test done for technology on the rehabilitation of the saline-sodic soils of a farm in the Souihla region. The specific objectives are:
Presentation of the SAG The SAG, “Sulfuric Acid Generator,” is a machine developed by Sweetwater International, Inc. in Utah, USA. A prototype was offered by the company to the IAV Hassan II to be tested in Morocco by treating the alkaline irrigation water in order to treat the alkaline soils. A diagram of the machine and its function is shown in Figure 1. The Function of the SAG From a container (A) (Figure 1), elemental sulfur feeds by gravity into the burning chamber (B). At a temperature of more than 205˚C, the elemental sulfur is oxidized creating sulfur dioxide (SO2). This gas is transferred towards a second chamber (C1) where it mixes with irrigation water. The SO2 is a very soluble gas in water and the product of this chemical reaction is sulfuric acid (H2SO3), which is a strong acid. Water charged with H2SO3 exits through the discharge (S1). When the H2SO3 comes in contact with the oxygen of the air it transforms into H2SO4. The excess SO2 goes to another chamber (C2). The S1 and S2 discharges are connected to the irrigation network which can either be gravitational, sprinkler, or localized (drip). In the case of the Souihla farm, drip irrigation was used. The desired pH of the irrigation water can be adjusted by altering the mixing rate of the treated water (maximum discharge of the machine is 6 liters per second) with the untreated water. The Action of the SAG on Water and Soil In alkaline irrigation water, the bicarbonate and the calcium of the soil have a general tendency to form calcium carbonate. Thus, following the precipitation of CaCO3, the Na/Ca ratio of the soil structure increases, which increase occurs due to an accumulation of Na (Sodium) in the soil exchange compound. This causes diminished porosity and infiltration capacity as well as causing a tendency to accumulate salts in the soil. The SAG allows precipitated calcium to return to the soil structure following the solubililization of the carbonates. The action of the SAG is two fold:
H2SO4 + NaHCO2 → H2SO4↓ + 2H2O + CO2↑ Work on the ground started with the choice of location for the SAG installation. We chose an experimental plot situated in one of the Souihla Company farms in the Haouz region. This plot was planted with citrus fruits (sour variety). The soil samples taken of the farm’s underdeveloped alluvial supply for initial classifications were done through the use of a drill at five levels of depth: 0-20, 20-40, 40-60, 60-80, 80-100 cm. Collection of water samples was carried out before and after treatment by the Sulfuric Acid Generator. The plot was irrigated daily by a drip system. The dates of the soil sampling are depicted in Table 1. The water quantities used daily are presented in Table 2. The vertical permeability of the soil surfaces and the hydraulic conductivity in the soil depths were measured. Three areas were measured for permeability (Muntz) and hydraulic conductivity (Porchet) during both the initial and final states. The climatic figures for the Souihla Company show that there was no rain during the period of experimentation from April 21 to may 26 1999. Light rain (2 mm on April 6) was recorded before the test began. The average temperatures recorded during the months of April and May were between 18.8˚ and 22.4˚ C. The temperature varied between a minimum of 5.5˚ C and a maximum of 36˚ C during the month of April, and between a minimum of 7.5˚ C and a maximum of 41˚ C during the month of May. Laboratory Analysis In order to assess the quality of the irrigation water the following analyses were completed:
To characterize the physical and chemical properties of the studied soils the following analyses were done: pH, EC from saturated soil mixture extract, total calcium, CEC (Cationic Exchange Capacity), exchangeable bases, ionic assessment from saturated soil mixture extract, granulometry, and reserved utility (RU). The evolution of the soil quality in the plot irrigated with treated water was characterized by measuring the pH, the ECps (Electric Conductivity of the saturated mixture), the SARps (the Sodium Absorption Ratio of the saturated mixture), and the ESP (Exchangeable Sodium Percentage). Quality of the Irrigation Water The quality of the dam water, used for irrigation, is shown in Table 3. From this table we can conclude that:
Initial Quality of the Soil The soil in the experimental plot had a balanced to fine texture and an average CEC (11.5<CEC<14.0). It also had a generally low amount of calcium (<5%). It was basically average and showed little variation in depths. The permeability of the plot was weak to very weak (K: 0.37 to 0.63 cm/h), because of the large amount of exchangeable Na+ (ESP>15%) on the surface. Therefore, the accumulation of salts in this plot was essentially due to the climb in salts from evaporation and to the weak permeability of the soil. The initial state, presented below, does not show the true state of the salinity and sodicity of this farm. The samples used to characterize the initial state before irrigation was taken from beneath drippers, therefore from the center of the humectation bulb (watering area). The samples taken from outside of this area showed a higher concentration of salts as well as a higher ESP (Table 4) in the same plot. In fact, the EC varied from between 6.2 dS/m on the surface to 2.7 dS/m underground. The ESP ranged from 31% in the first 60 cm to 18.4% in between 80 to 100 cm of depth. Note that the treated plot was modified with furrows to help evacuate excess salts and sodium. This modification of the land effectively reduced the salinity and the sodicity but did not eliminate them. One unmodified plot showed values of sodicity and salinity even more serious (Table 4). The ESP was very high, varying from 39.7% on the surface to 26% underground. The EC was also higher since it ranged from10.6 dS/m on the surface to 5.3 dS/m underground. Ground observations showed that water infiltration is very weak in these zones. Consequently, the problem of salinity and sodicity in this farm were very serious. Stagnation of water after rain or irrigation is a recurring incident. Action of the SAG on the Quality of Irrigation Water This regards the treatment of irrigation water with a machine that generates sulfuric acid from pure sulfur. The machine treats an average of about 6 liters per second. The treated water is injected into a principal irrigation conduit which creates a tie between the head unit and the irrigation pipes through two discharges. The primary discharge is the principally water treated, and the secondary discharge is a complementary utilization of excess non-solubilized sulfuric acid. The characteristics of the two treated waters are represented in Table 5. The treated water evacuates into the canal through the two discharges and then mixes with untreated water making a total debit of approximately 30 l/s. The exchange ration is therefore about 6 l/s of treated water with 24 l/s of untreated water, a volume ratio of about ¼. A comparison between the characteristics of the waters: initial, treated, and mixed; are presented in Table 6. To reduce the pH of the irrigation water is the principal objective of using the SAG. According to the obtained results, the water, which was moderately basic (pH 8.4), became slightly acidic (ph 6.0) after being treated and mixed (Table 5). This reduction in pH is very beneficial for very basic water because it facilitates the solubilization and the assimilation of certain nutritive elements, particularly trace elements. Also, use of this slightly acidic water is especially recommended for fertigation, in which it is essential for preparation of the nutritive solution. Water with slightly acidic pH constitutes a favorable medium for element solubulization. The pH of the irrigation water can be adjusted by changing the ratio of treated water to non-treated water. From Table 6 it is also inferred that there has been an increase in electric conductivity. Variation in the water’s ionic composition during treatment is the origin of this slight increase. Note that the two waters belong to the same class of salinity (C2). In conclusion, treatment of the irrigation water, which is initially alkaline (high bicarbonate content and pH of 8.4), corrects the alkalinity and reduces the pH. A slight increase in EC is also recorded following the addition of SO42-. This change in the quality of the water will necessarily have an impact on the soil. Action of the SAG on Soil Quality Five parameters have been retained to follow and control the effect of treated irrigation water on the physical and chemical characteristics of the soil. Effect on Salinity The global salinity strongly diminished in the whole of the soil profile (Figure 2). Starting at 3 dS/m on the surface, the EC stabilized at about 1 dS/m. These reductions of salinity are explained by the fact that treating the irrigation water with sulfuric acid acts on the soil by improving the permeability (see the effect on permeability). The soil becomes more humid from the previous irrigation and the surplus high quality water allows desalting of the soil, particularly in the first two levels (0-20 and 20-40 cm). Effect on the ESP of the Soil The most spectacular effect is that of the exchangeable sodium percentage reduction (ESP), especially in the first 40 cm of soil (Figure 3). In fact, it was recorded that on the surface (0-20 cm) the initial ESP was reduced considerably after only one week of irrigation with treated water. The value of the ESP passed from 17.38% to 5.48%. This diminution is a direct consequence of the Na-Ca exchange and sodium leaching in depths more than 100 cm. For the rest of the profile, the variation of ESP was not very significant because the values recorded always varied about 5%. The diminution of the ESP in the plot irrigated with treated water (SAG) can be explained by the following reactions: H2 SO4 + Ca CO3 → Ca CO4 + H2O + CO2↑ Therefore, it can be concluded that the diminution of the ESP is due to the continual enrichment of the soil solution with solubilized calcium from the calcium deposits of the soil thanks to the action of slightly acidic water. Effect on the SARps of the Soil A significant reduction in the sodium absorption ratio (SAR) (Figure 4) of the soil, especially in the soil profile, was recorded. This reduction is consequent of a relatively more significant reduction of Na+ in comparison to Ca++ and Mg++. The solubilization of CaCO3 (see the final state, Figure 6) by treated water comes about by a relative enrichment of calcium and magnesium ions in the soil. During the experiment it was observed that in the first 20 cm the SARps diminished considerably after the first week of irrigation with treated water, while throughout the rest of the soil profile the change was not as significant. This phenomenon, widely observed after the first week, is momentary and is the result of the soil solution being enriched by Na+ coming from the absorption complex on the surface level. Pursuing irrigation with treated water will lower the SARps, even in deep underground. We can therefore conclude that water treated by the SAG allows precipitated calcium to return to the soil solution and to control the Na/Ca ratio of the solution. We should also note that this ratio, which was initially more than 1 (1.06), had been reduced to 0.61 after the last week of irrigation. Effect on the pH of the Soil The pH did not record significant reductions after 5 weeks of irrigation with treated water (Figure 5). The protons brought by the water were neutralized by the dissolving calcium carbonate. This result shows that even though the soil was irrigated with pH 6 water during 5 weeks the pH of the soil did not significantly diminish. The excess proton H+ brought by the irrigation water was continually neutralized by the carbonates. Effect on the Permeability of the Soil Irrigation with treated water had a very significant effect on the permeability of the soil. The average value found at the initial state was 0.46 cm/h. At the final state this value had increased to an average of 21.5 cm/h. Otherwise said, the vertical permeability went from an extremely weak state to an extremely high state (K>20 cm/h). The variation of the hydraulic conductivity showed a significant improvement. It went from an average of 0.63 m/j at the initial state to a value of 2.38 m/j at the final state after irrigation with treated water. After all this, we construe that treatment of irrigation water by a sulfuric acid generator has highly significant effects on the following physical and chemical properties of the soil: electric conductivity, ESP, ionic composition of the soil solution (SARps), and permeability. This effect is due in grand part to the quality of the treated irrigation water. Irrigation with treated water produced rapid desalting and desodifying of the soil while improving the permeability and aggregation of the soil which facilitated salt leaching. However, under drip irrigation, only the moistened part is desalted and desodified (Figures 7, 8, and 9). Salts accumulate outside of the moistened zone. Irrigation by submersion or strong rain is necessary to leach the accumulated salts and sodium from the areas untouched by the irrigation water. Summary The sulfuric acid generator is a practical method to treat irrigation water. It was specially created to improve the quality of water and soil contaminated by sodium. Analysis of the experiment results of this study allows certain conclusions to be drawn concerning the effect of this treatment on the evolution of water and soil quality.
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