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Factors affecting Aquasorb Polyarylamide Hydration in Soil
1Gorgan University of Agricultural Sciences and Natural Resources, Golestan, Iran, Email: 2Gorgan University of Agricultural Sciences and Natural Resources, Golestan, Iran, Email: Abstract
The weigh percentage of water absorption by Aquasorb PR3005A hydrophilic polymer (a salt copolymer polyacrylamide) at tensions 0, 0.05, 0.1, 0.3 and 1.0 bar were measured using Tempe Pressure Cells and at 5.0 and 15.0 bars using Pressure Plate Vessels. Aquasorb with two rates of 0.0007 and 0.0014 mass of Aquasorb to soil mass ratio (about commercial recommendation) and garden waste compost (equivalent to 50 t/ha) were incorporated to a loam sand and a loam soil and their water retention were measured at same potentials with same method. To have a notion of Aquasorb hydration capacity when incorporated, the hydration in distilled water and saturated paste extracts of loam and loam sand soils were determined (mass of solution absorbed per mass of Aquasorb). Using a similar method, the effect of valence and concentration of cations on water absorption by Aquasorb was investigated. Gel hydration by CaCl2 and NaCl solutions with 11 levels of electrical conductivity (0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 4.5 ds/m) and 3 replications were also determined. The water stored between field capacity and permanent wilting point is used by a crop and the quantity absorbed at greater potentials is lost to drainage. The hydration capacity at
different potentials is a function of soil solution electrolyte concentration and the hydrophilic
polymer chemical structure. The hydration capacity of Aquasorb was reduced as the
electrolyte concentration and electrical conductivity increased. Increased electrolyte
concentration in a soil solution through drying, may result in gel dehydration and water
release at potentials greater than field capacity which maybe lost to drainage. This might be a
useful consideration for gel chemical formulation. Rather slight and insignificant increased
water absorption at different pressure steps by Aquasorb incorporation was due to electrolytes
in soil solution, requiring greater application than commercial recommendation.
Key words: hydrophilic polymer, Aquasorb, evaporation, water retention.
In arid and semi arid climates annual rainfall is limiting with an uneven distribution. Increasing water holding capacity in soils with limited water retention (such as sandy soils) using hydrophilic polymers or through improving soil physical properties (with different amendments such as compost) may reduce water loss through leaching and improve efficiency of its application. Water holding capacity for subsequent use by a plant may differ by different hydrophylic polymers based on their chemical structures. These solid materials normally absorb distilled water hundreds of times of their own weight as a gel (Al-Omran, Mustafa and Shalaby,1997; Peterson, 2002). They are small particles with different sizes when dry and remain as individuals when wet (Bowen and Evans, 1991; Peterson, 2002). Hydrophilic polymers (HPs) are either natural or synthetic. Some natural hydrophlilic polymers are polysacharides, humus, polyuronids and Aljinic acids. Synthetic polymers with net type chemical bonds are not dissolvable in water. HPs have an intensive hydrophilic character owing to presence of polar groups within polymer chains (Wallace and Terry,1998). Synthetic HPs usually are either polyveneyl alcoholes (-CH2OHOH-) n or polyacrylamids (-CH2CHCONH2-) n. HPs used in agriculture are usually formulations commonly made of starch polyacrylamid graft copolymers (starch copolymers: SCP), venylalcohol-acrylic acids (copolymers: PVA) and acrylamids sodium acrylate copolymers (Polyacrylamides: PAM) (Peterson, 2002). Synthetic polymers are used more than natural polymers because they are more resistant to biological degradation (Peterson, 2002). PAMs such as Aquasorb do not pose any treat to human life and environment (Saybold, 1994). Under higher magnification the detailed framework of the polymer can be seen as a matrix of vacuoles connected by polygonal bridges of cross-linked. A greater proportion of water, around 80 to 85 percent are stored within vacuoles as numerous minute reservoirs. The remaining 15 to 20 percent is bound with a greater tenacity. This water is still plant available and it persists under a tension of 9.8*104 Pa at which point the vacuoles are air filled (Johnson and Veltkamp, 1984). Addition of HP to growing media has been shown to increase water holding capacity by up to 400 percent (Johnson, 1984a) and to decrease water stress and delay the onset of wilting (Gehring and Lewis, 1980). Gel storage of water provides a buffer against temporary drought stress and reduces the risk of failure of certain crops at establishment (Johnson and Leah, 1990). When lettuce and barely were grown under limited irrigation on a coarse sand substrate the interval between field capacity and the onset of wilting was increased by up to three-fold in the presence of polymer. Water use efficiency and dry matter production also responded positively to the presence of both starch copolymer and polyacrylamides (Woodhous and Johnson, 1991). The common hydrophylic polymers are sensitive to electrical conductivity and their absorption capacity is strongly reduced even with low electrical conductivity and this might affect their application in soils with variable electrical conductivity. Starch co-polymers have a greater hydration capacity relative to other types of hydrophilic polymers but hydration capacity is less affected by polyacryamides with same salinity levels (Johnson,1984b). Due to low cationic exchange capacity of coarse textured sandy soils and hence electrical conductivity the addition of HP to these soils had the best results compared to other soils (Peterson, 2002). Range of potentials at which the water is retained by hydrophilic polymers is also important. Water retained at potentials greater than field capacity and lower than permanent wilting point is not available for plant use. Evaporation in a soil treated with these polymers must also be considered when determining their efficiency. The reducing of evaporation from soil surface with using HP has been reported by Johnson (1984a) and Chaudhary, Shalaby and Al-Omran (1995), Whereas Tue, Armitage and Vines(1985) and Blodgett, Betti, White and Eliott (1993) reported that HP was not effective on evaporation. Compost incorporation also might reduce evaporation and increase soil water storage in available range (Opara-Nadi & Lal, 1986). A comparison of water retention by compost incorporation and hydrophilic polymers could be informative for proper recommendation. Compost mulch reduces evaporation and increases water storage (Unger, Parker and Jessie, 1976; Shekour, Brathwate and Mc David, 1987; Todd, Klocke, Herger and Parkhurst, 1991; Bussier & Cellier, 1994; Movahedi Naeini & Cook, 2000). There are many reports about the enhancing effect of organic materials and also HP on the yield of the different plants which is strongly dependent to soil and plant and climatic conditions and the rate of application. Wofford (1989) reported increased yield, fruit size, flower number and early maturation in different plants with HP in USA. In a field experiment Silberbush, Adar and De-Malach (1993a and 1993b) reported that in a sandy soil water storage and the yield of cabbage and maize were increased using HP. All the yield indices were positively correlated with reduction in water salinity and HP increments. Different plants respond differently when organic residues are used as a mulch or incorporation (O-Paranadi and Lal, 1987). Cassava and Yam showed different responses with pine needle mulch and incorporation. This research has the following objectives: (1) to relate soil texture and water holding properties of soils and the hydrophilic polymers and composted material; (2) to investigate the shortcomings of common hydrophilic polymers for agricultural use such as Aquasorb as a guide for further improvement of their chemical structure. MATERIALS AND METHODS
To have a notion of Aquasorb hydration capacity in soils, the hydration in distilled water and saturated paste extracts of a loam and a loam sand soil were compared. A constant weight of HP (0.20 g) were added to a 50 cm3 saturated paste extract of each soil in a glass container with 8 replications. Two hours later the contents of each container were filtered through a Watmann filter paper. Water absorption capacity was considered the difference between the volume of added and filtered extracts. Using a similar method, the effect of valence and concentration of cations on water absorption by HP was investigated. The two factors of electrolyte with 2 levels (CaCl2 and NaCl) and electrical conductivity with 11 levels and 3 replications (0, 0.25, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0 and 4.5 ds/m) were randomized and gel water absorption capacities were compared statistically. The effect of Aquasorb and a garden waste compost on water retention by the above soils was also determined. The coarse textured topsoil was a flovent (Entisols) loam sand soil (from Ziarat river bank, south of Gorgan-Iran) and the finer textured topsoil was a calcixerolls (Mollisoils) loam soil (Jahanabad series, Banaei, 1973). This was sieved using a 2 mm mesh and packed to an initial dry bulk density of around 1.88 g/cm3 for loam sand soil and 1.46 for loam. Treatments were bare soil (B), compost incorporation (I) (equivalent to 50 t/ha), and Aquasorb hydrophilic polymer with two rates of 0.0007 and 0.0014 dry mass to unit soil mass (Hp1 and Hp2 respectively), all with three replications. Soil water content at tensions 0, 0.05, 0.1, 0.3 and 1.0 bar were measured using Tempe Pressure Cells and at 5.0 and 15.0 bars using Pressure Plate Vessels. Ultimate dry bulk densities were also measured for respective treatments in cores of Tempe Pressure Cells. Tap water retention by Aquasorb (mass of water per unit mass of Aquasorb) for two replications at same pressure steps were also determined when unmixed with soil. Data were analyzed by the analysis of variance and co-relations techniques (SAS Inst, 1996). The physical properties such as particle size distribution (Klute, 1986), dry bulk density and chemical properties (Ali Ehyaei, 1997) and cation exchange capacity (Polemeo and Rhodes in Page, Miller and Keeney, 1986) of soils were determined (Table 1). The cation Exchange capacity for Aquasorb PR3005A was 178.3 meq/100 g. In loam soil, Na, K, Ca and Mg concentrations in saturation extract (meq/lit) were 8.9, 2.6, 20 and 15.2 respectively. These values for loam sand were 6.1, 1.5, 15.2 and 11.6 respectively. The available water capacities (AWC) averaged 7.1% for control and 8.6% for incorporation in loam sand and 5.5% for control and 5.6% for incorporation in loam and their differences with their controls were not significant (p>0.05 for incorporation versus control). According to Table 2 incorporation increased soil water retention relative to control at all potentials below zero bars, in both soils but this increase was only significant at 0.1 bar for loam soil (p<0.05) and at saturation (zero bar) for loam sand (p<0.05). Available water capacity for control in loam sand was 7.1% and in loam 5.5%. The available water capacities averaged 8.7% for HP1 and 12.2% for HP2 in loam sand and 8.6% for HP1 and 5.6% for HP2 in loam with no statistically significant increased water retention relative to their controls (Table 2). Water retention was only significant for HP1 at potentials greater than -0.1 bar for loam soil (p<0.05). The effect of HPs on increasing soil water holding capacity has been reported by Miller (1979), Johnson (1984b), Johnson and Veltkamp (1984), Johnson and Lea h(1990), Bowman and Evans (1991), Al-Harbi, Al-Omran, Shalabi and Choudhary (1999), Huttermann, Zommorodi and Reise (1999), Sivapalan (2001) and Peterson (2002). The quantity of this increment depends on the quantity of HP used (Huttermann et al., 1999). Average water retention for two replications of Aquasorb at potentials 0, -0.05, -0.1, -0.3, -1.0, -5.0 and -15.0 bars were 160.5, 131.3, 127.7, 112.2, 93.3, 76.2 and 20.6 gram of water per gram of Aquasorb respectively. The quantity of water stored between potentials zero and -0.1 bars (field capacity) was 32.8 gram per unit mass of Aquasorb and between -0.1 and -15 bars was 107.1. Most of water absorbed by Aquasorb is stored at potentials available for plant use (107.1 g vs. 32.8 g per unit mass of Aquasorb). Johnson and Veltkamp (1984) stated that with a correct reaction condition at least 95 percent of moisture held by the acryamide polymers at full expansion is stored at tensions within the range of pF 2 to 4.2 and is therefore available to plants. In this experiment however, only 66.7 percent of moisture held by Aquasorb at full expansion was stored within the available range (-0.1 to -15 bars) and 20.4 percent could be lost to drainage in a field soil (at potentials greater than -0.1 bars). The rest is retained at potentials greater than –15 bars. The capacity of Aquasorb for absorbing both soils extract was significantly less than distilled water (p<0.001). Its absorption capacity for distilled water (EC=0 ds/m) was 230 times of its own mass. Its absorption capacity was diminished to 66 and 56 times of its own mass with saturation extracts of the loam (EC=1.52 ds/m) and loam sand soils (EC=2.36 ds/m) respectively. Table 3 shows in both NaCl and CaCl2 solutions water absorption by Aquasorb was reduced as the electrical conductivity increased from 0 to 4.5 ds/m. Maximum hydration occurred in distilled water and the minimum in CaCl2 solution with an electrical conductivity of 4.5 ds/m. There was a considerable reduction in hydration within the range 0 to 1 ds/m electrical conductivities in CaCl2 solutions (p<0.001). Hydration reduced with a reducing acceleration at any further increment of electrical conductivity from 1 to 4.5 ds/m. By analogy with same increment in electrical conductivity in a soil solution due to drainage, water release is expected to be greater with the lower previous electrical conductivity. For electrical conductivities (1 and 1.5), (1.5 and 2), (2 and 2.5), (2.5, 3 and 3.5) and (3.5, 4 and 4.5) no significant difference was observed in water absorption. At electrical conductivities 0.25 and 0.5 ds/m there was no significant difference between NaCl and CaCl2 solutions for absorption by Aquasorb (p>0.05). With Ecs greater than 1 ds/m the absorption capacity for two electrolytes differed with same electrical conductivities (p<0.0001). Water absorption by Aquasorb in NaCl solution was greater than CaCl2 at same conductivity. Therefore valence and concentration of cations in a soil solution are both determining factors in water absorption by Aquasorb. Hydration of HP is reduced in the presence of cations specially divalent cations (Johnson, 1984a; Wang and Gregg, 1990; Bowman, Evans and Paul, 1990). Bowman and Evans (1991) expressed valence of the accompanying anion does not affect hydration. Sequential rinses of the hydrated gels with de-ionized water completely reversed the inhibition of water absorption due to mono-valent cations with very slight effect on divalent cations. Johnson (1984a) reported that in a saline water (EC=3.2 ds/m) absorption by HP was diminished to %75 of its maximum capacity in de-ionized water. In our Experiment however, absorption by Aquasorb in CaCl2 solution (EC=3 ds/m) was diminished to 18.2% of its maximum capacity in de-ionized water and in a NaCl solution (EC=3 ds/m) to 29.9%. In the presence of fertilizer salts physical properties of growth media was not affected by HP additions (Bowman et al., 1990). HPs have many –COO-K+ groups that may behave as salts increasing their affinity for water. Multivalent cations actively dislodge and replace water molecules at polarized sites upon and within polymers (Wang and Gregg, 1990). However in present study using Aquasorb at the recommended commercial dose faced failure in measurable enhancing available water capacity. Therefore a greater available water capacity in these soils requires possibly a greater quantity of Aquasorb. DISCUSSION
Available water capacity for both soils were increased by compost incorporation and both rates of Aquasorb but differences with their respective controls were not significant. A greater rate of Aquasorb or compost incorporation could possibly increase soil available water capacity. For each soil texture a different particle size distribution for compost could be effective in this respect. As the electrical conductivity in a soil solution is increased the hydration capacity of Aquasorb is diminished and a greater quantity of Aquasorb is required for same increment of available soil water capacity. Contrary to an unmixed expanded Aquasorb, as a soil dries the electrical conductivity of its solution is gradually increased. According to Hillel (1980, page 246) the first increments of the solution extracted from the soil often have a lower concentration of solutes than the remaining solution. The subsequent increased electrical conductivity due to drainage causes a considerable reduction in water absorption by Aquasorb (Table 3) and increases water release. The water that is released at potentials greater than field capacity is lost to drainage and the rest is available to plants. Since with same increment in electrical conductivity due to drainage, water release is greater with the lower previous electrical conductivity (at higher potentials) in a soil solution, a greater release by Aquasorb is expected at potentials greater than field capacity and within the upper range of available soil water. Also a greater release is expected in soils with low electrical conductivity. Consequently more than 20.4 percent water release at potentials greater than –0.1 bars (as mentioned in results) or upper range of soil available water is expected from Aquasorb when mixed with soil. In a field, in addition to increased salt concentration due to drainage it is also increased within upper soil layers after infiltration through evaporation. Conclusions
The hydration capacity of Aquasorb 3005A (and also other current hydrophilic polymers) is highly sensitive to electrical conductivity of soil solution. The commercial recommendations for using Aquasorb in soils are normally based on water retention at different pressure steps for a pure Aquasorb (not mixed with a soil). Proper recommendations must consider soils with different electrical conductivities in saturation extract. Even in a soil with a normal electrical conductivity a greater quantity of Aquasorb is needed for increasing soil available water content than commercial recommendations. These recommendations must also consider soil solution concentration variations and the respective soil water release at potentials greater than field capacity. In pots with small height, the drainage water is considerably less than field condition and therefore the release of water to drainage by Aquasorb (and compost incorporation) is minor relative to field. By this analogy even a greater quantity of Aquasorb is required in field relative to pots. Table 1. Physical and Chemical Properties of loam and loam sand soils Table 2. Mean loam soil moisture (%v/v) at different pressure steps (bar) and the plant available water between F.C. and P.W.P. Soil Texture Alternation 39.4b 37.1b 35.8b 33.7a 29.9a 29.2a 28.2a 48.5a 42.8a 41.2a 37.0a 30.1a 29.6a 28.5a 46.9ab 42.2ab 39.7ab 34.4a 31.1a 29.8a 28.7a 46.8ab 42.3ab 40.7a 37.3a 32.7a 31.7a 30.4a 38.1b 32.3a 29.6a 24.3a 18.3a 17.9a 17.2a 43.9b 37.6a 33.2a 26.9a 21.2a 19.8a 18.1a 44.0ab 37.7a 35.5a 30.9a 25.7a 20.5a 18.7a 12.2a 45.2a 37.0a 33.5a 26.7a 20.0a 18.8a 18.0a Mean separation within columns by Duncan multiple range test, 5% level; with two individual statistics for loam and loam sand soils Different letters (a and b) indicate significant difference at 5% level P= pressure (bar) Table 3. Absorption capacity by HP (w/w) as a function of electrical conductivity (ds/m) and cationic valance Mean separation within rows by Tukey multiple range test, 5% level Different letters (a,b and …) indicate significant difference at 5% level ACKNOWLEDGEMENT
The authors wish to thank Mr. Mohammad Zaman Alaodin and Mr. Mohammad Ajami for their technical assistance; and Dr. Saeed Hassani for statistical advice. This research was funded by Gorgan University of Agriculture (Iran). REFERENCES
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