effect of aquasorb and organic compost amendments on soil water retention and evaporation with...
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Effect of Aquasorb and Organic CompostAmendments on Soil Water Retention andEvaporation with Different EvaporationPotentials and Soil TexturesM. Taban a & S. A. R. Movahedi Naeini aa Gorgan University of Agricultural Sciences and Natural Resources ,Gorgan , IranPublished online: 05 Feb 2007.
To cite this article: M. Taban & S. A. R. Movahedi Naeini (2006) Effect of Aquasorb and Organic CompostAmendments on Soil Water Retention and Evaporation with Different Evaporation Potentials and Soil Textures,Communications in Soil Science and Plant Analysis, 37:13-14, 2031-2055, DOI: 10.1080/00103620600770383
To link to this article: http://dx.doi.org/10.1080/00103620600770383
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Effect of Aquasorb and Organic CompostAmendments on Soil Water Retention andEvaporation with Different Evaporation
Potentials and Soil Textures
M. Taban and S. A. R. Movahedi Naeini
Gorgan University of Agricultural Sciences and Natural Resources,
Gorgan, Iran
Abstract: Aquasorb PR3005A, a hydrophilic polymer (a salt copolymer polyacryl-
amide), and garden waste compost were added to a loamy sand and a loam soil in
pots to assess their impact upon soil physical properties at two different evaporation
potentials. Compost was mulched and incorporated, the Aquasorb was incorporated,
and their effect on temperature and amelioration of soil water content and evaporation
was investigated. Mulching with compost reduced evaporation and increased soil
temperature. Maize (Zea mays var. single cross 704) was sown in the same pots
later, and growth indicator factors (plant height, fresh and dry weight, root weight,
and leaf area) were compared. It was concluded that compost mulch application is
beneficial to soil water retention whereas compost incorporation did not show these
benefits. Compost mulch advances seedling emergence and enhances early growth
through hydrologic soil amelioration. High rates of Aquasorb were also beneficial in
advancing the emergence and early growth of maize seeded in loamy sand. The
hydration capacity of Aquasorb is reduced as the electrolyte concentration and
electrical conductivity are increased. Increased electrolyte concentration in soil
solution, through drying, may result in gel dehydration and water release at potentials
greater than field capacity, which may be lost to drainage. Furthermore, it is concluded
that pot experiments with amendments fail to simulate field conditions.
Keywords: Aquasorb, compost incorporation, compost mulch, evaporation, field
simulation with pots, hydrophilic polymer, water retention
Received 18 February 2004, Accepted 14 December 2005
Address correspondence to S. A. R. Movahedi Naeini, Soil Science Department,
Agricultural College, Gorgan University of Agricultural Sciences and Natural
Resources, Gorgan, Iran. E-mail: [email protected]
Communications in Soil Science and Plant Analysis, 37: 2031–2055, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0010-3624 print/1532-2416 online
DOI: 10.1080/00103620600770383
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INTRODUCTION
The average rainfall is less than 250mm per year in Iran. More than 90% of
the whole country is covered by arid and semiarid zones. In these climates,
annual rainfall is limited with an uneven distribution. The main user of
water resources in Iran is the agricultural sector, which pays little attention
to efficient application. 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 evaporation and
improve efficiency of its application.
Hydrophylic polymers are produced in Iran and other countries of the
world, but their water-holding capacity for subsequent use by a plant may
differ according to their chemical structure. 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). Hydrophylic polymers
are small particles with different sizes when dry and remain as individuals
when wet (Bowman 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 polyacrylamides
(–CH2CHCONH2–)n. HPs used in agriculture are usually formulations
commonly made of starch polyacrylamid graft copolymers (starch copoly-
mers: 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 threat to human life or environment (Seybold
1994). Under higher magnification, the detailed framework of the polymer
can be seen as a matrix of vacuoles connected by polygonal bridges. A
greater proportion of water, around 80 to 85%, is stored within vacuoles as
numerous minute reservoirs. The remaining 15 to 20% is still plant
available and is bound with a greater tenacity, persisting 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% (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 barley were grown under limited irrigation on a coarse
sand substrate, the interval between field capacity and the onset of wilting
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was increased by up to threefold in the presence of polymer. Water-use effi-
ciency and dry matter production also responded positively to the presence
of both starch copolymer and PAMs (Woodhouse and Johnson 1991).
The common hydrophylic polymers are sensitive to electrical conduc-
tivity, and their absorption capacity is strongly reduced even with a low
electrical conductivity. This might affect their application in soils with
variable electrical conductivity. Starch copolymers have a greater hydration
capacity relative to other types of hydrophilic polymers, but hydration
capacity is less affected by PAMs with the same salinity levels
(Johnson 1984b). Because of the low cationic exchange capacity of coarse-
textured sandy soils and hence low electrical conductivity, the addition of
HPs to these soils had the best results compared to other soils (Peterson
2002). A range of potentials at which the water is retained by HPs 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. Reducing evaporation from soil surfaces by using HPs has
been reported by Johnson (1984a) and Choudhary, Shalaby, and Al-Oman
(1995), whereas Tue, Armitage, and vines (1985) and Blodgett et al. (1993)
reported that HPs were not effective in reducing evaporation.
Compost incorporation also might reduce evaporation and increase soil
water storage in available range to plants (Opara-Nadi and Lal 1987). A com-
parison of water retention and evaporation by compost incorporation and
hydrophilic polymers could be informative. Compost mulch reduces evapor-
ation and increases water storage (Unger, Parker, and Jessie 1976; Shekour,
Brathwate, and McDavid 1987; Todd et al. 1991; Bussier and Cellier 1994;
Movahedi Naeini and Cook 2000). Compost mulch reduces evaporation
during the energy-limited stages of drying and extends its duration (stage 1
and transitional stage).
Soil amendment studies have developed the notion of soil evaporation
stages to describe alterations to the hydrological and thermal balance in exper-
imental soils. Three main stages of evaporation are usually recognized when a
wet soil dries (Hillel 1980). Stage 1 is purely energy limited when water
supply to the soil surface is not limited. The effect of evaporation demand
of the atmosphere driving a given potential evaporation would be expected
to end at the onset of stage 2, when soil limiting factors manifest. Stage 2
shows a rapid decline in evaporation over time as the soil dries until stage
3, when the residual rate is low and constant. During stage 3, water
movement in soil is mainly through vapor diffusion, and with its slow rate
it may last for months. Stage 1 drying ends at the point at which the curve
of accumulated evaporation deviates from that of free water. Container
filled soils such as pots, however, experience a long transitional stage when
both energy supplied through sides of the pots and the transmissional proper-
ties of the soil determine evaporation rate (Movahedi Naeini and Cook 2000).
In field (but not in container filled soils), due to diurnal alteration in
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evaporative demand of atmosphere and nocturnal vapor transfer to the soil
surface and subsequent distillation, a diurnal transition from stage 1 to stage
2 and vice versa may be recognized. This stage is also called the transitional
stage and is very short as compared to that in pots. In pots, the contemporary
method for measuring duration and magnitude of the efficiency of amend-
ments on controlling soil evaporation is using cumulative evaporation
reduction curves (CER). CERs are the difference between cumulative water
loss from the bare and treated soil. Maximum evaporation reduction (MER)
is the peak CER with a descending trend afterward.
There are many reports about the enhancing effect of organic materials
and HPs on the yield of plants, which is strongly dependent on soil, plant,
climatic conditions, and the rate of application. Wofford (1989) reported
increased yield, fruit size, flower number, and early maturation in different
plants with HPs in the United States. In a field experiment, Silberbush,
Adar, and DeMalach (1993a, 1993b) reported that in a sandy soil water
storage and the yields of cabbage and maize were increased using HPs. 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 incorporated (Opara-Nadi and Lal 1987).
Cassava and yam showed different responses with pine needle mulch and
incorporation. Density of feeder roots for cassava were maximum with
mulch, lower with compost incorporation, and minimum with control.
However, mulch did not affect feeder root density with yam. Tuber yield
was not affected by mulch for cassava but it was increased for yam.
Mulching reduced bulk density under cassava but did not affect it for yam.
Compost incorporation reduced bulk density for cassava and yam both.
Acharia and Sharma (1994) observed increased maize and wheat growth
with pine needle mulch. Badaruddin, Reynolds, and Ageeb (1999) showed
that straw mulch enhanced tillering, advanced harvest date, and increased
harvest index and yield of wheat.
This research has the following objectives: 1) to relate evaporative
behavior to soil texture, soil temperature, and water-holding properties of
soils, the HPs, and composted material; 2) to determine the extent at which
an albedo change with a white color on a pot surface is effective on simulating
energy transaction in field soils; 3) to investigate plant response to the quantity
of Aquasorb according to the commercial recommendations; and 4) to inves-
tigate the shortcomings of common HPs for agricultural use such as Aquasorb
as a guide for further improvement of their chemical structure.
MATERIALS AND METHODS
Two pot experiments were conducted in January–June 2002 to establish the
effect of compost cover, incorporation, and Aquasorb HP on evaporation
from loam and loamy sand soils in different stages of drying and in the
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absence of crops under different temperature regimes. In a third experiment
(August–October 2002) maize (var. single cross 704) was sown in the same
pots used in experiments 1 and 2, for a comparison between these treatments
with plants present.
Experiments 1 and 3 were completely randomized in an open glasshouse,
whereas experiment 2 was completely randomized in a large temperature-
controlled chamber. In experiments 1 and 2, the treatments were open water
(W), bare soil (B), compost incorporation (I) (equivalent to 50 t/ha),compost mulch (M) (equivalent to 50 t/ha on the surface), and Aquasorb
HP with two rates of 0.0007 and 0.0014 dry mass to unit soil mass (Hp1
and Hp2, respectively). In the third experiment with maize, all these treat-
ments were used except open water. For all three experiments, all the treat-
ments were applied once to a coarse textured soil and once to a finer
textured soil in pots with three replications. In compost dry matter, nitrogen
(N), phosphorus (P), potassium (K), and carbon (C) were 72, 0.63, 0.13,
4.2, and 14%, respectively, giving a C:N ratio of 22.2.
The pots were 20.5 cm wide and 18 cm deep. The coarse-textured topsoil
was a flovent (Entisols) loamy sand soil (from Ziarat river bank, south of
Gorgan, Iran), and the finer textured topsoil was a calcixerolls (Mollisoils)
loam soil (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 the loamy sand soil and
1.46 for the loam. The water treatment acted as a control with no soil
surface constraints on evaporation. The physical properties such as particle-
size distribution (Klute 1986), dry bulk density, chemical properties (Ali
Ehyaei 1997), and cation exchange capacity (Page, Miller, and Keeney
1986) of the soils were determined (Table 1). The cation exchange capacity
for Aquasorb PR3005A was 178.3 meq/100 g. In the loam soil, sodium
(Na), K, calcium (Ca), and magnesium (Mg) concentrations in a saturation
Table 1. Physical and chemical properties of loam and
loamy sand soils
Soil subgroup Calcixerolls Flovent
Texture Loam Loamy sand
Sand (%) 31 78
Silt (%) 48 18.5
Clay (%) 21 3.5
Db (g/cm3) 1.4 1.8
pH 7.6 7.4
CaCO3 (%) 12 25.75
OM (%) 3.53 3.8
CEC (meq/100 g) 22.28 8.15
CaSo4 (meq/100 g) 0 0
ECe (ds/m) 2.8 1.2
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extract (meq/lit) were 8.9, 2.6, 20, and 15.2 respectively. Sodium (Na), K, Ca,
and Mg concentrations for the loamy sand were 6.1, 1.5, 15.2, and 11.6,
respectively.
To recover drainage water and reduce the effect of heat conduction
through the sides of the pots, each pot was painted the color white, placed
within a second vessel, and mounted on rubber stoppers. Soils were then
saturated, covered with aluminum foil, and left to drain. When drainage was
observed to be minimal, the cover was removed and the pots were exposed
to allow evaporation and weighed regularly. Drainage water was included
in water balance calculations. The soil pots were handshifted daily around
the bench.
Data was obtained as follows. 1) Evaporation rates were derived from pot
weights collected from 8:00 to 17:00 h and from 17:00 to 8:00 h
(overnight) for about 2763 h at low evaporation potential (experiment 1)
and 1083 h at high evaporation potential (experiment 2). For the first 2 days
in both experiments, pots were weighed every 2 h between the 8:00 and
17:00 h. 2) Soil temperature was taken at 2- and 10-cm depths using a
digital portable thermometer for experiments 1 and 2. 3) Maximum and
minimum daily air temperatures were taken in laboratory. 4) Soil volume
moisture percentage at potentials 0, 0.05, 0.1, 0.3, 1, 5, and 15 bars were
measured using Tempe pressure cells and pressure plates. 5) Water-
absorbing capacity of HP at different potentials and electrical conductivities
were calculated. 6) Growth indicator factors for maize including plant
height, plant fresh and dry weight, plant moisture content, and root dry
weight were figured. Data were analyzed by the analysis of variance and
corelations techniques (SAS 1996).
Experiment 1 experienced ambient air temperatures averaging 22.58C(ranging from 20 to 248C) between the 17:00 and 8:00 h; daytime mean temp-
erature was 23.5 (ranging from 21 to 258C). For experiment 2, the nighttime
air temperature averaged 32.28C (ranging from 30 to 348C), daytime mean
temperature was 34.5 (ranging from 32 to 388C). The purpose of increasing
the ambient temperature was to identify its effect on evaporation from the
amendments.
Experiment 3 was conducted to assess the effect of two rates of Aquasorb
polymer and compost (mulch and incorporation) on growth of maize under
moderate drought stress. The soils were wetted by standing the pots in
water (with the same height as soils in pots) for 48 h to ensure complete
polymer expansion and soil saturation. The pots were then allowed to drain
freely for 48 h. After drainage, weight of the pots were determined. The
pots were irrigated every week with equal amounts of water, and only one
pot with the lowest weight reduction was selected as the irrigation scale. To
minimize the risk of any temporary limiting aeration due to usual limited
drainage of pots, an arbitrary value of 100 cm3 of irrigation water was sub-
tracted from the weight reduction, and a quantity equivalent to the product
of this calculation was applied to each pot. A few seeds of maize were
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sown in each pot, and after emergence, they were thinned to three seedlings
per pot. No fertilizer was used in the third experiment. The first plant height
was obtained 17 days after sowing (H1), and three further measurements
(H2 to H4) were obtained at 7-day intervals. The two last heights (H5 and
H6) were obtained at 5-day intervals. Two months after sowing, the plants
were harvested, and plant fresh and dry weight, root dry weight, leaf area,
and plant water percentage at harvest were determined.
For estimation of Aquasorb hydration capacity in soils of this experiment,
the hydration in distilled water and saturated paste extracts of loam and loamy
sand soils were compared. A constant weight of HP (0.20 g) were added to a
50-cm3 saturated paste extract of each soil and a distilled water in a glass
container with eight replications. Two hours later, the contents of each
container were filtered through a Whatman filter paper no. 42. Water-
absorption capacity was considered to be 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. In
a factorial design, CaCl2 and NaCl with 11 levels of 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 three replications
were compared statistically.
Soil samples were obtained from pots when all three experiments were
finalized, and soil water content at tensions 0, 0.05, 0.1, 0.3, and 1.0 bar
was 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
all 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 was also determined when unmixed with soil.
RESULTS
Diurnal and nocturnal time-averaged temperatures at 2- and 10-cm depths for
loam and loamy sand soils are presented in Tables 2 and 3 for low and high
evaporation potentials. In loamy sand, mulch was found to increase soil temp-
erature (p , 0.05 for M. versus B & Hp1 & Hp2 & I, both depths, both exper-
iments) with one exception; the mentioned differences were not significant at
2- and 10-cm depths for the treatments under the diurnal high ambient temp-
erature. In loam, mulch significantly increased diurnal and nocturnal soil
temperatures at 2-cm depth relative to bare soil for the low ambient tempera-
ture (p , 0.05). Also, at 2-cm depth and high ambient temperature, the
addition of mulch increased the soil temperature with a significant difference
for nocturnal temperatures only (p , 0.05). Temperature differences for
the loam soil, with mulch at the 10-cm depth, were significant (p , 0.05)
with high nocturnal ambient temperature, whereas with high diurnal
ambient temperatures, temperature rises of the soil were not significant. For
low diurnal and nocturnal ambient temperatures, soil temperature values
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were almost the same for mulch and bare soil at the 10-cm depth. The amount
of HP applied and the treatment where compost was incorporated did not show
any significant effect on temperature. The mean temperatures for the two soils
at 10-cm were significantly greater than the temperatures at 2-cm (Table 4).
This was true at both ambient temperatures. There was a positive and signifi-
cant correlation between soil temperature and evaporation rate at low evapor-
ation potential for both depths (p , 0.0001). For high evaporation potential,
nocturnal correlations were positive and diurnal correlations were negative
but not always significant (Table 5).
Treatment-averaged stage 1 drying (defined as time zero to deviation
from the evaporation curve for water) was 76 h for loam soil in experiment
1 and 81 h for loamy sand. These values for experiment 2 were 47 h for
loam and 25 h for loamy sand. The duration for the first stage of evaporation
with high ambient temperature was shorter than low ambient temperature.
From the beginning of experiment 1, 179 h onward for loam (Figure 1)
and 359 h onward for loamy sand (Figure 2), the evaporation rate for bare
soil, incorporation of compost, and both rates of Aquasorb were greater
than 50 t/ha mulch (p , 0.05 for M versus B & I & Hp1 & Hp2). From
204 to 720 h for loam soil (Figure 3) and from 106 to 778 h for loamy sand
(Figure 4), experiment 2 showed a similar pattern to experiment 1 with a
Table 2. Soil temperature at 2- and 10-cm depths with low
and high ambient temperature for loam soil
Treatment
Day Night
10 cm 2 cm 10 cm 2 cm
Low ambient temperature
Control 22.8ab 22.3b 22.5a 21.8c
HP 0.07% 22.7ab 22.3b 22.5a 22.0b
HP 0.14% 22.7b 22.3b 22.5a 21.9b
Incorporation 22.9a 22.4a 22.5a 21.9b
Mulch 22.8ab 22.4a 22.4a 22.3a
Mean 22.8 22.3 22.5 22.0
High ambient temperature
Control 32.3a 30.5a 32.1b 30.7b
HP 0.07% 32.3a 30.5a 32.1b 30.7b
HP 0.14% 32.3a 30.9a 32.1b 30.7b
Incorporation 32.1a 30.7a 32.1b 30.7b
Mulch 32.4a 31.5a 32.6a 31.1a
Mean 32.3 30.8 32.2 30.8
Notes: Mean separation within columns by Duncan’s
multiple range test, p � 0.05, with two individual statistics
for low and high ambient temperatures. Different letters
(a and b) indicate significant difference at p � 0.05.
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statistically significant lower cumulative evaporation for mulch (p , 0.05).
Cumulative evaporation for mulch was less than the control, 95 h onward
for loam and 83 h onward for loamy sand in experiment 1 (p , 0.05 for M
versus B). According to statistical analysis (p , 0.05) from 204 to 720 h for
loam and from 106 to 874 h for loamy sand, experiment 2 showed a similar
Table 3. Soil temperature at 2- and 10-cm depths with low and
high ambient temperature for loamy sand soil
Treatment
Day Night
10 cm 2 cm 10 cm 2 cm
Low ambient temperature
Control 22.9b 22.4b 22.4b 21.8b
HP 0.07% 22.8b 22.4b 22.4b 21.9b
HP 0.14% 22.8b 22.3b 22.4b 21.9b
Incorporation 22.8b 22.3b 22.4b 21.9b
Mulch 23.1a 22.6a 22.6a 22.1a
Mean 22.9 22.4 22.4 21.9
High ambient temperature
Control 32.2ab 30.7a 32.0b 30.7b
HP 0.07% 32.2ab 30.7a 32.1b 30.8b
HP 0.14% 32.2ab 30.7a 32.1b 30.7b
Incorporation 32.0b 30.7a 32.1b 30.8b
Mulch 32.9a 31.6a 32.6a 31.2a
Mean 32.3 30.9 32.2 30.8
Notes: Mean separation within columns by Duncan’s multiple
range test, p � 0.05, with two individual statistics for low and
high ambient temperatures. Different letters (a and b) indicate
significant difference at p � 0.05.
Table 4. Two experiments’ mean temperature for 2- and 10-cm depths of soil
Evaporation
potential
Alteration
sources
depth Control
HP
0.07%
HP
0.14%
Compost
incorporation
Compost
mulch
Low 2 21.0b 21.8b 21.8b 21.8b 22.0b
10 22.3a 22.3a 22.3a 22.3a 22.4a
High 2 30.5b 30.6b 30.6b 30.6b 30.9b
10 31.7a 31.8a 31.8a 31.8a 32.2a
Notes: Mean separation within rows by Duncan’s multiple range test, p � 0.05, with
two individual statistics for low and high ambient temperatures. Different letters (a and
b) indicate significant difference at p � 0.05.
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pattern to experiment 1 for evaporation (Figures 3 and 4). However, after
1083 h the cumulated amount for the mean treatments was the same as the
control for loam soil and even less for loamy sand. Compost mulch effectively
reduced evaporation rates in the beginning of experiment 2 but its rate relative
to other treatments was gradually increased, giving a cumulated evaporation
identical to other treatments after about 874 h in loam soil and 1011 h in
loamy sand (Figures 3 and 4).
Table 5. Correlations between evaporation rate (mm/h) and diurnal and nocturnal
soil temperatures (centigrade degrees) at 2- and 10-cm depths with low and high
ambient temperatures
Soil Parameter
Day Night
10 cm 2 cm 10 cm 2 cm
Loam soil Low ambient temperature
Correlation coefficient 0.4297 0.4724 0.5930 0.5439
P value 0.0001 0.0001 0.0001 0.0001
High ambient temperature
Correlation coefficient 20.1848 20.0361 0.1237 0.1932
P value 0.0300 0.6745 0.1485 0.0232
Loamy sand Low ambient temperature
Correlation coefficient 0.5189 0.5437 0.6168 0.5916
P value 0.0001 0.0001 0.0001 0.0001
High ambient temperature
Correlation coefficient 20.2695 20.1670 0.0730 0.1495
P value 0.0014 0.0502 0.3951 0.0801
Figure 1. Cumulative evaporation over time for treatments at low evaporation
potential in loam soils.
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For loam soil, plotting the CER over time (Figures 5 and 6) for treatments
(derived by cumulating the difference between treatment and bare soil evap-
orations over time) shows a dramatic difference after 1565 and 407 h for
mulch in low and high evaporation potentials respectively. The MERs were
16.8 and 9.7mm at low and high ambient temperatures respectively. Other
treatments showed no significant difference over time. For loamy sand, the
MER (of 11.4mm) for mulch occurred after 1183 h in low ambient tempera-
tures (Figure 7) and the MER (of 11.8mm) occurred after 346 h in high
ambient temperatures (Figure 8). For both soils, increasing ambient tempera-
tures in the second experiment reduced the time it took to reach the MER
under all treatments. With increasing the magnitude of the ambient tempera-
ture, the magnitude of MER for mulch was reduced in loam soil (16.8mm
Figure 2. Cumulative evaporation over time for treatments at low evaporation
potential in loamy sand soils.
Figure 3. Cumulative evaporation over time for treatments at high evaporation
potential in loam soils.
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versus 9.7mm) but in loamy sand it was almost identical for both ambient
temperatures (11.4 versus 11.8). CERs were not significantly different
among control, compost incorporation, and both rates of Aquasorb over
time (p . 0.05 for both soils; both ambient temperatures). Movahedi Naeini
and Cook (2000) reported increasing ambient temperatures reduced the time
for cumulative evaporation reduction under their treatments to reach MER
and increased the magnitude of MER.
Cumulated evaporation for compost incorporation and both rates of
Aquasorb were not significantly different from the control for both soils and
both experiments. With high ambient temperature, the cumulated evaporation
Figure 4. Cumulative evaporation over time for treatments at high evaporation
potential in loamy sand soils.
Figure 5. Cumulative evaporation reduction (CER) for treatments at low evaporation
potential in loam soils.
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for high rate of Aquasorb for loamy sand from 1011h onward (1083 h) was
greater than bare soil (p, 0.05) and from 821 h onward (p , 0.05) was
greater than incorporation and mulch. According to Figures 5–8, Aquasorb
treatments had caused a high evaporation relative to bare soil, giving negative
CER values. Negative CER values for compost-incorporated treatments is
observed with high evaporation potential and loam soils (Figure 6). The
maximum negative value was 4mm for the HP1 treatment for loam and high
ambient temperature. Blodgett et al. (1993) also stated there is no significant
difference between Aquasorb and control treatment in cumulative evaporation.
Incorporation of compost did not change cumulative evaporation in
experiments 1 and 2 significantly. Opara-Nadi and Lal (1987), Movahedi
Figure 6. Cumulative evaporation reduction (CER) for treatments at high evapor-
ation potential in loam soils.
Figure 7. Cumulative evaporation reduction (CER) for treatments at low evaporation
potential in loamy sand soils.
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Naeini and Cook (2000), and Shanging and Unger (2001) also reported that
mulching compost was more effective for evaporation control than incorpor-
ating the mulch. The available water capacities (AWC) averaged 7.1% for the
control and 8.6% for incorporation in loamy 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). Incorporation
increased soil water retention relative to the control at all potentials below 0
bars in both soils. However, this increase was only significant at 0.1 bar for
loam soil (p , 0.05) and at saturation (zero bar) for loamy sand (p , 0.05)
(Table 6). Increased water retention by incorporation with loamy sand at
potentials about 0 bars is expected to increase saturated and unsaturated
hydraulic conductivity at these potentials. Increased water retention by this
treatment at 0.1 bars in loam is expected to increase unsaturated hydraulic
conductivity in these potentials (Table 6).
AWC for the control in loamy sand was 7.1% and in loam 5.5%. The
AWCs averaged 8.7% for HP1 and 12.2% for HP2 in loamy sand and 8.6%
for HP1 and 5.6% for HP2 in loam with no significant increase in water
retention relative to their controls (Table 6). Water retention was only signifi-
cant for HP1 at potentials greater than 20.1 bars for loam soil (p , 0.05)
(Table 6). The effect of HPs on increasing soil water-holding capacity has
been reported by Miller (1979), Johnson(1984b), Johnson and Veltkamp
(1984), Johnson and Leah (1990), Bowman and Evans (1991), Al-Harbi
et al. (1999), Huttermann, Zommorodi, and Reise (1999), Sivapalan (2001),
and Peterson (2002). The quantity of this increment depends on the quantity
of HP used (Huttermann, Zommorodi,and Reise 1999).
Average water retention for two replications of Aquasorb at potentials
0, 20.05, 20.1, 20.3, 21.0, 25.0, and 215.0 bars were 160.5, 131.3,
127.7, 112.2, 93.3, 76.2, and 20.6 g of water per gram of Aquasorb
Figure 8. Cumulative evaporation reduction (CER) for treatments at high evapor-
ation potential in loamy sand soils.
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respectively. The quantity of water stored between potentials zero and 20.1
bars (field capacity) was 32.8 g per unit mass of Aquasorb and between
20.1 and 215 bars was 107.1 g. Most of the water absorbed by Aquasorb
is stored at potentials available for plant use (107.1 g versus 32.8 g per unit
mass of Aquasorb). Johnson and Veltkamp (1984) stated that with a correct
reaction condition, at least 95% of moisture held by the acrylamide
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, 66.7% of
moisture held by Aquasorb at full expansion was stored within the available
range (20.1 to 215 bars) and 20.4% could be lost to drainage in a field
soil (at potentials greater than 20.1 bars). The rest is retained at potentials
lower than –15 bars.
The capacity of Aquasorb for absorbing both soils’ extract was significantly
less than for distilled water (p,0.001). Its absorption capacity for distilled water
(EC ¼ 0 ds/m) was 230 times its own mass. Its absorption capacity was dimin-
ished to 66 and 56 times its own mass with saturation extracts of the loam
(EC ¼ 1.52 ds/m) and loamy sand soils (EC ¼ 2.36 ds/m).
Table 7 shows that 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. A considerable
reduction in hydration occured within the EC range of 0 to 1 ds/m in CaCl2solutions (p , 0.001). Hydration reduction was greater with any further
increment of electrical conductivity (solute concentration) within 0 to
Table 6. Mean loam and loamy sand soils moisture (% v/v) at different pressure steps(bar) and the plant-available water between F.C. and P.W.P.
Treatment
Pressure steps
P0 P0.05 P0.1 P0.3 P1 P5 P15
Available
water
Loam soil
Control 39.4b 37.1b 35.8b 33.7a 29.9a 29.2a 28.2a 5.5a
HP 0.07% 48.5a 42.8a 41.2a 37.0a 30.1a 29.6a 28.5a 8.6a
HP 0.14% 46.9ab 42.2ab 39.7ab 34.4a 31.1a 29.8a 28.7a 5.6a
Incorporation 46.8ab 42.3ab 40.7a 37.3a 32.7a 31.7a 30.4a 6.9a
Loamy sand soil
Control 38.1b 32.3a 29.6a 24.3a 18.3a 17.9a 17.2a 7.1a
HP 0.07% 43.9b 37.6a 33.2a 26.9a 21.2a 19.8a 18.1a 8.7a
HP 0.14% 44.0ab 37.7a 35.5a 30.9a 25.7a 20.5a 18.7a 12.2a
Incorporation 45.2a 37.0a 33.5a 26.7a 20.0a 18.8a 18.0a 8.6a
Notes: Mean separation within columns by Duncan’s multiple range test, p � 0.05,
with two individual statistics for loam and loamy sand soils. Different letters (a and b)
indicate significant difference at P � 0.05. p ¼ pressure (bar).
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4.5 ds/m range. By analogy, with the same increment in solute concentration
in a soil solution due to water evaporation, water release is expected to be
greater with the lower previous electrical conductivity. Only 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. With
ECs greater than 1 ds/m, the absorption capacity for two electrolytes
differed when they had the same electrical conductivities (p , 0.0001).
Water absorption by Aquasorb in the NaCl solution was greater than the
CaCl2 solution 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 especially
divalent cations (Johnson 1984a; Wang and Gregg 1990; Bowman, Erans, and
Paul 1990). Bowman and Evans (1991) expressed that the valence of the
accompanying anion does not affect hydration. Sequential rinses of the
hydrated gels with deionized water completely reversed the inhibition of
water absorption due to monovalent cations with very slight effects on
divalent cations. Johnson(1984a) reported that in a saline water
(EC ¼ 3.2 ds/m), absorption by HP was diminished to 75% of its maximum
capacity in deionized water. However, in the present experiment, absorption
by Aquasorb in CaCl2 solution (EC ¼ 3 ds/m) was diminished to 18.2% of
its maximum capacity in deionized water and to 29.9% in a NaCl solution
(EC ¼ 3 ds/m). In the presence of fertilizer salts, physical properties of
growth media were not affected by HP additions (Bowman, Erans, and Paul
1990). HPs have many –COO2Kþ groups that may behave as salts, increasing
Table 7. Absorption capacity by HP
(w/w) as a function of electrical con-
ductivity (ds/m) and cationic valance
NaCl CaCl2
4.5 61 hmor 24 pq
4 63 ghmr 31 pq
3.5 66 ghm 35 opq
3 71 ghm 43 op
2.5 80 gf 50 nor
2 82 f 6 hmn
1.5 99 e 74 fhm
1 120 d 91 efl
0.5 142 c 130 cdk
0.25 171 b 157 bcj
0 237 a 236 a
Notes: Mean separation for all means
within table by Tukey multiple range
test, p � 0.05. Different letters indicate
significant difference at p � 0.05.
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their affinity for water. Multivalent cations actively dislodge and replace water
molecules at polarized sites upon and within polymers (Wang and Gregg
1990). In this study, Aquasorb was used at the recommended commercial
dose with no significant effect on available water capacity. Therefore, a
greater available water capacity in these soils requires a greater quantity of
Aquasorb.
For all treatments in both soils, the plant mean fresh and dry weights and
leaf area were greater than controls but with no significant difference
(Table 8). Plant water percentage at harvest was not significantly changed
by any treatment. Root mean dry weights for the incorporation treatment
(1.4 g/pot) was significantly greater than the control (0.4 g/pot) in loamy
sand (p , 0.05), but this difference was not significant in loam (2.3 versus
2.8 g/pot, respectively). Plant height under the mulch treatment in loam soil
was significantly greater than the other treatments only for 31 days after
sowing (p , 0.05). In loamy sand, plant height under the mulch treatment
was significantly greater than the control throughout the experiment
(p , 0.05). Mulch advanced emergence for at least 4 days with both soils.
Because of an early harvest, maize ears only emerged at harvest with this
treatment in loamy sand. The high rate of Aquasorb significantly increased
plant height in loamy sand relative to the control for 24 days after sowing
(p , 0.05). Huttermann, Zommorodi, and Reise (1999) reported increased
root and plant growth with a 0.4% HP (w/w) application with Aleppo Pine.
Al-Harbi et al. (1999) reported increased cucumber growth using 0.3% HP
application in a loamy sand soil. Austin and Bondari (1992) expressed
additions of HP had no effect on blueberry because of high salts in soil.
DISCUSSION
A greater soil temperature at the 10-cm depth relative to the 2-cm depth shows
that contrary to field condition, thermal diffusion through the soil surface is
not the only source of energy for evaporation. In fields with one interface
for energy exchange, diurnal soil temperature at the surface is greater than
at the 10-cm depth (Movahedi Naeini 1998). In pots, thermal diffusion
takes place from the soil surface and sides. Because of low water filled
porosity, low tortuosity, and a short distance to soil surface for vapor
diffusion, latent heat transfer from upper soil layers is possibly greater than
depth. Greater latent heat transfer at or near the soil surface makes it cooler
than at the 10-cm depth. Increased ambient temperature makes this difference
even greater (compare mean treatment nocturnal or diurnal temperatures for
each soil with two different ambient temperatures in Tables 2 and 3).
With high evaporation potential, superficial layers of a soil (within a pot)
air dry quickly, and thermal diffusion is concentrated within a shallow surface
layer (raising surface layer temperature), impeding its unrestrained pen-
etration through deeper layers (2- and 10-cm depths), reducing the ratio of
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Table 8. Mean treatment average plant height per pot at 6 different stages (cm), total plant dry and fresh weigh per pot at harvest (g), total leaf area
per pot at harvest (cm2), total root dry weight per pot at harvest (g), plant moisture percentage, soil moisture percentage at harvest, and soil final dry
bulk density (Db) (after running three experiments) for loam and loamy sand soils
Treatment
Alteration sources
H1 H2 H3 H4 H5 H6
Plant
dry
weight
Plant
fresh
weight
Leaf
area
Root
dry
weight
Plant
moisture
(w/w)
Soil
moisture
(%v/v) Db
Loam soil
Control 39.3b 50.5b 79.9b 90.5a 95.2a 101.0a 6.0a 58.8a 1440a 2.8a 877a 20.2ab 1.26a
HP 0.07% 44.6b 55.6b 83.0b 94.6a 100.5a 108.4a 6.7a 66.0a 1579a 2.5a 878a 19.8ab 1.24a
HP 0.14% 43.4b 56. 4b 80.1b 92.3a 99.1a 105.2a 6.6a 61.9a 1565a 2.7a 841a 18.5b 1.23a
Incorporation 45.8b 55.7b 80.3b 92.4a 99.37a 106.9a 6.3a 59.0a 1446a 2.3a 841a 24.8ab 1.17a
Mulch 56.0a 75.0a 96.1a 103.4a 109.3a 111.7a 7.1a 66.4a 1685a 3.9a 867a 31.6a —
Loamy sand soil
Control 37.5b 44.0b 59.8b 68.2b 76.3b 84.5b 2.3a 25.3a 556a 0.4bc 1015a 28.2ab 1.33a
HP 0.07% 42.9ab 50.8ab 62.2b 69.6b 76.3b 84.6b 2.5a 26.4a 702a 0.4c 935a 25.8b 1.29ab
HP 0.14% 43.7a 52.4a 62.7b 72.1b 77.4b 86.5b 2.7a 28.6a 564a 0.2c 947a 29.2ab 1.26ab
Incorporation 42.5ab 50.8ab 66.8ab 79.0ab 84.7ab 92.7ab 3.1a 33.9a 809a 1.4a 1004a 25.5b 1.25b
Mulch 46.0a 52.2a 71.7a 82.2a 92.7a 102.8a 3.8a 40.3a 930a 1.2ab 950a 36.4a —
Note: Mean separation within columns by Duncan’s multiple range test, p � 0.05, with two individual statistics for loam and loamy sand soils.
Different letters indicate significant difference at p � 0.05. H ¼ plant hight (cm); subscripts indicate different stages in plant growth.
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energy entrance through soil surface relative to the sides. Therefore, a greater
ratio of total energy is lost through sensible heat from the surface relative to
latent heat (greater Bowen ratio), and this reduces the correlation between
latent heat transfer and soil temperature fluctuation when evaporation
potential is increased. Therefore, the correlation between the temperatures
at 2- and 10-cm and the evaporation rate might be minimized or even
reversed when the evaporation potential is increased. For latent heat transfer
through a deeper air-dry surface layer (by a high evaporation potential), a
greater ratio of energy is conducted through the sides of the pots relative to
the soil surface. However, with low evaporation potential, the surface soil
layers air dry later. A more moist surface soil layer causes a deeper thermal
diffusion through soil surface, a greater latent heat relative to sensible heat
transfer, and a more significant and positive correlation between soil tempera-
ture and evaporation rate. Movahedi Naeini and Cook (2000) in a similar
experiment using black pots found an inverse but insignificant correlation
between soil temperature and evaporation rate for both depths (2- and
10-cm) even with a low evaporation potential. In this experiment, painting
pots a white color (increasing albedo) reduced thermal adsorption through
the sides of the pots. Low energy enterance through sides of the pots
possibly increases the ratio of energy adsorbed through the soil surface for
latent heat transfer, resulting in a low Bowen ratio and, therefore, a greater
correlation between evaporation rate and soil temperature. White pots with
increasing ratio of energy exchange through soil surface provide a better
simulation for evaporation conditions in the field.
Mulch, by reducing latent heat transfer, increased soil temperatures at
2- and 10-cm depths. Mulch reduced cumulative evaporation during the transi-
tional stage (energy-limited stage) with both soil textures and both ambient
temperatures. Mulch, by reducing turbulent transfer of water vapor, reduces
the evaporation rate (Oke 1978). At the second stage of evaporation, mulch
effect is lost when MER is reached. The maximum influence of compost is
achieved at MER. After MER, the evaporation would be expected to
continue depending on water transmission properties of soil. Todd et al.
(1991) reported that straw mulch reduced evaporation from soil. This result
is in agreement with Opara-Nadi and Lal (1987), Acharia and Sharma
(1994), Bussier and Cellier (1994), Tolk, Howell, and Evett (1999),
Movahedi Naeini and Cook (2000), and Shangning and Unger (2001). The
occurrence of dry surface layers with evaporation is called self-mulching.
Similar to an organic mulch, self-mulching presents a barrier to water vapor
loss. Therefore, as the self-mulching grows deeper through evaporation, the
relative contribution of organic mulch curbing evaporation is decreased.
Increased ambient temperature advances the development of self-mulching
and hence MER, shortening the effect of organic mulch on evaporation
control. With a deeper self-mulching, the evaporation rate for the control
equals the evaporation rate for the mulch treatment at MER and lags mulch
treatment with lower water content and transmissivity after MER. The
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cumulative evaporation from soils with the mulch treatment were equivalent
to bare soil when CER equaled zero and exceeds bare soil when CER gained
negative values (Figure 6). Compost incorporation and Aquasorb did not
influence evaporation rate and latent heat transfer and hence soil temperature
in this experiment.
With a greater hydraulic conductivity, contrary to a coarse-textured soil
(loamy sand), the zone of evaporation is close to the soil surface for a
longer time in the fine-textured soil (loam). Zone of evaporation in pots
with greater energy enterace per unit soil volume was considered a
boundary between soil surface and the shifting seat of evaporation with
maximum rate. Therefore, with a low ambient temperature, most of the evap-
oration and latent heat transfer took place probably from a layer shallower
than the 10-cm depth with a loam soil (above the 10-cm depth where ther-
mometer sensor was placed for resding temperature at this depth). Conse-
quently, mulch, by reducing latent heat transfer, increased diurnal and
nocturnal soil temperature at the 2-cm depth as was sensed by a thermometer
located at this depth (Table 2). Increased ambient temperature lowered the
zone of evaporation (below the 10-cm depth) and hence mulch increased
soil temperature at 2- and 10-cm depths, whereas in loamy sand, at both
ambient temperatures, mulch increased diurnal and nocturnal soil tempera-
tures at 2- and 10-cm depths relative to other treatments. In a coarse-
textured soil with less water content at different potentials (Table 6), overall
for the period of the experiment, the zone of evaporation is possibly a soil
layer deeper than the 10-cm depth. Therefore, mulch, by reducing evaporation
rate and latent heat transfer within this layer, increased soil temperatures at 2-
and 10-cm depths.
The initial bulk density for loamy sand was 1.88 and for loam was
1.46 g cm23. Final bulk densities at the end of experiment 3 were 1.33 and
1.26 g cm23 for loamy sand and loam respectively. Even with slightly
greater bulk densities, coarse-textured soils exhibit a considerable greater
thermal conductivity relative to fine-textured soils at comparable soil water
contents (Hillel, 1980). Because there is no redistribution after saturation of
a soil within pots, soil water loss due to drainage is minor for different
textural classes. Therefore, thermal flux is expected to increase within
loamy sand greater than in loam with identical increments in ambient tempera-
tures. Increased ambient temperatures advances the development of self-
mulching and hence the MER, but because of enhanced greater thermal flux
within coarse-textured soils (loamy sand) especially through the sides of the
pots, latent heat transfer and the respective inhibition by mulch is enhanced
before MER. Mulch presents a barrier to water vapor loss. According to
Ficks’ first law for vapor diffusion, considering a constant diffusion coefficient
by mulch (as a porous medium), the greater the rate of vapor concentration
gradient (latent heat transfer) the greater the fall in vapor flux relative to the
control treatment, resulting in a greater magnitude for MER. With a low
ambient temperature, the loam soil, with greater unsaturated hydraulic
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conductivity, exhibited the longest and greatest MER, whereas Movahedi
Naeini and Cook (2000) reported MER increased but the duration was
reduced with lowering ambient temperatures in a loam soil. They used
black pots in their experiment (with a low albedo), which easily adsorbed
higher quantities of energy from their sides. Contrary to white pots, in their
experiment a lower ratio of energy was absorbed from the soil surface
relative to the sides and thus the effect of the depth of self-mulching was
not as significant as in the white pots. This difference in magnitude of MER
could be due to a greater quantity of energy entering black pots with the
same increment in ambient temperatures.
Movahedi Naeini and Cook (2000) reported that incorporation of compost
increased the evaporation rate early in their pot experiment and reduced the
evaporation rate afterward. They suggested the early increase could be due
to both a high unsaturated hydraulic conductivity and water retention by incor-
poration during earlier parts of the energy-limited period. In their experiment,
incorporation increased soil water retention at potentials greater than –5 kPa
and reduced water retention in potentials below –5 kPa. They stated that high
water retention and the respective pore volume might be expected to increase
unsaturated hydraulic conductivity at potentials above –5 kPa (and hence
evaporation rate at the early stages of their experiment) and the reduced
water retention and respective pore volume to reduce unsaturated hydraulic
conductivity at lower potentials (and hence evaporation rate at the later
stages of evaporation). In experiments 1 and 2, evaporation rate with incorpor-
ation was not changed significantly. At potentials with significantly greater
water retention than control and hence unsaturated hydraulic conductivity, a
greater evaporation is expected. Organic compost could block soil pores
and cancel some of these opposite effects. Jalota and Prihar (1990) also
stated that reduction in bulk density by tillage (and by analogy incorporation)
increased porosity, which presented less resistance to vapor diffusion relative
to an untreated soil. According to Opara-Nadi and Lal (1987), Movahedi
Naeini and Cook (2000), and Shangning and Unger (2001), in most
instances organic mulch more effectively reduces evaporation relative to
incorporation.
As the solute concentration in a soil solution is increased, the hydration
capacity of Aquasorb is diminished and a greater quantity of Aquasorb is
required for the same increment of available soil water capacity. As a soil
dries, its solution solute concentration is gradually increased, resulting in a
considerable reduction in water absorption by Aquasorb (Table 7) that is
released. The water that is released at potentials greater than field capacity
is lost to drainage, and the rest is available to plants. Because quantity of
water release is decreased with any further equal increment in soil solute
concentration through evaporation, 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, when mixed with soil, a greater water
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release is expected at potentials greater than –0.1 bars or the upper range of
soil-available water (greater than 20.4% for unmixed Aquasorb as
mentioned in the results). In a field, in addition to increased salt concentration
due to drainage, salt concentration is also increased within upper soil layers
after infiltration through evaporation.
Maize seedling emergence was advanced with mulch probably because of
hydrologic amelioration in both soils. Within the first 31 days after sowing in
loam and throughout experiment 3 in loamy sand, mulch increased plant
height relative to other treatments with no effect on yield. Compost mulch
did not supply enough nutrition to have an effect on plant height. A high
rate of Aquasorb increased plant height after emergence up to 24 days.
With a more rapid drainage in loamy sand, the soil surface dries more
quickly, and therefore Aquasorb absorption and subsequent release of water
advances germination and emergence and enhances early seedling growth.
Root weight with the compost incorporation treatment was greater than the
other treatments in loamy sand, possibly as a result of reducing bulk density.
CONCLUSIONS
The hydration capacity of Aquasorb 3005A (and also other current HPs) is
highly sensitive to the electrical conductivity and solute concentration of
the 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 a saturated extract.
Even in a soil with a low electrical conductivity (less than 1 ds/m), 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, the drainage
water and also the soil water tension at field capacity are considerably less
than field condition. Therefore, the release of water to drainage by
Aquasorb (and compost incorporation) is minor relative to the field. By this
analogy, even a greater quantity of Aquasorb is required in the field than in
pots.
In general, pot experiments fail to simulate field conditions. Energy trans-
actions take place from the sides and the soil surface in pots, resulting in
diurnal and nocturnal latent heat transfer at the surface and a warmer depth.
In a field, there is no nocturnal evaporation due to temperature, and vapor
pressure inversion and diurnal temperature gradients are downward. In
addition to nocturnal evaporation in pots, the higher energy transaction per
unit volume of soil in pots enhances evaporation relative to a field soil with
low transmissivity. Increasing the albedo of pot surface by painting the pots
white and also increasing pot volume reduced the rate of energy transfer
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through the sides of the pots relative to their surface, closing the gap between
pot and field for energy transaction per unit soil volume. When water rises
from subsoil to upper layers or downward redistribution of water after infiltra-
tion are significant, the height of the pots is also important in comparing
overall outcomes of pot and field experiments. In a short column of soil (in
pots), the difference in transmissional properties of different textural classes
is minimized.
ACKNOWLEDGMENTS
The authors thank Mohammad Zaman Alaodin and Mohammad Ajami for
their technical assistance and Saeed Hassani for statistical advice. This
research was funded by Gorgan University of Agriculture (Iran).
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