SLOPE AND PHOSPHOGYPSUM'S EFFECTS ON SEAL AND CRUST FORMATION AND INFILTRATION RATE
Mücahit Karaoğlu Mustafa R. Çanga
Ankara Üniversitesi, Ziraat Fakültesi Toprak Bölümü 06110 ANKARA ABSTRACT
In this study, slope and phosphogypsum's (PG) effects on seal and crust formation and infiltration rate (IR) have been investigated. Clayey soil of Samsun (Typic Udorhent) and silty loam soil of Ankara (Typic Calciorthid) were used.
Using a rainfall simulator, control and PG (at a rate of 5 and 10 t.ha-1) applied soils samples that have six different slopes angles (0, 2, 6, 15, 24 and 30 %) were subjected to two simulated rainfalls with an intensity of 45 mm for 60 minutes leaving seven days dry interval between two treatments. As a result of this study, it was found that in the PG-treated soils samples seal and crust formation were thinner and later than the control samples, and the increases of the infiltration rate were significant as statistically at level P≤0.01
INTRODUCTION
Several studies have been conducted on the effect of rainfall on the structure and hydraulic properties of soil crusts (Agassi et al., 1981; Morin et al., 1981). McIntyre (1958) found the crust to consist of two distinct parts: (a) an upper skin seal attributed to compaction due to raindrop impact and (b) a "washed in" region of decreased porosity, attributed to fine particles movement and accumulation. He measured thickness of 0.1 and 2 mm for the skin seal and the "washed in" zone, respectively.
Formation of a crust at the soil surface, generally due to the beating action of raindrops but also as a result of sprinkler irrigation (Aarstad and Miller, 1973), is a common feature of many soils, particularly in the arid and semiarid regions. Surface crusts are thin (< 2-3 mm) and are characterised by greater density, finer pores, and lower saturated conductivity than the underlying soil (Morin et al., 1981).
The rapid drop in the IR of soils during rainstorms is mainly due to seal formation on the soil surface. Breakdown of the soil structure and formation of a seal are enhanced by the impact energy of the raindrops and the low concentrations of electrolytes in rain water (Agassi et al., 1981; 1985).
PG powder present at the soil surface may affect the crust properties and the IR curves by two additional physical mechanisms: by interfering mechanically with the organisation of the skin crust and thus disturbing the formation of a continuous crust; and by a mulching effect of the PG powder which protects the soil surface from the beating action of the raindrops (Agassi et al., 1986).
It has been shown (Gal et al., 1984) that PG reduces surface sealing by raising the electrolyte concentration in the solution at the soil surface, thus reducing the dispersion of the soil aggregates and the soil clays and preventing the formation of the "washed in" layer. Thus, the crust forms more slowly on the surface of soil treated with PG, and is more permeable than the crust of untreated soil, reducing the rate of runoff.
MATERIAL AND METHODS
A clayey soil material from Samsun, where the mean annual precipitation is 710.9 mm and a silty loamy soil material from Ankara, where the mean annual precipitation is 385.5 mm were used in this study. Each soil exposed to erosion. Soil samples were taken from the Ap horizon of each soil series (0-20 cm). Disturbed soil samples were collected, air dried, crushed to pass a 2-mm sieve for laboratory analysis and a 7-mm sieve for experiments.
Six slope angles were examined (0, 2, 6, 15, 24 and 30%) for both untreated soil samples and for soil samples treated with PG. After spread over of PG at a rate of 5 and 10 t.ha-1., at the ground of the
The samples air-dried were subjected to two simulated rainfalls with an intensity of 45 mm h-1 for 60 minutes having 7 days intervals. Distilled water was used to simulate rainwater Typical mechanical parameters of the applied rain were: median raindrop diameter, 5.09±0.03 mm; median drop velocity, 5.5 m.s-1; fall height, 2.75 m; and kinetic energy 15.07 J.mm-1.m-2.
As soon as soil samples were saturated, the volume of the effluent was measured at 5-min intervals, and the IR was calculated. The samples exposed to first simulated rainfall had been dried during seven days. At the same time seal and crust formation were observed on the level samples (0 %) and then 2nd simulated rainfall was applied at 8th day.
In order to verify; the differences between the values of the infiltration rates values belong to control and PG treatments for each simulated rainfall whether significant or not, Analysis of Variance and Duncan tests were applied.
RESULT AND DISCUSSION
Kurupelit series has a clay texture (48.4%) and Çiftlik series has a silty loam texture (55.1 %) in the Ap horizon. According to soils analysis (Table 1), the Kurupelit series is slightly acid, and has moderately high permeability, lower CaCO3 content, the moderate exchangeable sodium, high organic
carbon and aggregate stability percentage. The Çiftlik series is slightly alkaline, and has high permeability, higher CaCO3 content, lower exchangeable sodium, organic carbon and aggregate
stability percentage.
Table 1. Physical and chemical properties of the soils used
Soil series and site
International classification
Mechanical
composition (%) Organic Matter (%) Aggregate Stability (%) pH (1:5) Exc. Na+ (%) Hydraulic Conductivity (cm/h) CaCO3 (%) Sand Silt Clay
Kurupelit Samsun Typic Udorhent 23.2 28.4 48.4 4.09 90.40 6.58 1.97 6.25 0.10 Çiftlik Ankara Typic Calciorthid 22.0 55.1 22.9 1.60 45.85 7.85 0.52 9.90 19.97
PG's effects on seal and crust formation
Seal formation at the soil surfaces exposed to rainfall is due to two mechanisms: (a) breakdown of the soil aggregates caused by the impact of raindrops, and (b) a physiochemical dispersion of the clay, which can then migrate and clog pores immediately beneath the surface (Warrington et al., 1989).
An untreated soil will, at the beginning of a rainstorm, form pits as a result of the impact of the raindrops (Hardy et al., 1983). As storm proceeds, a crust is formed, and runoff and erosion take place (Warrington et al., 1989). Agassi et al. (1984) showed that crust formation processes on a given soil can be limited by preventing the impact of the raindrops (e,g. mulching) or by increasing the electrolyte concentration of the rainwater. It was proposed (Keren and Shainberg, 1981) that the release of electrolytes by PG dissolution reduced clay dispersion and crust formation. Gal et al. (1984) showed that PG reduced surface sealing by raising the electrolyte concentration in the solution at the soil surface, thus reducing the dispersion of the soil aggregates and the soil clays and preventing the formation of the ''washed-in'' layer. Thus the crust forms more slowly on the surface of soil treated with PG, and is more permeable than the crust of untreated soil reducing the rate of runoff.
In order to observe seal and crust formation on the level soil samples caused by the impact energy of raindrops, control and PG treatments were applied for each series. After the seal formation the soil permeability and IR reduced rapidly. Owing to absence of slope, rain water accumulated to soil surface, and a puddle occurred. After seven days, a crust structure was formed on each level soil sample exposed to air dry. The PG application delayed the beginning of the seal formation, and reduced thickness of a seal and crust structure. The results of seal and crust formation are presented in Table 2.
Table 2. Results related to seal and crust formation. Events Simulated rainfall Kurupelit Çiftlik C PG C PG Puddle occurrence (mm) 1 50 55 40 45 2 45 50 40 40 Avarage seal thickness (mm) 1 2.0 1.4 1.5 1.2 2 1.8 1.5 1.5 1.2 Avarage seal thickness (mm) 1 1.5 1.1 1.3 0.9 2 1.8 1.5 1.5 1.2
Slope and PG's effects on infiltration rates
The effects of slope angle (0 and 30'%) and the PG treatment on the IR are illustrated in Fig. 1 for the Kurupelit series and in Fig. 2 for the Çiftlik series. There is a drop in the IR of the control soil for each soil series, however the drop in the IR of the Çiftlik series is more evident than IR of the Kurupelit series. The Çiftlik series is more susceptible to surface scaling but more permeable than the Kurupelit series. Thus the infiltration rate from the silty loam was higher than the infiltration rate from the clay for the first rainfall. Similar observations were made by Ben Hur et al. (1985) and Warrington et al. (1989) who found that soils with moderate (15-20%) clay percentage and low content of organic matter are most susceptible to crusting. The infiltration curves for the other slopes ranged between those of the 0 and 30% slopes.
The IRs of the soils samples treated with PG (10 t.ha-1) dropped much less rapidly than those of
the control soils samples. The drop began after a greater cumulative rainfall, and the final IRs were higher (Fig. 1 and 2).
Infiltration rates increase 1.4 (30%, 2nd rainfall, PG 5 t.ha-1); 1.7 (30%, 2nd rainfall, PG 10 t.ha-1) fold for the Kurupelit series and 2.0 (30%,2nd rainfall, PG 5 t.ha-1); 2.2 (30%, 2nd rainfall, PG 10 t.ha-1) fold for the Çiftlik series in respect of control treatments at the different slopes.
Figure 2. Infiltraiton rates for Çiftlik series. Table 3. Duncan test for infiltration rates.
Slope % Mean S.Dev. PG Mean S.Dev. Soil Mean S.Dev. Rainfall Mean S.Dev
0 28,621a 0,858 C 17,484b 0,607 K.pelit 19,146b 0,496 1 24,246a 0,231 24 18,096b 0,858 10 25,106a 0,607 Çiftlik 23,691a 0,496 2 18,591b 0,602
0.01 0.01 0.01
According to analysis of variance and Duncan test for infiltration rates (Table 3), differences between slopes, phosphogypsums, soils and rainfalls were significant as statistically at level P≤0.01.
The final IRs for the six slopes and each soil series are presented in Fig. 3 and 4. Differences between slopes ≤15% are significant. As slope angle increased to 24 and over, there was no decrease in the final IR, besides the evidently increase in the final IR of the PG-treated soils samples occurred in the 2nd rainfall for each soil series.
Figure 3. Final infiltration rates for the Kurupelit series.
Figure 4. Final infiltration rates for the Çiftlik series.
The effect of slope angle on the properties of the seal, as determined by the final IR, was similar in both PG-treated and untreated soils samples, in spite of the fact that PG-treated soils are less susceptible to sealing and are less erodible than untreated soils.
Final infiltration rates increase 1.6 (24%, 2nd rainfall, PG 5 t.ha-1); 2.2 (30%, 2nd rainfall, PG 10 t.ha-1) fold for the Kurupelit series and 3.3 (30%, 2nd rainfall, PG 5 t.ha-1); 3.7 (30%, 2nd rainfall, PG 10 t.ha-1) fold for the Çiftlik series with respect to control treatments (Table 4).
According to analysis of variance and Duncan test for final infiltration rates, differences between slopes, PGs, soils (P≤0.01) and rainfalls (P≤0.05) were significant as statistically (Table 5).
CONCLUSION
Breakdown of the soil aggregates caused by the impact of raindrops and a physiochemical dispersion of the clay cause seal and crust formation at the soil surface. After the seal formation IRs decrease sharply, and a puddle occurs When PG was spread over the soil sample especially at a rate of 10 t.ha-1 it dissolved and prevented clay dispersion, seal and crust formation were thinner and later than the control samples, approximately increased infiltration rates and final infiltration rates twofold and fourfold, respectively.
Table 4. Final infiltration and final infiltration rates (mm.h-1).
Slope
Kurupelit series Çiftlik series
1st rainfall 2nd rainfall 1st rainfall 2nd rainfall
C 5PG 10PG C 5PG 10PG C 5PG 10PG C 5PG 10PG 0 1.5 18.0 1.7 20.4 1.7 20.4 1.2 14.4 1.6 19.2 1.6 19.2 1.7 20.4 1.8 21.6 2.2 26.4 1.3 15.6 1.5 18.0 1.3 15.6 2 1.3 15.6 1.5 18.0 1.7 20.4 1.0 12.0 1.3 15.6 1.5 18.0 1.5 18.0 1.7 20.4 2.0 24.0 0.9 10.8 1.3 15.6 1.3 15.6 6 1.0 12.0 1.3 15.6 1.5 18.0 0.8 9.6 1.2 14.4 1.3 15.6 1.2 14.4 1.7 20.4 1.8 21.6 0.7 8.4 1.0 12.0 1.1 13.2 15 0.8 9.6 1.0 12.0 1.1 13.2 0.6 7.2 0.9 10.8 1.0 12.0 1.2 14.4 1.3 15.6 1.4 16.8 0.5 6.0 1.0 12.0 1.0 12.0 24 0.8 9.6 1.0 12.0 1.0 12.0 0.5 6.0 0.8 9.6 0.9 10.8 1.0 12.0 1.2 14.4 1.3 15.6 0.4 4.8 1.1 13.2 1.2 14.4 0.7 0.8 0.9 0.4 0.6 0.9 1.0 1.2 1.2 0.3 1.0 1.1
Table 5. Duncan test for final infiltration rates.
Slope % Mean S.Dev. PG Mean S.Dev. Soil Mean S.Dev. Rainfall Mean S.Dev 0 19,117a 0,364 C 11,183c 0,258 K.pelit 13,164b 0,210 1 15,391a 0,141 2 17,042b 0,364 5 14,779b 0,258 Çiftlik 14,828a 0,210 2 12,091b 0,283 6 14,625c 0,364 10 16,025a 0,258 15 11,842d 0,364 24 11,233d 0,364 30 10,117e 0,364 0.01 0.01 0.01 0.05 REFERENCES
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