Introduction

Due to anthropogenic activities, Soil Organic Carbon (SOC) in many parts of the world is declining and this is likely to cause deterioration of soil structural stability [1,2,3]. The continuous decomposition of soil organic matter (SOM) in cultivated lands of arid and semiarid regions may lead to soil degradation with a consequence of inability to ensure a sustainable production. Land application of organic waste materials obtained from different sources has the advantage of preventing the accumulation of wastes in the environment and providing organic matter and nutrients for soils. In recent years, the use of organic amendment other than chemical fertilizers has become popular. SOM can improve a range of soil physical and mechanical properties which include compactibility [4, 5], soil strength and soil water content range for tillage operations [6] and in general the soil physical quality. Addition of organic fertilizers results in increased SOC [7]. Previous investigations have consistently found that incorporating farm yard manure (FYM) [8] increased SOC and has reduced soil bulk density (BD) [9] and has direct and indirect effect on soil physical properties [10, 11].

Infiltration is the process by which the reservoir of water in the soil is replenished, it is very important in irrigation to meet the plant needs. Water movement in soil is governed by capillarity and gravity. Capillary forces dominate water movement into the soil, with time the capillarity will be dominated by gravity. Factors influencing the infiltration rate are soil texture, surface sealing and soil cracking. If the surface sealing does not occur, the steady-state infiltration rate would be equal to the permeability of the soil. Surface sealing occurs due to shearing effect of the flowing water that causes aggregates on the soil surface to decompose into smaller aggregates and individual particles which tend to form a thin layer with low permeability on the soil surface forming a surface seal. Soils that contain clays, shrink when dried and swell when wet leading to formation of cracks. These cracks pronounce the initial infiltration rate, as the soil wets the cracks swell and shut, leading to rapid decrease in infiltration rate. The degradation of soil physical conditions and the risk of erosion are strongly related to a gradual decrease of the soil organic matter content in intensively cultivated soils [12]. Arid soils usually have low SOC content [13,14,15]. In such soils there is hardly any relation between SOC and soil stability [16, 17]. Erosion often causes changes in the biolog-soils and mask the effects of soil loss [12, 18,19,20] leading to permanent reductions in crop measure.

Chemical properties can be offset with fertilizer by bringing the changes in soil physical properties especially water retention [21,22,23,24,25]. The ability of soil to infiltrate and retain water is critical for plant production. Limited water retention can lead to insufficient water for plant uptake unless supplemental irrigation is used. Retention is affected by particle and pore size distribution. Therefore, in eroded soils, one can expect decreased water retention because of the preferential removal of clay and silt size particles that occur with erosion. However, if the lower soil horizons have greater clay content than the removed surface layers initially, soil water retention may increase, but not all this water will be plant-available [26]. Under such eroded conditions, soil water is held strongly, thus extra energy is required for plant uptake when compared with uneroded conditions. Eroded soils are usually shallower and plant roots have less volume to exploit for water and nutrients compared with uneroded soils. For these reasons, some researchers have observed a decrease in the available soil water capacity of eroded soils, whereas others have reported no differences, and even increases, in the water retention ability of eroded soils [21,22,23,24]. Therefore, by improving the physical properties of an eroded soil, it may be possible to ameliorate the harmful effects of erosion.

Previous studies [27, 28] have justified the use of SOC to improve the water holding capacity and infiltration capacities of soils. SOC also plays the role in binding the soil particle resulting in reduced erosion [12].

Considering the importance of SOC in the soil health this study was taken up with the intention of utilizing the organic based wastes, as a source of SOC to artificially restore the lost SOC in the locally available soils and explore its effects on perviousness and conservation property. Various studies have been carried by incorporating the statistical technique called response surface methodology (RSM), for designing the experiments. This technique evaluates the effects of factors and searches the optimum conditions for desirable responses and reduces the number of experiments. A pioneer attempt is made in this paper, to the study of soil properties using this technique.

Materials and Methods

The study was conducted in Raichur, a district head quarter located in the northern region of Karnataka state (16.2°N 77.37°E), which is drought prone and falls in the arid tract of India. The climate of the district is characterized by dryness for the major part of the year and a very hot summer. Locally available BC, marshy, red and mountainous soils that prevail in this area were collected from different locations. Four amendments namely Class “F” category fly ash procured from Raichur thermal Power plant of Karnataka, called Raichur Fly ash (RFA), pressmud and bagasse ash were the wastes from nearby sugar industry and humus from the agricultural fields were collected and analyzed for SOC [29], particle size and field density. The SOC in the native soils were 0.44, 0.35, 0.34 and 0.40% respectively for BC, red, marshy and mountainous soils.

Properties of Soils and Amendments

Ten soil samples from the fields for each of the soil were extracted by cutting the cores clearing the top five centimeters of soil from the ground. They were then used for the determination of field capacity and bulk densities. Sieve analyses were carried for disturbed soil samples. BC soil was clayey sand with high plasticity, having 38% sand and 62% silt and clay. Red soil was clayey sand with intermediate plasticity, having 41% sand and 59% silt and clay. Mountainous soil was silty sand with low plasticity, having 42% sand and 58% silt and clay and marshy soil was non Plastic, with 77% sand and 23% silt and clay. The grain size distribution curve of the soils is shown in Fig. 1.

Fig. 1
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Grain size distribution curve

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Similarly hydrometer tests were carried for the particle size analysis of amendments. Bagasse ash particles were uniform non-granular and average particle sizes ranged between 7 and 12 μm, fly ash had 1% clay sized, 12% of silt sized and 87% of sand sized particles in it. Pressmud was coarser than rest of the amendments with 20% of its particles ranged from 0.1 μ to 1 mm, 80% of its particles ranged from 1 to 10 mm. Humus had 38% of fine sand fraction, 35% silt sized fraction and 27% clay sized fraction. The SOC in the native amendments were 0.24, 0.285, 0.277 and 0.30% respectively for pressmud, bagasseash, humus and flyash.

Micro structure of soils and amendments were also analysed by scanning electronic microscope (SEM) images as seen in Fig. 2. The SEM images of the SOC amended soils are shown in Fig. 3.

Fig. 2
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SEM images of soils and amendments

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Fig. 3
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SEM images of amended soils

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The black cotton soil particles were nearly angular in their shape with corrugated and protruded surface. Red soil particles were rough textured in a randomly arranged cluster with the surface fractured into irregular shape, size and length. Marshy soil particles though had the random shapes, the surface was nearly smooth and non angular. Mountainous soil particles were absolutely angular in their shapes with the mixed sized cluster.

Humus in its microstructure was filamentous. Bagasseash was the cluster of nearly spherical, filamentous and angular shaped particles. Pressmud in its microstructure was angular in shape; surface of the particle was rough with presence of many cracks. Fly ash in its microstructure was spherical in shape with very smooth surface resembling a sphere. Pores in the natural form will be usually angular or slit-shaped than having the shape of cylindrical capillaries [30, 31]. The angular shape of the soil particles created the angular pores. Due to angularity, the pore volume will also be reduced because of the idealistic arrangement of particles. This shabby arrangement of sharp edges, rough and corrugated surface leads to the formation of better aggregation of the soil particles. The spherical particle arrangement as in the case of fly ash leads to the formation of spherical or cylindrical pores. Angular pores offer additional advantages over cylindrical tubes in terms of liquid behavior, when angular pores are drained a fraction of the wetting phase remains in the pore corners. This aspect of dual occupancy of the invaded portion of the tube, is not possible in cylindrical tubes, more realistically represents liquid configurations and mechanisms for maintaining hydraulic continuity in porous media [32].

Laboratory Experiments

Experiments were performed in the laboratory to determine runoff and infiltration capacities of the amended soils and also on the soils without amendment application henceforth called the controls. A channel of size 20 cm × 60 cm × 35 cm was fabricated with the allowance of 5 cm free board at the top, this channel had two compartments as seen in Fig. 4. The soils were mixed with amendments, compacted to their field densities and placed in one of the compartments of the channel in unsaturated condition and in the other compartment the control (soil in natural state without external addition of SOC) was placed and compacted to its field density. Water of 20 L volume was discharged at the rate of 1 L/min through a shower and was made to flow as a sheet to avoid the formation of channelized paths. Care was taken to avoid the water loss due to spill.

Fig. 4
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Laboratory setup to study infiltration, runoff and erosion

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Arrangements were made to collect the runoff water from surface and infiltrated water from the base of the channel separately from both the compartments and quantified. Similarly collected sediment from runoff was also separated and quantified.

Soil samples were extracted from the channel, bulk densities (BD) were determined by dividing the oven dried mass of soil by its corresponding volume these values were then used to determine (i) Porosity as \(Po\left( \% \right) = 1 - \frac{BD}{Particle\, Density}\). The results of control acted as the datum to ensure the betterment in the property of perviousness and conservation of soil. Erosion was minimized and infiltration was enhanced in amended soils when compared with the control.

Experimental Design

It was intended to study the effect of SOC on the property of soil conservation and perviousness. This property was influenced by the factors like porosity, infiltration, erosion and runoff which are interrelated. Overall effect of SOC on these factors is addressed in this study. The numbers of experiments required to obtain the reliable results was determined by response surface methodology (RSM). It is a statistical technique for designing experiment, evaluating the effects of factors and searching optimum conditions for desirable responses and reducing number of experiments [33]. RSM has a dual aim to find the optimum settings for the variables and to see how the variables perform over the whole experimental domain, including any interactions. It is assumed that screening has been done to reduce the number of factors to four or less. RSM uses an experimental design such as the central composite design (CCD) to fit a model by least squares technique [34].

CCD is a fractional factorial design with center points, augmented with a group of axial points also called star points that allows one to estimate the curvature. It consists of ‘2k’ factorial runs, ‘2k’ axial runs and ‘nc’ center runs; it is represented as \(2^{k} + 2k + n_{c}\), where k represents the number of factors. This property had five factors SOC, Po, In, Er and Ru. CCD with uniform precession provided 32 factorial runs, 10 axial runs and 10 center runs mounting a minimum of 52 runs for this property, where as CCD with two center points suggested 28 runs only.

Since the initiative was to understand the role of SOC, the study was diverged to realize whether SOC could have any influence on the independent factors apart from the property under consideration. Therefore experimental work was bifurcated into phases to understand the combined influences of SOC and mode of amending, 18 runs were set for each factor independently, which altogether resulted 72 runs.

Phase-I (P1): Blending SOC Only to the Top 15 cm Soil Depth

Soil-amendment combinations were made by replacing 0–40% volume of soil in the channel by the amendments and blending it with the top 15 cm soil. Thus every gram of soil contained the SOC dosage of 0.09–0.55, 0.1–0.62, 0.2–1.19 and 0.08–0.5 g for BC soil when respectively amended with humus, bagasseash, pressmud and fly ash. Similarly every gram of red soil had the SOC of 0.07–0.39, 0.07–0.45, 0.14–0.86 and 0.06–0.36 g. For marshy soil it was 0.07–0.41, 0.08–0.47, 0.15–0.9 and 0.06–0.38 g for marshy soil. For mountainous soil it was 0.08–0.47, 0.09–0.53, 0.17–1.02 and 0.07–0.42 g.

Phase-II (P2): Blending SOC with Soil to Complete 30 cm Depth

Soil-amendment combinations were made by replacing 0–70% volume of soil in the channel by the increments of 10% with waste (SOC) and blending it with the complete soil depth (30 cm). This resulted in the SOC dosage of 0.09–1.92, 0.1–2.18, 0.2–4.18 and 0.08–1.74 g for BC soil when respectively amended with humus, bagasseash, pressmud and fly ash. Similarly for red soil it was 0.07–1.37, 0.07–1.56, 0.14–2.99 and 0.06–1.25 g. For marshy soil it was 0.07–1.45, 0.08–1.65, 0.15–3.16 and 0.06–1.32 g. For mountainous soil it was 0.08–1.63, 0.09–1.86, 0.17–3.57 and 0.07–1.49 g.

Phase-III (P3): SOC is Stacked in the Column Without Blending with Soil

Soil-amendment combinations were made by removing 0–70% volume of soil in the channel and the amendments were just stacked as a layer on top of soil in proportion to the removed soil without blending.

To understand the exclusive role of SOC on the factors, the outcomes of the factor wise runs were individually addressed by plotting SOC as abscissa and factor as ordinate using graph builder which generated RMSE (Eq. 1) and mean (Eq. 2) values for individual factors. To decide the best suited soil amendment combination for a particular factor; the obtained phase wise RMSEs as ordinate and means as abscissa were tried for the best fit, with polynomial curve (linear, quadratic and cubic), logistic and sigmoid curves. The Quadratic (Eq. 3) curve provided the finest regression for all the factors when they are to be individually addressed.

$${\text{RMSE}} = \sqrt {\frac{{\mathop \sum \nolimits_{{{\text{n}} = 1}}^{\text{N}} \left( {\left( {{\text{Mean}} - {\text{Factor}}_{\text{n}} } \right)} \right)^{2} }}{\text{N}}}$$
(1)
$${\text{Mean}} = \frac{1}{\text{N}}\mathop \sum \limits_{{{\text{n}} = 1}}^{\text{N}} \left( {{\text{Factor}}_{\text{n}} } \right)$$
(2)
$${\text{y}} = {\text{a}} + \left[ {{\text{b }} \times {\text{mean}}} \right] + \left[ {{\text{c }} \times {\text{mean}}^{2} } \right]$$
(3)

The parameters which were directly dependent on SOC were grouped in the response surface model through analysis of variance (ANOVA). The mathematical model and evaluation of the subsequent regression analyses were done using JMP®10.0.Service Pack software (version 6.0.6, Stat-Ease, Inc., Minneapolis, USA).

Preliminary experiments were conducted on factors of soil perviousness i.e. SOC, porosity, infiltration, runoff and erosion. The maximum and minimum values for soil-amendments and the factors obtained are listed in Table 1, which functioned as inputs for Design of Experiments (DOE).

Table 1 Maximum and minimum values for the factors
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Physical experiments were also conducted for all the soil amendment combinations for perviousness and conservation property of soil. Since quadratic models were developed by relating SOC alone with the factors, while SOC was associated with the microstructure of soil and amendments in the physical experiments. Hence experimental and the quadratic model outcomes were contradictory. Outputs of quadratic models and the experimental results for all the factors were considered independently to decide the best soil amendment combination for the property under consideration. The soil amendment combination that had its prime role either through SOC or the microstructure for majority of the factors was decided as the best suited soil amendment combination in benefiting the property of soil conservation and perviousness. Fly ash for BC soil and bagasseash for rest of the soils were decided as the best suited soil amendment combinations.

Results and Discussion

The Quadratic (Eq. 3) curve provided the finest regression for all the factors against SOC for BC soil, turned the highest R2 value of 0.9 for flyash amendment. This regression suggested the exclusive role of SOC was found only in the fly ash when compared with the rest of the amendments. The response variable (betterment in the property under study) was fitted into an empirical model by correlating the response of soil perviousness property to the individual variables using a polynomial equation as given below:

$$y = 28.88 + 5.07 \times \left( {\frac{{SOC {-} 1.005}}{0.92}} \right) - 2.23 \times \left( {\frac{{Po {-} 47.15}}{25.39}} \right) + 3.13 \times \left( {\frac{{In {-} 35.90}}{12}} \right) + 1.1 \times \left( {\frac{{Er {-} 16.01}}{14.16}} \right) - 0.71 \times \left( {\frac{{Ru {-} 78.74}}{16.1}} \right)$$

The values of BC soil’s control for porosity, infiltration, runoff and erosion were respectively 27.28% and 18.33, 16.97 and 81.67%, amending flyash in optimum dose of 1.005 g per every gram of soil enhanced respectively porosity and infiltration to 35.7 and 48.2% also reduced runoff and erosion to 4.67 and 51.3%, recommending phase-III mode of amendment application.

Similarly for red soil highest R2 value of 0.86 was obtained for bagasse ash amendment. The empirical model for red soil is given below:

$$y = 17.34 + 4.49 \times \left( {\frac{{SOC {-} 0.72}}{0.65}} \right) - 6.09 \times \left( {\frac{{Po {-} 37.49}}{25.77}} \right) - 1.43 \times \left( {\frac{{In {-} 52.70}}{24.70}} \right) + 0.2 \times \left( {\frac{{Er {-} 16.91}}{14.32}} \right) - 0.79 \times \left( {\frac{{Ru {-} 58.72}}{36.08}} \right)$$

The values of red soil’s control for porosity, infiltration, runoff and erosion were respectively 30.57, 31.18, 19.83 and 68.74%, amending bagasse ash in optimum dose of 0.72 g per every gram of soil enhanced porosity and infiltration to 61.75 and 62.13% respectively also reduced runoff and erosion to 1.12 and 37.28% respectively, recommending phase-II mode of amendment application.

For marshy soil R2 of 0.67 was obtained with bagasse ash amendment this reduction in regression value indicate the influence of microstructure of soil and amendment was pronounced than the effect of SOC. Since both bagasseash and marshy soil particles are coarse with sharp edges in their micro structure as seen in Fig. 1. There definitely have been a nexus between the micro structure of soil and amendment on infiltration [30, 31]. The empirical model for marshy soil is given below given below:

$$y = 38.56 - 3.99 \times \left( {\frac{{SOC {-} 0.76}}{0.69}} \right) + 1.79 \times \left( {\frac{{Po {-} 47.63}}{14.83}} \right) - 2.88 \times \left( {\frac{{In {-} 43.29}}{34.11}} \right) - 0.28 \times \left( {\frac{{Er {-} 12.84}}{11.14}} \right) - 3.05 \times \left( {\frac{{Ru {-} 67.61}}{23.21}} \right)$$

The values of marshy soil’s control for porosity, infiltration, runoff and erosion were respectively 41.39, 28.61, 39.16 and 71.39%, amending bagasse ash in optimum dose of 0.76 g per every gram of soil enhanced respectively porosity and infiltration to 62.73 and 55.84% also reduced runoff and erosion to 1.78 and 44.32% respectively, which Recommends phase-III mode of amendment application.

For mountainous soil highest R2 of 0.89 was obtained when amended with bagasse ash. The empirical model for mountainous soil is given below.

$$y = 17.09 - 6.34 \times \left( {\frac{{SOC {-} 0.86}}{0.78}} \right) + 6.94 \times \left( {\frac{{Po {-} 34.38}}{24.95}} \right) - 2.22 \times \left( {\frac{{In {-} 28.70}}{8.2}} \right) - 1.52 \times \left( {\frac{{Er {-} 5.53}}{4.97}} \right) - 2.14 \times \left( {\frac{{Ru {-} 41.84}}{25.96}} \right)$$

The values of mountainous soil’s control for porosity, infiltration, runoff and erosion were respectively 31.28, 24.72, 5.23 and 75.20%, amending bagasse ash in optimum dose of 0.86 g per every gram of soil enhanced respectively porosity and infiltration to 67.35 and 84.66% also reduced runoff and erosion to 1.56 and 15.53% respectively, in phase-II mode of amendment application.

“Y” is the result/parameter/property under study. “Y” predicted is the expected result the model predicts and “Y” actual is the result that is actually noticed in experimental observation. The response predicted by DOE and the responses obtained by experimental output were quite similar. Experimental responses obtained were well within the boundaries as predicted by DOE. Table 2 lists the predicted and actual responses of soil perviousness for all soils.

Table 2 Predicted and actual responses (Y) for soil perviousness and conservation property
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The coalition of micro structure and SOC together functioned in the betterment of soil perviousness and conservation property. Coarse and rough textured micro enhanced soil porosity because they physically formed the clusters of aggregates by their rough textured micro structure which physically arrested the fines from motion, thus forming the stable structural matrix. Soil perviousness of amended soils increased because of the reduction in BD related to the mixing of soil with less dense organic material, resulting in better aggregation and a consequent increase in volume of micropores.

Response Surface Contours

For the graphical interpretation of the interactions, the use of three-dimensional plots of the regression model is highly recommended [35,36,37]. The three-dimensional response surface curves were plotted to understand the interaction of the components as seen in Figs. 5, 6, 7, and 8.

Fig. 5
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Response surface curves for BC soil

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Fig. 6
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Response surface curves for Red soil

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Fig. 7
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Response surface curves for Marshy soil

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Fig. 8
figure 8

Response surface curves for Mountainous soil

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The graphical representations of the models facilitate an assessment of the effects of the experimental factors on the responses by 3D surface graphs and contour plots between the factors. The plots are symmetrical in shape with circular contours. The response plots show clear mountain, which imply the maximum values of the responses endorsed to SOC in the design space. I-Optimal Design for central composite rotatable design (CCRD) with axial value of 2.00 had D-Efficiency of 71.82 G-Efficiency of 92.31 and A- Efficiency of 49.83 with average Variance of Prediction of 0.35. For all the soils, Variance of Prediction were less than 0.5, indicating the generated response model is significant.

Conclusions

A series of experiments were conducted to find the effect of SOC on soil perviousness and conservation property. The number of experiments was determined based on the factors considered in RSM for design of experiments. Channel studies were performed for the analyses of infiltration and runoff volumes for each soil amendment combination. RSM with CCRD was successfully employed for determining the optimum SOC dosages for the betterment of the property under study. The proposed model fitted very well to the experimental data. The responses obtained by experimental output were well within the boundaries as predicted by DOE. SOC enhances the binding between the soil particles, resulting in the formation of stable soil aggregates due to which the porosity in the soil increases admitting much of rainwater inflow and minimizing the runoff. The rough and corrugated surfaces of soil and amendment combination could break the momentum of flowing water thus reducing runoff and enhancing infiltration, thereby avoiding the self movement of soil and amendments. Collective role of SOC, microstructure of soil as well as amendments and the mode of blend have their influence on this property. Fly ash for BC soil and bagasse ash for red, marshy and mountainous soil were found to be the best suited soil-amendment combinations. Amending the arid soils improves the structure of the soil. Utilizing the organic wastes as amendments can be a better alternative to waste handling, which helps to maintain the soil health thereby facilitating proper soil management.