Introduction

In India, about 50 % of the groundwater is either marginal or poor in quality. Of this, 37 % has high sodicity, 20 % has high salinity and remaining 43 % has high salinity as well as sodicity [26]. Irrigation water of category IV (RSC > 4, DCR < 0.25) required addition of gypsum to neutralize alkalinity, whereas category II water (RSC < 4, DCR > 0.25) could be used safely for crop production [17]. Long-term use of water containing excess of sodium and carbonate, bicarbonate and chloride for irrigation increased soil pH, electrical conductivity (EC) and exchangeable sodium percentage (ESP), and as a result, physical properties of soil such as aeration and permeability were adversely affected [8, 16, 20]. Additionally surface sealing, crusting and clay migration leading to clogging of pores and deposition of clay below the plough layer in soil took place and created hard pan in subsurface soil, particularly where rains followed [11, 19, 21, 23], and these adversely affected the yield, quality and profitability of field crops.

Use of sodic water with application of gypsum alone or in combination with organic amendments for irrigating cotton, wheat, paddy, sunflower, has been reported by several workers [4, 7, 9, 18]. For amelioration of sodic water irrigation effects, application of gypsum was commonly recommended [3]. Also, mixing of good quality water in varying quantities or cyclic use of canal/pond water was recommended [24]. However, limited information was available on the effect of frequency of deep tillage along with gypsum application for sustainable management of sodic water. Keeping this in view, present field study was carried out to investigate the frequency of deep tillage and RSC neutralization of sodic water through gypsum application on clusterbean—wheat crop sequence grown on sandy loam soil.

Materials and Methods

Experiment and Weather Condition

A field experiment was conducted at the Research Farm, Banasthali University, Rajasthan, India (23°3′–30°12′N latitude and 69°30′–78°17′E longitude, 264.3 m above mean sea level) during 2004–2008. The experiment was laid out in a split plot design with three replications. The main plot treatments consisted of frequency of deep tillage while sub-plot treatments represented different qualities of irrigation water (good quality and RSC neutralized sodic water). Total rainfall received during the experimentation period was 396, 473, 361 and 506 mm in 2004–2005, 2005–2006, 2006–2007 and 2007–2008, respectively. Out of total, 95.6 % rainfall occurred in monsoon season from June end to mid-September and maximum intensity of rain was observed during July and August. Except monsoon season, potential evaporation was higher than rainfall every year. Maximum evaporation occurred in May and June when the fields were fallow.

Soil

The physico-chemical characteristics of the soil profile (0–0.90 m) at the start of the experiment are given in Table 1. Sodic water had been used to irrigate different crop rotations from last 5 years before initiation of experiment. The soil electrical conductivity (ECe), pH, sodium adsorption ratio (SAR) and bulk density were slightly higher in 0.10–0.20 m horizon compared to upper (0.0–0.10 m) and lower (0.60–0.90 m) horizons of the soil. The water table was 13 m below the surface. The experimental plots (5.0 × 5.0 m2) were effectively isolated from one another by polyethylene sheet laid to 1.0 m depth on all the four sides of a plot and walkways of 1 m width between the plots to avoid lateral fluxes of water and salt.

Table 1 Initial physicochemical characteristics of soil profile
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Treatments

Sixteen treatments consisted of combinations of four frequencies of deep tillage in summer viz. farmer’s practice of tillage (FPT), every year deep tillage (EYDT), 1 year gap in deep tillage (OYGDT) and 2 years gap in deep tillage (TYGDT) and four qualities of irrigation water viz. 0, 50, 100 % of RSC neutralization of sodic water through gypsum and good quality irrigation water. The depth of tillage in farmer’s practice was 12 cm, whereas that of deep tillage was 23 cm. The tillage operations were carried out in the month of May after harvest of wheat by cultivator in FPT and by disc plough in case of deep tillage. The composition of sodic underground water and good quality rain water conserved in farm pond is given in Table 2.

Table 2 Composition of irrigation waters
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The quantity of gypsum for neutralization of each mmolc L−1 of RSC was 86 kg ha−1 per 1,000 m3 (for 10 cm deep irrigation) of water. The agricultural grade gypsum used was 70 % pure and 7 cm deep irrigation was applied to wheat crop, which was measured by V-notch. The quantity of gypsum required was calculated by the formula: neutralizable RSC (4.5) × 86 × number of irrigation (6) × depth of irrigation (7 cm) × purity of gypsum [17]. The quantity of gypsum required for the treatment was:

$$ 4.5 \times 86 \times 42/10 \times 100/70 = 2,322\,{\text{kg or }}2.32 \; {\text{tonnes}}\,{\text{ha}}^{ - 1} $$

The quantity of gypsum for 50 and 100 % RSC neutralization was 1.16 tonnes and 2.32 tonnes ha−1, respectively. Application of gypsum as per treatment was broadcasted and mixed before rainy season in each year of experimentation.

Crops

Clusterbean (cv. RGC 986) and wheat (cv. Raj 3765) were grown during rainy and winter season, respectively, for 4 years (2004–2005 to 2007–2008). A recommended basal dose of 30 kg N, 17.2 kg P and 30 kg S ha−1 for clusterbean and 90 kg N, 25.8 kg P, 33.2 kg K and 5 kg Zn ha−1 for wheat was applied using urea, diammonium phosphate and muriate of potash, as sources for N, P and K, and gypsum and zinc sulphate as sources of S and Zn, respectively. Clusterbean was sown during first week of July after receiving first monsoon rain, while wheat was sown during first week of December every year. The clusterbean was rainfed every year, whereas wheat crop was irrigated as per treatment and six irrigations were applied including pre-sowing one.

Measurements

Soil samples were collected initially and after harvest of wheat crop from each plot from 0 to 0.10, 0.10 to 0.20, 0.20 to 0.30, 0.30 to 0.60 and 0.60 to 0.90 m depths with an auger 4.0 cm in diameter. The soil samples were air dried, ground to pass through a 2-mm sieve, and analyzed for sand, silt and clay fraction [10], pH (saturation paste), ECe and SAR of saturation paste extracts [25], bulk density (Soil core method), available N (Kjeltec-II auto analyzer) and P, K, Cu, Fe, Zn and Mn [12]. After harvest of wheat infiltration rate was measured in situ every year from all treatments plot by double ring infiltrometer method [5]. Root density of wheat crop at flowering stage was computed by dividing the dry weight of roots by the volume of core. Crude protein and gum content of clusterbean and protein, starch and lysine content of wheat grain were determined by standard methods [15].

Results and Discussion

Yield

Deep tillage treatments significantly increased the grain and straw yield of clusterbean and wheat (Table 3) as compared to farmer’s practice of tillage (FPT). Non-significant difference was found in grain and straw yield of clusterbean and wheat during first and second year, whereas, during third year of experimentation grain and straw yields of both crops were significantly reduced in the treatment of two years gap in deep tillage (TYGDT) compared to every year deep tillage (EYDT) and one year gap in deep tillage (OYGDT). As compared to FPT, EYDT, OYGDT and TYGDT increased the yield (pooled for four years) of clusterbean grain by 20.4, 17.3 and 15.2 % and that of straw by 21.5, 18.4 and 17.2 %, respectively. Similarly, EYDT, OYGDT and TYGDT increased the yield of wheat grain by 14.0, 13.1 and 11.4 % and that of straw by 12.8, 14.2 and 10.8 %, respectively, compared to FPT. Similarly deep tillage significantly increased the yield of corn grown in sandy loam soil [2].

Table 3 Effect of frequency of deep tillage and RSC neutralization of sodic water on seed and straw yield (kg ha−1) of clusterbean and wheat crops
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Sodic water irrigation to wheat crop significantly reduced grain and straw yield of clusterbean (rainfed) and wheat during each successive year compared with good quality water irrigation to wheat and minimum yields were recorded in the fourth year (Table 3). The 50 and 100 % RSC neutralization of sodic water through gypsum significantly increased the grain and straw yield of clusterbean and wheat during each year as compared to no neutralization of RSC. Neutralization of RSC of sodic irrigation water through gypsum application @ 50, 100 % of gypsum requirement and use of good quality irrigation water increased the average yield of clusterbean grain by 84.9, 137.7 and 151.5 %, and that of clusterbean straw by 73.3, 116.0 and 133.6 %, that of wheat grain by 32.8, 48.1 and 55.3 % and that of wheat straw by 34.5, 44.8 and 51.8 %, respectively, compared to yields obtained with sodic water irrigation. The minimum yield obtained under sodic water irrigated condition may be ascribed to Ca-deficit in the soil solution [17, 22], deteriorating effects due to high pH on other soil properties [6] and poor aeration due to higher ESP level [4, 13]. Except grain yield of clusterbean in first year (2004–2005), grain and straw yields of both crops were significantly higher with good quality irrigation water than 100 % RSC neutralization of sodic water. This was further clear from pooled analysis of yield-data.

Quality

Quality parameter of clusterbean grain (crude protein and gum) and wheat grain (protein, starch and lysine) was significantly and positively influenced by different frequencies of deep tillage as compared to FPT (Table 4). This was attributed to proliferation of root to a greater depth and consequently, a greater plant growth. Deeper and denser rooting system helped to extract more water and nutrients from the soil and thus resulted in high yield and quality of clusterbean and wheat. However, differences between different frequencies of deep tillage were not significant. The quality parameters of clusterbean grain (crude protein and gum) and wheat grain (protein, starch and lysine) were significantly influenced by RSC neutralization through gypsum. The effect on quality parameters brought about by 50 and 100 % RSC neutralization of sodic water and good quality irrigation water was not significant. Neutralization RSC through gypsum had favourable effect on physico-chemical property of soil affected more extraction of nutrients due to improvement in aggregate stability [1, 14].

Table 4 Effect of frequency of deep tillage and RSC neutralization of sodic water on grain quality of clusterbean and wheat (four years mean)
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Root Mass Density of Wheat

The data on profile distribution of root mass at flowering stage (Table 5) revealed that maximum root mass was located in the 0–0.15 m soil horizon and it decreased with depth. Deep tillage significantly influenced the root mass density and maximum root mass density 654 g m−3 was observed in EYDT followed by 619 g m−3 in OYGDT and 585 g m−3 in TYGDT. Maximum rooting depth was beyond 1.2 m under deep tillage treatments, and only 0.90 m under FPT. The root mass density of wheat increased from 26.1 to 40.9 % with different frequencies of deep tillage as compared to FPT. Deep tillage improved physical environment of soil, which facilitated better root proliferation and penetration and consequently resulted in higher root mass density. Increase in root density resulted in greater extraction of water and nutrients from the lower soil layer which were relatively less deteriorated due to sodic water irrigation.

Table 5 Effect of frequency of deep tillage and RSC neutralization of sodic water on soil profile distribution of root density (g m−3) of wheat at flowering stage (four years mean)
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As shown in Table 5, the rooting depth of wheat was greater than 1.2 m under 50 and 100 % RSC neutralization of sodic water and under good quality irrigation water, whereas rooting depth was only 0.60 m under sodic water irrigation. The root density in 0–1.2 m soil profile was maximum (689 g m−3) under good quality irrigation water followed closely by 100 % RSC neutralization of sodic water (672 g m−3) and minimum (335 g m−3) under sodic water irrigation.

Infiltration Rate

At the end of winter crop every year, final infiltration rate (FIR) was measured in all the treatment plots (Table 6). Deep tillage improved the infiltration rate and the highest mean infiltration rate was 2.23 cm h−1 under EYDT treatment, although the difference between various frequencies of deep tillage was not significant. Minimum mean infiltration rate 1.16 cm h−1 was observed under FPT. Different frequencies of deep tillage EYDT, OYGDT and TYGDT increased the infiltration rate by 92.2, 77.6 and 70.7 %, respectively, as compared to FPT. The higher infiltration rate in deep tillage plots was due to the fact that deep tillage shattered the plough sole and thereby made the soil more pervious.

Table 6 Effect of frequency of deep tillage and RSC neutralization of Sodic water on infiltration rate (cm h−1) after harvest of wheat
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In sodic water irrigated plots the infiltration rate decreased every year and it was minimum (0.37 cm h−1) at the end of fourth year. The infiltration rate possibility decreased due to clay dispersion, its migration and the resultant clogging of soil pores under irrigation with sodic water which also appeared to cause structural problems and thereby decline in infiltration rate [11, 13, 23]. Maximum mean value infiltration rate (2.61 cm h−1) was observed in good quality water irrigated plots but it was not significantly different from infiltration rate under 100 % RSC neutralization of sodic water. The treatment 100 % RSC neutralization resulted in significantly higher infiltration rate as compared to 50 % RSC neutralization of sodic water. Favourable effect of RSC neutralization on infiltration rate might be due to improvement in aggregate stability [1, 14].

Soil Properties

After four consecutive year cultivation of clusterbean and wheat significant changes in pH, ECe and SARe of soil were observed (Table 7). Deep tillage significantly reduced the pH, ECe and SARe compared to FPT but the differences between different frequencies of deep tillage were not significant. Beneficial effects of deep tillage are likely due to increased soil-aeration and improved water penetration resulting in more leaching.

Table 7 Effect of frequency of deep tillage and RSC neutralization of Sodic water on soil properties after the harvest of crops
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Soil pH was reduced significantly on irrigation with RSC-neutralized water and 100 % neutralization of RSC was as effective as good quality water for irrigation. Even more contrasting effects of RSC-neutralization were observed on SARe values which were considerably reduced by RSC-neutralization. Significant reduction in ECe was observed under good quality irrigation water and 50 and 100 % of RSC neutralization of sodic water compared with sodic water irrigated plots. Beneficial effects of RSC neutralization on pH and SAR can be ascribed to beneficial effect of Ca++ supplied through gypsum.

Conclusions

Long-term use of sodic water for irrigation significantly decreased the grain and straw yields and quality parameters of clusterbean and wheat grains due to increase in soil pH, ECe and SARe. On 100 % neutralization of RSC of sodic water with gypsum, grain and straw yields increased 137.7 and 116.0 %, respectively, in case of clusterbean and 48.1 and 44.8 % in wheat, compared with sodic water irrigation. Deep tillage in 1 year gap could sustain grain and straw yields of both clusterbean and wheat. Similarly, quality of clusterbean grain (crude protein and gum content) and wheat grain (protein, starch and lysine content) improved due to neutralization of RSC and deep tillage. Reduction in pH (0.44), ECe (0.59 dS m−1), SARe (15.55) and increase in infiltration rate (1.44 cm h−1) were also observed on 100 % RSC-neutralization as compared with sodic water irrigation. Similarly, deep tillage reduced the pH, ECe, SARe and increased the infiltration rate. Concludingly, 1 year gap in deep tillage in the month of May and 100 % RSC neutralization through gypsum application at the beginning of monsoon (last week of June) had a positive effect on yield and quality of clusterbean and wheat with sodic water irrigation on sandy loam soil in arid region.