Influence of Crop Growth Stages and Management Practices on Soil Water Content at Different Soil Depths under Dryland Conditions

Soil water loss through evaporation plays a role on low crop productivity and this is due to poor cropping systems and soil surface coverage. The study was carried out at three locations of North-West province of South Africa, which were Potchefstroom, Taung and Rustenburg during 2011/12 and 2012/13 planting seasons. The experimental design was a factorial experiment laid out in a randomised complete block design (RCBD) with three replicates. The experiment consisted of five cropping systems, which were monocropping cowpea, monocropping maize, cowpea followed by maize in rotation, maize followed by cowpea in rotation and intercropping maize-cowpea. The three crop growth stages compared in this study were before tasselling/flowering, during tasselling/pod formation and during physiological maturity of maize and cowpea. Soil was sampled for the 0-0.15, 0.15-0.3, 0.3-0.6 and 0.6-0.9 m depth increments and soil water content determined using the Gravimetric method. The crop growth stage before tasselling/flowering in maize/cowpea had significantly (P < 0.05) higher water content of 10.2, 10.8, 12.5 and 13.3% at the depth of 0-0.15, 0.15-0.3, 0.3-0.6 and 0.6-0.9 m respectively. Soil collected at Rustenburg and Potchefstroom had significantly (P < 0.05) higher water content of 13.5 and 10.2; 15.9 and 10.9; 18.3 and 12.8; 18.4 and 14.5% at the depths of 0-0.15, 0.15-0.3, 0.3-0.6 and 0.6-0.9 m respectively. Monocropping cowpea plots had significantly (P < 0.05) higher water content of 12.4% than other cropping systems at the soil depth of 0.3-0.6 m. Monocropping plots of cowpea had the ability to hold soil water and this depends on the type of cowpea cultivar and canopy cover. The stage before tasselling/flowering of maize-cowpea (V10/Vn) was found to have high soil water content. Soil water content differs across locations due to different soil physical properties.


Introduction
Soil and water conservation is one of the cardinal principles of land management in rainfed areas with considerable potential for increased productivity.Soil water availability is controlled by matric potential at which the water is held (Panday & Nkongolo, 2015).Matric potential depends on soil water content, the size of soil pores, the surface proportion of soil particles and surface tension of soil water (Whalley et al., 2013).Soil texture has profound effect on water retention and is considerably most important among physical properties (Schoonover & Crim, 2015).Sandy soil has low water retention, low organic matter and high infiltration (Abdel-Nasser et al., 2007).It was further indicated that soil with high percentage of organic matter and natural deposits rich in clay content caused an increase in water holding capacity and reduction in evaporation (Parikh & James, 2012).The soil's ability to retain water is strongly related to particle size (Leeper & Uren, 1993).Water retention is positively correlated with the clay content because of the occurrence of micropores and menisci that generate capillary forces (Reichert et al., 2009).Clay increases specific area of soil matrix and water adsorption (Hillel, 1998).
Crops such as maize (Zea mays) have different responses to water deficit according to their developmental stages (Cakir, 2004).During stem elongation of maize (after floral initiation), leaves and stems grow rapidly, requiring adequate supplies of water to sustain rapid organ development (Muchow, 1989).Soil water shortage was damaging to grain yield if it occurred early in the growing season, at flowering and during grain filling (Heisey & Edmeades, 1999).Cowpea can tolerate drought stress at the vegetative stage, and recover when water is available at the reproductive stage to produce seed yield equivalent to that of unstressed plants.Drought stress at the flowering or pod filling stage of cowpea reduced yield (Akyeampong, 1985).Excess water may limit yields through nutrient losses from soil (Nandwa & Chege, 1996).
Crop canopy coverage conserves soil water, since shaded soil surface receives very little radiation and its temperature becomes lower than exposed soil (Hsiao & Xu, 2005).An intercrop of two crop species such as legumes and cereals may use water more efficiently than a monoculture of their species through exploring a larger total soil volume for water, especially if the component crops have different rooting patterns (Ofori et al., 2014).It was further indicated that, the intercrop recorded the highest water use efficiency followed by the maize sole crop and lastly the cowpea sole crops (Ofori et al., 2014).The soil water depletion can be minimized by the use of cover crops (Unger & Vigil, 1998).The effective of cover crops in maintaining or improving soil water storage depends on factors such as method of planting and stage of growth.The methods of planting such as rotation have been reported to improve soil water content (Roder et al., 1989).The influence of cropping systems in terms of maize/legume rotation, intercropping and monocropping on soil water content is not known to most farmers.The stages of crop growth which play a role on soil water conservation is also not identified by most farmers.Therefore, the objective of this study was to determine the effect of cropping system, crop growth stages and location on soil water content.

Site Description
The study was conducted at three dryland localities in South Africa; Department of Agriculture Experimental Station in Taung situated (27º32′0″S 24º47′8″E), Agriculture Research Council-Grain Crops Institute (ARC-GCI) experimental station in Potchefstroom (26º42′54″S 27º06′12″E) and Agriculture Research Council-Institute for Industrial Crops (ARC-IIC) experimental station in Rustenburg (25º40′51.4″S27º13′58.4″E)Taung experimental station is situated in grassland savannah with annual mean rainfall of 1061 mm that begins in October.The average clay content of this location is 8%.The soil at Taung is described as Hutton, deep, fine sandy dominated red freely drained, eutrophic with parent material that originated from Aeolian deposits (Soil Classification Working Group, 1991).The ARC-GCI experimental station soil has a clay content of about 30% and the location receives annual mean rainfall of 622.2 mm (Macvicar et al., 1977).Potchefstroom (ARC-GCI) has plinthic catena soil, eutrophic, red soil widespread (Pule-Meulenberg et al., 2010).The ARC-IIC experimental station soil has a clay content of approximately 49.5% and receives an annual rainfall of 661mm.The soil at Rustenburg (ARC-IIC) has dark, olive grey and clay soil, bristle consistency, medium granular structure (Botha et al., 1968).Baseline soil analysis performed before planting indicated amounts of 5, 8 and 6.5 kg of N ha -1 available at Potchefstroom, Rustenburg and Taung, respectively.The soil physical properties analysed from three locations before planting are indicated in Table 1.The monthly mean temperature and rainfall data for Potchefstroom, Taung and Rustenburg for the duration of experimental period is indicated in Table 2 as described by Sebetha et al. (2015).

Experimental Design and Treatments
The experimental design was a factorial experiment laid out in randomised complete block design (RCBD) with three replicates per location.The study was carried out at three locations of North-West province of South Africa, which were Potchefstroom, Taung and Rustenburg during 2011/12 and 2012/13 planting seasons.The experiment consisted of five cropping systems, which were cowpea monocrop, maize monocrop, cowpea followed by maize in rotation, maize followed by cowpea in rotation and intercropping maize-cowpea.These cropping systems are more popular and practised by small scale and commercial maize/legume farmers in Southern Africa.The three growth stages compared in this study were V10/Vn, VT/R4 and R6/R8 stages of maize/cowpea respectively as described in Table 3.These stages are the most critical period of crop development and the water demand by crops is high during those stages.The lack of soil water during those stages results in reduction of yield in both crops.

Agronomic/Field Practices
Based on previous studies performed on the selected locations, the optimum N rate to be applied on maize was 100 kg ha -1 at Potchefstroom and Rustenburg and 120 kg ha -1 at Taung.For cowpea, the optimum N rate to be applied was 25 kg ha -1 at Potchefstroom and Rustenburg, and 30 kg ha -1 at Taung.Based on the above information, two levels of N fertilization to the amount of 0 and 95; 0 and 92; 0 and 113.5 kg of N ha -1 were applied during planting on maize plots at Potchefstroom, Rustenburg and Taung, respectively.The amount of 0 and 20; 0 and 17; 0 and 23.5 kg of N ha -1 were applied during planting on cowpea plots at Potchefstroom, Rustenburg and Taung respectively.Maize (Zea mays L. cultivar PAN 6479) and cowpea (Vigna unguiculata L. cultivar Bechuana White) were used as test crops.

Soil Sampling and Analysis
The experiment commenced in November/December of 2010/11 planting season, and the data were collected during the 2011/12 and 2012/13 planting seasons.In this study, the soil water content up to 0.9 m deep was evaluated in five cropping systems at three different growth stages at different locations.The evaluation was performed at different growth stages of maize and cowpea.Soil samples were collected at the depth of 0-0.15, 0.15-0.3,0.3-0.6 and 0.6-0.90m during V10/Vn, VT/R4 and R6/R8 stages of maize/cowpea respectively.The samples were collected from each plot using soil auger.Soil samples were put inside plastic bags during collection at the field, sealed and kept at cold room to avoid water loss.The gravimetric water content (GWC) method was used to determine the soil water content (Black, 1965).Each porcelain tin was weighed and tarred before weighing of 10 grams of soil.Samples were oven dried at 105 °C for 24 hours.The samples were returned to the oven to dry for several hours, until there was no difference between any two consecutive measurements of the weight of dry soil + tare.GWC is calculated using the formula as: (1)

Statistical Analysis
Analysis of variance was performed using GenStat 15 th edition (2012).Least significant difference (LSD) was used to separate means.A probability level of less than 0.05 was considered to be statistically significant (K. A. Gomez & A. A. Gomez, 1984).

The Influence of Crop Growth Stages and Management Practices on Soil Water Content at the Depth of 0-0.15 m
Crop growth stages had significant effect (P < 0.001) on soil water content at the soil depth of 0-0.15 m as indicated in Table 4.The crop growth stages V10/Vn and R6/R8 of maize/cowpea had significantly (P < 0.05) higher water content of 10.2 and 9.1% respectively than soil collected during VT/R4 stage.Location also had significant effect (P < 0.001) on soil water content.Soil collected at Rustenburg and Potchefstroom had significantly (P < 0.05) higher water content of 13.5 and 10.2% respectively than soil collected at Taung.Season also showed significant effect (P < 0.001) on soil water content.The soil collected during 2012/13 planting season had significantly (P < 0.05) higher soil water content of 9.9% than soil collected during 2011/12 planting season.The interaction of crop growth stages × location x season (P < 0.001) had significant effect on soil water content.Note.Inter M/C = Intercropping of maize and cowpea; Monocowpea = Monocropping cowpea; Monomaize = Monocropping maize; Maize-cowpea Rot = Maize followed by cowpea in rotation; Cowpea-maize Rot = Cowpea followed by maize in rotation; Potch = Potchefstroom; Rust = Rustenburg.

The Influence of Crop Growth Stages and Management Practices on Soil Water Content at the Depth of 0.15-0.3 m
Crop growth stages had significant effect (P < 0.001) on soil water content at the depth of 0.15-0.3m as indicated in Table 5.The crop growth stages V10/Vn and R6/R8 of maize/cowpea had significantly (P < 0.05) higher soil water content of 10.8 and 10.7% than soil collected during VT/R4 stage.Location also had significant effect (P < 0.001) on soil water content.Soil collected at Rustenburg and Potchefstroom had significantly higher water content of 15.9 and 10.9% than soil collected at Taung.The interaction of crop growth stages × location × season (P < 0.01) had significant effect on soil water content.Note.Inter M/C = Intercropping of maize and cowpea; Monocowpea = Monocropping cowpea; Monomaize = Monocropping maize; Maize-cowpea Rot = Maize followed by cowpea in rotation; Cowpea-maize Rot = Cowpea followed by maize in rotation; Potch = Potchefstroom; Rust = Rustenburg.

The Influence of Crop Growth Stages and Management Practices on Soil Water Content at the Depth of 0.3-0.6 m
Cropping system had significant effect (P = 0.029) on soil water content at soil depth of 0.3-0.6 m as indicated in Table 6.Monocropping cowpea plots had significantly (P < 0.05) higher soil water content of 12.4% than other cropping systems.Crop growth stages had significant effect (P < 0.001) on soil water content.The crop growth stage V10/Vn of maize/cowpea had significantly (P < 0.05) higher soil water content of 12.5% than VT/R4 and R6/R8 stages.Location also had significant effect (P < 0.001) on soil water content.Soil collected at Rustenburg and Potchefstroom had significantly (P < 0.001) higher water content of 18.3 and 12.8% respectively than soil collected at Taung.Season also showed significant effect (P < 0.001) on soil water content.The soil collected during 2011/12 planting season had significantly (P < 0.05) higher water content of 12.3% than soil collected at 2012/13 planting season.The interaction of cropping system × location × season (P = 0.014) had significant effect on soil water content.Note.Inter M/C = Intercropping of maize and cowpea; Monocowpea = Monocropping cowpea; Monomaize = Monocropping maize; Maize-cowpea Rot = Maize followed by cowpea in rotation; Cowpea-maize Rot = Cowpea followed by maize in rotation; Potch = Potchefstroom; Rust = Rustenburg.

The Influence of Crop Growth Stages and Management Practices on
Soil Water Content at the Depth of 0.6-0.9m Crop growth stages had significant effect (P < 0.001) on soil water content at the depth of 0.6-0.9m as indicated in Table 7.The crop growth stage V10/Vn had significantly (P < 0.05) higher soil water content of 13.3% than VT/R4 and R6/R8 stages.Location also had significant effect (P < 0.001) on soil water content.Soil collected at Rustenburg and Potchefstroom had significantly (P < 0.05) higher water content of 18.4 and 14.5% respectively than soil collected at Taung.Season also showed significant effect (P < 0.001) on soil water content.The soil collected during 2011/12 planting season had significantly (P < 0.05) higher water content of 13.1% than soil collected during 2012/13 planting season.The interaction of crop growth stage × location × season (P = 0.034) had significant effect on soil water content.
Table 7.The influence of crop growth stages and management practices on soil water content in percentages at 0.6-0.9m soil depth Note.Inter M/C = Intercropping of maize and cowpea; Monocowpea = Monocropping cowpea; Monomaize = Monocropping maize; Maize-cowpea Rot = Maize followed by cowpea in rotation; Cowpea-maize Rot = Cowpea followed by maize in rotation; Potch = Potchefstroom; Rust = Rustenburg.

Correlation between Soil Water Content and Maize/Cowpea Grain Yield
There was no correlation between soil water content and maize grain yield during both planting seasons at physiological maturity as indicated in Figure 1a up to Figure 4b.There was also no correlation between soil water cont Figures 5a during 201

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The time physiologi findings by water cont V10/Vn gr rainfall du led to high (2004) wh water cont soil water crops was as compar water in hi

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The Influence of Cropping System on Soil Water Content at Different Soil Depths
The higher water content in soil collected on cowpea plots planted on monocropping system may have been attributed to lower evaporation from sole cowpea plots.This corroborated the findings by Kemp et al. (1997) who reported that evapotranspiration is strongly controlled by radiation and vapour pressure deficit and crop canopy relates to the planetary boundary layer, which was the case under sole cowpea plots in this study.Ghanbari et al. (2010) found that soil water content in the soil was reduced dramatically in the sole crop of maize due to high evapotranspiration potential, on contrary soil water content in the soil was increased dramatically in the sole crop of cowpea due to low evapotranspiration potential for growth period.This implied that, canopy cover of dense cowpea cultivar played a significant role in soil water retention due to decreased evaporation rate from soil surface.It was expected for intercropping to play a role in soil water content based on the previous studies.Adiku et al. (1998) found that intercropping has the benefits to use water from different soil layers by the companion crops and enhances overall water use efficiency.In this study, intercropping had no significant role on soil water content.It was assumed that, the cowpea cultivar (Bechuana white) which was indeterminate cultivar and covering large soil surface of the plots played a role in soil water content.

The Interaction of Crop Growth Stages × Location × Season on Soil Water Content at Different Soil Depths
The interaction effects of crop growth stages × location × season on soil water content had significant contribution on water conservation, since such interactions was under 0-0.15,0.15-0.30and 0.60-0.90m depths.This implied that, soil water availability depends on the rate of evaporation during stages of crop growth and this was affected mainly by season and the type of location due to different soil types and climatic factors.Badel et al. (2013) found that, the effect of soil water depletion, growth stages and their interaction effect on evapotranspiration at vegetative stage were highly significant.The growth stages which reduced evaporation because of canopy cover to the soil surface, the location with good soil physical properties with sufficient organic matter and season with good climatic factors were the main factors contributing significantly to high soil water retention.In this study, the different in soil water across the locations and seasons was due to the fact that, under high humidity and cooler temperature, atmospheric evaporative demand was low, and this resulted in more soil water content as indicated in Table 2.

The Correlation between Soil Water Content and Cowpea Grain Yield
The positive correlation between soil water content and cowpea grain yield during the physiological maturity period may have been attributed to occurrence of rainfall during that stage of April and May of 2012/13 planting season as indicated in Table 2.The increase in soil water with increase in soil depth at Potchefstroom and Rustenburg may have been attributed to decrease in evaporation in deeper layer.This corroborated the findings by Schwinning and Sala (2004) who reported that the rates of plant water uptake increase and evaporation and vapour diffusion rates decrease in deeper soil layers.The similar soil water content in different layers of soil samples collected at Taung may have been attributed to the uniformity of soil texture (sand) from upper layers to lower layers.

Conclusions
Crop growth stage V10/Vn had higher soil water content at all different depths as compared to other growth stages.Soil collected at Rustenburg and Potchefstroom had higher water content at all different depths as compared to Taung.Cropping system had the influence on soil water content at the depth of 0.3-0.6 m only.There was no consistency in terms of season on soil water content.
Monocropping plots of cowpea had the ability to hold soil water and this depends on the type of cowpea cultivar and canopy cover.It is then recommended that, legumes should be included in cropping systems for the purpose of soil water conservation.The critical stage for high soil water content was at ear/pod formation stage (VT/R4) and this was not expected since crops use high amount of soil water during that stage.In this study, it was found that locations with high percentage of clay content were able to hold soil water during different stages of sampling.Soil water content was found to be location dependent.The production of crops such as cereals and legumes in this study are recommended to be on the area with average clay percentage.This is due to the ability of clay soil to hold water that will be accessible to crops.

Table 1 .
Soil texture properties of the three experimental locations

Table 2 .
The monthly mean temperature and rainfall data for Potchefstroom, Taung and Rustenburg for the duration of experimental period

Table 3 .
Description of plant growth stages in maize and cowpea

Table 4 .
The influence of crop growth stages and management practices on soil water content in percentages at 0-0.15 m depth

Table 5 .
The influence of crop growth and management practices on soil water content in percentages at 0.15-0.3m soil depth

Table 6 .
The influence of crop growth stages and management practices on soil water content in percentages at 0.3-0.6 m soil depth