Introduction

According to the USDA National Agricultural Statistics Service (USDA-NASS, 2016), ten states grow 99% of the peanuts in the United States of America with Georgia and Alabama combining to harvest 60% of that production. In the Southeast, soils are highly weathered and are prone to drought conditions (Faircloth et al., 2012). With drought prone soil, irrigation is an important part of peanut production from the setting of pods to final yield not just in the US, but across the globe (Faircloth et al., 2012; Tojo Soler et al., 2013; El-Habbasha et al., 2015; Rowland et al., 2007; Suleimam et al., 2013; Stansell and Pallas, 1985; Pahalwan and Tripathi, 1984). Irrigation in Georgia has been the focus of agricultural water use in the southern regions of the state and is presented in the form of management practices listed in the Regional Water Plans approved in 2011 for the Lower Flint-Ochlockonee Region, Upper Flint Region, Altamaha Region, and Suwannee-Satilla Region (Anonymous, 2011). Some of the management practices referenced in the Regional Water Plans deals with increased efficiencies of irrigation systems as well as the use of conservation tillage to name a few.

In the southeast, the cropping rotation will include peanuts and cotton. Within the rotation, there are two methods of managing soil, conventional and conservation tillage. Conventional tillage (CT) methods may use a cover crop but prior to planting the commercial crop, the cover crop will be tilled into the soil. Conservation production systems (CPS), also known as conservation tillage, will plant the commercial crop into the residue remaining on the soil surface, with at least 30% soil coverage. Both methods are used by farmers with CPS use in Georgia at approximately 30% (Tubbs and Gallaher, 2005; Rowland et al., 2007). However, Reeves et al. (2005) states the Conservation Technology Information Center estimated that less than 11% of the peanut acres in Georgia were planted in a CPS. There have been numerous studies conducted comparing CT systems to CPS. Faircloth et al. (2012) states that five studies give CPS a clear advantage as it relates to yield, improved quality or net economic returns, seven studies favored CT systems and seven studies showed no difference in the two systems. Faircloth et al. (2012) further states that some authors of research on CPS supports the practice for peanuts, others give it no clear advantage over CT, and some oppose the practice for peanuts. Based on the weather conditions during the growing season, peanut yields can even be comparable for the two methods of CPS and CT (Tubbs and Gallaher, 2005; Bosch et al., 2005; Faircloth et al., 2012).

The use of a CPS can have environmental benefits, including reduced water runoff, increased water infiltration, and building of soil organic matter when including a winter cover crop such as rye ( Secale cereale L.) (Jemia et al., 2013; Bosch et al., 2005; Balkcom et al., 2003; Arriaga and Balkcom, 2005). Studies have been conducted where peanuts are grown under reduced irrigation amounts in both CT and CPS (Stansell and Pallas, 1985; Pahalwan and Tripathi, 1984; Faircloth et al., 2012; Bosch et al., 2005; El-Habbasha et al., 2015; Bosch et al., 2012; Rowland et al., 2007). The results from this research indicates as water availability decreases that yield also decreases. However, some of these studies and others (Ohu et al., 2009; Reeves et al., 2005) also indicate that by increasing the organic matter in the soil, water is made more available to the plants in that specific study.

With increased focus on water resources and water management in Georgia as well as the Southeast, it is important to understand how soil moisture and moisture loss occurs in both tillage practices. Previous studies have reduced water applications but the final measured parameter was peanut yield. This leads to a knowledge gap on how quickly applied water dissipates through the soil profile. Retention of soil moisture and the rate of water movement in the soil is governed by the pressure head of the water, hydraulic conductivity, soil particle size and other physical factors of the soil (Lal and Shukla, 2004; Miyazaki, 2006). From a soil physical property aspect, water potential is the main driver of water movement in a soil profile as explained by basic physical soil properties and laws. Soil water movement at a theoretical level will follow the water retention curve for a given soil type. These curves can be produced in the laboratory under controlled environmental and physical conditions. Groenevelt and Grant (2004), van Genuchten (1980), Grant and Groenevelt (2015) and others have conducted research to demonstrate how soil water moves within a column. In production fields, soil moisture retention can be related to the amount of soil organic matter. As soil organic matter increases, the potential to hold or retain water in the soil increases (Sullivan et al., 2007; Ohu et al., 2009; Bosch et al., 2012; Jemai et al. 2013; Kahlon et al., 2013; Strickland et al., 2015; Balkcom et al., 2003). This increased water holding capacity of the soil, or increased water retention, can have major benefits to producers in times of drought, or short duration rainfall events (Sullivan et al., 2007). With increased water holding capacity of the soil based on soil management, this study was designed to measure the water loss rate in a soil profile where CPS and CT were used in peanut production.

Materials and Methods

Results and Discussion

Water retention in the soil profile is important in many different aspects from meeting plant needs to scheduling irrigation, if available. The data collected from the experiment sites was used to determine a rate of soil moisture loss in the CPS and CT systems. The rate of water loss was calculated and is presented in terms of kPa loss per hr (kPa/hr).

There were differences in soil moisture loss rate at different depths under different moisture conditions (Table 1). The highest moisture loss was the dry soil at 10 cm in the CT system, and the slowest moisture loss was the CT system at 30 cm. There was no significant difference in the moisture loss rate between the CPS dry and the CT wet soils. The rate of soil moisture loss for each location and soil moisture condition averaged over depth is presented in Figure 1. Further, the rate of soil moisture loss is greatest in the CT field when the soil moisture condition was dry. Interestingly, the lowest soil moisture loss occurred in the CT field when the soil was saturated. Overall the CT field loses water the least when the profile is wet, but loses moisture the greatest when the soil profile is drying out. The CPS field, when wet, has a higher soil moisture loss rate when compared to the CT wet soils, but not significant. There is however, a significant difference in the loss rates in the CT soils under different soil moisture levels. As described, one of the benefits of CPS is the increased infiltration rates associated with soil organic matter and greater open pore spaces. Therefore, when the soil profile is wet, as would be the case directly after a saturating wetting event, the water infiltrates rapidly and leads to a higher soil moisture loss rate in the wet CPS soils as compared to the wet CT soils. When the fields are dry, the rate of soil moisture loss in the CPS field is doubled as compared to a wet CPS soil. When compared to that of the CT field, the rate of loss in the CPS field is two and a half times less under dry conditions. Based on known benefits of increased soil organic matter and as referenced by different authors (Jemai et al., 2013; Kahlon et al., 2013; Strickland et al., 2015), it is expected that in this experiment, water infiltrates faster in the CPS field when wet and has more available water as the soil dries. With more available water, the CPS soil holds water longer than that of the CT soils. The increased water holding capacity increases the time between needed water application from either rainfall or irrigation (Figure 2). When the soil profile is wet, the need for a wetting event is approximately 8 or 14 d for a CPS or CT field, respectively. When the soil is dry, the d between water application for the CT and CSP fields are 1.5 and 3.9 d respectively.

Summary and Conclusions

Soil moisture data was collected from two fields operated under either a CPS with residue management or a CT practice. The overall objective was to determine if there was a difference in the soil moisture loss rate down to 30 cm. The data indicated that when the soil was wet, as measured below 50 kPa, the CPS soil loss soil moisture at a faster numerical rate than that of the CT soils, but there was no significant difference. However, as the soils dried, measured above 80 kPa, the CT soil had significantly faster soil moisture loss rates than the CPS soils. The CT soils loss soil moisture approximately 2.5 times faster than that of the CPS soils. These loss rates indicates that the soil needs to receive some form of precipitation every 1.5 d for the CT soils and 3.9 d for the CPS soils to maintain soil moisture conditions for peanut growth. The study, provides further information that agrees with others that increasing soil organic matter through the use of CPS increases infiltration and holds water longer in the soil profile. This retention of soil moisture allows the farmer to extend the time between irrigation applications of water or provides some storage of water within the soil profile, thereby reducing the risk if farming without irrigation.

The authors would like to thank the Southern Peanut Research Initiative for funding this research under agreement #25SPRI-NPB2013. They would also like to thank Mr. Gary Murphy for his assistance with installation and data collection as well as the farmers that allowed us to use their fields for this research.

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