December 13, 2006
Effects of Composted Yard Waste On Water Movement in Sandy Soil
By Pandey, C; Shukla, S
A two-year study was conducted in 2002-2003 (season one) and 2003- 2004 (season two) at a seepage irrigated vegetable farm in south Florida to investigate the effects of soil organic amendment (composted yard waste) on movement of water in a sandy soil. Season one result showed that for the same water table depth, soil moisture content in the compost field was higher than the noncompost field in the root zone (top 20 cm). The increased soil moisture was attributed to the increased upflux due to increased capillary rise. Increased capillary rise was a result of increased organic matter content of the soil from compost application. After a rainfall event, soil moisture at 10 cm depth in the compost field responded rapidly, suggesting a higher extent of capillary fringe in the compost field compared to the noncompost field, which did not show the similar response. Another addition of compost further enhanced the soil moisture effect in season two. Season two results showed a higher difference between compost and noncompost soil moisture from the previous season. Results from the study showed that the addition of compost can help in maintaining the same level of soil moisture with a lower water table compare to the noncompost field. A lower water table in the compost field can result in higher retention of rainfall in the soil compared to the noncompost field, which in turn can reduce runoff, deep percolation, and seepage losses and achieve water conservation.
Agricultural production in the U.S. relies on inorganic fertilizers and continued supply of surface and ground waters for irrigation. For the long-term sustainability of agricultural production in the U.S., judicious use of water resources is needed. In several states including Florida, agricultural areas are shrinking at the expense of urban development. Increased urban activities have resulted in generating large amounts of waste which has created an ever increasing need for landfills for its disposal. In recent years, part of the waste from urban areas has been used in agricultural areas to provide soil fertility and other potential benefits such as improved water storage and retention. Increased water retention is especially beneficial for areas with sandy soils such as Florida. If managed properly, use of composted waste has the potential to act as a water conservation measure.
Florida ranks second in fresh vegetable production in the U.S. Vegetables are harvested on more than 88,000 ha (National Agricultural Statistics Service 2002) of mostly sandy soils. South Florida, a major vegetable growing area, can be characterized by sandy soils with low water-holding capacity. Sustaining agriculture on these soils requires frequent irrigation inputs. Rapid water movement in recently fertilized sandy soils can potentially result in loss of nutrients through leaching. Eventually, leached nutrients reach shallow groundwater, from where it can laterally move to ditches and canals, adversely impacting the water quality of downstream water bodies.
Water quality challenges are further compounded by future water shortage issues due to increasing rate of urban growth in the coastal areas of Florida. Finite water resources in the state are being stressed in meeting the water supply needs of urban expansion, especially in the coastal areas of south Florida. In the west coast region of south Florida, the water use demand is expected to increase by 28% by the year 2020 (SFWMD 2000). Continued urbanization is also generating wastes that include yard trimmings (YT), sewage sludge, and municipal solid waste (MSW). In recent years, composts produced from a wide range of waste materials (MSW, YT and biosolids (BS)) have become available in Florida on a large- scale (Smith 1995). Use of urban organic waste in agricultural production has the potential to improve rainfall retention in sandy soil, which in return can reduce the irrigation input for vegetable production.
Several studies have investigated potential advantages of using composts in agriculture (OzoresHampton et al. 1998; Li et al. 2000; and Ozores-Hampton and Deron 2002). Reported advantages of compost include enhanced soil physical, chemical, (Tester 1990; and McConnell et al., 1993) and microbial properties (Debosz et al. 2002; and Speir et al. 2004), increased crop yield (Smith 1995; Stoffella 1995; and Ozores-Hampton et al. 2000), reduced crop disease (Hoitink and Fahy 1986; and Hoitink et al. 1991), and weed control (Aparbal-Singh et al. 1985; FAO 1987; and Roe et al. 1993).
Several studies have attributed improved soil physical properties such as decreased bulk density of sandy sou (Gupta et al. 1977; Khaleel et al. 1981; Tester 1990; Turner et al. 1994; and Mamo et al. 2000), decreased infiltration rate (Stamatiadis et al. 1999), and increased water content and soil water retention (Tester 1990; and Mamo et al. 2000) to the increased organic matter due to organic amendment. Most of these effects of compost such as increased water content and soil water retention were observed in lab conditions and have not yet been tested under field conditions. Furthermore, these effects have not been reported for Florida's sandy soil. Although it has been postulated that amending Florida's sandy soils with compost may reduce the frequency and rate of irrigation (Ozores-Hampton et al. 1998; and Li et al. 2000), these inferences have not been substantiated by an actual field study.
Seepage irrigation is a prominent irrigation method in south Florida for vegetable production. seepage irrigation involves maintaining a high water table under raised bed with plastic mulch to provide soil moisture in the root zone through upflux and capillary rise. Upflux involves upward movement of water from a shallow water table within a soil profile. Upflux is caused by negative pressure developed above the water table within a soil profile. Capillary rise is caused by the capillary pressure (difference between the nonwetting fluid pressure and wetting fluid pressure). Capillary pressure depends on the interfacial tension and contact angle of the soil particle and pore size of the soil (Mercer and Waddell 1993).
Traditionally, vegetable fields have been irrigated to keep a high water table at around 45 cm depth (Stanley and Clark 1991). Upflux and capillary rise provides soil moisture for crop growth. The extent of the capillary rise above the groundwater surface varies with the soil physical characteristics (e.g., pore size) (Gillham 1984). If application of organic amendments to Florida's sandy soils can increase the extent of the capillary rise and increase the upward flux, it can sustain vegetable production with a lower water table compared with a non-amended soil. Furthermore, use of organic amendments may increase rainfall retention in the soil. Maintaining a low water table can result in water savings due to reduced percolation, runoff, and lateral losses due to seepage of groundwater to ditches and canals. Use of a low water table can reduce groundwater concentration of nutrients by increasing the travel time from soil to the groundwater.
The objective of this study was to quantify the effects of composted yard waste use on the water movement and retention in a south Florida sandy soil. The water movement and retention were investigated for the vegetable production systems with raised bed and plastic mulch.
Materials and Methods
The study was conducted at a commercial vegetable farm located in Hendry County in south Florida for the periods of 08/15/02 - 05/05/ 03 (season one) and 08/15/03 - 04/26/04 (season two). Two fields of 30.5 m x 274 m were selected for the study (Figure 1). Both fields had similar topographical characteristics. These fields had a subsurface irrigation and drainage (SID) system for maintaining a uniform water table. The SID system used in this study consisted of drain tiles installed at regular intervals of 25 m at a depth of approximately 0.70 m from the soil surface. It facilitated better control of the water table in the field compared with the traditional open ditches seepage systems.
One field received YT compost at 100 Mg-ha^sup -1^ at the beginning of each season in August and is termed from this point forward as compost (CO) field. The physical and chemical properties of the compost are provided in Table 1. Standard testing procedure (TMECC 2000) recommended by the US Composting Council Research and Education Foundation (CCREF) was used for data collection and measurement of the physical and chemical properties of the compost. The maturity of compost was measured using Solvita maturity test (TMECC 2000). The compost was uniformly broadcasted using a manure spreader. A tractor-mounted 41 cm disc was used to incorporate the applied compost. The other field, termed as noncompost (NC), did not receive any compost application.
FIGURE 1. Experimental field layout (not to scale).
Physical and chemical properties of the composted yard waste
The plastic mulch beds (height = 0.22 m width = 0.81 m) were made with tractor-driven equipment. The beds were 1.80 m apart (center to center distance). Field slopes, measured from center of the field along the field length, were maintained at 0.03% for both the north and the south sides of the field. Pepper was planted at the beginning of each season in both the fields. The growing s\eason for both years was approximately 8 months.
Inorganic fertilizer application rates (N-P-K) for the two fields were the same. Due to the high soil P, the inorganic P fertilizer was not applied in the two fields. Fertilizer was applied in third week of August at 356 (N), O (P^sub 2^O^sub 5^) and 400 (K) kg- ha^sup -1^. Approximately 15 and 25 % of the N and K respectively were incorporated into the bed and the remainder was applied in two shallow grooves at the top of the beds.
Description of the hydrologic monitoring system
The soil moisture in the CO and the NC fields was monitored at a frequency of 10-min with four fixed type capacitance probes (EnviroScan, Sentek PTY Ltd., Australia) installed at four different locations in the middle of the four sections of the two fields, termed CO-1, CO-2, NC-1, and NC-2 (Figure 1). The probes were connected to a datalogger. Each probe had four sensors located at 10, 20, 30 and 40 cm depths from the top of the plastic mulch bed to measure the soil moisture at 10-min intervals. To provide backup data and capture the spatial variability in soil moisture within the fields, a portable capacitance-based instrument (Diviner, Sentek PTY Ltd., Australia) was also used to manually take the data. Four access tubes, one at each locations (CO-1, CO-2, NC-1, and NC-2), were installed for the manual soil moisture measurements at 10, 20, 30 and 40 cm depths (Table 2).
At each of the fixed type capacitance probe locations, pressure transducers (Levelogger, Solinst Canada Ltd., Canada) were installed in a monitoring well (well depth = 104 cm, screen length = 70 cm from the bottom) to record the water table depth at 10-min interval (Figure 1). Due to differences in irrigation inputs and drainage management, the water table on the north side of the two fields (CO- 1 and NC-1) was lower than the south side (CO-2 and NC-2). To monitor the rainfall, a weather station was installed near the experimental field (Table 2).
To evaluate the changes in the organic matter as a result of compost application, three soil samples using soil core sampler from each field were collected at two different depths (0-10 cm and 10- 20 cm) from top of the beds. The loss on ignition (LOI) method (Dellavalle 1992) was used to estimate the organic matter (OM) content in the soil. The effect of compost application on water movement was evaluated by analyzing the soil moisture and water table depths data for the CO and the NC fields (CO-1 vs. NC-1 and CO- 2 vs. NC-2). The two sample t-test was used to test the statistical differences in OM content and soil moisture between the two fields.
Mean organic matter (%) and p-values for the compost and noncompost fields
Results and Discussion
The application of compost increased the OM content in the soil. However, the differences in OM content in the CO and the NC fields were not significant in season one (Table 3). For season two, the CO field had statistically higher OM content than the NC field indicating that the additional application of compost during the second year resulted in significant increase in the OM.
Despite the fact that there was no statistical difference in OM content between the NC and the CO soils, it seems that around 20% higher OM in top 10 cm depth in the CO soil affected the soil moisture content in the CO field. The soil moisture data taken at 10 (Figure 2a) and 20 cm (Figure 2b) showed consistently higher soil moisture in the CO-1 compared the NC-1 during most of the season. Similar effect of compost application on soil moisture was also observed for the CO-2 and the NC-2 locations (data not shown).
FIGURE 2. Water table depth and soil moisture during season one at 10 cm (a) and 20 cm (b), and during season two at 10 cm (c) and 20 cm (d) depth from the top of the bed at the compost (CO-1) and noncompost (NC-1) locations.
It can be argued that the introduction of finer particles through compost addition increased the soil water movement to the upper soil profile through higher upflux. Since water input from the top of the bed through rainfall can cause differences in soil moisture, the rain-free periods (Figure 3) were examined to confirm the effect of YT compost on upflux. A period of near static water table of 40 cm from top of the bed, with no rainfall, was identified from 02/01/03 to 02/07/03 (Figures 2a and 2b) to examine the effect of compost on capillary rise. Under static water table condition, it can be assumed that the soil moisture in the root zone was mostly affected by the capillarity and depth to the water table.
FIGURE 3. Daily rainfall during the seasons one (a) and two (b).
Average moisture values (from the 10-min interval data) for the 10, 20, 30, and 40 cm depths were computed for the period of constant water table (Figure 4 a). Similar or higher soil moisture at 30 and 40 cm depths suggest that the capillary fringe (tension saturation zone) above the water table was extended to 30 cm depth in both the fields. The decrease in the soil moisture from 30 cm to 20 cm was much larger in the NC field compared to the CO field (Figure 4a). Gradual decrease in soil moisture for the CO field suggests the presence of finer capillary in the soil compared to the NC field which caused higher rise of water in the capillary for the former. To confirm the effect of compost on capillary rise, the bi- weekly soil moisture data collected using a portable capacitance probe at two separate locations in the CO and the NC fields on 02/ 01/03 (Figure 4c) were also examined. Portable probe soil moisture data (Figure 4c) clearly show the same capillary fringe effect that was observed for the fixed probe locations for 30 cm depth (Figure 4a) under the static water table of 40 cm.
FIGURE 4. Soil moisture measured by the fixed (a and b) and portable (c and d) probes at 10,20,30, and 40 cm depth during the constant water table periods for the compost (CO-1) and noncompost (NC-1) locations. The data shown for the fixed probe are the soil moisture values computed from 10-min data during the period of constant water table for season one (a; 02/01/03 - 02/07/03) and season two (b; 11/22/03 -12/02/03). The data from portable probe during the constant water table period for season one (measured on 02/01/03) and season two (measured on 12/01/03) are shown in c and d, respectively.
Higher soil moisture in the CO field than in the NC field was due to higher capillary rise. If the capillary rise on the CO field was higher, it can be argued that the capillary fringe on the CO will also be higher and will result in much higher soil moisture after a rainfall event. The soil moisture, water table, and rainfall data were examined further to validate the above-mentioned effect of compost on capillary rise.
FIGURE 5. Soil moisture at 10 cm (a) and 20 cm (b), water table depth (from the top of the bed), and rainfall (10 minute) data for the compost (CO-1) and the noncompost (NC-1) locations in season one.
A 0.74 cm rainfall occurred on 02/17/03 (Figure 3). The water table depth and the soil moisture responses to this rainfall are shown in Figures 5a and 5b for 10 and 20 cm depths, respectively. An immediate and large increase in the 10 cm soil moisture was observed in the CO-1 following the rainfall, suggesting that the capillary fringe for the CO field was closer to the surface than for the NC field (Figure 5 a). For the NC-1, this effect was not observed, and the volumetric water content after the rain event never increased to more than 10% (Figure 5 a). The soil moisture at 20 cm was more responsive to rainfall in the NC-1 (Figure 5b) compared to the 10- cm, indicating rapid movement of water in the NC field. Higher extent of capillary rise and fringe for the CO field compared to the NC field has important implications for water as well as nutrient movement. Under the same water table depth, addition of compost seems to pull more water from the water table compared to the noncompost conditions indicating that the use of compost may facilitate lowering the water table and yet provide the same soil moisture as in the noncompost fields.
The above mentioned differences in the soil moisture for the CO and the NC fields can be attributed to differences in OM content as shown in Table 3. The OM percentage for the CO field was higher than for the NC field. Increased soil moisture due to compost addition has also been shown by Bauer and Black (1992), who showed that a unit increase in organic carbon concentration in soil can cause a relatively larger increase in soil moisture in sandy soil. The effect of increased OM on soil moisture is further evident by examining the soil moisture data for the second season (season two).
Average water table depth and soil moisture in the compost and noncompost fields for seasons one and two
For season two, another addition of compost prior to bed preparation in September 2003 raised the OM in the CO field from 2.26% in the season one to 3.16% in the season two. The actual difference in the OM % between the CO and the NC fields was 0.45 in season one, while it increased to 1.16 in the season two. More than 250% increase in difference in % OM between the CO and the NC fields during the second season resulted in a greater soil moisture difference at all depths between the CO and the NC fields (Figures 2c and 2d). Consider for example, the time series soil moisture data at 10 cm at the CO-1 and the NC-1 for the season two (Figure 2c) which shows the increased soil moisture effect. It can be seen from Figure 2c that while the soil moisture in the NC-1 for the season two was almost the same as observed in the season one, it was much higher at the CO-1 (Figures 2a and 2c). This result can be further confirmed by examining the average soil moisture and water table data for the seasons one and two (Table 4). The average soil moisture at 10 cm in the NC-1were same for both the seasons (season one = 9.16 vs. season two = 9.17) (Table 4), but soil moisture in the CO field in season two was 42% higher than season one (season one = 12.14 vs. season two = 17.21). A similar response of soil moisture can also be seen for the 20 cm depth (Figure 2d).
Similar to the season one, the average soil moisture for the period 11/22/03 - 12/02/03 of constant water table depth of 40 cm with no rainfall was examined. Compared to the previous season (Figure 4 a), it can be seen that the differences in soil moisture at the 10, 20, 30, and 40 cm depths between the CO and the NC for the constant water table period increased by threefold for the second season (Figures 4a and 4b). Such large differences in soil moisture during the second season indicate that repeated application of compost enhanced the soil moisture effect by increasing the extent of finer capillaries present in the soil in the CO field. Portable capacitance probe data taken on 12/01/03 (Figure 4d) for the CO-1 and the NC-1 and on 04/23/04 (data not shown) for the CO-2 and the NC-2 locations under the constant water table of 40 cm depth and no rainfall within the 7 to 10 days, further confirmed the enhanced soil moisture effects of compost.
The most important finding from the portable capacitance probe data was that the soil moisture gradient in the CO field (Figure 4d) did not decrease from 30 to 20 cm depths. This was due to the fact that the capillary fringe for the CO extended up to the 20 cm depth resulting in the near saturation soil moisture up to the 20 cm depth. If the capillary fringe is extended by 10 cm due to compost addition, it can help in lowering the water table by 10 cm and yet maintain the same soil moisture in the root zone as in the noncompost field. In a separate study conducted on the same field and same seasons, Pandey and Shukla (2006) showed that lowering the average water table by 13 cm can result in saving 36 % of irrigation water. The study also showed that lowering the water table decreased the number of runoff events by 50 % and reduced the nutrient concentration in groundwater.
Compost application has the potential to conserve water by reducing water use. In addition, lowering the water table can help in reducing nonpoint source pollution to the downstream water bodies by reducing runoff and offsite nutrient discharges. Compost application can help in increasing rainfall retention by reducing the water table depth and improving the soil physical property to hold more water in the root zone.
No yield differences between the two fields were reported (Pandey and Shukla 2006) and therefore, the differences in upflux were mainly caused by the finer capillaries. Overall, results from this study showed that application of compost resulted in: 1) higher soil moisture in the root zone compared with the noncompost field for the same water table depths; 2) compost application resulted in more retention of rainfall in the bed compared with the noncompost field; and 3) compost application has the potential to help lower the water table and yet maintain the same soil moisture as in the noncompost field.
Summary and Conclusions
The effect of compost application on movement of water was investigated in this study for two vegetable growing periods: 08/15/ 02 - 05/05/03, and 08/15/03 - 04/26/04. seepage irrigation was used to supply water to the plants. The amended field received compost application since 2002. Soil moisture and the water table response to compost addition were examined by monitoring the two parameters at several locations. Results from this study indicate that soil moisture in the compost field was consistently higher than in the noncompost field. Repeated application of compost resulting in considerable increase in the soil moisture indicated that increase in organic matter content of soil increases the capillary rise and the water retention capacity of the soil.
Addition of compost to soil not only increased the soil moisture in the root zone but also increased the extent of capillary fringe, which in turn can help in lowering the water table depth. Compost application retained the water input from rainfall in the bed for a longer period of time compared with the noncompost field. Use of compost has the potential for lowering the water table and yet maintain the same moisture that would be present for shallower water table under the noncompost conditions. Lowering water table will result in reducing irrigation input and nutrient transport to the groundwater.
The authors express their gratitude to Mr. Chuck Obern, owner of the C and B farms, who generously offered his vegetable farm to be used for this research and for his willingness to help financially and physically. Funds for this study were also provided by Florida Fruit and Vegetable Association (FFVA) and Southwest Florida Vegetable Research Fund. Authors wish to thank Mr. Eugene McAvoy, UF- IFAS for his help.
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C. Pandey1 and S. Shukla2
1. Former Graduate Research Assistant and Engineer
2. Assistant Professor, Agricultural and Biological Engineering Department, Southwest Florida Research and Education Center, University of Florida, Immokalee, Florida
Copyright J.G. Press Inc. Autumn 2006
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