Compost Impacts on Sodicity and Salinity In a Sandy Loam Turf Grass Soil

By Wright, Alan L Provin, Tony L; Hons, Frank M; Zuberer, David A; White, Richard H

Compost application to turf grass may influence soil sodicity and salinity and eventually the establishment and growth of turf grass. The objectives of this study were to determine effects of compost source and application rate on soil sodicity and salinity during 29 months after a one-time application to Saint Augustine grass and Bermuda grass turf grown on a sandy loam soil. Extractable soil Na, electrical conductivity (EC), and pH did not differ among compost sources having variable Na and nutrient levels. However, compost application decreased soil Na, EC, and pH compared to unamended soil likely due to high applications of Ca, Mg, and K, which occupied cation-exchange sites on soil particles, minimizing adsorption of Na and enhancing Na leaching losses during precipitation events. Furthermore, high application of composts increased soil dissolved organic C (DOC) levels, which may have coated soil particles and limited adsorption of Na. Complexation of extractable cations with DOC, followed by potential leaching of DOC-associated cations, tended to decrease soil EC. Thus, composts may actually serve to alleviate soil sodicity and salinity problems. Seasonal variation of extractable soil Na and EC were related to growth stages of turf grass, which influenced DOC levels, and precipitation patterns, which influenced vertical movement of DOC-associated cations. Introduction

Composted organic materials are commonly applied to agricultural lands. Much research has been conducted on the fate of nutrients in compost-amended agricultural soils, but limited information is available for turf grass (Wright et al. 2007a). Application of composts and organic amendments alter soil organic matter and nutrient cycling (Chang et al. 1991; Eghball 2002), and increase soil nutrient levels (Gregorich et al. 1998; Chantigny et al. 2002). Immediate increases are often attributed to the presence of soluble material in composts (Chantigny et al. 2002). Application of organic amendments increases soil DOC which adsorbs to soil particles and reduces the ability of soils to retain nutrient cations, ultimately increasing their leaching potential (Qualls and Haines 1992; McCracken et al. 2002; Wright et al. 2007b).

Seasonal variation of soil sodicity and salinity may be related to plant growth stages and decomposition of soil organic matter and compost, as well as precipitation and irrigation patterns (Mercuri et al. 2005). Accumulation of Na and salts in soils may cause undesired effects, such as inhibition and reduction of plant growth by toxicity or reduced water absorption (Stevenson and Cole 1999; Adriano and Weber 2001). However, Na and soluble salts readily leach from surface soil (Jakobsen 1996; Mercuri et al. 2005), except when organic amendments are applied at high rates in low-rainfall areas (Wahid et al. 1998; Stevenson and Cole 1999)

Nutrient and salt content of composts are often moderate but vary among sources due to differences in waste materials and processing methods (Smith et al. 1998). Therefore, assessment of the fate of nutrients and salts from different composts is important, as application at rates above plant requirements or soil assimilative capacity may pose leaching or runoff hazards (Gross et al. 1990; Vietor et al. 2002; Easton and Petrovic 2004). Thus, information on the effects of compost on long-term changes in sodicity and salinity in turf grass systems is needed. The purpose of this study was to determine the influence of compost source and rate on changes in extractable soil Na and EC under Saint Augustine grass and Bermuda grass turf.

Materials and Methods

Site Description

A field study was established in July 2001 in Central Texas on a Boonville fine sandy loam (fine, smectitic, thermic Chromic Vertic Albaqualfs) formerly under pasture. Average annual rainfall and temperature at this location is 980 mm and 20[degrees]C. Total monthly precipitation and growth stages of turf grass for this location are provided in Figure 1.

FIGURE 1. Monthly precipitation for 29 months after compost application in July 2001. General turf grass growth and dormant periods are noted. Soil sampling occurred at 0, 3, 8, 11, 16, 23, and 29 months after compost application.

A completely randomized experiment with three compost sources and an unamended control, two application rates, and two turf grass types was established on field plots (20 m each) replicated four times. Soil was chiseled to a depth of 350 mm and tilled to 150 mm prior to compost application. Composts were applied by thorough mixing into the top 150 mm of soil in July 2001. Unamended and compost-amended plots were then plugged with Saint Augustine grass [Stenotaphrum secundatum (Walt.) Kuntze] or seeded with common Bermuda grass [Cynodon dactylon (L.) Pers.]. Compost sources were DilloDirt (City of Austin, Texas), Bryan compost (City of Bryan, Texas), and Nature’s Way compost (Nature’s Way Resources, Conroe, Texas) applied at 80 and 160 Mg ha^sup -1^, which were equivalent to 25 and 50 mm depths. DilloDirt and Bryan Compost are cocomposted landscape wastes and municipal biosolids, while Nature’s Way is composted landscape wastes and manure. Turf grasses received 72 kg N ha^sup -1^ in 2001 and 49 kg N ha^sup -1^ in 2002 as NH^sub 4^NO^sub 3^, but no N fertilizer was applied in 2003. Fertilizers were applied only when soil tests indicated N deficiencies. Turf grasses were mowed to maintain a canopy height of 35 mm, with clippings returned to plots. To minimize moisture stress, supplemental irrigation was provided at 12 mm d^sup -1^ for 60 d after plant establishment followed by 12 mm every 3 d until November 2001. Thereafter, 6 mm of water was applied every 3 d during the growing season upon onset of symptoms of drought stress.

TABLE 1.

Soil and compost properties and extractable nutrient concentrations of soils prior to compost application.

Soil Sampling and Analysis

For each soil sampling event, 15 soil cores (5 cm diam.) were taken from each plot to a depth of 150 mm and composited. Samples were taken in July 2001 before compost application, and in Oct. 2001, March, June, and Nov. 2002, and June and Dec. 2003, corresponding to 0, 3, 8, 11, 16, 23, and 29 months after compost application. Soil samples were dried at 65[infinity]C and passed through a 2-mm sieve before analysis.

Soil pH was measured using 2:1 water:soil (Schofield and Taylor 1955), as was EC (Rhoades 1982). Soil organic C and total N were measured by dry combustion using an Elementar VarioMax CN analyzer (Elementar Americas, Inc., Mt. Laurel, NJ). Nitrate was extracted with 1 M KCl and analyzed by cadmium-reduction (Dorich and Nelson 1984). Soil Na, P, K, Ca, Mg, and S were extracted with acidified NH^sub 4^OAc-EDTA (Hons et al. 1990), while soil Mn, Fe, Cu, and Zn were extracted with DTPA (Lindsay and Norvell 1978) and analyzed by ICP (Spectro Analytical Instruments, Marlborough, MA). Soil DOC was extracted with water and analyzed by persulfate oxidation (Wright et al. 2005). Chemical characterization of composts was determined using the same procedures as soil. Initial soil properties under Saint Augustine grass and Bermuda grass and for compost sources are presented in Table 1.

Data were analyzed using CoStat (CoStat Statistical Software 2003). An ANOVA model was used to determine the main effects of compost source, rate, sampling time, and turf grass type. A one-way ANOVA model was used for determination of differences among treatments at each sampling time after compost application. Separation of means was accomplished using LSD, and Pearson’s correlation coefficients were calculated at P < 0.05.

Results and Discussion

Extractable Na

The three compost sources exhibited variable nutrient, organic C, and extractable soil Na levels (Table 1). Compost application failed to increase extractable Na above levels of unamended soil (Figure 2). In fact, composts applied at 80 and 160 Mg ha^sup -1^ significantly decreased extractable Na (401 mg kg^sup -1^) to levels lower than observed for unamended sou (424 mg kg^sup -1^). Unamended soil had higher extractable Na than compost-amended soil starting at 16 months for all treatment combinations and extending to 29 months for all but two Bermuda grass treatments. Extractable soil Na generally increased during the 29-month period for both compost- amended and unamended soils. An exception was a decrease in extractable Na at 16 months for Saint Augustine grass. High rainfall levels preceding the 16-month sampling (Figure 1) event may explain this decrease, which probably leached Na from the surface soil. In fact, total monthly precipitation preceding sampling events was significantly negatively correlated with Na (r=-0.37) (Table 2).

FIGURE 2. Extractable soil Na under Bermuda grass and Saint Augustine grass turf up to 29 months after application of three compost sources at 0, 80, and 160 Mg ha^sup -1^.

TABLE 2.

Statistically significant correlation coefficients (P < 0.05, n = 98) among soil properties and extractable nutrients under compost- amended Saint Augustine grass and Bermuda grass turf.

Irrigation water applied during the growing season contained approximately 270 mg L^sup -1^ of Na. Supply of Na to soil by irrigation explains the increasing trend in extractable Na up to 29 months for both unamended and compost-amended soil. Extractable Na levels in soil represent Na contribution from composts, residual Na in soil, Na from irrigation, and Na in leachate, but determination of differences among treatments can still be made since all plots received the same irrigation rates. Averaged across rates, extractable soil Na did not differ between turf grass or compost sources. Extractable Na was significantly positively correlated with most soil nutrients, in addition to soil organic C (r=0.51) and DOC (r=0.74), but was negatively correlated with NO^sub 3^, Fe, and Zn (Table 2). Soil Electrical Conductivity

Soil EC exhibited different seasonal trends than extractable soil Na during the 29 months after compost application (Figure 3). Soil EC significantly increased for 8 months after application, then decreased until 16 months at winter dormancy, then increased from 16 to 23 months. These trends occurred for all treatment combinations. Soil EC under Bermuda grass significantly increased from 23 to 29 months but decreased under Saint Augustine grass during this time frame. For Bermuda grass, EC was generally higher for unamended than compost-amended soils, especially soon after compost application. In contrast to our study where EC increased after compost application, EC in soils amended with compost decreased by 24 months due to the leaching of salts during precipitation and irrigation (Mercuri et al. 2005). However, in our study, application of soluble salts by irrigation was the probable cause of EC increases from 0 to 29 months. Likewise, increases in soil EC after compost addition were attributed to saline irrigation water (Johnson et al. 2006).

FIGURE 3. Soil electrical conductivity under Bermuda grass and Saint Augustine grass turf up to 29 months after application of three compost sources at 0,80, and 160 Mg ha^sup -1^.

Averaged across treatments, unamended soil had higher soil EC (0.32 dS m^sup -1^) than soils receiving 80 and 160 Mg ha^sup -1^ compost (0.29 dS m^sup -1^). For Saint Augustine grass, no effects of compost on soil EC were observed. Increases in soil EC in unamended soil were likely due to addition of salts by irrigation but may also be indicative of seasonal variation due to precipitation patterns and turf grass growth stages. Extractable Na was significantly correlated with EC under Saint Augustine grass (Figure 4) but not Bermuda grass. Similar to extractable Na, EC was positively correlated to soil organic C (r=0.58) and DOC (r=0.56) in addition to soil nutrients (Table 2).

FIGURE 4. Relationship between extractable Na and electrical conductivity in soils under Saint Augustine grass turf.

Soil pH

No differences in soil pH were observed between turf grass and compost sources when averaged across treatments (Figure 5). Soil pH significantly increased after compost addition for the unamended control and all compost sources and application rates in the first 3 months. Beyond 3 months, no further increases in soil pH occurred for turf grass soils receiving composts. However, the pH of unamended soil was significantly higher at 23 and 29 than at 3 months. Compost-amended soil had significantly lower pH (7.7) than unamended soil (7.9). Even though compost-amended soils received the same quantity of irrigation as unamended soils, composts apparently functioned to buffer the soil pH against irrigation water. Soil pH often decreases in compost-amended soils due to effects of nitrification (Harrison et al. 1994). However, soil pH increases in our study were likely a result of soluble salt additions from irrigation, as well as high levels of basic cations in compost (Table 1). Soil pH was significantly correlated with extractable soil Na (r=0.42) and EC (r=0.27) (Table 2).

FIGURE 5. Soil pH under Bermuda grass and Saint Augustine grass turf up to 29 months after application of three compost sources at 0, 80, and 160 Mg ha^sup -1^.

Relationships Among Soil Properties

Composts decreased extractable soil Na, EC, and pH compared to unamended soils. Since all treatments received the same irrigation rates, these results were likely related to effects of compost on concentrations of basic cations, soil organic matter dynamics, and leaching. High organic C and nutrient loading rates in compost- amended soils can occupy cation-exchange sites on soil colloids, or coat these particles with organic matter (McCracken et al. 2002), resulting in a decrease in the number of exchange sites capable of reacting with Na. In fact, DOC adsorption onto particle surfaces can significantly influence DOC concentrations in soil (Qualls and Haines 1992). Calcium, Mg, and K preferentially occupy cation- exchange sites in preference to Na (Stevenson and Cole 1999), leading to increases in soil solution Na concentrations and enhancing leaching potential. The lack of available exchange sites for Na and soluble salts in soils receiving high organic C additions may further enhance Na leaching losses, especially after precipitation events, as occurred in this study for Saint Augustine grass. Thus, high levels of basic cations in compost, in addition to DOC increases caused by compost application, may have decreased Na adsorption onto soil particles and enhanced leaching losses. These mechanisms likely explain how compost additions decreased extractable soil Na and EC relative to unamended soil.

Short-term increases in soil EC commonly occur after irrigation with water containing soluble salts, but most salts eventually leach from the surface soil following high levels of precipitation (Mercuri et al. 2005). In our study, decreases in extractable soil Na and EC occurred at 16 months, which followed high precipitation levels (Figure 1). Monthly precipitation preceding sampling times was significantly negatively correlated with Na and EC (Table 2). Seasonal variation in soil EC may also be caused by growth stages of turf grass (Figure 1), which influenced DOC levels (Wright et al. 2007a) and mobility of DOC-associated cations through seasonal variation in biomass production and decomposition. Compost application, concurrent with establishment of turf grass and their contribution of root residues and clippings, increased soil organic matter and ultimately DOC levels (Wright et al. 2005). Salts released from compost decomposition, along with those added from irrigation water, may have formed complexes with DOC that were leached from the soil surface during precipitation and irrigation events, thus lowering soil salinity relative to unamended soil. Therefore, greater soil EC for unamended soil may have been a result of lower organic matter inputs compared to compost-amended soil. Thus, composts did not appear to contribute to soil sodicity and salinity, and in fact may alleviate these potential problems in turf grass soils.

Significant relationships between extractable soil Na and EC were more evident for Saint Augustine grass (Figure 4) than Bermuda grass. In addition, extractable soil Na for Saint Augustine grass exhibited greater seasonal variability than Bermuda grass (Figure 2). Although few differences in extractable soil Na and EC existed between turf grass, different seasonal patterns, particularly during winter dormancy at 16 and 29 months, suggest that effects of compost on soil sodicity and salinity vary among turf grass.

Conclusions

In summary, extractable soil Na and EC did not differ for this sandy loam soil amended with different compost sources, although composts exhibited variable composition. Compost application decreased extractable soil Na, EC, and pH compared to unamended soil, likely due to high organic matter inputs which occupied cation- exchange sites and coated soil particle surfaces, limiting Na adsorption and enhancing leaching of Na and salts by precipitation. Furthermore, high concentrations of basic cations in composts may affect the potential of composts to alter extractable Na and salinity levels. Formation of DOCcation complexes, when leached from the surface soil during precipitation or irrigation events, may also have contributed to lower extractable soil EC in compost-amended than unamended soil. Because of lower soil organic matter and DOC levels in unamended soils, there was greater potential for Na adsorption onto soil colloids, resulting in higher extractable soil Na and EC relative to compost-amended soils. Thus, compost application did not contribute to sodicity or salinity in surface soil, and may actually alleviate these potential problems for sandy loam soils.

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Alan L. Wright1, Tony L. Provin2, Frank M. Hons2, David A. Zuberer2 and Richard H. White2

1. Everglades Research & Education Center, University of Florida, Belle Glade, Florida

2. Department of Soil and Crop Sciences, Texas A&M University, College Station, Texas

Copyright J.G. Press Inc. Winter 2008

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