Soil Texture Effect on Nitrate Leaching in Soil Percolates
Nitrate nitrogen (NO3-N), which is an essential source of nitrogen (N) for plant growth, is now also considered a potential pollutant by the Environmental Protection Agency (EPA). This is because excess applied amounts of N03-N can move into streams by runoff and into ground water by leaching, thereby becoming an environmental hazard. Soils have varied retentive properties depending on their texture, organic matter content, and cation exchange capacity (CEC) The purpose of this study was to determine the effect of soil texture on NO3-N retention to reduce NO3-N contamination in the environment. A sand, 85:15 sand:peat Greensmix, a loamy sand, and sandy clay loam soils were placed in 2x3 inch metal cylinders and soaked in a 240 ppm solution of NO3-N for seven days to saturate the soil with NO3 ions. The columns were leached with water to collect 10 soil percolate samples of 50 rnL each until a total volume of 500 mL was collected. NitrateN was measured in each 50 mL aliquot by automated colorimetry. The results showed that soil texture affected the retention of NO3-N in the sand, which adsorbed the least amount of NO3-N at 119 ppm, followed by the Greensmix at 125 ppm, loamy sand at 149 ppm, and sandy clay loam at 173 ppm. More NO3-N was released in the first 50 mL of the sand percolate at 63% followed by the Greensmix, loamy sand, and sandy clay loam at 58, 46, and 37% NO3-N released, respectively. Soils with more silt, clay, and organic matter retained more NO3-N than the straight sand. Therefore, a straight sand would be the poorest of soil types since NO3-N retention was low.
During the past 20 years, the N03-N form of fertilizer N. an essential source of N for plant growth, is now considered not only a fertilizer, but also a potential pollutant by the Environmental Protection Agency (EPA) (1). This is because excess amounts of applied N03-N applied as fertilizer to croplands, lawns, nurseries, orchards, gardens, and golf courses can move into streams by runoff and into ground water by leaching. Excess N03-N is considered as a contaminate and environmental hazard to water quality.
Soils have varied retentive properties depending on their texture and organic matter content. Soil texture refers to the relative proportion of particles of various sizes in a given soil. Sandy soils have less retention than finer clay soils because sandy soils have less silt and clay which gives rise to a lower cation exchange capacity (CEC) (2). The CEC is the sum total of exchangeable cations that a soil can adsorb. Although NO3 is an anion that can readily leach through the soil profile, soils with significant quantities of silt, clay, and organic matter will also retain more N03-N than soils without much silt and clay.
Soil texture also affects the water permeability or percolation rate of a soil. Percolation is the downward movement of free water and is often referred to in the laboratory as the saturated hydraulic conductivity rate. The coarser the soil, the faster the rate, and the finer the soil, the slower the rate. That is why sands, which have small amounts of silt and clay, have higher water permeability rates than loamy sands or sandy loams. Coarse medium and medium coarse sands that meet United States Golf Association (USGA) particle size recommendations for a putting green rootzone mixture (Greensmix) typically have water permeability rates between 30 and 50 in/hr when compacted using USGA Green Section Procedures (3) to simulate a compacted golf green. That is why a sand must be amended with an organic amendment, such as peat, to reduce the rate to a manageable level such as 6 to 12 in/hr. to develop a Greensmlx.
The best soils for golf course fairways are loamy sands and sandy loams because these soils have a moderate amount of drainage. Loamy sands and sandy loams typically have water permeability rates of 0.5 to 2.0 and 0.1 to 1.0 in/hr, respectively, when compacted by USGA Green Section Procedures (3) to simulate a golf course fairway (half the compaction of a golf green).
The purpose of this study was to determine the effect of soil texture on N03-N retention by comparing the N03-N retention of soils with varied sand, silt, clay, and organic matter concentrations for the purpose of reducing NO3N contamination through run off into streams and ground water.
MATERIALS AND METHODS
To determine the effect soil texture had on soil N03-N leaching, four soil samples with a diverse soil texture were studied. A particle size analysis was made to determine the soil texture. The soils were a medium sand (2% very coarse, 23% coarse, 50% medium, 18% fine, and 2% very fine sand particles), an 85:15 same medium sand:sphagnum peat putting green rootzone mixture (Greensmix), a loamy sand, and a sandy clay loam.
A chemical analysis was made on the four soils to determine organic matter, cation exchange capacity (CEC), pH, N03-N, potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and hydrogen (H) concentrations (Table 1) by chemical methods described by Gaines and Mitchell (4).
A physical analysis of the soils was made for the five physical property parameters considered important by the USGA for determining the optimum physical quality of a Greensmix (Table 2), and include water permeability rate, percent large and small pore space, bulk density, and percent water retention. These values were determined by USGA Green Section Procedures (3).
The soil samples were placed in 2x3 inch metal cylinders and soaked in a 240 ppm N03-N solution for seven days to saturate the soil sites with NO3 ions. The N03-N source was potassium nitrate. The concentration of 240 ppm N03-N was selected because this concentration is equivalent to 12 Ibs N/1000 sq. ft., which is the recommended N fertilizer rate for bermudagrass golf greens per year in Georgia (5).
After the seven days of soaking, a physical analysis was made on the four samples as described by the USGA Green Section Method (3). Samples were placed on a tension table until they reached field capacity at 40 cm tension, then they were compacted, saturated again in the 240 ppm N03-N solution, and placed into a permeameter to measure the water permeability rate. Ten soil percolate samples of 50 mL each were immediately collected into 50mL plastic screwcap vials until a total volume of 500 mL was collected. NitrateN was determined in each 50rnL aliquot of sample by automated colorimetry using a coppercadmium reductor column (4).
RESULTS AND DISCUSSION
Soil Texture: The four soil samples were diverse in soil texture as the percent silt and clay increased from 4 and 1%, respectively, for the sand and Greensmix (which was composed of the same medium sand) to 11 and 6% for the loamy sand, to 8 and 25% for the sandy clay loam (Table 1).
Chemical Analysis: Consequently, the CEC increased from 0.28, 1.63, 3.41, to 5.31 meq/100 g for the sand, Greensmix, loamy sand, and sandy clay loam, respectively (Table 1). Finer textured soils tend to have higher CEC than sandy soils (2). The chemical analysis showed that all four samples had low levels of N03-N (0.4 to 1.1 ppm) prior to the study. Only the Greensmix had an appreciable amount of organic matter, 1.12% by weight.
Physical Analysis: The physical analysis showed that water permeability rates were 21.30 in/hr for the sand, 15.20 in/hr for the Greensmix, 0.41 in/hr for the loamy sand, and 0.32 in/hr for the sandy clay loam when compacted by USGA procedures to simulate a golf green (Table 2). Based on these rates, it took only 24 minutes for 500 mL to percolate through the sand, compared to 33 minutes for 500 mL to percolate through the Greensmix, 20 hours for 500 mL to percolate through the loamy sand, and 26 hours for 500 mL to percolate through the sandy clay loam. Soil 'fines' (silt and clay) and peat had an effect on percent water retention of the four soils as percent water retention values increased from 9.5, 16.0, 14.3, and 25.1% for the sand, Greensmix, loamy sand, and sandy clay loam, respectively (Table 2). Percent water retention had a strong positive correlation with percent small pore space (capillary) and a strong negative correlation with percent large pore space (noncapillary). The bulk density had a strong negative relationship with percent total pore space but not with the other physical properties reported.
Nitrate-Nitrogen Concentrations: The N03-N concentration in the 50 rnL soil percolate aliquots to a volume of 500 mL for the four soil samples that varied in soil texture and were soaked in a solution of 240 ppm N03-N for seven days are shown in Table 3 and Figure 1.
Soil texture affected the retention of N03-N for the four soils as the sand, which had the coarsest texture and lowest CEC (Table 1), adsorbed the least amount of N03-N (119 ppm, Table 3) during the seven days the samples soaked in the 240 ppm N03-N solution. The sandy clay loam, which had the finest soil texture and the highest CEC (Table 1), adsorbed the highest amount of N03-N at 176 ppm during the seven days the samples soaked in the 240 ppm N03-N solution. The Greensmix, which was composed of 85% sand and 15% sphagnum peat moss, and the loamy sand were intermediate as they adsorbed 128 and 149 ppm N03-N, respectively. These results show that soils with less sand and with either more silt, clay, and organic matter adsorbed a greater amount of N03-N which corroborates findings recently summarized by the USGA Green Section on the relationship between soil type and subsurface loss of N for both turf and croplands (6).
The sand and Greensmix released more N03-N in the first 50 mL of the soil percolate at 63 and 58%, respectively (Table 3), than the loamy sand and sandy clay loam at 46 and 37%, respectively. These results show that soil samples with fewer 'fines' (silt and clay) and a low CEC, such as sands, retain less N03-N than finer textured soils that have higher silt and clay contents and CECs. NitrateN released in the first 100 mL of soil percolates was 88% for the sand compared to 82% for the Greensmix, 71% for the loamy sand, and 62% for the sandy clay loam. Little N03-N (approximately 67% of the total adsorbed) remained in all four soils after 200 mL had percolated through the soils.
NitrateNitrogen Release Curves: The N03-N release curves show the soil texture effect on N03-N movement graphically (Figure 1). The sandy clay loam shows higher N03-N retention, followed by the loamy sand, the Greensmix, and the sand. These results show that silt and clay have greater retention capacity for N03-N than organic matter, such as sphagnum peat.
NitrateN leaches faster through sands than through soils that have an appreciable amount of silt and clay, such as loamy sands, sandy loams, and sandy clay loams. Excessive rates of N03-N fertilizer should be avoided for sandy soils than for loamy soils due to their low N03-N retention. NitrateN fertilizer should be applied more frequently at low dose rates for sands than for loamy sands as more of the N03-N will leach through sand and into ground water due to low N03-N retention. Weekly low application rates of N03-N to plants, referred to as a spoonfeeding technique, is much more environmentally sound and a better means of fertilizing plants than one or two high rate applications of NO3-N. Several excellent points have been made by Petrovic (7) to golf course managers when applying N fertilizer, which are the frequent application of light rates, the use of slowrelease N sources, avoiding fertilizer applications at times of the year when turfgrass is slow growing (during cool weather), and irrigating conservatively to reduce leaching.
1. Federal Register. 1975. National Interim Primary Drinking Water Standards. Federal Register 40:59, 566588.
2. Brady, N. C. 1974. The Nature and Properties of Soil, pp. 4070. 8th Edition. MacMillan Publishing Co., Inc., New York, NY.
3. United States Golf Association Green Section Staff. 1960. Specifications for a method of putting green construction. USGA J. Turf Management 13(5):2428.
4. Gaines, T.P. and G.A. Mitchell. 1979. Chemical Methods for Soil and Plant Analysis. University of Georgia Coastal Plain Experiment Station Agronomy Handbook No. 1. Tifton, GA.
5. Plank, C.O. 1989. Soil Test Handbook for Georgia. Cooperative Extension Service, College of Agriculture, University of Georgia, Athens, GA.
6. United States Golf Association Green Section. 1992. Environmental Research Summary, 10.
7. Petrovic, A.M. 1989. Golf course management and nitrates in groundwater. Golf Course Management. 57(9):5464.