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The ease with which fertilizers are leached through soils is a growing interest of society, and one with which students should become knowledgeable. This exercise was developed to demonstrate the degree to which N, P, and K fertilizers move through soils of different adsorption and permeability characteristics and under different leaching intensities. Rainfall equivalent to 13 cm did not leach fertilizers through a 60-cm column of dry sandy loam soil. Addition of 54 cm water resulted in leaching 57% of the applied N, 0% P, and 42% of the K. In a related exercise, students are led through a series of calculations to estimate relative mobility of N, P, and K in soil based on routine chemical analysis of column leachate and a surface layer of soil after leaching is completed. On a scale of 1 to 10 (10 = most mobile), mobility was 9.9 for N, 0.1 for P, and 3.3 for K. The results support the common practices of broadcasting N, but banding P and K for maximum efficiency. S GROWING CONCERN of the effects of farming practices on groundwater quality creates an interest in leaching of agricultural chemicals from soil. The importance of movement of fertilizer nutrients with respect to soil type and rainfall environment must be stressed to students planning to work in the agronomic crops area. With this in mind, an exercise was designed to demonstrate leaching and relative mobility in soil of different fertilizer nutrients. Although dyes have commonly been used to demonstrate movement of materials through soils, the leaching of fertilizer nutrients in laboratory exercises has not been reported. Bowman et al. (1988) used a 0.01 M CaCI 2 saturated sand column and dyes to demonstrate miscible displacement and the rapid leaching of CrO42-, which they suggested was similar to movement of NO~.Williams (1985) used anionic (methyl orange) and cationic (methylene blue and basic fuchsin) dyes to demonstrate the effect of soil cation exchange capacity on movement of cations and anions through a sandy soil. Similarly, Butters and Bandaranayake (1993) used cationic and anionic dyes to demonstrate solute transport through soil columns. This exercise consists of placing two soils with differing permeability and nutrient retention capacities into several transparent tubes. Fertilizer compounds are added and columns leached with different amounts of water to represent high rainfall and low rainfall environments. The high rainfall amount is enough to readily leach mobile nutrients through the column of soil while the low rainfall amount is enough to move the wetting front to the end of the tube withD.S. Owens and G.V. Johnson, Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078. Contribution from the Oklahoma Agric. Exp. Stn. Received 23 May 1995. *Corresponding author (gvj@soilwater.agr. okstate.edu). Published in J. Nat. Resour. Life Sci. Educ. 25:i28-131 ( 996). out solution leaving the tube. Water passing through the soil (leachate) is collected in increments that represent fractions of a total pore volume for the soil used. Leachate samples are analyzed. For Part II of the exercise a surface layer of soil is removed for nutrient analysis. This exercise was used in an undergraduate soil fertility course at Oklahoma State University. Objectives for development of the teaching-learning exercise in nutrient mobility were: (i) develop a technique demonstrate fertilizer nutrient movement in the soil, (ii) devise a system of calculations to interpret data obtained from the exercise, and (iii) determine relative nutrient mobility. MATERIALS AND METHODS Part I. Fertilizer Nutrient Leaching Soil Column Preparation Approximate pore volumes for loam and sandy loam soils are determined by weighing a 100-mL graduated cylinder, and then adding dry soil while gently tapping the cylinder on the lab bench top until the cylinder is filled. The cylinder plus soil is then weighed and soil weight calculated. Bulk density (BD) is calculated by dividing the weight of the soil (g) by the volume (100 cm3). Assuming a particle density (PD) of 2.65 gcm-3, the percentage of soil volume occupied by solids is then given by (BD/PD)I00 = % solids. The percent porosity is determined by subtracting the percent solids from 100. The soil columns are created by filling capped transparent plastic tubes (100 cm long by 4.34 cm i.d.; part no. BL 1750; Giddings Machine Co., Ft. Collins, CO) to a depth of 60 cm from the bottom with dry soil. When filling the tubes with loam or sandy loam soils, the students should lightly tap the colunms to try to approximate the packing of soil in the 100-mL graduated cylinder when bulk density was determined. A filter paper disc slightly smaller than the tube diameter is placed on the soil surface to reduce soil mixing when solution is added to the columns. The tubes are placed on a display rack and labeled to identify the following fertilizer leaching conditions: L/LR/C, L/LR/F, L/HRJC, L/HR/F, S/LR/C, S/LR/F, S/HR/C, S/HR/F, where L = loam, S = sandy loam, LR = low rainfall, HR = high rainfall, C = control, and F = fertilized. The pore volume of each column will equal the volume of soil column (V = rcr2 L) times the porosity. Fertilizer Preparation and Addition The solution containing fertilizer nutrients is prepared to approximate a 224 kg ha-~ (200 lb acre-r) fertilizer addition of each nutrient. To 400 mL of distilled water (enough for 10 replications), 618 mg of (NH4)2HPO4, 573 mg Abbreviations: ICAP, induction coupled argon plasma. 128 d. Nat. Resour. Life Sci. Educ., VoL 25, no. 2, 1996 NH4NO3, and 528 mg of KCl were added,. For nitrogen (N), phosphorous (as P205), and potassium, (as K20) a total 33.2 mg of each will be contained in a 40-mL volume. In terms of the elemental concentration the 40 mL additions represent 23.1 mg NH4-N, 10 mg NO3-N, 14.5 mg P, and 27.8 mg K. Students add the fertilizer solution to each of the fertilized columns of soil and 40 mL of distilled water to the control columns then observe the downward movement of the wetting front. After the wetting front appears to have stopped (48 h), the front is marked by making a line on the tube. The distance from the soil surface is measured and recorded. Inverted beakers placed over the column tops minimizes evaporative loss of solutions. Development of the exercise to this point may be accomplished in a 2-h lab period. Leaching the Columns Distilled water is added to soil columns to simulate rainfall. An amount equal to one-half a pore volume is added to each of the low rainfall treatments. Funnels and leachate collection containers (marked to indicate one-fourth leaching volumes) are placed under the high rainfall treatment soil columns. Water is periodically added to maintain a "head" on the top of these columns until a total of two pore volumes have been applied. Students observe the collection of leachate in the containers below the soil columns. After each one-fourth pore volume leaches through, another empty container (calibrated) is placed under the column and the collected leachate is poured into a graduated cylinder, the volume recorded, and a portion transferred to an analysis sample container and labeled. Anticipate rapid leaching in the sandy loam soil and much slower leaching in the loam soil. A collection of four or five one-fourth pore volumes will be sufficient. Technically there is enough water for eight one-fourth pore volumes but, about one-half pore volume will remain in the columns at field capacity. Because the loam soil will leach much slower, someone will have to monitor the columns throughout the day until enough onefourth pore volumes leach through. It is not highly critical that the collected samples measure exactly one-fourth pore volume but they should not vary greatly. When all the samples are obtained and labeled appropriately, they are sent to the laboratory for analysis. The laboratory used automated flow injection analysis (Lachat, 1989, 1990) for NH~-and NO]-N, colorimetric (phosphomolybdenum blue) analysis of P and atomic emission spectrophotometric analysis (ICAP) of K for solutions. Soil was extracted using a saturated CaSO4 solution for NOy-N, and Mehlich-3 (Mehlich, 1984) for P and K. However, these extraction and analytical methods are not unique to the success of this exercise. Any reliable analytical method, and any extraction method that is capable of extracting the chemical form of the nutrient added, will be acceptable. Interpreting the Data When the laboratory analysis is completed, laboratory data sheets (reporting nutrient concentrations in ppm) together with several tables and graph paper are made available to the students. The students must follow stepwise calculations through the series of tables to determine how Table I. Concentration of nutrients leached through fertilized (F) and |
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