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Potassium Leaching from a Sandy Loam Soil Johnston, A.E., Goulding, K

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Potassium Leaching from a Sandy Loam Soil Johnston, A.E., Goulding, K Publisher: International Potash Institute, P.O. Box 1609 - CH4001 BASEL (Switzerland), Phone (41) 61 26129 22/24 - Telefax (41) 61 261 29 25 Subject 12 No. 4/1993 * Miscellaneous 14th suite Potassium leaching from a sandy loam soil Johnston, A.E., Goulding, K. W. and Mercer, E. Soil Science Dept., AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts A15 21Q, Great Britain Summary The risk of potassium (K), applied as KCI, K2SO4, or in farmyard manure (FYM) and animal slurry, leaching from an uncropped sandy loam soil was tested in uncropped lysimeters and field plots. Even in the absence of applied K, some K was leached from lysimeters. No additional fertilizer K was lost even after heavy rain augmented by irrigation, and most K was absorbed in the top 10 cm of soil; none moved below 23 cm. Within these limits, K in FYM and slurry was somewhat more mobile than K from KCI and K 2 S0 4. An existing potassium leaching model predicted the distribution in the soil of the applied K quite well. Results from this study were used to modify and improve the model. Introduction There is a very low Maximum Admissible Concentration (MAC) of 12 mg 1-1 for K in potable water imposed by the EC Drinking Water Directive (80/778/EEC), and a marginally lower guide limit of 10 mg 1-1 K. This presents a possible threat to the agricultural use of potassium if it can be shown or even implied that water supplies in breach of the admissible levels have been polluted by the excessive use of K in fertilizer or manures. The study described here had two aims. The first was to investigate the potential for K leaching by measuring both downward movement through the profile, and leaching losses of K applied in four forms commonly used in agriculture, namely KCI, K2SO4, animal slurry and farmyard manure (FYM). A second aim was to use the experimental data to validate and modify a model for assessing the risk of K leaching from soil. Potassium in soils and losses by leaching Potassium is found in the soil in the mineral matrix, adsorbed on clay surfaces and in solution. Chemical weathering of soil minerals provides K in ionic form (K+) which is held in soil in pools (or categories) of differing availability (Johnston and Goulding, 1990). Four pools are often considered, water soluble, exchangeable, non-exchangeable and matrix K. Amounts of K in the first three pools tend to be in equilibrium, so whatever the reserves of soil K, there is always some in solution. Warren and Johnston (1962) found a strong relationship between exchangeable K and water soluble K in a Rothamsted soil over a wide range of values, and showed that, above 70 mg kg-' K, approximately 15% of the K was water soluble. Fertilizer K applied in water soluble forms increases the concentration of K in the soil solution leading to adsorption as exchangeable and non-exchangeable K. Plant uptake, on the other hand, removes K from solution, reducing the concentration and K is released into solution from the adsorbed phases. Only water-soluble K will be leached from surface soils. However, when water soluble K in fertilizers and manures is applied to soil it is likely that much of this K will be held as both exchangeable and non-exchangeable K in both surface and subsoils. The distribution of K down the profile depends on both the amount of K applied and the amount and type of clay present and the amount of organic matter. For example, on a sandy loam soil at Woburn, heavily manured with K, each horizon down to 61 cm was equally enriched with exchangeable potassium, but the extent of enrichment depended on the quantity of K applied. By contrast, on a silty clay loam at Rothamsted, also heavily manured with K, an exchangeable K profile developed, with amounts of exchangeable K decreasing with depth down to 54 cm (Johnston, 1988). 2 12/14 While K remains within root range in the subsoil it will be available to plants, readily if in solution, more slowly if as exchangeable and non- exchangeable K. The uptake of subsoil K may, in part, explain poor relationships between yield and exchangeable K (Johnston and Goulding, 1990). Subsoil K may also enhance the uptake of nitrate from depth by providing a readily available cation to balance the nitrate taken up by plant roots (Johnston and McEwen, 1984). The concentration of K in leachate from agricultural soils depends on the amount of K lost and on the volume of through drainage. Averaging data from many different cropping systems on a wide range of soil types with varying rainfall suggested that I kg ha- K was lost for each 100 mm through drainage (Johnston and Goulding, 1992) which would give a concentration well below the EC guide limit for K in potable water. Soil erosion can also lead to loss of K but much of this K will be in the mineral lattice and little is known of the release of exchangeable and non- exchangeable K to water in streams, rivers and lakes. Experimental details Site, lysimeters and field plots A site on a sandy loam soil at the Woburn Experimental Farm was chosen because monolith lysimeters and plots with porous ceramic cups at various depths were available. Also, this type of soil with 10% clay is within a critical range of soil texture with regard to its ability to hold K, being between the very light textured sands and the loamy clays. It is also a soil type often used for intensive vegetable production, with large inputs of fertilizer, and where irrigation is used. The soil was a loamy sand (Cottenham series) maintained at pH 6. Lucerne had been grown on the field since 1987 and this was killed at the start of nitrate leaching studies in 1989. The only K applied to the lysimeters over the three years 1989-91 was a small amount (38 kg ha-I K) as potassium bromide applied as a nitrate tracer. The monolith lysimeters (0.8 m diameter and 1.35 m deep) had been collected in summer 1988 from a trench on an adjacent site by driving fibreglass cylinders into the ground (Webster et al., 1993). The cylinder and soil core were then lifted, a baseplate and drainage system fitted and three ceramic probes were inserted in each monolith at a depth of 100 cm, enabling "suction" drainage to be sampled. The monoliths were transferred to the lysimeter installation some 20 m away where they were positioned so that they were surrounded by soil and the surface soil was level with that of the surrounding soil. All access and drainage tubes from the lysimeters were 3 taken to a central pit where drainage could be collected. Four other lysimeters of the same diameter, but only 65 cm deep, had been constructed from similar but repacked soil in 1988. By 1991 the soil had settled to give similar drainage rates to those of the undisturbed lysimeters and these were also available for the K leaching study. Ceramic probes were inserted into field plots 10 m from the lysimeters in 1989. Each plot had four probes, one at each of four depths, 15 cm, 40 cm, 80 cm and 135 cm. Unfortunately, several of those at shallower depths (particularly at 15 cm) had been damaged and were inoperable. Experimental method and design The fertilizers were applied at several rates representative of field applications, plus a nil treatment. It was impractical to apply a low rate of organic manure because even spreading was not possible without artificially breaking up the FYM or diluting the slurry; such manures are usually applied in quite large amounts anyway. Inorganic K fertilizers, FYM and slurry were applied on 27 February 1992 as follows: A. The eight monolith (1.35 m deep) lysimeters were treated with either 1 KCI or K2SO4 to supply 0, 75, 150 and 225 kg ha- K, only the no K treatment was duplicated. The four 65 cm deep repacked lysimeters 1 were treated with either 125 or 250 kg ha- K2 SO 4 ; there were two replicates of each treatment. B. The sixteen 2 m x 2 m field plots containing ceramic probes received KCI at 0, 75, 150 and 225 kg ha-I K, FYM to supply 150 and 300 kg ha-1 K, and slurry to supply 145 and 290 kg ha-1 K. The organic manures were equivalent to 25 and 50 t ha- 1 slurry and 12.5 and 25 t ha-1 FYM. There were duplicate plots of each treatment, one of which was irrigated. The FYM, 30 kg, was semi-rotted cattle manure from Rothamsted Farm roughly chopped into small pieces. The cattle slurry was collected from a neighbouring farm; it was taken directly from the cowshed. The KCI and K2SO4 were applied in pure chemical form, diluted up to 100 times with air- dried, sharp sand to aid even spreading. Soil sampling Soil samples were taken at the beginning of the experiment (21 February 1992) to determine exchangeable K in the soil before treatments were 4 12114 applied. Five cores of 2 cm diameter were taken from each field plot (no cores were taken from the lysimeters) to a depth of 23 cm and bulked. Extra cores were taken from one plot at 0-7.5, 7.5-15 and 15-23 cm. Holes were refilled with surrounding soil. Soil samples were taken again on 8 June 1992, at the end of the experiment.
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