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This paper presents a parameterisation and test of an algorithm to calculate distributions of the major nutrient cations between the solution and exchangeable phases of soil when cation exchange capacity (c) may vary. Two contrasting soils were considered: a volcanic subsoil where c is dominated by variable-charge surfaces, and an alluvial silt loam topsoil with stable c. Experimental treatments consisted of applying either water or solutions of Ca[Cl.sub.2], KC1, Mg[Cl.sub.2], or NaCl. Solution concentrations of [Ca.sup.2+], [K.sup.+], [Mg.sup.2+], and [Na.sup.+] varied by up to two orders of magnitude, and were simulated well, particularly when using logi0-transformed data. The ratios of the solution concentrations of [K.sup.+], [Mg.sup.2+], and [Na.sup.+] to [Ca.sup.2+] also were generally simulated well. However, the algorithm's description of soil acidity needs further checking. For the variable-charge soil, cation concentrations were strongly influenced by fitted parameters associated with anion exchange. For the alluvial soil, fitted parameters had little influence, and the cation calculations were dominated by information gathered front the initial (equilibrium) distributions between phases. The algorithm has strong potential for forecasting changes in solution concentrations of the major nutrient cations, using relatively small amounts of fitting data. Keywords: anion exchange, cation exchange, model, soil solution, variable-charge soil., Introduction Although plant uptake of the major nutrient cations is strongly affected by their concentrations in the solution around the roots, soil fertility testing rarely involves analysis of the soil [...]

Commercial cropping farms in New Zealand (NZ) showed highly variable rates of nitrate-N leaching losses (13–148 kg/ha/year) across sites, cropping systems and seasons. Losses were measured at 1.2 m below the soil surface using a network passive-wick drainage fluxmeters (DFMs) which were installed across nine commercial farms between August 2014 and June 2016. Sites were located in the Canterbury, Manawatu, Hawke's Bay, Waikato and Auckland regions and monitored for a period 51 to 72 months. Twelve DFMs were installed at each site and drainage volumes were validated using an established soil water balance model. At seven sites, losses were calculated using measured drainage and measured concentrations, and at two sites losses were calculated using modelled drainage and measured concentrations. Nitrate-N was the predominant form of inorganic N (96.7–99.9%) in drainage water and annual losses averaged 52 kg/ha/yr for mixed cropping systems with livestock grazing (n = 6) and 101 kg/ha/yr for mixed cropping systems with a focus on vegetable production (n = 3). While not broadly representative of all NZ cropping land uses, results from this study do, nevertheless, suggest that when drainage occurs, nitrate losses may be considerable under the land uses represented here (> 70 kg/ha/y). [ABSTRACT FROM AUTHOR]