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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 [...]

Difficulties forecasting cation exchange processes may limit the use of soil solution analyses in routine soil testing for agriculture. An important part of those difficulties is the need for much data and expertise to parameterise existing models that can be used for variable charge soils. This paper proposes an algorithm to simulate concentrations of the major nutrient cations in the soil solution. It is designed to have only modest data requirements, be applicable for variable charge soils, and be simple to implement in crop and environment simulation packages. It is based on a Gaines-Thomas type approach to cation exchange, adapted to allow for variations in cation exchange capacity. It includes interactions with anions and anion exchange, which may affect cation behaviour in variable charge soils. Assumptions and limitations of the approach are described, and some issues associated with parameterisation, implementation, and future extension are discussed. Keywords: algorithm, cation exchange, soil acidity, soil solution, variable charge soils., Introduction Plant uptake of the major nutrient cations is strongly affected by their concentrations in the soil solution (Tinker and Nye 2000; White and Greenwood 2013). These in turn may [...]

The hypothesis that soil anion storage capacity (ASC) directly influences the response of onion yield to soil Olsen P status was tested. Onions were grown to maturity in three different soils in the same tunnel house. Each soil received three rates of phosphorus (P) fertiliser 1 month before planting, and at planting soil ASC and Olsen P status was measured. Although bulb yield increased with Olsen P (over the range 15–95 μg g−1), ASC (which varied from 10 to 70%) had no influence on this response. This supports recent findings with young onions grown in small pots, but contradicts previous advice to vegetable growers. The results show that low to moderate soil Olsen P values and low initial availability of N, slow early plant growth and development. Delays in maturity due low P input may allow growth to catch up, at least partially, later in the season. This has implications for the design and execution of field experiments to test P fertilisers for onions and suggests that visible responses to P fertiliser in young onion crops may be misleading. [ABSTRACT FROM AUTHOR]

Conflicting reports of the responsiveness of carrots (Daucus carota L.) to nitrogen (N) fertiliser have been rationalised on the basis of the climatic zones in which the experiments were conducted. We tested this explanation with two experiments under temperate conditions, where yield increases were predicted to be unlikely, yet growers frequently apply N fertiliser. Our results show that in temperate regions carrot yields may indeed respond to N fertiliser when the yield potentials are c. 140 t ha−1 of fresh roots, if soil mineral N to 0.6 m depth and readily mineralisable N add to less than c. 190 kg N ha−1. With normal experimental precision, such responses may be undetected unless soil water deficit is kept small and variations in plant population are accounted for. Carrots have strong potential for luxury uptake of N. This has implications for marketing and the possible use of carrots as catch crops. [ABSTRACT FROM AUTHOR]

AbstractIncreased below-ground carbon allocation in forest ecosystems is a likely consequence of rising atmospheric CO2 concentration. If this results in changes to fine root growth, turnover and distribution long-term soil carbon cycling and storage could be altered. Bi-weekly measurements were made to determine the dynamics and distribution of fine roots (< 1 mm diameter) for Pinus radiata trees growing at ambient (350 μmol mol–1) and elevated (650 μmol mol–1) CO2 concentration in large open-top chambers. Measurements were made using minirhizotrons installed horizontally at depths of 0.1, 0.3, 0.5 and 0.9 m. During the first year, at a depth of 0.3 m, the increase in relative growth rate of roots occurred 6 weeks earlier in the elevated CO2 treatment and the maximum rate was reached 10 weeks earlier than for trees in the ambient treatment. After 2 years, cumulative fine root growth (Pt) was 36% greater for trees growing at elevated CO2 than at ambient CO2 concentration, although this difference was not significant. A model of root growth driven by daily soil temperature accounted for between 43 and 99% of root growth variability. Total root loss (Lt) was 9% in the ambient and 14% in the elevated CO2 treatment, although this difference was not significant. Root loss was greatest at 0.3 m. In the first year, 62% of fine roots grown between mid-summer and late-autumn disappeared within a year in the elevated CO2 treatment, but only 18% in the ambient CO2 treatment (P < 0.01). An exponential model relating Lt to time accounted for between 74 and 99% of the variability. Root cohort half-lives were 951 d for the ambient and 333 d for the elevated treatment. Root length density decreased exponentially with depth in both treatments, but relatively more fine roots grown in the elevated CO2 treatment tended to occur deeper in the soil profile. [ABSTRACT FROM AUTHOR]

The increase in number of fine (< 0.5 mm diameter) roots of one-year-old clonal Pinus radiata D. Don trees grown in large open-top field chambers at ambient (362 μmol mol−1) or elevated (654 μmol mol−1) CO2 concentration was estimated using minirhizotron tubes placed horizontally at a depth of 0.3 m. The trees were well supplied with water and nutrients. Destructive harvesting of roots along an additional tube showed that there was a linear relationship between root number estimated from the minirhizotron and both root length density, Lv, and root carbon density, Cv, in the surrounding soil.Root distribution decreased with horizontal distance from the tree. At a depth of 0.3 m, 88% of the total Cv was concentrated within a 0.15-m radius from tree stems in the elevated CO2 treatment, compared with 35% for trees in the ambient CO2 treatment. Mean Cv along the tubes ranged up to 5 × 10−2 μg mm−3 and tended to be greater for trees grown at elevated CO2 concentration, although the differences between CO2 treatments were not significant. Root growth started in spring and continued until late summer. There was no significant difference in seasonal rates of increase in Cv between treatments, but roots were observed four weeks earlier in the elevated CO2 treatment. No root turnover occurred at a depth of 0.3 m during the first year after planting.Mean values of carbon dioxide flux density at the soil surface, F, increased from 0.02 to 0.13 g m−2 h−1 during the year, and F was 30% greater for trees grown at elevated CO2 concentration than at ambient CO2. Diurnal changes in F were related to air temperature. The seasonal increase in F continued through the summer and early autumn, well after air temperature had begun to decline, suggesting that the increase was partly caused by increase in Cv as the roots colonized the soil profile. [ABSTRACT FROM PUBLISHER]