To investigate a rapid, nondestructive way of characterizing solute transport properties, time domain reflectometry (TDR) and disk permeametry have been used in combination. Calibration measurements had previously related TDR measurements to both the volumetric water content and the pore water concentration of Cl‐. Laboratory measurements from a horizontal TDR probe were used to estimate transport parameters in a soil column by applying a one‐dimensional numerical model in an inverse sense. A vertical TDR probe was used to provide independent verification of these parameters. A repacked column of Ramiha silt loam (an Andic Dystrochrept) was used under unsaturated, transient flow conditions. The disk permeameter, set to a pressure head of −50 mm and containing a solution of 0.032 M KCl, was placed straight onto the repacked soil column, which had an initial water content of 0.32 m3 m‐3. The soil wet to 0.60 m3 m‐3. However, in the columns only an envelope of Cl‐ concentration could be obtained, due to exchange between the initially resident Ca2+ and the invading K+. This illustrates why cation exchange needs to be considered when TDR is used to infer solute dispersivity and the retardation were found to be 2.3 mm and 1.2, respectively. The retardation is shown to be due to the anion‐exchange capacity varying with the concentration of the invading soil solution.

Undisturbed cores were removed from the surface of 14 New Zealand soils with a wide range of textures. The sorptivity to ethanol and water was measured with a 'sorptivity tube' to determine the repellency index (Rl) of each soil. Texture and gravimetric water content were measured, and the water drop penetration time (WDPT) and molarity of ethanol droplet (MED) tests for water repellency were conducted on the soils. The RI measured all soils water repellent (RI> 1.95) at field moisture conditions, and was more sensitive than the WDPT or MED tests. The RI was used to demonstrate that water repellency reduced short-time water infiltration of all soils by approximately an order of magnitude. Actual and 'potential' infiltration was then compared with rainfall and irrigation intensities. This illustrated the hydrological significance of the phenomenon, even in soils which appeared to wet normally (low WDPT). In all soils the curves of cumulative infiltration versus the square root of time for both water and ethanol stayed linear long enough for sorptivity evaluation. However, at longer times the slope of the curve tended to increase for water sorption in the more repellent soils, but decreased consistently for ethanol.

Nitrogen (N) fertiliser is an important and expensive input to oil palm in Papua New Guinea. Of about 3000 mm/year of rainfall, about 1300 mm is lost as evaporation. This leaves an excess of >1000 mm/year lost as surface runoff and/or deep drainage, and with it the potential for N loss. Approximately 11% of rainfall reached the ground as stem flow. Throughfall was generally lowest near the trunk and highest where canopies overlapped, but random spatial variability was large. The difference between the measured rainfall and stem flow plus throughfall was 6%, indicating relatively little interception. Surface runoff from the volcanic ash soils was 6% of rainfall at one site, but only 1.4% at the other. Less than 2% of the applied N was lost in the surface runoff after an ammonium chloride application. Calculations of N leaching losses made using suction cup data and the water balance indicated that significant losses occur, but the estimates were not reliable due to the huge spatial variability in the suction cup and throughfall data. Therefore, another technique is needed to study N leaching in oil palm plantations.

There is little information available on the magnitude of nutrient losses to surface water from the two-pond and daily irrigation treatment systems for farm-dairy effluent (FDE). A research site has been established on a mole-pipe drained Tokomaru silt loam at Massey University's No. 4 Dairy Farm (475 cows) to investigate some of these issues. The site consists of four plots (40 × 40 m) that have been instrumented to allow the continuous monitoring of drainage and surface runoff. The research was conducted over three lactation seasons (2000/01-2002/03). Based on data collected at the study farm it was calculated that in the past 1500 kg N yr -1 and 250 kg P yr -1 were potentially discharged from the two-pond system directly to a stream. A simulation exercise suggests that approximately 108 kg N yr -1 and 18 kg P yr -1 would be lost to surface waters if daily irrigation was practised at the farm. The problems of daily irrigation, particularly A04050; those related to surface runoff, were further quantified in an experiment in which a single 25-mm FDE irrigation was applied to a soil near field capacity. Approximately 40% of the applied effluent left the soil profile as mole and pipe drainage and 30% as surface runoff. These losses equated to 12 kg N ha -1 and 2 kg P ha -1 . To minimise nutrient losses from land application of FDE, a system called "deferred irrigation" was designed. Deferred irrigation involves storing effluent in a two-pond treatment system and then applying it strategically when there is a suitable soil water deficit, i.e., the irrigation volume does not exceed the potential soil-water storage. The evaluation of deferred irrigation over three lactation seasons showed that direct losses of nutrients to surface waters were almost eliminated and resulted in the drainage of only approximately 1% of the total effluent nutrients applied. The successful adoption of the deferred irrigation system would require only the capability to store effluent and model or measure soil moisture status within the active root zone.

Knowledge of where roots are active is crucial for efficient management of nutrients in tree crops but measurement of root activity is problematic. Measurement using soil water depletion is an approach that has not been tested in a humid climate. We hypothesised that the three dimensional distribution of root activity of a tree crop in the humid tropics (a) can be determined by measuring soil water depletion during rain-free periods, and (b) is influenced by environment (soil type and climate) and surface management. A field study was carried out in which soil water content was measured and water uptake calculated (by difference between soil water content at beginning and end of rain-free periods) for different surface management zones and depths (0.1 m intervals to 1.6 m depth) under oil palm (Elaeis guineensis Jacq.) at a loam-clay site and a sandy site. Significant differences were measured between sites and between surface management zones at each site. At both sites water uptake was highest under the weeded zone close to the palm stem, slightly lower under the zone where pruned fronds are placed, and lowest under the path used for removing harvested fruit. Vertical distribution of root activity differed between the sites, with higher activity near the surface at the finer textured site. Total water uptake values were lower than estimates of evapotranspiration made using climate data. The difference was probably largely due to water uptake from deeper than 1.6 m. This study showed that the spatial distribution of tree root activity in a humid climate could be quantified using a relatively simple method.

No abstract

The effect of lime (CaCO,) and phosphate additions on surface charge characteristics and their effect on the leaching of sulphate were examined for two soils (Patua loam and Tokomaru silt loam) which differed in their adsorption capacities for sulphate.Incubation of soils with either CaCO, (0-600 mmol kg-') or phosphate (0-208 mmol kg-') resulted in a two-to five-fold increase in the net negative charge and a similar decrease in the adsorption of sulphate. The effect of either lime or phosphate addition on both the surface charge and sulphate adsorption was more pronounced for the allophanic Patua soil than for the Tokomaru soil containing mainly vermiculite.In a column experiment, liming induced the leaching of sulphur either by the desorption of adsorbed sulphate or by the mineralization of organic sulphur. During a miscible displacement study, addition of either CaCO, or phosphate resulted in an early breakthrough of sulphate in the leachate. In a pulse experiment, in which soils were incubated with sulphate (3.12 mmol kg-') for 1 week and subsequently leached with water, more added sulphate was lost in the leachate of the soils previously incubated with either CaCO, or phosphate.

Field trial data relating pasture growth to measures of soil fertility are confounded by many site-specific environmental factors, particularly the weather. One approach to accommodate this is to express fertiliser responses in terms of relative rather than absolute yields, but this approach places constraints on trial design and is unhelpful when attempting to extrapolate data to estimate actual yields at other sites or in other years. We suggest an alternative approach that includes the soil moisture, and so the effect of climate, as it influences evapotranspiration. The model assumes that pasture growth is proportional to evapotranspiration, and that the proportionality constant (k) depends on soil fertility. Evapotranspiration is calculated from a simple daily soil water balance. Values for k varied from 11 to 19 kg DM ha-1 mn-1 of evaporation. The greatest divergence between the measured and modelled production occurred during a prolonged dry period. Possible reasons for this are discussed. With simulated weather data, the model can be used to generate probability-density functions of pasture production. The advantage of