Search

Sustainable horticulture depends on the integrity of soil functions, which directly depend on soil architecture affecting aggregation, root growth, as well as liquid and gas permeability. We hypothesised that changes in soil architecture resulting from feedback mechanisms between management, soil organic carbon contents (SOC), biota and vegetation can be captured with X-ray computed tomography (CT), and that these affect the soil filtering function, which thus, can be manipulated through orchard management. We compared the transport of copper, a widely used fungicide, through intact soil cores from vine rows of kiwifruit orchards under organic and integrated management. We first derived 3D-macropore characteristics from CT-images, followed by leaching a pulse of copper and a tracer through the same cores. The organic orchard soil had a significantly higher SOC content than the integrated orchard soil, and this was positively correlated with total porosity. Macropores (>92µm) were larger with a higher connectivity, but significantly fewer in the organic than the integrated orchard soil. This resulted in a lower macroporosity and a better copper filtering capacity of the organic than the integrated orchard soil. Copper distribution was reasonably predicted when combining SOC contents, pH and macropore characteristics. Significant relationships between soil parameters and indicators of the strength of preferential flow verified that CT-derived macropore characteristics can be used to predict functional solute transport parameters. The relevance of our results and relationships observed between macropore characteristics, functional indicators of preferential flow and the fate of copper needs verification with samples representing more soils and sites.

Field applications of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) are commonly formulated as esters, amine salts, or alkali, but studies on the environmental impact focus on the 2,4-D acid form. We hypothesized that the formulation would affect its transport. To better understand 2,4-D transport through allophanic soils, leaching experiments with 2,4-D acetic acid, 2,4-D ethylhexyl ester, and tritium were conducted in 100 mm of repacked soil columns. Three of the four silt loam soils were allophanic. The 2,4-D sorption isotherms and kinetics of sorption were determined from batch experiments. The symmetrical tritium breakthrough curves (BTCs) showed that water transport was in physical equilibrium, and these were used to estimate the dispersivity of the soils. Tritium was not inert in the allophanic soils. The BTCs of both 2,4-D formulations were shifted to the right of tritium and showed extended tailing characteristic typical for sorption nonequilibrium. The 2,4-D BTCs were successfully fit with a two-site chemical nonequilibrium model. Batch and leaching experiments showed the sorption of 2,4-D ethylhexyl ester to be higher and characterized by stronger nonequilibrium than 2,4-D acetic acid sorption in all soils. The sorption coefficient values derived from the BTCs were higher than those from 24-h batch experiments. For the two allophanic topsoils, the predicted ester BTCs were significantly worse when using the independently derived Kdcompared with the BTCs using the fitted Kd. 2,4-D sorption was correlated to pH and organic matter content. Transport of 2,4-D was slower than expected in the allophanic soils and was attributed to retention of dissociated 2,4-D by positively charged soil surfaces.

On the South Pacific islands, the change from traditional to more intensive agricultural practices is leading to more fertilizer use, both extensively and intensively. Nutrient leaching should be minimized to avoid plant deficiencies and ground water pollution. The fate of nitrate and potassium under corn (Lea maysL.) and perennial grass (Rhodes grass, Chloris gayanaKunth.) was monitored during three wet seasons on a Ferralsol soil from Mare in the Loyalty Islands. In 1995,130 and 41% of the applied NO−3and K+, respectively, leached beyond the root zone. Split application of the fertilizers in 1996 decreased the amounts leached to 48 and 11%, respectively. This reduction occurred even though the rainfall nearly doubled from 1995 to 1996. Nitrate and potassium transport, however, can be decreased due to their retention by this Ferralsol. Nitrate can be adsorbed on the positively charged surface of aluminum and ferric oxides. Due to the rapid drainage under tropical rainfall, however, this phenomenon is not very effective. Potassium is usually tightly fixed on 2:1 clay minerals, but these are nonexistent in our soil. Potassium also is bound to humic substances and the organic matter reaches 15% in the subsoil of this Ferralsol. Leaching of K+was retarded compared with NO−3, but was still quite high compared with others studies on tropical soils.

Time domain reflectometry (TDR) possesses potential for determining solute‐transport parameters, such as dispersion coefficients and retardation factors for reactive solutes. We developed a simple method based on peak‐to‐peak measurements of water and solute velocities through the soil using TDR. The method was tested by carrying out unsaturated leaching experiments in the laboratory on two soil columns packed with a South Pacific soil from Maré, which is a ferrasol with variable surface charge. One column was left bare and the other was planted with mustard. Pulses of CaBr2and Ca(NO3)2were applied to the surface of either wet or dry soil and then leached by water from a rainfall simulator applied at a steady rate of between 30 and 45 mm h−1Water and solute transport were monitored by collecting the effluent. Contemporaneous in situ measurements of the water content and electrical conductivity were made using TDR. Transport parameters for the convection–dispersion equation, with a linear adsorption isotherm, were obtained from the flux concentration and the solute resident concentrations measured by TDR. Anion retardations between 1.2 and 1.7, and dispersivities between 1 and 9 mm, were found. Retardations also were calculated using our simple approach based on TDR‐measured water and solute front velocities. These used TDR measurements of soil water content and bulk soil electrical conductivity with time, and were similar to those obtained from the effluent. The agreement suggests TDR could be a valuable in situ technique for obtaining the parameters relating to reactive solute transport through soil.