Loamy Topsoil Research Articles

Concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content

Concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover (Rc) on sediment yield for an initially wet and an initially air-dry loamy topsoil. The experimental results indicate that Rc reduces concentrated flow erosion rates ( E) in an exponential way (i.e., E= e − bRc ), which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet-rill erosion measured on runoff plots. The decay rate ( b) of this exponential relationship increased throughout the experiments because of scour-hole development and bed armouring. The concentrated flow erosion rates and b-values also depend on the initial moisture content of the topsoil. Depending on Rc, mean concentrated flow erosion rates were 20% to 65% less on initially wet compared to initially air-dry topsoils. The mean value for b was 0.032 for the initially wet, but only 0.017 for the initially air-dry topsoil, indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air-dry than when it is initially wet. The results help explain the data scatter in reported relationships between Rc and interrill–rill erosion rates. They also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils, which are very common in Mediterranean environments. Event-based water erosion models should incorporate effects of antecedent soil moisture content as well as those of Rc on concentrated flow erosion rates.

Aliphatic hydrocarbons in an oil-contaminated soil

Microbial decontamination of hydrocarbon-polluted soil was paralleled with soil respiration measurements. About 1,500 tons of a loamy top soil were found to be contaminated with approximately 2000 mg/kg of aliphatic hydrocarbons, mainly oleic (C18:1) and linoleic acid (C18:2) found in the vicinity of a linoleum manufacturing and then a car dewaxing plant. The contaminated soil was analysed for dry matter, pH, dehydrogenase activity, electrical conductivity and nutrient content viz. nitrate, phosphorus and potassium, as well as a number of indigenous microbes. The soil was low in salt and nutrients. This paper describes the procedure and measures to decontaminate this bulk soil on site from approx. 2,000 to 500 mg of aliphatic hydrocarbons/kg dry matter by use of a nutrient emulsion, indigenous micro-organisms and aeration over 13 months. This 75% reduction in aliphatic hydrocarbons resulted in a concomitant carbon efflux, measured as soil respiration, and was used to calculate carbon fluxes.

Decomposition of 14C-labelled plant material in a salt-affected soil

Sodicity and salinity are each known to influence the decomposition of plant residues, but the effects of their interactions are not well understood. In this study, a loamy topsoil was adjusted to 3 values of exchangeable Na (0, 3 and 25%), for each of 3 values of salinity (0.1, 0.5 and 1.5 dS m −1 in a 1:5 soil-to-water extract), by equilibration with solutions containing appropriate amounts of NaCl and CaCl 2. 14C-labelled Trifolium subterraneum shoots, either finely ground or unground, were added to the soils at a rate of 2.22 g kg −1 soil. Samples were kept for 82 days at −50 kPa water potential, or 97 days with drying-rewetting cycles. Mineralisation of ground plant C increased with increasing sodicity, and decreased with increasing salinity. The influence of both sodicity and salinity were less with unground than ground plant material because of reduced interaction of substrates with the soil matrix. The effects of the treatments on mineralisation of plant C were established within the first 20 days of incubation. Mineralisation of native soil C was increased by sodicity throughout the incubation at low salinity, but was unaffected by sodicity at high salinity. At the end of the incubation, neither plant- nor soil-derived microbial biomass C were greatly affected by sodicity or salinity. Total microbial biomass C was significantly higher in the soils kept at constant water content than in those submitted to drying-rewetting, in which it was approximately the same as in the soils before incubation. Of the total microbial biomass C, 50–93% was derived from the plant material. Drying-rewetting cycles decreased mineralisation and microbial biomass C (both plant- and soil-derived), but did not influence the effects of sodicity and salinity. At each value of salinity, mineralised plant C was positively correlated with water-extractable organic C derived from plant material. This was also true for soil-derived C at low, but not at high values of salinity. Mineralisation generally resulted in a reduction in the amount of carbohydrate C relative to other forms of organic C, as determined by 13C CP-MAS nuclear magnetic resonance.

Interactive effects of exchangeable sodium and water content on soil modulus of rupture

The interactive effect between exchangeable sodium (sodium adsorption ratio, SAR = 1, 7, 15, 30 and 50) and water content (water saturated and field capacity) on soil hardsetting behaviour was investigated in a loamy topsoil of a Typic Natraquoll. Hardsetting was evaluated by the modulus of rupture (MOR) of dry aggregates, previously subjected to a static pressure simulating that imposed by an adult cow (400 kPa). A control receiving no pressure was included. In both control and water saturated treatments, MOR increased steadily (from 6.5 to 10.4 MPa) with SAR. This indicates that the effect of exchangeable sodium prevailed over that of water content when the soil was stressed at saturation. Exchangeable sodium and water content interacted significantly (F ratio = 4.56; α < 0.002) during the applied pressure, but this interaction was not synergistic. Soil stressing at field capacity prevented MOR increases above a threshold of SAR ≥ 7. As a consequence, an attenuation of hardsetting behaviour can be expected when the studied soil is trampled in moist field conditions.

Growth of Perennials and Leaching of Heavy Metals in Media Amended with a Municipal Leaf, Sewage Sludge and Street Sand Compost

Abstract Municipal leaves, sewage sludge, and street sand were composted by a modified Beltsville aerated pile technique. Rooted cuttings of Aster novi-belgii L. ‘Peter Harrison,’ Gaura Lindheimeri Engelm. &amp; A. Gray and Sedum purpurem (L.) Link ‘Autumn Joy’ were grown for 4 months in 2 1 (2 qt) containers filled with media containing 0, 10, 30, 60, 80 and 100 (percent by vol) compost. The medium with 0% compost contained loamy sand topsoil, peat, sand and Styrofoam pellets (1:5:2:2 by vol). The pH of all media was between 6.0 and 7.0. Half the containers in each treatment received liquid fertilization in mid-season and half did not. Leachate from the containers was collected at monthly intervals and analyzed for the heavy metals Cd, Cr, Cu, Mn, Ni, Pb and Zn to determine their potential for leaching. Growth of perennials was equal or greater in all media containing compost, compared to the medium containing no compost. Liquid fertilizer further improved growth. Adding compost increased media aeration, available plant nutrients and heavy metals and decreased moisture retention and bulk density. Heavy metal concentrations in leachate were low, probably due to the near neutral media pH, and not likely to pose an environmental risk.

Laboratory experiments on the effects of simulated tractor wheelings on linear soil erosion

The influence of the orientation of simulated tractor ruts on soil-erosion processes on a silty loamy topsoil with varying moisture content is studied. A sediment-transporting capacity equation is used to calculate the erosion efficiency in different experimental situations. The experiments show that the erosion resistance of wheel tracks is strongly limited. A compacted topsoil, that is rewetted at once after a long dry period, is very sensitive to severe erosion. In particular, tracks formed by tractors that have driven upslope can provoke linear erosion. Rill development in a wheel track can proceed very rapidly owing to the formation of numerous headcuts.

Potato irrigation scheduling and straw mulching

The effect of mulching and irrigation frequency on tuber yield was investigated during three seasons at the Glen Agricultural Research Institute The soil on the experimental site consisted of a loamy sand topsoil and sandy clay subsoil with 94 mm total available water in the top 775 mm of the soil profile. A wheal straw mulch of 14 t ha−1 reduced soil temperature and increased tuber yield consistently (averaged over all irrigation treatments in all three seasons, mulched plots produced 30% more tubers than bare plots). Tuber yield also responded well to increased irrigation frequencies it was concluded that the cooling effect ot frequent irrigation could be obtained much cheaper by mulching It is recommended that from commencement of stolonization, potatoes should be irrigated with 50 mm of water as soon as 50 mm of water has evaporated from a Class A pan for an unmulched crop and 65 mm for a mulched crop. S Afr J. Plant Soil 1984. 1: 111–116