No abstract

A field experiment was conducted to evaluate the influence of root diameter on the ability of roots of eight plant species to penetrate a compacted subsoil below a tilled layer. The soil was a fine sandy loam red-brown earth with a soil strength of about 3.0 MPa (at water content of 0.13 kg kg-l, corresponding to 0.81 plastic limit) at the base of a tilled layer. Relative root diameter (RRD), which was calculated as the ratio of the mean diameters of roots of plants grown in compacted soil to the mean diameters of those from uncompacted soil, was used to compare the sensitivity of roots to thicken under mechanical stress.Diameters of root tips of plants grown in soil with a compacted layer were consistently larger than those from uncompacted soil. Tap-rooted species generally had bigger diameters and RRDs than fibrous-rooted species. A higher proportion of thicker roots penetrated the strong layer at the interface than thinner roots. There were differences between plant species in the extent to which root diameter increased in response to the compaction. The roots which had larger RRD also tended to have higher penetration percentage.The results suggest that the size of a root has a significant influence on its ability to penetrate strong soil layers. It is suggested that this could be related to the effects which root diameter may have on root growth pressure and on the mode of soil deformation during penetration.

The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2kgkg -I water content and mean penetrometer resistance of 4.2MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).Differences in both root elongation and root diameter were observed among plant species. Generally, the roots of dicotyledons (with large diameters) penetrated the strong medium more than graminaceous monocotyledons (with smaller diameters). There was a significant positive correlation (r--0.78, p < 0.05) between root diameter and elongation over all the species in the stressed plants. The species were ranked according to the relative root elongation and relative root thickening. Based on this ranking, lupin (Lupinus angustifolius), medic (Medicago scutelata) and faba bean (Vicia faba) were the species with the greatest thickening and elongation while wheat (Triticum aestivum), rhodesgrass (Chloris gayana) and barley (Hordeum vulgate) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.

Ten soils varying widely in chemistry and mineralogy were used to examine the physical and chemical properties that influence As sorption processes in soils. The sorption of As was measured using a batch technique with Asv and AsIII concentrations added in background solutions of NaNO3 (0.003–0.3 mol L−1) and a range of pH values (2.0–8.5). The soils generally sorbed more Asv than AsIII at equivalent As concentrations. In general, highly oxidic soils sorbed three times more Asv than soils containing small amounts of oxidic minerals. The effect of pH on As sorption varied considerably among the soils. In soils with low concentrations of oxidic minerals, increasing pH had little affect on the amount of Asv sorbed while in highly oxidic soils, sorption of Asv decreased with increasing pH. This decrease was attributed to two interacting factors, the increasing negative surface potential on the plane of sorption and increasing amount of negatively charged Asv species present in soil solution. In contrast to Asv, sorption of AsIII increased with increasing pH. The effect of ionic strength on As sorption varied between the As species. In general, there was a pH (∼3) below which Asv sorption decreased with increasing ionic strength and above which the reverse occurred. For AsIII species, increasing ionic strength had little effect on the amount of As sorbed.

Summary The axial root growth force exerted by seedlings of pea (Pisum sativum cv. Greenfeast), cotton (Gossypium hirsutum cv. Sicot 3) and sunflower (Helianthus annuus cv. Hysun) was measured.Effects of different seedling age and different batches of seeds on axial root growth pressure were investigated.Mean values of the maximum axial root growth pressure (Pa) estimated from the maximum axial root growth force (Fmax) and root diameter were 497, 289, and 238 kPa respectively for pea, cotton and sunflower seedlings of same size. Pa and Fma x were significantly influenced by seedling age and for pea seedlings of same age they varied with the seed batch.A new technique was developed for estimating radial root growth pressure and was tested on pea seedlings. Each pea root was confined both in the axial and radial directions in a cylindrical chalk sample at a constant water potential. The roots exerted radial stress which caused tensile failure in a proportion of the chalks. The measurement of tensile strength of duplicate chalks enabled estimation of the maximum radial pressures exerted by the roots. The maximum axial and radial root growth pressures were of comparable magnitude.

No abstract

Wheat (Triticum aestivum L. cv. Warigal) was grown in a glasshouse in deep pots (0.125 x 0.125 x 1.2 m) containing sieved solonized brown soil (calcixerollic xerochrept) comprising 0.2 m sandy loam topsoil above 0.6 m treated calcareous sandy loam subsoil and a base layer of light clay 0.26 m thick. The subsoil was treated with a mixture of salts (0, 13, 39, 75 mmolc kg-1) and with boric acid (0, 20, 38 and 73 mg B kg-1) in factorial combination. The soil was initially watered to field capacity and water use was determined by regularly weighing the pots. The soil was allowed to dry gradually during the season, but the weights of the pots were not permitted to fall below that corresponding to 17% of the available water holding capacity of the soil. Tillering, dry weight of shoots and grain, and root length density were determined. Water-use efficiency was calculated with respect to total dry weight and grain production. Salt decreased tillering, dry matter production, grain yield, root length and water-use efficiency (total dry weight): it increased sodium and decreased boron concentrations in the plants. Boron decreased dry matter production (but not tillering), grain yield, root length and water-use efficiency (total dry weight and grain yield): it increased the concentrations of boron and decreased the concentration of sodium in the plants. At the concentrations of salt and boron used (which cover the range normally encountered in subsoils in much of Upper Eyre Peninsula), boron had more deleterious effects on wheat than did salt. Yield was depressed by salt at concentrations of sodium in the tissue commonly found in field-grown plants.

The sulfonylurea herbicides comprise a group of compounds designed to control broad-leaved weeds and some grasses in a variety of crops. The herbicides have become popular because of their low application rates (10-40 g/ha), low mammalian toxicity, and unprecedented herbicidal activity. We present a review of the fate and behaviour of these herbicides in soils with particular reference to alkaline soils of Australia. The review shows that the low application rates of sulfonylurea herbicides continue to present an analytical challenge, although in recent years a number of new methods capable of detecting them at very low concentrations have been developed. A range of analytical methods is available, including high performance liquid chromatography, gas chromatography, immunoassay, and bioassay. However, analytical sensitivity required to detect trace levels of these herbicides continues to pose problems in routine detection of herbicide residues in soils. The review reveals that there are no reports of studies of the behaviour of sulfonylureas in soils with pH >8·2. This is of particular significance to Australian conditions because a number of Australian soils are even more alkaline, and the pH(water) in subsoils can be as high as 10· 2. Sorption of sulfonylureas is pH-dependent and has a strong negative correlation with pH. At pH >8·0 sorption is very low. In acid soils, however, sorption of chlorsulfuron, metsulfuron-methyl, and triasulfuron is strongly influenced by the soil temperature, clay content, and, particularly, organic matter content. The principal modes of degradation of the herbicides are acid hydrolysis and microbial degradation with the latter being the only major pathway in alkaline soils. Hydrolysis of the sulfonylureas is more rapid under acidic conditions (pH 4{7), and the data suggest that hydrolysis is likely to be very slow in alkaline soils. Data from other countries suggest that the half-life of chlorsulfur on increases exponentially with pH, and that it is also influenced by variations in the temperature and water content of the soil. Being acidic in nature, the herbicide molecules become anionic at high pH and can move to a considerable depth in the soil profile by leaching. Movement of the sulfonylureas in soil is largely influenced by organic matter content and soil pH and the reviewed data show that sulfonylureas have substantial leaching potential in the sandy alkaline soils of Australia. This is likely to result in increased persistence in alkaline subsoils lacking in organic matter and biological activity. Computer models to predict the persistence and movement of the sulfonylureas are available; however, additional input parameters are required to predict accurately the behaviour of specific herbicides in alkaline soils under Australian conditions. Since new herbicides with chemistry similar to existing sulfonylureas are increasingly likely to be available for use, there is a need to develop comprehensive understanding of their fate, behaviour, and impact on Australian cropping and ecological systems.