A. M. Alston

Penetration of very strong soils by seedling roots of different plant species

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 0.2 kg kg−1 water content and mean penetrometer resistance of 4.2 MPa. 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 vulgare) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.

Influence of root diameter on the penetration of seminal roots into a compacted subsoil

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-1, 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.

Effect of soil compaction on root growth and uptake of phosphorus

SummaryZea mays L. andLolium rigidum Gaud. were grown for 18 and 33 days respectively in pots containing three layers of soil each weighing 1 kg. The top and bottom layers were 100 mm deep and they had a bulk density of 1200 kg m−3, while the central layer of soil was compacted to one of 12 bulk densities between 1200 and 1750 kg m−3. The soil was labelled with32P and33P so that the contribution of the different layers of soil to the phosphorus content of the plant tops could be determined. Soil water potential was maintained between −20 and −100 kPa.Total dry weight of the plant tops and total root length were slightly affected by compaction of the soil, but root distribution was greatly altered. Compaction decreased root length in the compacted soil but increased root length in the overlying soil. Where bulk density was 1550 kg m−3, root length in the compacted soil was about 0.5 of the maximum. At that density, the penetrometer resistance of the soil was 1.25 and 5.0 MPa and air porosity was 0.05 and 0.14 at water potentials of −20 and −100 kPa respectively, and daytime oxygen concentrations in the soil atmosphere at time of harvest were about 0.1 m3m−3. Roots failed to grow completely through the compacted layer of soil at bulk densities ≥ 1550 kg m−3. No differences were detected in the abilities of the two species to penetrate compacted soil.Ryegrass absorbed about twice as much phosphorus from uncompacted soil per unit length of root as did maize. Uptake of phosphorus from each layer of soil was related to the length of root in that layer, but differences in uptake between layers existed. Phosphorus uptake per unit length of root was higher from compacted than from uncompacted soil, particularly in the case of ryegrass at bulk densities of 1300–1500 kg m−3.

Measurement of phosphorus in the soil microbial biomass: A modified procedure for field soils

Abstract A range of gaseous, liquid and vapour biocides was tested in combination with seven extradants for their ability to release P from soil microorganisms in situ . The biocides tested were chloroform, ethanol, propan-1-ol, hexan-1-ol, β-propiolactone, formaldehyde, glutaraldehyde, ethylene oxide and methyl bromide. The extradants tested were 0.5 M NaHCO 3 (pH 8.5), 0.1 M NaHCO 3 (pH 8.5), 50 mM NaOH, 10 mM CaCl 2 , 50 m m H 2 SO 4 , 30 m m NH 4 F + 0.1 m HC1, and an anion exchange resin in the bicarbonate form suspended in distilled water. An incubation technique using 32 P ensured that only microbial P was measured. Chloroform and hexanol were the most effective biocides: the latter is preferred because of its less hazardous nature. The best extradant was 0.5 M NaHCO 3 (pH 8.5). Mixed populations of soil organisms were used for calibration, and the K p factors obtained with hexanol and 0.5 m NaHCO 3 (pH 8.5) were 0.33, 0.40 and 0.57 for the three soils studied. Since microflora differ from soil to soil, as do the amounts and form of P released, calibration is necessary for each soil. Incubation is not recommended as a prior treatment for samples used to measure biomass P: the soils should be treated with hexanol or extracted immediately after sampling to avoid quantitative or qualitative changes in the biota or biomass. Errors associated with the inclusion of roots in the sample can be minimized by removing the bulk of the roots before fumigation.

Maximum axial and radial growth pressures of plant roots

SummaryThe 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 Fmax 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.

Phytotoxicity of aluminium-fluoride complexes and their uptake from solution culture by Avena sativa and Lycopersicon esculentum

AbstractAvena sativa (oats) and Lycopersiconesculentum (tomatoes) were grown in dilute nutrientsolutions supplemented with Al only, F only or acombination of both. In solutions containing Al andF, shoot growth was limited when predicted Alr(Al3+, AlOH2+ and Al(OH)

Formation of aggregates by plant roots in homogenised soils

_2^ +

The effects of soil contamination with copper, lead and arsenic on the growth and composition of plants

)activities were < 0.1 μM, activities three orders ofmagnitude lower than the critical value determinedwith Al only. The data suggest that at the activitiesused in these experiments, Alr is most toxic,AlF2+ and AlF