On moist incubation the equivalent of 50 t ha-1 or more of ground, readily decomposable organic materials greatly increased the proportion of stable aggregates of Shepparton fine sandy loam within 1-4 weeks; the aggregates remained stable for up to 32 weeks if left undisturbed. Severe restriction of microbial activity in aggregates of Shepparton fine sandy loam by sterilization or dryness increased the effect of physical disruption associated with intermittent wetting and drying, and simulated cultivation. The results suggest that microorganisms in non-sterile moist soil can produce bonding substances which compensate partially for those bonds broken physically.

Penetration tests were conducted in a saturated clay using fine probes and pea radicles. The point resistance encountered by fine probes, Qp, was related inversely to the rate of penetration of the probe. The resistance increased exponentially with decreasing voids ratio, ew,; at any given value of ew more resistance was offered by clay that had been remoulded than by finely aggregated clay. When radicles grown through a bed of soil crumbs encountered cores of moderately consolidated saturated clay, they merely indented the clay before turning and growing horizontally. When their tips were inserted in channels that had been formed in the cores, they were unable to penetrate directly through moderately consolidated remoulded clay (ew < 1.10), but grew along horizontal fissures that developed in the clay. No fissures developed in the aggregated clay, and the radicles were unable to elongate at ew < 0.81. The results of the penetration tests are interpreted with the aid of a mathematical model taking into account the strength and compressibility of the clay. In addition, multiple exposure photography was used to show the paths of motion followed by clay particles near the tip of the probe and the radicle. Qualitative information on friction between the root tip and the clay was obtained by matching the local deformation of the clay around the root tip with the deformations produced by probes of known roughness. The root tips tended to compress the clay 'cylindrically', whereas the probe tips compressed the clay 'spherically'. There appeared to be little friction between the root tip and the clay.

No abstract

The effect of soil water supply on peach root production in summer was studied in the Goulburn Valley area of northern Victoria. Under commercial irrigation practice roots do not grow in summer. Field experiments showed that when the soil was kept moist by frequent irrigation (every 3–4 days), the concentration of roots in the surface soil nearly doubled in a single season. Furthermore, observation through glass plates showed that this enhanced root growth occurred throughout summer. When roots were kept inactive in dry soil until midsummer and thereafter irrigated frequently, they immediately responded and grew rapidly. The increased root growth did not impair fruit production. In a glasshouse experiment, with glass-fronted chambers, a relationship between root elongation, root concentration and the drying rate of the soil was found. Each experiment supported the hypothesis that soil drying rate determines the growth of roots in the surface soil. Where there was a high concentration of roots, a low irrigation frequency, and high transpiration, the rate of soil drying was rapid and roots grew slowly. It is suggested that manipulation of root growth by regulation of soil water supply may be an important method of studying root-top interactions.

A survey of root distribution of peach and pear trees has been made in shallow, medium, and deep soils in Goulburn Valley orchards. Root patterns were similar in both species, and shallow (less than 36 in.) in all soils. A further survey in soil management trials showed that the influence of soil management treatments was on roots in the surface soil. Roots grew as close to the actual ground surface as was allowed by such factors as cultivation depth, competition, and soil temperature.