The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6-week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between ~600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore-size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore-size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth

winter oilseed rape growing seasons, numbers of air-borne ascospores of Leptosphaeria maculans were often > 4 m − 3 from autumn (September/October) to spring (April / May), while few or no ascospores were detected during the summer. Mature pseudothecia were generally not observed on debris of the previous crop until September. One-year-old debris (harvested in July 1998) had 95% discharged and 5% mature pseudothecia in August 1999, but by 15 September new pseudothecia (of which 30% were mature) were observed and the first increase in air-borne ascospores (> 4 m − 3 ) occurred. Phoma leaf spotting appeared in untreated field plots 14 -25 days after the first increase in air-borne ascospores in autumn. The fungicide mixture difenoconazole plus carbendazim decreased the incidence of new leaf lesions for 1 month after application in autumn and for 2 months in midwinter. When L. maculans was isolated from infected leaves, the growth rate of isolates from leaves to which fungicide was applied was less than that of those from untreated leaves. Foliar applications of fungicide to field plots in the autumn and winter not only decreased the incidence of crown cankers but also reduced the rate of canker development on stem bases in the spring and early summer (when severity of crown cankers increased linearly with time). In untreated crops, when phoma leaf spots appeared early in the autumn, crown cankers developed early in the spring but only became severe enough before harvest to reduce yield greatly in 1997/98. Yield loss was associated with crown cankers that girdled more than half of the stem by harvest (mean severity > 3 on a 0 -5 scale). Infections of new leaves produced after stem extension, from January onwards, led to phoma stem lesion development above the crown. In the three seasons, phoma stem lesions became moderately severe (> 2) by harvest only in untreated plots in 1997/98.

No abstract

No abstract

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water-filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ÿ85 kPa. In this paper, we report the development of a porous-matrix sensor to measure matric potential in the approximate range ÿ50 to ÿ300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water-filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ÿ80 kPa. The porous-matrix sensors appeared to give reliable readings of matric potential in soil down to ÿ300 kPa and also responded appropriately to repeated wetting and drying. This porous-matrix sensor has considerable potential to help understand plant responses to drying soil.