A whole crop computer simulation model of winter wheat has been written in FORTRAN and used to simulate the growth of September-and October-sown crops of Hustler wheat at Rothamsted for the years 1978-9, 1979-80 and 1980-1. Results of the simulations, which are for crops with adequate water and nutrients, are compared with observations from experiments at Rothamsted. The model uses daily maximum and minimum temperatures and daylength to calculate the dates of emergence, double ridge, anthesis and maturity of the crops and the growth and senescence of tillers and leaves. In the simulations, the canopy intercepts daily radiation and produces dry matter that is partitioned between roots, shoots, leaves, ears and grain. Partial simulations, using observed LAI values, produced dry matter in close agreement with observations of late-sown crops, but consistently overestimated the total dry-matter production of the early-sown crops. Full simulation described satisfactorily the average difference in dry-matter production to be expected with changes in time of sowing, but did not give as close correspondence for individual crops. A grain growth submodel, that linked maximum grain weight to average temperatures during the grain growth period, correctly simulated the observed growth of individual grains in the 1981 crop. The benefits to be obtained by combining whole crop modelling with detailed crop observations are discussed.

Winter wheat growing on a silty clay loam soil was protected from rainfall by a mobile shelter for 100 days from tillering to maturity. During this time the crop was either irrigated according to demand or grew on stored soil water. The effects of this high and low water supply, in combination with a high and low N supply, on root and shoot growth and water uptake were studied. The crop given both N and water yielded 9.7 t/ha of grain (85 % DM), drought reduced this to 7.9 t/ha, low N to 4.3 t/ha and drought and low N to 3.8 t/ha. Yield reductions were mainly due to fewer grains being produced. Little root growth occurred in the topsoil during the drought but there was compensatory growth in the subsoil provided that N fertilizer was given. The droughted crops rooted to 160 cm, about 20 cm deeper than the irrigated crops, but the amount of root in the deep subsoil was very small, less than 0.1 cm/cm3 ai 140–160 cm, compared with 5–9 cm/cm3 in the topsoil. The crop demand for water at any given time was partitioned throughout the root system but atmospheric demand was only met whilst the topsoil was wet. The fertilized, droughted crop extracted all of the potentially available water to a depth of 80 cm and a mean rooting density of 1 cm/cm3 was necessary to achieve this. Uptake from below this depth was limited by root growth. The limiting value of the potential soil water deficit was 170 mm, and weather records showed that this would be exceeded one year in ten, on average. The likelihood of yield reduction due to drought could be reduced on this soil by improving root growth below 80 cm depth, although the chances of achieving this are low as root growth was probably limited by poor soil structure.

Abstract--Mineralogical and chemical analyses of fine clay fractions from in and around Lake Abert, Lake County, Oregon, show that the pyroclastic rocks supplying detritus to the lake weather to a suite of layer silicate clay minerals varying from high-charge dioctahedral montmorillonite to montmorillonite/intergrade smectite-chlorite interstratifications. In the lake these clays extract K, Mg, and Si to form authigenic interstratified illite and a trioctahedral, Mg-rich mineral resembling stevensite in composition. Both the neoformed interstratifications contribute little unambiguously to X-ray powder diffraction patterns, which are dominated by the reflections of detrital clays. From limited data it appears that the illite occurs below 0.8 m depth in sediments of a possibly somewhat fresher (brackish) lake and the trioctahedral interstratification between 0.4 and 0.2 m depth in sediments of a lake of about the same size and salinity (about 30-90 g/kg) as that of the present take.

Avalon winter wheat was grown in 1983 on a light-textured, sandy loam (Cottenham series) which had a subsoil pan with a maximum dry bulk density of 1-8 g/cm 3 at 35 cm depth. This was destroyed on part of the site with a 'Wye Double Digger' so that crop growth in panned and pan-free soils could be compared. The interaction of the pan with soil water supply was studied by sheltering the crops during May, June and July and either withholding water completely or irrigating weekly back to field capacity.The pan had a major effect on the vertical extension rate of the root system as monitored both by coring and from observation tubes. Roots were largely confined above the pan until March, but compensatory growth occurred within this soil layer and the total length of root was unaffected. At anthesis, roots had reached a maximum depth of 100 cm in the panned soil compared with 140 cm in the pan-free soil.Early shoot growth and N content were substantially reduced by the pan because of the inaccessibility of mineral N in the subsoil. However, both the growth of the crop and N uptake recovered following top dressings of N fertilizer and, when water was not limiting, the pan had a negligible effect on grain yield.Root and shoot growth were reduced by the fixed shelter, but the imposed drought did not affect water use by the crops until after anthesis when the root systems were already fully developed. Without irrigation, the crop growing on the double-dug soil yielded 5% more than that growing on the panned soil, but there was no evidence for extra water use from the subsoil by the former crop. The best treatment (double-dug with irrigation) outyielded the worst (panned soil with drought) by 8%.

A B S T R A C T: Investigation of the clay mineralogy of forty-seven samples of sediments from boreholes in the western N lie Delta, an area little studied hitherto, and from surface sites on the mouth of the Nile and adjacent coast shows that the clay fractions consist of dominant iron-rich, dioctahedral, randomly interstratified smectite-illites together with kaolinite, illite and chlorite.Amounts of the constituent minerals of the clay fractions are estimated from their X-ray diffraction intensities, supported by selective dissolution chemical data, and a new method is used to estimate the proportion of expanding layers in randomly interstratified smectite-illite. The results, which confrm and extend the work of previous investigators, also show that there is little correlation between the clay mineral composition and texture of the sediments, only kaolinite being weakly linearly correlated with clay content. Transformation of 2: 1 layer silicate minerals occurs within the buried sediments; chlorite is transformed and smectite and illite interlayers redistributed within randomly interstratified smectite-illites.

No abstract

An experiment to measure the variation in the phenological and apical development of winter wheat (cv. Avalon) in England and Scotland is described. Ten sites which ranged from Aberdeen (57-2° N), the most northerly, to Newton Abbot (50-6° N), the most southerly, were included in the survey, and at each site seed was hand-sown in midSeptember, October and November 1983. Developmental stages and sampling procedures were precisely defined to ensure uniformity in scoring by the observers at each site. Temperatures during the growing season were in line with the long-term means, though spring was cooler at all sites and summer warmer at most. The range of monthly-mean temperatures between sites was about the same as the difference between consecutive months. The method of analysis of development rates and durations was in terms of thermal time, modified by sensitivity to photoperiod and a vernalization requirement that slowed early development until a number of days of low temperatures had been experienced.In general, crops at northern sites developed more slowly than those in the south and particularly the south-west of England. There was less variation in the timing of apical stages for later sowings. Developmental rates responded linearly to temperature and photoperiod, with the base temperature increasing for later phases of development. The effect of photoperiod in modifying the rate of development was apparent for all developmental phases from emergence to anthesis, longer days accelerating development, but there was no effect on the duration of the grain-filling period. Vernalization exerted its effect solely within the phase from emergence to double ridge, and had a major influence on the variation between sites only for the first sowing.

Uptake of nitrogen from the subsoil (30–200 cm) by winter wheat has been measured in field experiments on deep loess‐parabrown soils in northern Germany and at Rothamsted (England) for different crop rotations and manuring schemes. The results can be summarised as follows: The mineral nitrogen content of the subsoil varies widely depending on farming practice. The effective depth limit for N uptake by winter wheat appears to be 150 cm. Averaged over 22 sites, 33% of the total N uptake was from the subsoil (range 9–75%); 25% was from the 30–90 cm soil layer and 8% from the 90–150 cm soil layer. Decreasing the N supply to the topsoil increased N uptake from the subsoil. N uptake from the subsoil is not dependent on water uptake from the subsoil; nitrate is readily transported to absorbing roots by diffusion. When deciding on the rate of fertilizer N to apply in early spring, soil mineral N to a depth of 90 cm should be taken into account. For subsequent dressings, the soil mineral N between 90–150 cm depth needs to be considered.