Studies of single soil organisms, while useful in specialized cases, e.g. Rhizobia and mycohrrizae, do not yield information on the functioning of the soil ecosystem. This is because most important soil processes, e.g. carbon and nitrogen mineralization, depend upon interactions between entire suites of organisms, many of which still await identification and most of which remain unculturable.

For many purposes, treating the soil microbial community as a single, undifferentiated unit, the soil microbial biomass (defined as all soil organisms <5000 μm3 volume), has much to commend it. It is analogous to studying the forest rather than an individual tree and uniquely permits an understanding of the soil–plant–microbe system as a whole, rather than studying only a small part. The present paper reviews the development of methods to measure microbial dynamics over the last century, the evolution of biomass methodologies and how they have helped the study of crucial soil processes such as nutrient and carbon cycling. Possible future directions for this research are also discussed and an explanation set forth of why the manipulation of this huge population (easily comprising 10 tonnes per hectare of living microbial cells in UK arable soils) has, so far, proved elusive.

The results of recent trials for winter wheat (Triticum aestivum L.) have influenced farming practice in the UK by encouraging the use of lower seed rates. Spink et al. (2000) have demonstrated that, particularly if sown early, wheat can compensate for reduced plant populations by increased tiller production.

Results from seed-rate trials are usually analysed separately for each environment or each combination of environment and variety, and not combined into a single model. They therefore address the question of what the best seed rate would have been for each combination, rather than answer the more relevant question of what rate to choose for a future site. The current paper presents a Bayesian method for combining data from seed-rate trials and choosing optimum seed rates: this method can incorporate information on seed and treatment costs, crop value and covariates. More importantly, for use as an advisory tool, it allows incorporation of expert knowledge of the crop and of the target site.

The method is illustrated using two series of trials: the first, carried out at two sites in 1997–99, investigated the effects of sowing date and variety in addition to seed rate. The second was conducted at seven sites in 2001–03 and included latitude and certain management factors. Recommended seed rates based on these series vary substantially with sowing date and latitude.

Two non-linear dose-response functions are fitted to the data, the widely used exponential-plus-linear function and the inverse-quadratic function (Nelder 1966). The inverse-quadratic function is found to provide a better fit to the data than the exponential-plus-linear and the latter function gives estimated optimum rates which are as much as 40% lower. The economic consequences of using one function rather than the other are not great in these circumstances.

The method is found to be robust to changes in the prior distribution and to other changes in the model used for dependence of yield on sowing date, latitude, variety and management factors.

Turkey is one of the most important durum wheat (T. turgidum L. var. durum Desf.) producers in West Asia and North Africa (WANA). There appears to have been a lack of progress in wheat yields in WANA over recent decades. Most of the modern cultivars grown in WANA are related to crosses produced by the International Maize and Wheat Improvement Center (CIMMYT). During 1999 and 2000, a chronological set of 11 cultivars related to CIMMYT crosses was evaluated using two irrigation regimes near Şanlıurfa, Turkey.

Average yields ranged from 386 g/m2 (local cultivar, Karakılçık-33) to 614 g/m2 (Ege-88). When wheat cultivars were grown with irrigation until physiological maturity, cessation of irrigation near anthesis reduced average grain yields of all cultivars by 6·2–25·7%. Since the introduction of the first generation of CIMMYT cultivars in 1975, CIMMYT grain yields have increased by about 0·60% per year compared with that of Karakılçık-33. This rate increased up to 1·08% per year until 1988. No further progress in yield was measured for varieties released in the 1990s. The average rate was 0·76% per year. After the introduction of the CIMMYT cultivars, yield changes fitted better in a quadratic function. Compared with Karakılçık-33, the yield increases of CIMMYT cultivars have resulted from an increase in harvest index associated with reduced height and the development of more and heavier grains. Grain quality traits decreased with the introduction of the first CIMMYT cultivar but recovered slightly thereafter with no clear trend. The results are discussed with consideration of breeding priorities and specific WANA conditions.

Motes are cotton (Gossypium spp.) ovules that fail to ripen into mature seeds. These aborted ovules represent a loss in yield and can cause imperfections in yarn and cloth quality. The present study was conducted to determine whether 10 widely grown cotton cultivars differ in some boll characters that are undesirable for the textile industry. The bolls at the first positions of fruiting branches formed at 1st (bottom), 6th (middle) and 11th (top) nodes of the main stem were compared in 10 varieties in 2 years for the number of motes and their weights, mote production frequency, and long fibre mote frequency within a boll.

Mote frequencies and mote weights were affected by varieties and years. Moreover, mote frequencies were higher at the bottom and middle plant positions compared with the top position.

Fibre length for long fibre motes ranged from 18 to 21 mm. In both years, cultivars Deltaopal, Sayar 314 and Suregrow 125 had consistently lower mote and long fibre mote frequencies. Thus, these cultivars could be useful in breeding programmes to improve seed-set. Cotton breeders also need to consider long fibre motes in plant improvement studies.

Climatic and soil conditions in Uruguay are marginal for white clover (Trifolium repens L.). Plants were collected from four sites, in different parts of the country, multiplied vegetatively, and the populations planted into each of the four sites. Plant survival and internode length (indicating growth potential) were recorded at intervals during the 55-week period after planting. On the site with the highest annual rainfall and lowest summer water deficit, one fifth of the plants survived at least 55 weeks, whereas on the site with the highest annual evaporation almost all the plants had died within 20 weeks. There was some evidence of adaptation to a ‘home’ site in both plant survival and growth potential. Thus, on three of the four sites, populations from the ‘home’ site survived better than the average of the non-‘home’ populations. Also, internode length was greatest (9·5 mm on average, compared with a mean of 5·6 mm for the experiment as a whole) in a population from a site with soil relatively high in available P, when planted into its ‘home’ site. Plant breeders could consider producing different white clover cultivars to suit different conditions of moisture supply and available P.

The Erosion and Productivity Impact Calculator (EPIC) has been used to determine the effect of different cropping systems and management practices on soil productivity in the Loess Plateau of China. However, its crop growth and soil water balance submodels have not been verified in this region. The objective of the present study was to evaluate the ability of EPIC to estimate soil water content (θ in m3/m3), seasonal evapotranspiration (ET in mm/season) and crop yield (Y in t/ha) for winter wheat and maize. A 20-year field experiment was conducted at the Changwu Agro-ecological Experimental Station of the Loess Plateau, and divided into a calibration period and a validation period. Data from calibration (1984–94) were used to optimize the four most sensitive parameters of the EPIC crop yield submodel, whereas data from 1994 to 2004 were used for validation. For both crops, there were no significant differences between measured and estimated long-term means of the three variables (P=0·05) for either the calibration or validation periods. EPIC estimated all three variables with a small relative root mean square error (RRMSE), i.e. the ratio of root mean square error to the mean value. For wheat and maize, the calibration period resulted in respective RRMSE values of 0·112 and 0·100 for θ, 0·121 and 0·116 for ET, and 0·135 and 0·147 for Y. During the validation period, the RRMSE values obtained were 0·090 and 0·085 for θ, 0·129 and 0·135 for ET, and 0·169 and 0·149 for Y, for wheat and maize, respectively. The performance of EPIC in estimating annual values of θ, ET and Y was variable. For validation, EPIC explained 65, 79 and 64% of the measured variations of θ, ET and Y, respectively, for wheat, and 60, 70 and 67% for maize. The EPIC-estimated long-term average values of the three variables were not significantly different from measured values for winter wheat and maize during the calibration and validation periods. It can therefore be used in the gully region of the Loess Plateau to define alternative cropping systems and management practices.

Increasing concern about non-point source pollutants released from grazing livestock, a worldwide problem, motivated the present study on the effects of vegetative filter strips (VFS) for controlling pollutants (nutrients, micro-organisms and sediment loading) from grazed, irrigated pastures. Flood-irrigated pastures are an important source of forage for livestock during summer months in California, USA when the surrounding rangelands are dry and dormant. Significant amounts of runoff can be generated from these pastures during irrigation events.

Nine plots on an irrigated pasture were assigned randomly to one of three treatments: Control (no VFS), Treatment VFS-1 (8·3×7 m, 0·0058 ha VFS) and Treatment VFS-2 (17·1×7 m, 0·012 ha VFS). In 2000, two grazing events (in April and June/July) occurred during the irrigation season prior to the experiment; further, the experimental plots were grazed between irrigations 2 and 3. Attenuation of runoff loads by VFS treatment was measured during four irrigation events (between 1 August and 3 October 2000) for total suspended solids (TSS), ortho-phosphate (Ortho-P), inorganic phosphate (Inorg-P), total phosphate (Total-P), organic phosphate (Org-P), polyphosphate (Poly-P), total Kjeldahl nitrogen (TKN), NH3, NO3 and presumptive faecal coliforms (FC).

On average, approximately 0·43 of the applied water left the plots as runoff. Treatment effects approached significance for TSS and TKN and were significant (P<0·05) for Poly-P and NH3. Irrigation number effects were significant for all but TSS, NO3 and FC. The effects of VFS treatments were not consistent. Treatment VFS-2, although representing the largest buffer strip, did not always produce the lowest pollutant loads in runoff. Slope, relatively high runoff volumes and some channelled flow were probably responsible for the limited effectiveness of VFS in the present study. These results suggest that effectiveness of VFS for reducing sediment and nutrient transport from irrigated pastures may be questionable.

Two 2-year field trials were conducted in Nigeria to investigate the effectiveness of tomato intercropped at different times with cowpea or okra in controlling the field densities of cowpea pod-sucking bugs and okra flea beetles. The two experiments were arranged as split-plots within a randomized complete block design. The main plots were 4-week-old tomato seedlings transplanted into a field, with cowpea or okra intercropped at 0, 2 or 4 weeks after transplanting tomatoes, while the two sub-plots were cowpea or okra sole crop and tomato/cowpea or tomato/okra intercrop. The densities of the pod-sucking bugs Clavigralla tomentosicollis, Riptortus dentipes, Anoplocnemis curvipes and Nezara viridula were significantly lower (P<0·05) on cowpea intercropped at 2 weeks after transplanting tomatoes than those obtained in the sole crop. Similarly, cowpea seed damage was significantly lower (P<0·05) on cowpea planted at 2 and 4 weeks after transplanting tomatoes than in the monocrop and when cowpea and tomato were planted simultaneously. Cowpea yields were influenced by both time of intercropping and cropping system; in the intercrop, the highest yield was obtained at 2 weeks after transplanting tomatoes. In the tomato/okra intercrop, the numbers of flea beetles (Podagrica spp.) were significantly influenced (P<0·05) by cropping system alone; their density was significantly lower (P<0·05) in the intercrop than in the sole crop irrespective of time of intercropping. Similarly, damage was significantly higher (P<0·05) in the sole crop than in the intercrop while yield comparable to that of the monocrop was obtained at 2 WAT. The highest land equivalent ratio (LER) for cowpea (1·8 and 1·6) or okra (0·7 and 0·8) was obtained at 2 weeks after transplanting tomatoes in both years. Two weeks after tomato establishment appeared to be the best time to introduce either cowpea or okra into the cropping system.

In order to assess the influence of nutrient elements on the accumulation of β-N-oxalyl-L-α, β-diaminopropionic acid (β-ODAP; the probable cause of lathyrism) in Lathyrus sativus L. (grass pea), it was first examined under field conditions during the lifespan of a grass pea plant using high-performance liquid chromatography (HPLC). β-ODAP mainly accumulated in young seedlings, in developing and mature seeds and in young leaves, especially in young seedlings. In contrast, all mature leaves, roots, and stems showed a low level of β-ODAP. The β-ODAP accumulation pattern in seedlings grown in a nutrient-deficient solution was highest (3·57 mg/g) in shoots at 7 d growing in the nitrogen-deficient solution and higher compared to the control (2·31 mg/g) in zinc-, calcium-, phosphorus- and molybdenum-deficient shoots (P<0·05). The contents of β-ODAP in seedlings growing in other element-deficient solutions were similar to controls. When the content of β-ODAP in grass pea seedlings fertilized with different forms of organic nitrogen was assayed the results indicated that amino acids such as glutamine and serine, as well as nucleotide nitrogen, all significantly enhanced the accumulation of β-ODAP in young seedlings relative to controls (P<0·05). Taken together, these data suggest that β-ODAP accumulation in grass pea might be related to the level of total free nitrogenous compounds and that nitrogen and phosphate may be the crucial nutrient factors influencing β-ODAP content under field conditions. Thus, the application of appropriate nitrogen and phosphorus fertilizers to the soil could decrease the content of β-ODAP in the seeds and leaves of grass pea.