Uncropped Plots Research Articles

Effects of rare arable plants on flower-visiting wild bees in agricultural fields

Arable plants and wild bees are negatively affected by agricultural intensification, one of the major drivers of global biodiversity loss. However, it remains unclear how endangered and low competitive arable plants (rare arable plants) contribute to the persistence of flower-visiting wild bees by providing additional flower resources in agricultural landscapes. Thus, the effects of sowing 10 rare arable plant species on wild bees were investigated in an experimental field and on 10 different arable farms on nutrient-poor soils. Sowing of rare arable plants on cropped and uncropped plots was compared to annual and perennial wildflower strips. Results showed that rare arable plants on uncropped plots attracted as many wild bees as wildflower strips. Wild bee abundance and species richness increased in the autumn-sown crops in the second year, likely because winter annual rare arable plants were preferred. In particular, rare arable plants provided flowers preferred by long-tongued bumblebees, which are often lacking in intensively managed arable fields. Our study shows that sowing of rare arable plants can increase niche diversity and therefore resource availability for wild bees, and it can also conserve diversity of arable plants in degraded agricultural landscapes. Conservation of arable plants through sowing can also support wild bee communities and may become an important tool in pollinator-friendly management of arable land.

Repetitive land application of urban sewage sludge: Effect of amendment rates and soil texture on fertility and degradation parameters

The agricultural reuse of urban sewage sludge in degraded croplands has seen a wide acceptance for biowaste recycling and for the improvement of soil fertility. However, soil degradation and health issues may arise from the fact that sludge addition causes chemical and biological contamination if mismanaged. To closely monitor the long term soil-sludge interactions, a field study was conducted over a three-year period under a semi-arid climate. More precisely, changes in topsoil properties were assessed in 4-m2 uncropped plots amended with sludge rates equivalent to 0, 40, 80 and 120 t ha−1 year−1 for two soil textures: soil A (sandy loam) and soil S (sandy). Results showed that all studied parameters changed significantly in a dose-dependent manner for both soils as compared to untreated controls. As such, sludge addition improved consistently TOC, N, P and K content up to soils treated with 120 t ha−1 year−1. The enhancement of soil nutrient status impacted positively on biological properties, including microbial biomass and soil enzyme activities (dehydrogenase, protease and phosphatase). However, the variation of soil properties was more important in soil A characterized by a higher fine fraction thus stronger retention capacity. Accordingly, XRD analysis of soil A revealed several identifiable peaks representing kaolinite and illite clays in contrast to soil S. Unexpectedly, high to excessive sludge doses of 80 and 120 t ha−1 year−1 did not provoke soil degradation after three repeated annual amendments. In fact, pH values in both soils remained within neutral to alkaline range (7.76–8.63); total heavy metals (Cu, Zn, Ni and Pb), soil salinity and fecal coliforms were all below threshold values for contaminated soils.

Performance of multipurpose trees and the associated crops in agroforestry system of Mizoram

An evaluation was made on the performance of three tree species viz. Alnus nepalensis D. Don. Melia azadirachta A. Juss. and Gmelina arborea Roxb. and their interactive effects on crop yield during establishment period. The seedlings of all tree species survived better under cropped than un-cropped plots. At the age of 36 months, the maximum height and collar diameter (CD) were attained by Melia azadirachta A. Juss. (356.53 cm and 9.54 cm) and minimum by Alnus nepalensis D. Don. (106.16 cm and 3.91 cm). The seedlings under cropped area became significantly (P<0.05) taller and thicker in comparison to un-cropped area. The yield of Ginger and Turmeric was found better under tree-crop association than pure crops after a 3-yr period. Our results depict that tree-crop association may provide better micro-site conditions like soil moisture and nutrients than control (pure crop) and may favour better crop productivity.

Land‐use and fire history effects on post‐fire vegetation dynamics in eastern Spain

AbstractQuestion: Is post‐fire, medium‐term vegetation dynamics determined by land‐use or fire history prior to fire?Location: South‐facing slope in the Gallinera valley, Alicante province, eastern Spain.Methods: After mapping the land‐use and fire history of the study site using photo‐interpretation, we sampled vegetation structure on a set of plots representing the most frequent land‐use and fire history combinations on an area burned six years before sampling. We studied the effects of land‐use history, comparing the one‐fire land‐use trajectories. We analysed the effects of fire history; comparing one‐ and two‐fire plots for both previously cropped and uncropped areas.Results: Most variables were not significantly different between the earliest abandoned plots (abandoned at least 38 years before the fire) and the uncropped plots. On the most recently abandoned plots (abandoned between one and four years before the fire), the therophyte richness and the ratio of seeder: resprouter richness were significantly greatest. Different fire recurrences did not determine different post‐fire vegetation on either the uncropped or the early abandoned plots (all dominated by fire‐recruited seeder shrubs). The most recently abandoned plots had a lower resilience to fire.Conclusions: Land‐use history and recent pre‐fire land use, in particular, determined the post‐fire vegetation in the medium term. The vegetation composition converged during secondary succession among land‐use histories. Increasing fire recurrence had a small effect on mature plant communities, due to the combination of life‐history traits determining the response to fire of the dominant species.

Land‐use history and fire effects on soil fertility in eastern Spain

SummaryChanges in land use and fire directly affect the physico‐chemical properties of the soil. In an eastern Spanish area, we analysed the effects of land‐use history and fire frequency prior to fire on soil fertility 9 years after fire. The results showed that long‐term cultivation caused decreases in soil organic matter, total nitrogen (N) and C/N. Between 20 and 40 years after cropping ceased, soil organic carbon (C) and total N had generally not recovered values similar to those found in soils that were never cropped. Total phosphorus and available phosphorus contents were larger in the latest abandoned plots than in the earliest abandoned plots and the uncropped ones, which was interpreted as a result of fertilization. Increasing fire frequency from one to two fires generally caused a decrease in soil organic C, total N, C/N, total phosphorus and available phosphorus. We observed that the losses of organic C in the soil caused by fire were larger among the uncropped plots and hypothesized that fuel loads and thus fire intensities were larger in these ecosystems. Our results also suggested that both long‐term cultivation and fire would tend to separate C and P cycles.

Synchrony of flowering between canola and wild radish (Raphanus raphanistrum)

Many conditions need to be satisfied for gene flow to occur between a transgenic crop and its weedy relatives. Flowering overlap is one essential requirement for hybrid formation. Hybridization can occur between canola and its wild relative, wild radish. We studied the effects of wild radish plant density and date of emergence, canola (glyphosate resistant) planting dates, presence of other weeds, and presence of a wheat crop on the synchrony of flowering between wild radish and canola (as a crop and volunteer). Four field experiments were conducted from 2000 to 2002 in St-David de Lévis, Québec. Flowering periods of wild radish emerging after glyphosate application overlapped with early-, intermediate-, and late-seeded canola 14, 26, and 55%, respectively, of the total flowering time. Flowering periods of early-emerging wild radish and canola volunteers in uncropped treatments overlapped from mid-June until the end of July, ranging from 26 to 81% of the total flowering time. Flowering periods of wild radish and canola volunteers emerging synchronously on May 30 or June 5 as weeds in wheat overlapped 88 and 42%, respectively, of their total flowering time. For later emergence dates, few flowers or seeds were produced by both species because of wheat competition. Wild radish density in canola and wild radish and canola volunteer densities in wheat did not affect the mean flowering dates of wild radish or canola. Increasing wild radish density in uncropped plots (pure or weedy stands) hastened wild radish flowering. Our results show that if hybridization is to happen, it will be most likely with uncontrolled early-emerging weeds in crops or on roadsides, field margins, and uncultivated areas, stressing the need to control the early flush of weeds, weedy relatives, and crop volunteers in noncrop areas.

Effects of municipal solid waste compost amendments on soil enzyme activities and bacterial genetic diversity

Municipal solid waste (MSW) composts have been used to maintain the long-term productivity of agroecosystems and to protect the soil environment from overcropping, changes in climatic conditions and inadequate management; they also have the additional benefit of reducing waste disposal costs. Since MSW may contain heavy metals and other toxic compounds, amendments cannot only influence soil fertility, but may also affect the composition and activity of soil microorganisms. The effects of MSW compost and mineral N amendments in a 6-year field trial on some physical–chemical properties, enzyme activities and bacterial genetic diversity of cropped plots ( Beta vulgaris– Triticum turgidum rotation) and uncropped plots were investigated. The compost was added at the recommended and twice the recommended dosage (12, 24 t ha −1). Amendments of cropped plots with MSW compost increased the contents of organic C from 13.3 to 15.0 g kg −1 soil and total N from 1.55 to 1.65 g kg −1 soil. There were significant increases in dehydrogenase (9.6%), β-glucosidase (13.5%), urease (15.4%), nitrate reductase (21.4%) and phosphatase (9.7%) activities. A significant reduction in protease activity (from 3.6 to 2.8 U g −1 soil) was measured when a double dose of compost was added to the cropped plots. No dosage effect was detected for the other enzymes. Changes in the microbial community, as a consequence of MSW amendment, were minimal as determined using denaturing gradient gel electrophoresis, rDNA internal spacer analysis and amplified ribosomal DNA restriction analysis of bacteria, archaea, actinomycetes, and ammonia oxidizers. This indicates that there was no significant variation in the overall bacterial communities nor in selected taxonomic groups deemed to be essential for soil fertility.

Runoff of herbicides from cropped and uncropped plots with different slopes

Losses of the selective herbicides alachlor, metolachlor and EPTC were monitored in runoff waters from experimentally controlled clay-soil plots cultivated with corn. Conditions were selected to simulate agricultural practices employed in the Mediterranean region. Plots were selected to have differing slopes, and uncultivated (control) areas were simultaneously monitored. The duration of the experiment was one year. The surface slopes of the plots, were 0, 1, 2.5, and 5%. Soil erosion was shown to increase with slope from 470 to 1350 kg/ha for slopes of 0 to 5% respectively and the corresponding runoff of surface water was from 6.5 to 26.4% of the total rainfall. Surface runoff herbicide concentrations were highest in the first flush events following the two herbicide applications. Initial concentrations were related to the time that had elapsed between the herbicide application and the runoff event. The maximum concentrations of herbicides in runoff were observed for the plots with the greatest slope (5%) reaching 142 and 154 µg/L for alachlor, 135 and 221 µg/L for metolachlor, and 765 and 1.071 µg/L for EPTC, in tilled and untilled plots, respectively. Cumulative losses of alachlor in surface runoff from tilled and untilled plots (with a slope of 5%) were estimated to be 0.27 and 0.47% of the initially applied active ingredient, respectively. For plots with a slope of 0%, the percentage loss dropped to 0.03 and 0.06% respectively. For metolachlor, the corresponding values from tilled and untilled plots were 0.42 and 1.09% (for a slope of 5%), and 0.08 and 0.16% (for a slope of 0%). In the case of EPTC, the corresponding values from tilled and untilled plots were 0.44 and 0.63% (slope 5%), and 0.04 and 0.07% (slope 0%). Analyses of soil samples from the plots during the period enabled dissipation kinetics to be studied. Results showed that corn cultivation decreased the half-lives of the herbicides [when compared to the control (uncultivated) fields] as follows: for alachlor from 16.7 to 10 days, for metolachlor from 22.3 to 13.3 days and for EPTC from 17.8 to 8 days. The slope of the soil surface, the compound's solubility and its sorption characteristics are identified as the major parameters that influence alachlor, metolachlor and EPTC transport in the present study.

Short-term effects of municipal solid waste compost amendments on soil carbon and nitrogen content, some enzyme activities and genetic diversity

Municipal solid waste (MSW) composts have been frequently used as N and C amendments to improve soil quality and to support plant growth, with the additional benefit of reducing waste disposal costs. However, attention has been paid to the risks of MSW use for the soil environment. The presence of heavy metals in MSW composts can affect some microbiological characteristics of soil such as the structure of the soil microbiota, which are responsible for the transformations making nutrients available to plants. The effects of MSW compost and mineral-N amendments in a 2-year field trial on some physical-chemical properties, some enzyme activities and the genetic diversity of cropped plots (sugar beet-wheat rotation) and uncropped plots were investigated. Variations of pH were not statistically related to MSW compost and mineral-N amendments, or to the presence of the crop. Amendment with MSW compost increased the organic C and total N contents, and dehydrogenase and nitrate reductase activities of soil. In cropped plots amended with MSW compost, dehydrogenase activity was positively correlated with β-glucosidase activity, and both enzyme activities with organic C content. No MSW compost dosage effect was detected. No effects were observed on denaturing gradient gel electrophoresis and amplified rDNA restriction analysis patterns, indicating that no significant change in the bacterial community occurred as a consequence of MSW amendment.

Availability of nitrogen after fertilizer applications to cereals

An experiment was conducted over seven sites in eastern England sown to winter cereals, whereby the soil mineral nitrogen (N) and N recovery in the crop were measured frequently during the active growth period. Plots received 100 kg/ha fertilizer N on one of three dates in spring (28 March, 11 April and 25 April) on each site in each of the three years (1994 to 1996), and these were compared with controls with no applied N. At one site, uncropped plots were included, and measurements of gaseous N emission, soil respiration, soil microbial biomass and root tissue N concentration were also made. The fertilizer applications boosted yields by at least 3 t/ha at all sites, but apparent recovery of fertilizer N varied throughout the previously known range of 45–85%. Soil type and timing of application had no effect on N recovery, or on final yields. N was mineralized from soil sources throughout the growth period, but mineralization was outweighed by ‘‘immobilization’’ of large amounts of N (around 30 kg/ha) in the soil, chiefly during May when crop growth was most rapid. This occurred on all sites with a crop, but not where the crop was absent. Measured losses of N2 and N2O were very small (<70 g/ha/dayN) on the site where they were measured, and no other evidence of loss or storage of N was found. The apparent recovery of fertilizer N at each site, was almost exactly explained by the amount immobilized during May. We conclude that the poor recovery of spring-applied N was due, not to losses as previously assumed but to temporary immobilization during the period of most active crop uptake. Immobilization was caused primarily by the presence of the crop.

603 Cover Crop Mulches for Tomato Production in South Florida as an Alternative to Methyl Bromide

In the quest to produce tomatoes without using methyl bromide, cover crops including sunnhemp, cowpea, hairy vetch, and sorghum sudan were planted on calcareous gravelly soils of southern Florida in Oct. 1998. These crops, singly or in mix, were grown on raised beds for 3 months before they were mowed down with no tillage. Sorghum sudan was plowed down and covered with plastic mulch, a conventional farming practice. In addition, uncropped plots fertilized with 6 N–2.6P–10K at 0 or 1124 kg·ha–1 were either treated with or without methyl bromide-chloropicrin and plowed down. `Sanibel' tomatoes (Lycopersicon esculentum Mill) were transplanted in two plant densities (one row vs. two rows on a bed) immediately after mowing. Tomatoes were fertigated with 112 N and 186 K kg·ha–1 during the growing season. Sunnhemp biomass alone or in mix with cowpea was higher than any other treatment. Biomass of sorghum sudan and hairy vetch were lowest. Canopy coverage, nutrient content of cover crops, and their effects on tomato growth, nutrient content, and yield will be discussed.

Influence of mulching on the pattern of growth and water use by spring wheat and moisture storage on a fine textured soil

Eight tonnes ha−1 of stubble were used to mulch spring wheat (Triticum aestivum) on a fine textured soil with the aim of controlling both transpiration and soil evaporation during the wet pre-anthesis phase to increase moisture supply during grain filling in the eastern wheatbelt of Western Australia. Mulching reduced leaf area per plant by reducing the culm number; consequently the green area index was reduced. Reduced culm number was associated with low soil temperature which at 50 mm depth averaged 7°C lower under the mulched crop relative to the control crop in mid-season. The smaller canopies of the mulched crop used 15 mm less water than those of the control before anthesis; this difference in water-use was due equally to reduced transpiration and soil evaporation. However, the mulched crop was unable to increase ET during grain filling, a response associated with the persistence of low soil temperature for most of the growth period. Hence, total ET for the season was significantly lower (18 mm) under the mulched crop than the control crop. At harvest, mulching did not have significant effects on total above-ground dry matter and grain yields, but it increased water use efficiency for grain yield by 18%, grain weight by almost 17% and available moisture in both uncropped and cropped plots by an average of 43 mm. To determine whether there was any residual effects of soil treatment on moisture storage during the summer fallow period, soil moisture was monitored both in cropped plots and uncropped plots, that were either mulched or unmulched during the growing season, from harvest in October 1988 until next planting in June 1989. Available moisture at next planting was correlated with moisture storage at harvest despite the differences in run-off, soil evaporation and fallowing efficiency (increase in moisture storage as a percentage of rainfall) between treatments during fallowing. Therefore, the mulched treatments had more moisture available (30 mm), mostly as a result of less water use during cropping in the previous growing season, than the unmulched treatment. The study shows that mulching may be used to restrain both transpiration and soil evaporation early in the season to increase availability of soil moisture during grain filling. Secondly, mulching during the previous growing season had little effect on soil moisture during the summer fallow period, however, the moisture saved by mulching during cropping was conserved for the following season. These results indicate the importance of evaluating mulching of winter crops in terms of crop yield in the subsequent growing season as well as in the current season in which the soil was treated.

Hydraulic Conductivity of Saline‐Sodic Soil after Gypsum Application and Cropping

AbstractSlow internal drainage is a major problem in the reclamation of saline‐sodic soils. This field experiment was conducted on a low‐permeability, saline‐sodic soil (a fine‐loamy, mixed, thermic Typic Natrustalf) near Lahore, Pakistan, to compare field‐saturated hydraulic conductivities (Kfs) after various treatments. Treatments were: (i) deep‐rooted perennial alfalfa (Medicago sativa L.), (ii) sesbania [Sesbania bispinosa (Jacq.) W.F. Wright]‐wheat (Triticum aestivum L.)‐sesbania rotation, (iii) incorporated wheat straw at 7.5 Mg Ha−1, and (iv) a fallow control. These four treatments were each combined with and without 25 Mg ha−1 of gypsum, subsoiling to 0.45 m, and open‐ditch drainage. The Kfs of the soil under each treatment was measured in each 20‐cm increment to 120 cm after 6 mo and 1 yr. Gypsum applications increased the Kfs of the top 20‐cm soil layers. Compared with uncropped plots, Kfs of the 0‐ to 20‐cm depth on cropped plots with gypsum increased about twofold after 6 mo and about fourfold after 1 yr. The crop rotation significantly increased the Kfs of the 0‐ to 20‐cm (with and without gypsum) when compared with wheat straw or fallow treatments. The Kfs on plots with alfalfa plus gypsum were significantly greater to 80 cm than the Kfs on fallow plots. Neither subsoiling nor open‐ditch drains improved soil permeability. Gypsum plus sesbania‐wheat‐sesbania rotation most increased permeability of the surface layer; gypsum plus alfalfa was best to improve the permeability of the deeper layers.

Dissipation of some organochlorine insecticides in cropped and uncropped soil

Dissipation of four organochlorine insecticides, viz. aldrin, HCH, chlordane and heptachlor was studied in a sandy loam soil with and without crops during a period of 10 cropping seasons. Dissipation of all chemicals followed first-order kinetics ( 2 = 0·537-0·976) with almost similar persistence in cropped and uncropped soils for all the insecticides. The average half-lives, ( t 1 2 values) for total residues of aldrin, HCH, chlordane, and heptachlor in cropped treatments were 80·7, 58·8, 93·2, and 110 days. Their respective values in fallow plots were 78·4, 83·8, 154, and 116 days. None of the parent compounds or their isomers could be detected below the 20 cm depth at the termination of the experiment. Highest residue concentrations were observed in the surface 10 cm layer in fallow plots, but in the deeper (10–20 cm) layer in cropped plots. Analysis of plants and grains showed significant residues of all the chemicals. Degradation of these compounds in cropped and uncropped plots is discussed with regard to their volatilization, microbial degradation, leaching, and plant uptake.

Effects of plant population on soil structure, soil moisture depletion and on yield of cassava (Manihot esculentd) on an Ultisol in Southeast Nigeria

AbstractThe effects of four cassava populations (8, 10, 13 and 20 × 103 plants per ha−1) on soil physical properties, moisture retention characteristics, moisture potential profiles and moisture depletion patterns were compared with uncropped plots receiving no cultivation of an Ultisol in the humid region in eastern Nigeria. The growing of cassava decreased the bulk density of 0–10 cm depth by 17% in comparison with the uncultivated soil (1.53 Mg m−3). The bulk density of the 10–20 cm depth was higher for the cropped than the uncropped control plots. The moisture retention below a suction of pF 2.4 for 0–10 cm depth was less for the uncultivated control than for soil growing cassava. The reverse was found in moisture retention characteristics of the 20–40 cm depth especially for low suction ranges of pF below 0.8. The 0–10 cm soil layer of uncropped control dried more rapidly than the cropped soil. Below 10 cm, however, cassava depleted moisture reserves more rapidly than in uncropped control. The effect of plant population on soil moisture depletion was more conspicuous within than between rows. Significant differences in water flux and in water uptake from different horizons were observed among various population densities. The total water loss was the highest for the least plant population for the between‐row zone, and for the highest population for the within‐row zone. Plant population had no effect on tuber and stalk yield.

Tests of soil fertility following repeated applications of MCPA, tri‐allate, simazine and linuron

Summary:The following herbicides were applied annually from 1963 to experimental plots of appropriate crops grown in monoculture: MCPA 1.7 kg ha−1, triallate 1.7 kg ha−1, simazine 1.7 kg ha−1 and linuron 0.84 kg ha−1 (applied twice per year). Before the eighth treatment in 1970, nutrient status, pH and growth in greenhouse tests of a range of plants were similar in soils from treated and control plots. There were no significant differences in yield when several test crops were grown in the field plots in 1977.In a similar experiment which ran for 6 years, the same herbicides were applied twice per year at twice the above rates on each occasion (three times a year at 1.7 kg ha−1 in the case of linuron) to uncropped plots. Three years after the last treatment, there were no differences in extractable nutrients, pH, soil structure and crop yield on treated or control plots. These results support the conclusion from the main monitoring of the experiments reported elsewhere that annual treatments with these herbicides have had no adverse affect on the soil.

Field Measurement of Denitrification: I. Flux of N2 and N2O

AbstractAbsolute amounts and rates of denitrification from a Yolo loam (Typic Xerorthents) field profile were studied in relation to the influence of soil‐water content, organic C source, and soil temperature. Field plots were intensely instrumented with soil atmosphere samplers, tensiometers, and thermocouples. Soil‐water contents were maintained constant with a spray irrigation system activated at frequent time intervals. The upper 15 cm of soil was maintained at soil‐water pressure heads (h) of −15 and −70 cm of water for the two different water regimes of the experiment conducted at a soil temperature (5 cm depth) of 23°C. Soil‐water pressure heads were maintained at −8 and −50 cm for the two water regimes conducted at 8°C. Plots cropped with ryegrass, uncropped plots, and plots to which manure was mixed in the top 10 cm of soil were used to establish three different C levels. Fertilizer was applied at the rate of 300 kg N ha−1 as KNO3 enriched with 20 and 40% 15N for the h = −15 or −8 and h = −70 or −50 cm treatments, respectively. The flux of gases at the soil surface was measured from the accumulation of N2O and 15N2 beneath an air‐tight cover placed over the soil surface for 1 or 2 hours per day. Denitrification from gas fluxes occurred for the 23°C experiment in order of decreasing magnitude in manure (h = −15 cm), manure (h = −70 cm), cropped (h = −15 cm), cropped (h = −70 cm), uncropped (h = −15 cm), and uncropped (h = −70 cm) plots. Approximately 70% of the fertilizer N was denitrified for the manure (h = −15 cm) treatment. Approximately 1% of the added fertilizer was denitrified in the uncropped (h = −70 cm) treatment. Denitrification from gas fluxes for the 8°C experiment occurred in the same order as that of the 23°C experiment except that rates and absolute magnitudes were much smaller. Approximately 11% of the fertilizer N was denitrified for the manure (h = −8 cm), and no measurable denitrification occurred in the uncropped (h = −50 cm) treatment. The amount of N2 produced was much greater than N2O for all plots. The N2O flux at the soil surface was dependent upon time and degree of anoxic development and varied between 5 and 26% of total denitrification.

Simulation of herbicide persistence in soil .II. Simazine and linuron in long‐term experiments

AbstractThe rates of degradation of simazine and linuron were measured in soil from plots not treated previously with these herbicides. Degradation of both compounds followed first‐order kinetics and soil temperature and soil moisture content had a marked effect on the rate of loss. With linuron, half‐lives increased from 36 to 106 days with a reduction in temperature from 30° to 5°C at 4% soil moisture, and from 29 to 83 days at 12% soil moisture. Similar temperature changes increased the half‐life of simazine from 29 to 209 days and from 16 to 125 days at soil moisture contents of 4 and 12% respectively. A computer program which has been developed for simulation of herbicide persistence was used in conjunction with the laboratory data and the relevant meteorological records for the years 1964 to 1968 in order to test the model against previously published field persistence data for the two herbicides. The results with simazine showed a close correspondence between observed and predicted residue levels but those for linuron, particularly in uncropped plots, were satisfactory for limited periods only.

THE PERSISTENCE AND PENETRATION OF LARGE DOSES OF SIMAZINE IN UNCROPPED SOIL

Summary. The residues remaining in the soil from repeated annual application of simazine at 2–8, 5–6, and 22–4 kg/ha to uncropped plots on a loam soil were measured by chemical or bioassay methods at various intervals after treatment.The total simazine residue present 12 months after the last of three treatments with 2–8 kg/ha and 8 months after the last of five treatments with 5–6 kg/ha was less than 10% of the annual dose. This rapid decomposition is considered consistent with the soil and climatic conditions. In contrast a much larger residue (a mean value of 1·7 kg/ha) was found on plots sampled 21/2 years after the last of two annual applications of simazine at 22·4 kg/ha and the reduction in the amount of residue during the next 12 months was only of the order of 25%.In all treatments the highest concentration of simazine was found in the surface layers of the soil but measurable residues were detected to 60 cm depth, 31/2 years after the last 22·4 kg/ha application.There was considerable variation in the total residues recovered between replicate plots and between different positions on the same plots in all treatments regardless of the depth of the sample. The possible causes of this variation are discussed.Persistance et pénétration de fortes doses de simazine dans un sol non cultivé

WEED COMPETITION IN TRANSPLANTED SPRING CABBAGE

Summary. The presence of weeds during autumn and winter had no effect on the growth of transplanted spring cabbage provided they were removed before rapid growth of crop and weeds began in early spring. Weeds left beyond this time competed with the crop, resulting in smaller marketable heads. Increasingly severe competition affected internal head quality, reduced the numbers of plants producing heads and resulted in the death of a proportion of the crop plants. The main weed species responsible for crop loss was Stellaria media, which survived winter frosts and grew rapidly in early spring to fill all available ground space, dominating the weed flora and shading the crop foliage.Application of propachlor at planting time failed to give sufficient control of S. media to avoid crop loss, although the onset of competition was delayed. Trifluralin gave excellent control of S. media and resistant species were kept in check by the crop.Comparison of cropped and uncropped plots showed that the crop exerted considerable competitive pressure on the growth and development of weeds particularly where this had been reduced or delayed by herbicide treatment. The presence or absence of the crop did not, however, affect the relative proportions of the weed species, except on plots treated with trifluralin, where the crop suppressed Capsella bursa‐pastoris. Compétition des mauvaises herbes dans les choux de printemps repiqués