Fallow Control Research Articles

Green manures and crop sequences influence alfalfa root rot and pathogen inhibitory activity among soil-borne streptomycetes

A two-year trial was conducted to determine the effects of green manures and crop sequences on plant disease, streptomycete and bacterial densities, and inhibitory activity of indigenous streptomycetes against four target pathogens. Green manure treatments, buckwheat (Fagopyrum esculentum L.), canola (Brassica napus L.), sorghum-sudangrass (Sorghum bicolor) (L.) Moench × Sorghum sudanense (Piper) Stapf.), and fallow control were tested in conjunction with three crop sequences in a Phytophthora-infested soil placed in containers. Alfalfa (Medicago sativa L.), potato (Solanum tubersoum L.), or corn (Zea mays L.) was grown in the first year, and alfalfa was grown in all containers in the second year. Compared to fallow controls, alfalfa grown in sorghum-sudangrass- or buckwheat-treated soil had significantly greater stand counts and total biomass, respectively. In addition, alfalfa grown in fallow-treated soils had the greatest Phytophthora root rot as a function of stand count. Crop rotation also had a significant effect on alfalfa root rot and yield. Potato scab disease intensity was greatest on tubers grown in fallow-treated soils, while tubers grown in canola-treated soils had the highest yields (total tuber weight). Green-manure-treated soils tended to have greater streptomycete and bacterial densities than fallow-treated soils. In addition, buckwheat- or sorghum-sudangrass-treated soils had greater proportions of streptomycetes that were antagonistic against the target pathogens than fallow-treated soils. The proportion of antagonists in soil was negatively correlated with alfalfa root rot, and positively correlated with alfalfa stand counts. Inhibitory activity of the streptomycetes was also negatively correlated with potato scab and positively correlated with potato yield. These data suggest that green manures may provide a strategy for increasing pathogen inhibitory activity within the streptomycete community in soil, and, in conjunction with crop rotation, may contribute to the control of a diverse collection of soil-borne plant pathogens on multiple crop species.

Cropping Sequence Effects of Four Broadleaf Crops on Four Cereal Crops in the Northern Great Plains

The productivity and cropping sequence effects of broadleaf crops are not well known within no‐till systems in the semiarid northern Great Plains. We compared productivity of cool‐ and warm‐season broadleaf crops with spring wheat (Triticum aestivum L.), measured cropping sequence effects of broadleaf crops on subsequent cereal crops, and tested if different cereal crops interacted with previous crops. During 1999–2001 we conducted 2‐yr cropping sequence experiments at five sites in Montana under climatic conditions ranging from severe drought to average rainfall. Grain yield and quality were measured for chickpea (Cicer arietinum L.), flax (Linum usitatissimum L.), pea (Pisum sativum L.), proso millet (Panicum miliaceum L.), sunflower (Helianthus annuus L.), and wheat in Year 1. In Year 2, four cereal test crops [barley (Hordeum vulgare L.), durum (Triticum durum L.), spring wheat, and winter wheat] were grown following the Year 1 crops and a fallow control to measure cropping sequence effects. Comparative productivity varied among crops by site, affirming that crop diversity mitigates production risk. Under average rainfall, the cereal test crop yields following flax, pea, and chickpea ranged from 84 to 101% of the fallow control and were greater than that following wheat in seven of nine cases. However, yields following sunflower were equal or less than those following wheat. Under severe drought, cereal test crop yields following broadleaf crops ranged from 21 to 41% of the fallow control and were equal or less than those following wheat. Previous crops affected four cereal test crop species similarly.

Cropping system and nitrogen dynamics under a cereal winter cover crop preceding corn

Cereal rye (Secale cereale L.) has been identified as a potential nitrogen (N) management tool when used as a winter cover crop (WCC). However, N deficient corn (Zea mays L.) has often been observed when preceded by a cereal rye WCC, resulting in yield reductions and deterring the integration of WCC into cropping systems of the Corn Belt. The objectives of this study were to assess soil N availability and plant N status throughout the corn growing season under various combinations of cereal rye kill date and N-fertilizer strategy in Illinois. Cereal rye WCC was killed three (KT1), two (KT2), and one (KT3) weeks prior to optimal corn planting, and N-fertilizer strategies included combinations of N splits (early and late) and N strategies (at planting, divided between planting and V6, or at V6). Although initial reductions in soil mineral N were observed in cereal rye WCC plots at planting of corn, soil mineral N among all cereal rye kill date and early N strategy plots was improved by the V6 stage and remained equal throughout the growing season. Corn under the latest cereal rye kill date in combination with its total N-fertilizer (160 kg N ha−1) allotted at V6 had lower N contents by the R1 stage than any other kill date, N strategy combination. Relative corn N deficiencies and grain yield reductions were not observed unless cereal rye kill date was delayed to one week before optimal corn planting in Illinois (KT3) and N-fertilizer applied in full at the V6 stage of corn development (late N split, V6 strategy). Residual soil nitrate (NO3-N) remaining post-harvest of corn varied between cereal rye WCC treatments and the fallow control depending on the N strategy employed throughout the season, indicating that N usage and demands of a winter fallow cropping system and cereal rye WCC systems under different residue loads require different N-fertilizer strategies to achieve more efficient N synchrony.

Evaluation of Conservation Tillage and Cover Crop Systems for Organic Processing Tomato Production

Field experiments were conducted in 2000 and 2001 in Meridian, Calif. to evaluate the effects of cover crop mixtures and reduced tillage on yield, soil nitrogen (N), weed growth, and soil moisture content in organic processing tomato (Lycopersicum esculentum) production. The trial was set up as a randomized complete-block design with eight treatments consisting of a 2 × 3 (cover crop × tillage) factorial design, a fallow control (F) and a single strip-till (ST) treatment. Cover crop mixtures were either legumes (L), common vetch (Vicia sativa), field pea (Pisum sativum) and bell bean (Vicia faba), or those legumes with grasses (GL), annual ryegrass/triticale (Lolium multiflorum/xTriticosecale) in 2000; cereal rye (Secale cereale)/triticale in 2001. Tillage treatments included an incorporation of the cover crop at planting (IP), a delayed incorporation (DI) (17 to 19 days after planting), and no-till (NT). Due to regrowth of the annual ryegrass in 2000, tomato fruit yields in 2000 were reduced by 50% to 97% within all GL treatments. However, regrowth of the cover crop was not a problem in 2001 and yields were not different among treatments. Total percent weed cover was 1.6 to 12.5 times higher in NT than IP treatments in 2000 and 2.4 to 7.4 times higher in 2001 as weed pressure was mainly affected by tillage practices and less by cover crop type. In 2000, available soil N was 1.7 to 9.4 times higher in L than GL treatments and was significantly influenced by tillage, but there were no treatment effects in 2001 due to a 60% reduction in weed pressure and minimal or no cover crop regrowth. Soil moisture content did not differ between treatments in either year. These results demonstrate the importance of appropriate selection and termination of cover crops for their successful adoption in organic conservation tillage systems.

Effects of Selected Marigold Varieties on Root-knot Nematodes and Tomato and Melon Yields.

Field experiments were conducted at two sites in California to evaluate the effects of marigold genotypes Tagetes patula var. Single Gold and Tagetes hybrid var. Polynema on Meloidogyne incognita infestation, root-galling, and yields of tomato grown immediately after marigold. Marigold cultivars were compared with a fallow control and with methyl iodide fumigation of soil prior to cultivation of M. incognita-susceptible and M. incognita-resistant tomato. Tomato yields after marigold were ca. 50% higher than after fallow. Marigold Single Gold consistently reduced nematode infestation and galling of tomato roots. Results were not significantly different between methyl iodide fumigation or marigold Single Gold at one site, but methyl iodide outperformed both marigold varieties at the other site. At one site, where melon var. Durango was grown during spring and summer of the year following cultivation of marigolds Single Gold and Polynema, melon yield increases of 95 and 45%, respectively, were still obtained.

Biomass and nitrogen traits of summer pigeon peas and winter wheat grown for three rotations in containers

Pigeon pea [Cajanus cajan (L.) Millsp.] cultivars, ‘Georgia-1’ and ‘ICPL-87’, were grown without inoculation and with Bradyrhizobium inoculation (multistrain, TAL 1127, or TAL 1132) to evaluate legume dry weight (DW) and nitrogen (N) content, soil mineral N, and subsequent wheat (Triticum aestivum L.) productivity. Pigeon peas were grown during summer and ‘TAM 101’ wheat was grown during winter, along with summer fallow controls fertilized with 0, 45, and 90 kg N ha− 1, in 36-cm diam. 20-L pots from 1992 to 1995. Representative pigeon peas were harvested in the fall and remaining plants were incorporated into the soil. Wheat was planted and soil cores were collected at 35 to 48 d after pigeon pea harvest. Wheat was harvested the following spring. Factors affecting DW and N content of both crops included length of growing season, environmental variation, and contribution of residual N. Among pigeon pea cultivars, Georgia-1 occasionally demonstrated higher DW and N content compared with ICPL-87. Estimation of N provided by pigeon pea to the last wheat crop in the third sequence of yearly rotations was 30 kg N ha− 1. Pigeon pea treatments demonstrating highest DW, N content, and contribution to soil N generally produced winter wheat with higher yield and N content compared with other treatments. While yield and N content of winter wheat fertilized at 90 kg N ha− 1 either decreased or stayed the same from 1993 to 1995, these same measurements in wheat following pigeon peas demonstrated a 3- to 4-fold increase over the same time period and warrant further research in field rotation systems of the southern Great Plains.

RESTORING ENRICHED GRASSLANDS: EFFECTS OF MOWING ON SPECIES RICHNESS, PRODUCTIVITY, AND NITROGEN RETENTION

Species-rich grasslands that become enriched with nitrogen often suffer decreases in species richness, increases in plant biomass, and invasion by weedy exotic species. Suitable techniques to restore enriched grasslands and reestablish native communities are increasingly needed. Here we report results of a 5-yr experiment in enriched coastal prairie grasslands (Bodega Marine Reserve, Bodega Bay, California, USA), to determine the combined effects of mowing and biomass removal on total soil nitrogen, net rates of mineralization, nitrogen retention, and species richness and biomass. We mowed and removed plant biomass from plots in areas where the N-fixing shrub, bush lupine (Lupinus arboreus), had greatly enriched the soil, and where the community was composed of weedy introduced plants. Our goal was to facilitate the establishment of the native grassland assemblage such as was found at nearby low soil nitrogen sites. Mowing and biomass removal resulted in a dramatic change in the species assemblage, from exotic annual grasses to a mixed exotic/native forb community composed primarily of perennials. Species richness was significantly greater in treated plots than in control plots; weedy exotic grasses diminished in abundance, and both native and exotic forb species increased. In mowed vs. control plots, there was significantly less mean aboveground biomass, but significantly greater belowground biomass. This shift in species composition had significant impacts on nitrogen retention. In late fall and winter when plant-available N was highest, much nitrogen leached from the effectively fallow control plots where germination of annual grasses did not peak until midwinter. In contrast, mowed plots retained substantially greater amounts of nitrogen, due to the presence of perennial plants possessing large amounts of belowground biomass early in the season. Despite the cumulative removal of 22 g N/m2 in biomass over 5 yr, there was no difference between mowed and control plots in total soil N, pool sizes of inorganic N, or net rates of N mineralization. The results indicate that removal of plant biomass by mowing shifted this plant community from an annual grass to a perennial forb assemblage. However, in doing so, N retention by vegetation was increased, making it more difficult to reduce soil N.

Use of Cover Crop Mulches in a No-till Furrow-irrigated Processing Tomato Production System

No-till processing tomato (Lycopersicum esculentum Mill.) production in four winter cover crop-derived mulches was evaluated in 1997 and 1998 in Five Points, Calif. The effectiveness of two medics, `Sava' snail medic (Medicago scutellata Mill.) (sava), and `Sephi' barrel medic (Medicago truncatula Gaertn.) (sephi), and two cereal/legume cover crop mixtures, triticale/`Lana' woolypod vetch (X Triticosecale Wittm./Vicia dasycarpa Ten.) (triticale/vetch) and rye/`Lana' woolypod vetch (Secale cereale L./V. dasycarpa) (rye/vetch), was compared with two conventionally tilled fallow controls (with and without herbicide) (fallow+h and fallow-h) in suppressing weeds and maintaining yields with reduced fertilizer inputs. The comparison was conducted as a split plot, with three N fertilization rates (0, 100, and 200 lb/acre; 0, 112, and 224 kg·ha-1) as main plots and cover crops and fallow controls as subplots. Tomato seedlings were transplanted 3 weeks after the cover crops had been mowed and sprayed with herbicide. There were no significant differences in weed cover in the no-till cover crop treatments relative to the fallow controls in 1997. Early season weed suppression in rye/vetch and triticale/vetch plots was similar to herbicide-treated fallow (fallow+h) in 1998, however, later in the 1998 season weed suppression was best in the fallow+h. Tissue N was highest in the fallow treatments in both 1997 and 1998. Yields were highest in the triticale/vetch and fallow and lowest in sephi treatments in 1997, but there were no differences among treatments in 1998. These results demonstrate the feasibility of no-till mulch production of furrow irrigated processing tomatoes and identify opportunities for further optimization of the system.

Testing CERES model predictions of N release from legume cover crop residue

Nitrogen release into soil from a legume cover crop (LCC) is affected by soil biotic and abiotic factors, quality of the LCC incorporated, and management practices. This study was conducted to test the ability of the CERES soil-process submodels to predict N release from an LCC, prior to growth of a subsequent crop. Field data were collected from 1995 to 1997 following spring incorporation of a mixed stand of woollypod vetch and field pea either early (C/N = 10) or late (C/N = 17) and in unfertilized fallow control plots (no-LCC). Soil temperature and water content by layer were predicted reasonably well with a few exceptions, although temperature errors of even a few degrees may be biologically significant. Inorganic soil N content during the fallow period with no-LCC and after late LCC incorporation was also realistically predicted. With early LCC incorporation, however, the N-transformation submodel substantially underestimated flushes in inorganic soil N immediately after rainfall in 1996 and irrigation in 1997. Second, the nitrification capacity factor, which should simulate a lag between a rapid increase in ammonium concentrations and slower nitrate increase after incorporation of LCC residues, performed poorly. Simulated N dynamics after incorporation of the maize residues (C/N ratio of 80) was also compared with field observations. A measured decrease in inorganic soil N contents was not correctly predicted. These discrepancies indicate the presence of some significant limitations in the soil-process submodels.

Integrated soil management for the savanna zone of W. Africa: legume rotation and fertilizer N

Integrated soil management with leguminous cover crops was studied at two sites in the northern Guinea savanna zone of northern Nigeria, Kaduna (190 day growing season) and Bauchi (150 days). One-year planted fallows of mucuna, lablab, and crotalaria were compared with natural grass fallow and cowpea controls. All treatments were followed by a maize test crop in the second year with 0, 30, or 60 kg N ha−1 as urea. Above ground legume residues were not incorporated into the soil and most residues were burned early in the dry season at the Kaduna site. Legume rotation increased soil total N, maize growth in greenhouse pots, and dry matter and N accumulation of maize. Response of maize grain yield to 30 kg N ha−1 as urea was highly significant at both sites and much greater than the response to legume rotation. The mean N fertilizer replacement value from legume rotation was 14 kg N ha−1 at Kaduna and 6 kg N ha−1 at Bauchi. W ith no N applied to the maize test crop, maize grain yield following legume fallow was 365 kg ha−1 higher than natural fallow at Bauchi and 235 kg ha−1 higher at Kaduna. The benefit of specific legume fallows to subsequent maize was mostly related to above ground N of the previous legume at Bauchi, where residues were protected from fire and grazing. At Kaduna, where fallow vegetation was burned, maize yield was related to estimated below ground N. The results show that legume rotation alone results in small maize yield increases in the dry savanna zone.

Effects of a copper‐tolerant grass (Agrostis capillaris) on the ecosystem of a copper‐contaminated arable soil

AbstractTo test how a dysfunctioning ecosystem of a severely metal‐polluted soil responds to renewed plant growth, a pot experiment was conducted with soil from an experimental arable field with pH and copper gradients imposed 13 years ago. In this experiment, four pH/copper combinations from this field were either planted with a pH‐ and copper‐resistant grass cultivar or remained fallow. During a 10‐week period, the dynamics of the microbial activity and of the abundances of bacteria, protozoa, and nematodes were measured, as were the dynamics of several chemical soil parameters. After 13 years of copper (750 kg copper per hectare) and/or pH stress (6.1® 4.7), which had resulted in severely reduced crop growth, no effects were observed on bacterial numbers, respiration, or protozoan numbers, but bacterial growth (measured as [3H]thymidine incorporation into bacterial DNA) was strongly reduced in the low pH plots, and even more so in low pH plots enriched with copper. Of the organisms, only nematodes were negatively affected under conditions of high copper load at low pH. In these plots, numbers belonging to all feeding categories were strongly reduced. Planting of a copper‐tolerant grass variety, Agrostis capillaris L. var. Parys Mountain, resulted within 10 weeks in faster bacterial growth and more protozoa and bacterivorous nematodes in comparison with fallow controls; these effects were markedly strongest in the acidic, copper‐enriched soils. During incubation, fungivorous nematodes increased in all treatments, in fallow and in planted pots and in the pots with high‐copper, low‐pH soil. The results of this experiment suggest that introduction of plant growth is one of the major causes of increased biological activity in acidic contaminated soils (approximately 1.2 times the current EC limits). Planting such soils with metal‐tolerant plant species can reestablish the necessary food base to support soil organism growth, and this can lead to numerous positive effects, reversing the loss of soil functions due to the high copper levels under acidic conditions.

Legumes as dry season fallow in upland rice-based systems of West Africa

Declining fallow length in traditional upland rice-based cropping systems in West Africa results in a significant yield reduction due mainly to increased weed pressure and declining soil fertility. Promising cropping system alternatives include the use of weed-suppressing legumes as short duration fallows. N accumulation, N derived from the atmosphere (Ndfa), weed suppression, and the effects on rice yield were evaluated in 50 legumes, grown at four sites in Cote d'Ivoire with contrasting climate, soils, and rice production systems. The sites were located in the derived and the Guinea savanna and in the bimodal and the monomodal rainfall forest zones. Legume and weed biomass during the fallow were determined at bimonthly intervals. Percent Ndfa by biological N fixation was determined by 15N natural abundance. Fallow vegetation was cleared and rice seeded according to the practice of local farmers and the cropping calendar. Weed biomass and species composition were monitored at monthly intervals. Legume fallows appear to offer the potential to sustain rice yields under intensified cropping. Biomass was in most instances significantly greater in the legume fallow than in the "weedy" fallow control, and several legume species suppressed weed growth. N accumulation by legumes varied between 1–270 kg N ha–1 with 30–90% Ndfa. Across sites, Mucuna spp., Canavalia spp., and Stylosanthes guianensis showed consistently high N accumulation. Grain yields of rice which had been preceded by a legume fallow were on average 0.2 Mg ha–1 or about 30% greater than that preceded by a natural weedy fallow control. At the savanna sites where fallow vegetation was incorporated, Mucuna spp. and Canavalia ensiformis significantly increased rice yield. In the bimodal forest zone, the highest rice yield and lowest weed biomass were obtained with Crotalaria anagyroides. In general, the effects of legume fallows on rice yield were most significant in environments with favourable soil and hydrological conditions.

Use of Cover Crop Mulches in a Processing Tomato Production System

Conventional processing tomato production in California relies on inputs of synthetic fertilizers, water, and tillage to achieve high yields. Recently, there has been interest in reducing off-farm inputs and increasing soil quality by reducing tillage and increasing organic matter. Thus, this experiment examined the use of cover crops and no-till tomato production. Experiments were conducted in 1996 and 1997 at two sites, Five Points and Davis, Calif. Winter-grown cover crops included barrel medics, subterranean clover, pea, vetch, triticale, ryegrass, oat, and fallow controls. Cover crop plots were divided into three parts and nitrogen fertilizer added at 0, 112 or 225 kg/ha. Cover crops were mowed, and 3 weeks later, tomatoes were no-till transplanted into each plot. Weed cover at all sampling dates was less than 5% on all plots at the Five Points site, with no significant differences among treatments. At Davis, weed cover was much higher, but was also not significant among treatments. Tomato yields were highest (93 ton/ha) on the fallow plots and lowest (58 ton/ha) on the barrel medic plots at the Five Points site. At Davis, tomato yields were generally very low due to competition from weeds. The barrel medic cover crops plots had the highest tomato yields at this site (34 tons/ha), with plots without cover crops having the lowest yields (12 tons/ha). Tomato yields were 18% and 29% higher when 112 kg/ha or 225 kg/ha of nitrogen was added, compared to no nitrogen.

Nitrogen Assimilation and Biomass Yield of Winter Cover Crops Used in Sustainable Horticultural Crop Production

Cover crops are planted during winter to prevent soil erosion, improve soil quality, and supply nutrients to the subsequent spring crops. In a 2-year study, three winter cover crops were compared for their nitrogen assimilation and biomass yielding ability. The experimental design was randomized complete block replicated four times with cereal rye, hairy vetch, crimson clover, and a fallow control comprising the treatments. Cover crop roots were well distributed from 1 to 50 cm of soil depth and increased from fall to spring as temperature increased. There was greater reduction in soil inorganic N during fall and winter in cover crop plots compared to control. Early season soil NO–3 concentration was lower in rye than crimson clover or hairy vetch. The amount of N assimilated by hairy vetch and crimson clover was significantly greater than cereal rye or control. There was no difference in the biomass yield of the three cover crops during the first year, but cereal rye and crimson clover produced significantly greater biomass than hairy vetch during the second year. The results suggest that cereal rye is more suited for preventing leaching of residual N from the preceding summer crop, while the two legumes can supply more N to the following crop.

Winter Cover Crops and Nitrogen Management in Sweet Corn and Broccoli Rotations

Cover crops hold potential to improve soil quality, to recover residual fertilizer N in the soil after a summer crop that otherwise might leach to the groundwater, and to be a source of N for subsequently planted vegetable crops. The objective of this 5-year study was to determine the N uptake by winter cover crops and its effect on summer vegetable productivity. Winter cover crops [red clover (Trifolium pratense L.), cereal rye (Secale cereale L. var. Wheeler), a cereal rye/Austrian winter pea (Pisum sativum L.) mix, or a winter fallow control] were in a rotation with alternate years of sweet corn (Zea mays L. cv. Jubilee) and broccoli (Brassica oleracea L. Botrytis Group cv. Gem). The subplots were N rate (zero, intermediate, and as recommended for vegetable crop). Summer relay plantings of red clover or cereal rye were also used to gain early establishment of the cover crop. Cereal rye cover crops recovered residual fertilizer N at an average of 40 kg·ha-1 following the recommended N rates, but after 5 years of cropping, there was no evidence that the N conserved by the cereal rye cover crop would permit a reduction in inorganic N inputs to maintain yields. Intermediate rates of N applied to summer crops in combination with winter cover crops containing legumes produced vegetable yields similar to those with recommended rates of N in combination with winter fallow or cereal rye cover crops. There was a consistent trend (P < 0.12) for cereal rye cover crops to cause a small decrease in broccoli yields as compared to winter fallow.

Chemical changes in a saline-sodic soil after gypsum application and cropping

Reclamation is needed on three million ha of slowly permeable saline-sodic soils in the Indus Plain of Pakistan. Previous studies measured an increased field-saturated hydraulic conductivity ( K fs) in the soil under study with cropping and gypsum application. This field experiment was conducted on a low permeability, saline-sodic soil (a fine-loamy, mixed thermic Typic Natrustalf) to compare the leaching of sodium and soluble salts and changes in chemical properties after various treatments. Treatments were: (i) perennial alfalfa ( Madicago sativa L.), (ii) a rotation of sesbania [ Sesbania bispinosa (Jacq.)W.F. Wright]-wheat ( Triticum aestivum L.)-sesbania, (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 and open-ditch drainage. Electrical conductivity (EC), pH, Na +, Ca 2+, Mg 2+ and Cl − of the soil in the saturated paste extract under each treatment were measured in each 20 cm increment to 120 cm after 6 month and 1 yr. Gypsum application increased the soluble Na + in the top 20 cm soil. Poor internal drainage of the soil caused the exchanged Na + to remain in the soil solution. However, one year after the treatments, the crop rotation with gypsum significantly decreased SAR, EC, pH and Cl − in the top 20 cm of soil. Alfalfa decreased these same parameters when compared to fallow in the top 80 cm of soil in gypsum-treated plots. The open-ditch drainage was not helpful in reclamation of this soil. In general, for surface soil improvement, a combination of added gypsum plus crop rotation was the best. For improvement of the deeper soil profile, gypsum plus alfalfa was the most effective of the treatments used.

Green manures of oat, rape and sweet corn for reducing common root rot in pea (Pisum sativum) caused by Aphanomyces euteiches

Green manure crops of sweet corn, soybean, alfalfa, snap bean, rape, pea and of the two oat cultivars ‘Dane’ and ‘Troy’ were incorporated into the same soil and containers in which the crop had grown for five weeks. The soil was then evaluated for suppression of common root rot (Aphanomyces euteiches) of pea grown in infested pasteurized and non-pasteurized soils in the greenhouse. Pea biomass reduction and a plant bioassay for A. euteiches were used to measure the green manure suppression of disease. Green manures of sweet corn cv.‘Jubilee’, oat cv.‘Troy’, and rape cv.‘Humus’ significantly reduced pea biomass losses over the non-amended control soil treatments. Oat cv.‘Troy’and sweet corn cv.‘Jubilee’ green manures significantly reduced inoculum density of A. euteiches over the corresponding fallow controls in inoculated pasteurized soil by 87% and 76%, respectively, and in inoculated non-pasteurized soil by 67% and 66%, respectively. Only the green manure of oat cv. ‘Troy’ reduced inoculum density significantly below fallow.

Winter cover crops in a vegetable cropping system: Impacts on nitrate leaching, soil water, crop yield, pests and management costs

Plant-soil relationships in the surface soil layer affect other processes in agroecosystems, including crop productivity, nitrate leaching and plant-pest interactions. This study investigated the effect of altering surface soil dynamics, using a winter cover crop rotation, on biotic and abiotic characteristics of the soil profile. Two cover crop treatments, phacelia and Merced rye ( Phacelia tanacetifolia cv. ‘Phaci’, and Secale cereale cv. ‘Merced’), with a fallow control, were planted in November after harvest of a broccoli crop on a commercial farm site, and were incorporated using reduced tillage techniques the following March. Changes in plant and soil N pools throughout the profile were described, emphasizing nitrate (NO − 3-N) leaching during winter, and N availability during the subsequent broccoli crop. Changes in other aspects of the ecosystem, such as plant-pest interactions and plant disease incidence, were monitored after cover crop incorporation. The on-farm economic costs of cover cropping were calculated. There was a 65–70% reduction in nitrate leaching from the cover-cropped plots compared with the fallow control during winter, because plant roots in the surface soil removed N and water that would have otherwise been lost from the profile. Incorporation caused sudden large surges in inorganic N pools, net mineralizable N, and microbial biomass N and C in the surface soil, which subsided within 6 weeks, by the time the broccoli crop was planted, but which did result in increased yield at harvest in the phacelia cover-cropped treatment. No insect or disease problems which threatened the cash crops were introduced or increased as a result of the cover crops. The economic analysis indicated that the costs of cover cropping were minor compared with conventional winter management of fallowed fields, and compared with the cost of producing broccoli. The cover crops therefore provided a clear advantage during winter by significantly reducing nitrate leaching, but the effects of one cover crop rotation on subsequent nutrient dynamics in the surface soil were mostly short-lived and possibly masked by large fertilizer applications.

Soil-plant nitrogen dynamics following incorporation of a mature rye cover crop in a lettuce production system

SUMMARYWinter non-leguminous cover crops are included in crop rotations to decrease nitrate (NO3-N) leaching and increase soil organic matter. This study examined the effect of incorporating a mature cover crop on subsequent N transformations. A field trial containing a winter cover crop of Merced rye and a fallow control was established in December 1991 in Salinas, California. The rye was grown for 16 weeks, so that plants had headed and were senescing, resulting in residue which was difficult to incorporate and slow to decompose. Frequent sampling of the surface soil (0–15 cm) showed that net mineralizable N (anaerobic incubation) rapidly increased, then decreased shortly after tillage in both treatments, but that sustained increases in net mineralizable N and microbial biomass N in the cover-cropped soils did not occur until after irrigation, 20 days after incorporation. Soil NO3-N was significantly reduced compared to winter-fallow soil at that time. A 15N experiment examined the fate of N fertilizer, applied in cylinders at a rate of 12 kg 15N/ha at lettuce planting, and measured in the soil, microbial biomass and lettuce plants after 32 days. In the cover-cropped soil, 59% of the 15N was recovered in the microbial biomass, compared to 21% in the winter-bare soil. The dry weight, total N and 15N content of the lettuce in the cover-cropped cylinders were significantly lower; 28 v. 39% of applied 15N was recovered in the lettuce in the cover-cropped and winter-bare soils, respectively. At harvest, the N content of the lettuce in the cover-cropped soil remained lower, and microbial biomass N was higher than in winter-bare soils. These data indicate that delayed cover crop incorporation resulted in net microbial immobilization which extended into the period of high crop demand and reduced N availability to the crop.

Management systems for conservation fallow on the southern Canadian prairies

Fallow continues to be a common agronomic practice on the southern Canadian prairies, but it has been associated with increased soil erosion and salinity and declining soil fertility. Field experiments were conducted at Lethbridge, Alberta, from 1987 to 1992 to determine the effects of various fallow treatments on weed control, conservation of surface crop residues, accumulation of soil water, and succeeding spring and winter wheat yields. Conventional cultivation during the fallow year with 168-cm sweeps controlled most spring-germinating weeds but did not adequately control overwintered flix-weed or downy brome. Repeated applications of glyphosate and 2,4-D effectively controlled most weeds. Paraquat did not control wild buckwheat or dandelion. Glyphosate alone often did not control wild buckwheat or Russian thistle. Treatments involving a combination of herbicides and tillage gave the best control of all weed species. Although not as effective in conserving surface crop residues as sole use of herbicides, many of the combined herbicide-tillage treatments maintained sufficient crop residue to keep the risk of soil erosion low. Soil water accumulation and succeeding wheat yields with the combined herbicide-tillage treatments were similar to, or greater than, those attained with repeated herbicides or repeated tillage. Management systems combining herbicides and tillage for fallow weed control may reduce costs and prevent or retard the development of herbicide resistance. Key words: Water conservation, herbicides, reduced tillage, soil erosion, stubble retention, weed control, wheat yield, no-till