The effect of the glyphosate, 2,4-D, atrazine e nicosulfuron herbicides upon the Edaphic collembola (Arthropoda: Ellipura) in a no tillage system

The use of herbicides is a common and intensive practice in no tillage systems. The herbicides can influence, directly or indirectly, the population of edaphic arthropods. Collembola is a group that functions as a bio-indicator of soil conditions. The degree of abundance and diversity of Collembola provides the level of soil disturbance provoked by agricultural practices. This experiment was designed to compare the influence of herbicides on the population fluctuation of Collembola in a no-till soil preparation system. The work was conducted in a non irrigated no-till area at the Núcleo Experimental de Ciências Agrárias of the Universidade Federal de Mato Grosso do Sul (UFMS), Campus de Dourados, in soil planted with corn as a surface covering, during the period of December, 2002 to December, 2003. The data were analyzed according to a completely randomized model, in a split plot design. The plots received four types of herbicides: glyphosate, atrazine, 2,4-D and nicosulfuron. A fifth plot did not receive any herbicide (control), for a total of five treatment types. The sub plots were represented by their collection times (10, 20, 30 and 40 days after the herbicide applications). Both the type of herbicide and the time of data sampling influenced the Collembola population fluctuaction. The treatments with atrazine and 2,4-D caused the most reduction of the population of Collembola, depending on the time of application.     

除草剂胁迫对空心莲子草叶甲种群的影响及应对策略

为探讨除草剂胁迫对空心莲子草叶甲种群的影响,寻求合理施用除草剂,有效保护叶甲,提高其控草效果的应对策略,利用叶片残留法测试了在草甘膦、农达、水花生净与35%苄·丁可湿粉等除草剂胁迫下空心莲子草叶甲卵孵化率,幼虫、成虫存活率以及蛹羽化率。结果表明被测试的4种除草剂均不同程度地降低空心莲子草叶甲卵孵化率,幼虫与成虫存活率及蛹羽化率,特别是水花生净28.0、35.0 g/L两种浓度处理,35%苄·丁可湿粉4.0 g/L浓度处理显著降低卵孵化率;农达13.4 g/L浓度处理显著降低1龄前期幼虫存活率,农达40.1 g/L与 53.4 g/L两种浓度处理显著降低2龄前期幼虫存活率;水花生净28.0 g/L浓度处理显著降低3龄前期幼虫存活率;草甘膦97.6 g/L与 122.0 g/L浓度处理、农达26.7 g/L与66.8 g/L浓度处理、35%苄·丁可湿粉2.0 g/L浓度处理显著降低后期蛹的羽化率;35%苄·丁可湿粉1.0 g/L浓度处理显著降低羽化后3 d雌虫的存活率。除草剂除直接杀死空心莲子草叶甲,降低其种群数量外,使叶甲失去食物与避护所,快速地引起叶甲种群崩溃。建议合理施药,并在施药时,随机留若干小斑块不施药,保护叶甲,可望提高其田间控草效果。     

Graminicide resistance in a blackgrass (Alopecurus myosuroides) population correlates with insensitivity of acetyl-CoA carboxylase

The appearance of biotypes of the annual grass weed black‐grass (Alopecurus myosuroides L. Huds), which are resistant to certain graminicides, is the most significant example of acquired resistance to herbicides seen so far in European agriculture. An investigation was perfomed into the basis of the specific cross‐resistance to cyclohexanedione (CHD) and aryloxyphenoxypropionoic acid (AOPP) herbicides in the ‘Notts A1’ population of A. myosuroides, which survived treatment of fields with recommended rates of AOPP herbicides. In comparison with the wild‐type ‘Rothamsted’ population, the resistant biotype showed over 100‐fold resistance to these herbicides in a hydroponic growth system. Biosynthesis of fatty acids and activity of crude extracts of acetyl‐CoA carboxylase (ACCase) were commensurately less sensitive to these herbicides in Notts A1 compared with the Rothamsted biotype. These data are consistent with the hypothesis that the highly resistant population has arisen through selection of a mutant ACCase which is much less sensitive to the AOPP and CHD graminicides. Rapidly growing cell suspension cultures established from the Notts A1 population also showed high resistance indices for CHD or AOPP herbicides compared with cultures from the Rothamsted biotype. Fatty acid biosynthesis and ACCase activity in the cell suspensions were similarly sensitive towards the graminicides to those in the foliar tissue counterparts of the resistant and sensitive populations. Moreover, purification of the main (chloroplast) isoform of acetyl‐CoA carboxylase showed that this enzyme from the Notts A1 population was over 200‐fold less sensitive towards the AOPP herbicide, quizalofop, than the equivalent isoform from the Rothamsted population. These data again fully supported the proposal that resistance in the Notts biotype is due to an insensitive acetyl‐CoA carboxylase isoform. Overall, cell suspensions were also demonstrated to be excellent tools for further investigation of the molecular basis of the high level herbicide resistance which is prone to occur in A. myosuroides.     

Evolution of herbicide resistance in weeds: initial frequency of target site-based resistance to acetolactate synthase-inhibiting herbicides in Lolium rigidum

The frequency of individuals resistant to two acetolactate synthase (ALS)-inhibiting herbicides in three previously untreated populations of Lolium rigidum was determined. The frequency of individuals resistant to the sulfonylurea herbicide sulfometuron-methyl varied from 2.2 x 10(-5) to 1.2 x 10(-4) and the frequency of individuals resistant to the imidazolinone herbicide imazapyr varied from 1 x 10(-5) to 5.8 x 10(-5) depending on the population. Application of sulfometuron-methyl selected individuals with a herbicide-insensitive ALS, which was also cross-resistant to imazapyr. The high initial frequency of individuals resistant to ALS-inhibiting herbicides in L. rigidumpopulations never previously exposed to these herbicides helps explain the rapid evolution of herbicide resistance in this species once ALS-inhibiting herbicides were used.     

荧光物示踪法测定除草剂对食虫沟瘤蛛摄食量的影响

采用一种全新的测定方法——荧光物示踪法,以稀土元素铕作为荧光示踪物质,以“水稻 白背飞虱Sogatella furcifera Horvath食虫沟瘤蛛 Ummeliata insecticeps Bosenberg et Strand”这一食物链中的水稻和白背飞虱作为铕的传递者,经萃取和反萃取等一系列化学分析方法,定量地测定了3种除草剂胁迫下食虫沟瘤蛛的摄食量。结果表明：施用除草剂西草净、使它隆和高效盖草能以后,食虫沟瘤蛛的存活个体12天内平均每天捕食白背飞虱分别为1.784、2.678和2.856头,均低于对照组的平均个体摄食量3.321头/d; 各组12天内个体摄食量的趋势线均为开口向上的抛物线,可用二次方程进行拟合; 同时除草剂的喷施亦对食虫沟瘤蛛造成一定的致死率,因而造成施药后食虫沟瘤蛛的种群摄食率减退,第2、5、8天的平均摄食率分别为对照组的52.11%、36.45%和50.27%; 药后第12天3个处理组的摄食量均超过了正常值,平均摄食率达到对照组的131.84%。因此在稻田中害虫大发生或天敌群落迁移时应慎用除草剂,以免食虫沟瘤蛛药后摄食量减少导致对目标害虫控制力减弱或由于除草剂的致死力而对食虫沟瘤蛛的群落重建构成威胁。     

Effects of three soil-applied herbicides on populations of plant disease suppressing bacteria in the cotton rhizosphere

The potential impact of three widely used herbicides, pendimethalin, prometryn, and trifluralin, on populations of five plant disease suppressing bacterial isolates (three isolates of Pseudomonas fluorescens and two isolates of Burkholderia cepacia) in the rhizosphere of cotton seedlings was investigated. All isolates are efficient cotton root colonizers and each is capable of suppressing a plant disease. In microcosm experiments, application of each of the test herbicides at the rates of 1, 2, and 4 µg active ingredient (a.i.) g^-1 soil caused significant (p<0.05) reductions in populations of most of the isolates in the rhizosphere, 14 days after the release of bacteria into the soil by seed coating. The responses of the isolates to the herbicides varied depending on the isolate and the type and concentration of the herbicides. In microcosm experiments the impact of pendimethalin, prometryn, and trifluralin at the respective concentrations of 2.4, 3.6, and 1.8 g a.i. g^-1 soil on the population of isolate D1 in the cotton rhizosphere declined with time during a four week period of monitoring following the release of the isolate into the soil by seed coating. The impact of soil applied test herbicides on the population sizes of D1 in cotton rhizosphere was also studied in two field experiments (Safford and Tucson, Arizona) where the bacteria were added as a soil drench. In the Safford experiment pendimethalin and prometryn, but not trifluralin, caused significant (p <0.05) reductions in the population of the bacterium 15 days after sowing. In the Tucson experiment a significant (p < 0.05) reduction in the population of the bacterium was observed 15 and 25 days after sowing in soils treated with pendimethalin and prometryn and 25 days after sowing in soils treated with trifluralin.     

Influence of four herbicides on the algal flora of a prairie soil

Summary Four herbicides (2,4-D, trifluralin, MCPA and TCA) were applied at two concentration levels to isolated cores of a grassland loam soil. After herbicide contact times of 1, 5, and 20 days, samples were taken and the algal population estimated both quantitatively and qualitatively using two selective mineral salts media. Thirty one genera of algae were identified as occurring in the soil. Of these, Chlamydomonas, Chlorococcum, Hormidium, Palmella, and Ulothrix proved to be so sensitive to the four herbicides that they were rarely isolated from the cores after treatment. Other algal genera were found to be less sensitive, and the theoretical percentile sensitivity of fifteen genera was calculated. Chlorella, Lyngbya, Nostoc, and Hantzschia were found to be the most resistant algae, having percentile sensitivity to all four herbicides of less than 50%. Some algal genera varied in their sensitivity to each of the herbicides. Scytonema was sensitive to all of the herbicides except 2,4-D, while Tolypothrix showed a greater tolerance to MCPA. In the top cm of the soil, the reduction in cell numbers experienced by many algal genera after herbicide treatment was offset by an increase in the population of Chlorella. Stichococcus, Oscillatoria, and Spongiochloris all exhibited the ability to recover rapidly after a reduction in cell numbers resulting from the application of one of the herbicides. An overall reduction in cell numbers was noted for the algae growing preferentially on a nitrogen-free medium (i.e. potential nitrogen-fixers). re]19760511     

Ethylene in plant growth and development, storage, soil microbiology and plant pathology

In field experiments on a sandy loam at Wellesbourne, England in 1972 and on a silt loam at Agassiz, British Columbia in 1973, combinations of herbicides and insecticides were applied at sowing to determine their effects on weeds and invertebrate populations and on the growth and yield of cauliflowers grown at high density There was good agreement between the results from the two locations. The two combinations of herbicides, 0.6 kg trifluralin/ha incorporated pre-drilling plus 2.2 kg propachlor/ha pre-emergence and 2.2 kg nitrofen/ha plus 2.2 kg propachlor/ha both applied pre-emergence, gave good weed control, their relative effectiveness depending on the species composition of the weed population. The insecticides isophenphos, carbofuran, chlorfenvinphos and fensulfothion were applied as bow-wave treatments. None of them, whether in combination with herbicides or not, adversely affected crop stand or yield. Yield was reduced when either weeds or root-fly maggots (Hylemya brassicae (Bouché)) were not controlled. Only in one experiment was there any evidence of any herbicide-insecticide interactions. Where trifluralin and carbofuran were applied together at Agassiz, the control of both weeds and maggots was less than that with the other combinations. None of the treatments affected the populations of predatory beetles, but the numbers of earthworms were greatly reduced by carbofuran and to a lesser extent by chlorfenvinphos. Except for carbofuran in one experiment, the treatments had no effects on the numbers of aphids, lepidopterous larvae or leaf miners present at harvest.     

Six amino acid substitutions in the carboxyl-transferase domain of the plastidic acetyl-CoA carboxylase gene are linked with resistance to herbicides in a Lolium rigidum population

The molecular basis of an acetyl-CoA carboxylase (ACCase) target-based resistant Lolium rigidum population (WLR 96) was studied here. The carboxyl-transferase domain of the plastidic ACCase gene from resistant individuals was amplified by PCR and sequenced. The DNA sequences were aligned and compared with a susceptible population. Six amino acid substitutions were identified in the resistant population. The substitution Ile-2041-Asn, known to confer resistance to ACCase-inhibiting herbicides aryloxyphenoxypropionate (APP) in Alopecurus myosuroides, was identified in most resistant plants but it is always linked with other amino acid substitutions. This was confirmed by a cleaved amplified polymorphism (CAP) marker and an allele-specific PCR. The sole amino acid substitution Ile-2041-Asn was not found in this population. It is likely this mutation evolved later among individuals already possessing the other substitutions. Three haplotypes were identified from the resistant population based on the six amino acid combinations, and two are linked with herbicide resistance in this population. The multiple amino acid substitutions including the Ile-2041-Asn form the molecular basis endowing a high degree of resistance to ACCase-inhibiting herbicides in this L. rigidum population.     

Acetolactate synthase mutation conferring imidazolinone-specific herbicide resistance in Amaranthus hybridus

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of branched-chain amino acids in plants and is the target of several herbicides. ALS inhibitors have enjoyed popularity as herbicides due to numerous attributes, although their current adequacy in weed control programs is hampered by herbicide resistance. Most cases of ALS-inhibitor resistance have resulted from selection of an altered target site. The study herein reports on an alanine by threonine amino acid substitution at position 122 of ALS as the basis for imidazolinone-specific resistance in an A. hybridus population from Illinois. In vitro inhibition of enzymatic activity (I(50)) required 1000-fold greater concentration of imazethapyr in the resistant population compared with a susceptible control. This mutation represents the second ALS alteration associated with herbicide resistance in a natural A. hybridus population.     

Genetic variation and population structure in black-grass (Alopecurus myosuroides Huds.), a successful, herbicide-resistant, annual grass weed of winter cereal fields

Black‐grass (Alopecurus myosuroides) is an allogamous grass weed common in cereal fields of northern Europe, which developed resistance to a widely used family of herbicides, the ACCase‐inhibiting herbicides. Resistance is caused by mutations at the ACCase gene and other, metabolism‐based, mechanisms. We investigated the genetic structure of 36 populations of black‐grass collected in one region of France (Côte d’Or), using 116 amplified fragment length polymorphism (AFLP) loci and sequence data at the ACCase gene. The samples were characterized for their level of herbicide resistance and genotyped for seven known ACCase mutations conferring resistance. All samples contained herbicide‐resistant plants, and 19 contained ACCase mutations. The genetic diversity at AFLP loci was high (H_T = 0.246), while differentiation among samples was low (F_ST = 0.023) and no isolation by distance was detected. Genetic diversity within samples did not vary with the frequency of herbicide resistance. A Bayesian algorithm was used to infer population structure. The two genetic clusters inferred were not associated with any geographical structure or with herbicide resistance. A high haplotype diversity (H_d = 0.873) and low differentiation (G_ST = 0.056) were observed at ACCase. However, haplotype diversity within samples decreased with the frequency of ACCase‐based resistance. We suggest that the genetic structure of black‐grass is affected by its recent expansion as a weed. Our data demonstrate that the strong selection imposed by herbicides did not modify the genome‐wide genetic structure of an allogamous weed that probably has large effective population sizes. Our study gives keys to a better understanding of the evolution of successful, noxious weeds in modern agriculture.     

Distinct non-target site mechanisms endow resistance to glyphosate,ACCase and ALS-inhibiting herbicides in multiple herbicide-resistant <Emphasis Type="Italic">Lolium rigidum</Emphasis>

This study investigates mechanisms of multiple resistance to glyphosate, acetyl-coenzyme A carboxylase (ACCase) and acetolactate synthase (ALS)-inhibiting herbicides in two Lolium rigidum populations from Australia. When treated with glyphosate, susceptible (S) plants accumulated 4- to 6-fold more shikimic acid than resistant (R) plants. The resistant plants did not have the known glyphosate resistance endowing mutation of 5-enolpyruvylshikimate-3 phosphate synthase (EPSPS) at Pro-106, nor was there over-expression of EPSPS in either of the R populations. However, [¹⁴C]-glyphosate translocation experiments showed that the R plants in both populations have altered glyphosate translocation patterns compared to the S plants. The R plants showed much less glyphosate translocation to untreated young leaves, but more to the treated leaf tip, than did the S plants. Sequencing of the carboxyl transferase domain of the plastidic ACCase gene revealed no resistance endowing amino acid substitutions in the two R populations, and the ALS in vitro inhibition assay demonstrated herbicide-sensitive ALS in the ALS R population (WALR70). By using the cytochrome P450 inhibitor malathion and amitrole with ALS and ACCase herbicides, respectively, we showed that malathion reverses chlorsulfuron resistance and amitrole reverses diclofop resistance in the R population examined. Therefore, we conclude that multiple glyphosate, ACCase and ALS herbicide resistance in the two R populations is due to the presence of distinct non-target site based resistance mechanisms for each herbicide. Glyphosate resistance is due to reduced rates of glyphosate translocation, and resistance to ACCase and ALS herbicides is likely due to enhanced herbicide metabolism involving different cytochrome P450 enzymes.     

Effect of four common herbicides on the population of microorganisms in Red Sandy soil

Summary A study was conducted to know the effect of four common herbicidesviz, Lasso, Nitrofen, Propinol, and Simazine on soil microorganisms in Red Sandy soil. In general the application of different herbicides did not adversely affect the microbial population. The bacterial population was decreased by the application of propinol and nitrofen. Simazine and lasso stimulated the fungal population. The stimulation in Azotobacter population was observed in lasso, nitrofen and simazine treated soil.     

Herbicidal activity of phosphonic,phosphinic, and phosphonous acid analogues of phenylglycine and phenylalanine

A series of phosphonic, phosphinic, and phosphonous acid analogues of phenylglycine and phenylalanine was synthesized and tested as herbicides against Lepidium sativum and Cucumis sativus. Aminobenzylphosphonic acids exhibited notable herbicidal activity and thus represent a group of the most active herbicides found among aminophosphonic acids.     

Evaluation of herbicide combinations for livid amaranth (Amaranthus blitum) control in tuberous begonia (Begonia x tuberhybrida)

In the past years livid amaranth (Amaranthus blitum) is observed increasingly in begonia production fields. Control of weeds in begonia is generally done by a combined application of the soil herbicides isoxaben + simazin followed 10 days later by application of the contact herbicide bentazone. This treatment usually controls the weed population sufficiently with exception of amaranth. In 2003 a field trial was conducted to evaluate control of livid amaranth in tuberous begonia with isoxaben, simazin. S-metolachloor, phenmedipham + desmedipham and bentazone. These herbicides were used as combinations of soil treatment and contact herbicides. The results suggest that a soil treatment of isoxaben + S-metolachloor significantly reduces livid amaranth compared to isoxaben + simazin, without a pronounced negative effect on tuber yield. Application of phenmedipham + desmedipham however did not improve control of livid amaranth compared to bentazone.     

Interaction of Fensulfothion and Phorate with Preemergence Herbicides on Soybean Parasitic Nematodes

The herbicides alachlor, linuron, vernolate, and metribuzin were applied to plots treated with the nematicide fensulfothion or the insecticide phorate and planted to soybean in two locations in North Carolina. In 1976 treatment with fensulfothion + alachlor or vernolate, phorate + alachlor or metribuzin resulted in greater nematode population densities than no treatment, or treatment with fensulfothion alone, or phorate alone. In 1977 fensulfothion and phorate alone and in combination with the preemergence herbicides effectively controlled Tylenchorhynchus cIaytoni. Late season population resurgence of Heterodera glycines occurred in fensulfothion + alachlor treated plots. Correlation coefficients for H. glycines vs. yield were -0.48 (P = 0.05) and -0.46 (P = 0.05) for 30 and 68 d after planting, respectively.     

Weed Management in Rice: Wheat Rotation by Allelopathy

Rice is major crop in India and its cultivation in northwest India started 25 to 30 years ago in assured irrigation areas during the summer rainy season. In this region, rice-wheat rotation became most popular owing to its high yields; however, these crops are highly infested by the weeds, thus farmers use herbicides for their control. Hence, this rotation consumes a maximum quantity of herbicides in this region, which has resulted in several problems (environmental pollution, human health hazards, development of herbicide resistance in weeds). Thus, serious ecological questions about the reliance on herbicides for weed control in this rotation have been raised. One of the alternatives to overcome these problems is with the use of allelopathic strategies, including the use of weed-smothering crops for weed management and for the sustainability of agriculture. The field, pot culture, and laboratory studies have shown that inclusion of weed-smothering crops in rotation considerably reduced the weed population in the current and succeeding crops. Early summer (April-June) fodder crops of sorghum, pearlmillet and maize drastically smothered the weed population and biomass. The residual suppression effect of peralmillet also persisted in the next crop up to 45 days. Thus, it is conceptualized that the inclusion of such summer fodder crops before the rice crop in the rice-wheat rotation may provide satisfactory weed control in the succeeding rice crop and may minimize the use of herbicides. Likewise, the replacement of wheat by winter fodder crops of oat and berseem (Trifolium alexandrinum) may also help in the control of winter weeds. Hence, further studies in this direction may provide satisfactory weed management in rice-wheat rotation and may minimize the use of herbicides and thereby help indeveloping sustainable agricultural practices.     

Role of a Novel I1781T Mutation and Other Mechanisms in Conferring Resistance to Acetyl-CoA Carboxylase Inhibiting Herbicides in a Black-Grass Population

Background

Knowledge of the mechanisms of herbicide resistance is important for designing long term sustainable weed management strategies. Here, we have used an integrated biology and molecular approach to investigate the mechanisms of resistance to acetyl-CoA carboxylase inhibiting herbicides in a UK black-grass population (BG2).

Methodology/Principal Findings

Comparison between BG2 phenotypes using single discriminant rates of herbicides and genotypes based on ACCase gene sequencing showed that the I1781L, a novel I1781T, but not the W2027C mutations, were associated with resistance to cycloxydim. All plants were killed with clethodim and a few individuals containing the I1781L mutation were partially resistant to tepraloxydim. Whole plant dose response assays demonstrated that a single copy of the mutant T1781 allele conferred fourfold resistance levels to cycloxydim and clodinafop-propargyl. In contrast, the impact of the I1781T mutation was low (Rf = 1.6) and non-significant on pinoxaden. BG2 was also characterised by high levels of resistance, very likely non-target site based, to the two cereal selective herbicides clodinafop-propargyl and pinoxaden and not to the poorly metabolisable cyclohexanedione herbicides. Analysis of 480 plants from 40 cycloxydim resistant black grass populations from the UK using two very effective and high throughput dCAPS assays established for detecting any amino acid changes at the 1781 ACCase codon and for positively identifying the threonine residue, showed that the occurrence of the T1781 is extremely rare compared to the L1781 allele.

Conclusion/Significance

This study revealed a novel mutation at ACCase codon position 1781 and adequately assessed target site and non-target site mechanisms in conferring resistance to several ACCase herbicides in a black-grass population. It highlights that over time the level of suspected non-target site resistance to some cereal selective ACCase herbicides have in some instances surpassed that of target site resistance, including the one endowed by the most commonly encountered I1781L mutation.     

Molecular Mechanisms Endowing Cross-resistance to ALS-Inhibiting Herbicides in Amaranthus hybridus from Argentina

Amaranthus hybridus L. is one of the most problematic weeds in summer crops in Argentina. However, 20 years after the detection of the first case of resistance to ALS-inhibiting herbicides in this country, no extensive reports of the molecular mechanisms endowing resistance were published. In this work, we sequenced the acetolactate synthase gene of resistant plants belonging to five different populations of A. hybridus from Santa Fe and Cordoba provinces. We found that every population presented at least one of the previously documented substitutions W574L and D376E in ALS amino acid sequence. These results explain the cross-resistance to ALS-inhibiting herbicides and should alert about the usage of herbicides with a different site of action after an ineffective control of this species. This is the first report of these target-site mechanisms endowing resistance to ALS-inhibiting herbicides in A. hybridus populations from Argentina.

     

Differential effects of the sulfonylurea herbicides chlorsulfuron and sulfometuron methyl on microorganisms

The sulfonylurea herbicides exert their effects on cells via their inhibition of the acetohydroxy acid synthase (AHS) enzymes. Although chlorsulfuron and sulfometuron methyl often affected microbial growth differently their effects on the AHS activities of toluenised cells were similar. Sulfometuron methyl was always a more potent inhibitor than chlorsulfuron. We have postulated that sulfometuron methyl penetrated into microbial cells more readily then did chlorsulfuron. The effect of the herbicides on microbial growth was altered by the composition of the medium and in particular by valine or valine plus isoleucine. Different microorganisms had different complements of AHS isoenzymes which together with differences in permeability were the most likely explanations for the different responses observed. It was pointed out that application of these sulfonylurea herbicides would have significant effects on the microbial ecological balance of soil, and particularly so in alkaline soils. The consequences would be most evident in agricultural situations where the microbial population played an important role in maximising the productivity of crops.