ORIGINAL DO NOT Remove from FILE

Reprinted from Weed Science, Volume 22, Number 6, November 1974. Purchased by the Forest Service, U.S. Department of Agriculture, for official use.

Effect of Cacodylic Acid and MSMA on Microbes in Forest Floor and Soil W. B. BOLLEN, LOGAN A. NORRIS, and KATHLEEN L. STOWERS

Abstract. Cacodylic acid (hydroxydimethylarsine oxide) and As in soil to plants, but interactions with soil microbes have MSMA (monosodium methanearsonate) gave slightly-visible in- been less extensively investigated. Low concentrations of As hibition of bacterial growth in pure culture at 1000 mg/L stimulate growth of bacteria, molds, and algae (6). Greaves and arsenic and moderate inhibition at 10,000 mg/L arsenic. There Carter (11) found that 200 ppm As (sodium arsenite) in soil were no effects at 100 mg/L arsenic. Carbon dioxide evolution greatly increased the number of bacteria that developed on from three kinds of forest floor declined with increasing concentrations of MSMA and cacodylic acid. Increasing concentra- agar plates. Covey (8) reported 10 ppm As (sodium arsenate) tions of MSMA caused an increase in carbon dioxide evolution reduced the growth of Phytophthora cactorum by 60% in from soil but cacodylic acid had no effect. Concentrations less potato dextrose broth cultures. Arsenic trioxide is more toxic than 10 mg/kg arsenic forest floor including L, F, and H hori- than arsenic pentoxide to bacteria, but equal in effect on zons, or soil had no pronounced effect on organic matter de- molds (20, 22). composition. Studies of the toxicity of organic pentavalent arsenic compounds to micro-organisms in forest floor and forest soil have INTRODUCTION not been previously reported. We have determined (a) the The arsenical silvicides, cacodylic acid and MSMA, are effec- toxicity of cacodylic acid and MSMA to pure cultures of sever- tive tools for precommercial thinning of overstocked stands. al common bacteria and molds found in agricultural and forest There are thousands of hectares of forest land which need soils (5, 21) and (b) the effect of these silvicides on the de- thinning now, and the magnitude of the problem will increase composition of organic matter by the native microbial popula- as forest regeneration techniques improve and the intensity of tions in forest floor material and underlying soil. land management increases. Foresters are concerned, however, about the safety to forest workers and animals and possible impact on forest floor and soil microbes of thinning with the MATERIALS and METHODS arsenicals (18). Usually, undiluted herbicide concentrate (50% Pure culture studies. For study of herbicide toxicity to organ- or more acid equivalent) is injected into axe cuts around the isms in pure culture, four bacteria, two higher bacteria, and stem of the selected tree, about 1 to 2 ml chemical/2.5 cm four molds were used: stem diameter are applied. When the tree dies and decays, Bacteria: Bacillus subtilis Cohn, Micrococcus caseolyticus chemical residues enter the forest floor and soil. Arsenic con- Evans, Enterobacter aerogenes (Kruse, Beijerinck) Ewing, centration in the forest floor and soil will depend upon the Pseudomonas florescens Migula. rate of decay. Arsenic (As) levels in forest floor and soil from Higher bacteria3 : Streptomyces antibioticus (Waksman) this source are likely to be less than 10 mg As/kg, but acciden- Waksman and Henrici, Streptomyces olivaceus (Waksman and tal spillage may cause "hot spots" of more than 1,000 mg Woodruff) Waksman and Henrici. As/kg in small areas. Our concern is whether or not these levels Molds: Penicillium4 claviforme Bainier, Penicillium restrict- of arsenic influence the microbiology of the forest floor and urn Gilman and Abbott, Aspergillus nidulans (Eidam) Winter, soil. Trichoderma viride Persoon ex Fries. The organisms were Arsenic occurs naturally in many soils. Greaves (10) found originally isolated from a variety of soils and identified and from 2 to 102 ppm of total As in 50 Utah soils. Jones and maintained in the culture collection of the senior author. All Hatch (13) reported 3 to 15 ppm "native" As in 10 Oregon are common to the forest floor and soil environment. soils, and soils from commercial orchards sprayed repeatedly Each bacterium was inoculated into triplicate sets of nutri- with inorganic arsenicals contained 17 to 439 ppm As in the 0- ent broth containing 10,000, 1,000, 100, 10, 1, and 0 ppmw to 20.3-cm zone. As as cacodylic acid or MSMA in 18-mm x 150-mm tubes. All The availability of As to plants and microbes is similar to tubes were incubated at 28 C for 7 days and the relative that of phosphorus. The formation of insoluble calcium salts growth of cultures estimated by visual comparison of turbidity as soil pH increases reduces availability (12). Soils high in clay, among treatments. The Streptomyces and molds were not iron, aluminum, or organic matter bind arsenates more extensively than light-textured soils (1). Aerobic actinomycetes, mold-like, but of bacterial dimensions. Woolson, Axley, and Kearney (20) reviewed the toxicity of Dense, compact subsurface colonies, aerial conidia. Usually they com- prise 30 to 60% of the soil bacterial flora. Streptomyces are particularly important because they attack residual organic matter and resistant Received for publication October 18, 1973. materials and also contribute to humus formation and decomposition. Princ. Microbiol., Princ. Chem., and Tech., For. Sci. Lab., Pacific Penicillium species are frequently predominant in the mold popula- Northwest For. and Range Exp. Sta., Corvallis, OR 97331. tion of soils and active in decomposing fresher organic materials. Volume 22, Issue 6 (November), 1974 557 ORIGINAL

DO NOT REmOVE FROM FILE WEED SCIENCE tested in broth because their habit of growth produces a sur- moisture to 50% of water-holding capacity; this amount of face pellicle but no turbidity. water generally being optimum for microbial activity and Additional tests with each organism were made using a roughly equivalent to field capacity. Bottles were connected to paper disc assay method (16). Sterile Whatman No. 1 filter a CO2 - free air manifold and each outlet air tube immersed in paper discs, 6-mm diameter, were saturated with different con- 11‘1 NaOH (4). Carbon evolved as CO 2 was absorbed in NaOH centrations of cacodylic acid, MSMA, 1,000 ppmw HgC1 2 , and and was measured by differential titration with H2 SO4 using a 1,000 ppmw crystal violet. Discs were placed on petriplates of Beckman automatic titrator and Coopers (7) end points. Bot- nutrient agar seeded with the test organisms. Triplicate plates tles were incubated at 28 C and CO2 evolution determined 7, of each treatment were incubated at 28 C until growth devel- 14, 21, and 28 days after addition of treatments. oped, and widths of clear zones (zones where test organisms did not grow) around each disc were measured to determine RESULTS toxicity. All the bacteria grew in liquid cultures with cacodylic acid and Forest floor and soil studies. Soil (0- to 15.2-cm depth) and MSMA although there appeared to be a slight reduction in overlying forest floor material (L, F, and H horizons com- growth of Bacillus subtilis and Micrococcus caseolyticus at bined) were used to determine the impact of cacodylic acid 1,000 mg/L As based on visual comparison of turbidity of and MSMA on carbon metabolism by native soil microflora. cultures among treatments. The turibidity of bacteria cultures Bulk samples of the following materials were collected: containing 10,000 mg/L As was less that turbidity of cultures Astoria silty clay loam soil and forest floor material containing less arsenic, but some bacterial growth did occur. In from the western hemlock [Tsuga heterophylla (Raf.) the paper-disc assay on solid media, all test organisms grew to Sarg.] Zone of northwest Oregon (9). the edge of the discs saturated with either cacodylic acid or Edds loam soil and forest floor material from the Doug- MSMA at concentrations of up to and including 10,000 mg/L las-fir [Pseudotsuga menziesii (Mirb.) Franco] Zone in As. Clear zones 2 to 11 mm in width were observed around all northeast Washington (9). discs saturated with either 1,000 mg/L HgC1 2 or crystal violet, 3. Klicker soil and forest floor material from the ponderosa indicating the potential of the paper-disc assay for detecting pine (Pinus ponderosa Laws) Zone in northeast Oregon inhibition of microbial growth. (9). The rate of CO2 evolution was much greater from forest Test materials were passed through a Tyler 10-mesh (2.362 floor than from soil (Tables 2 and 3). This undoubtedly re- mm) screen, placed in polyethylene bags, and stored at 2 C for flects the generally high carbon and microbial content of the use as required. Subsamples of the materials as received were forest floor (Table 1). The microbial counts, especially for analyzed for moisture, water-holding capacity, pH, Kjeldalil bacteria, reflect conditions when the samples were collected. nitrogen, total carbon, bacteria, Streptomyces, and molds (3) Upon incubation numbers increased, as reflected by CO 2 pro- (Table 1). duction. Thus, the ponderosa pine forest floor, initially lowest Triplicate samples of the soils or forest floor materials.were in microbial counts but highest in carbon, produced the most treated with MSMA or cacodylic acid to achieve concentra- CO2. tions of 0, 10, 100, or 1,000 ppm by weight As (dry weight). For statistical purposes, data for soils were analyzed sepa- For each treatment, 25 g of forest floor or 100 g of soil were rately from data for forest floor material because of this placed in a 453-ml glass bottle. The test material was added in marked difference in their rates of CO2 evolution. The experi- four equal portions with sufficient distilled water, containing ments with cacodylic acid and MSMA were not run at the the required amount of cacodylic acid or MSMA, to bring same time and, therefore, these data were not combined for

Table 1. Chemical and biological properties of test materials. Total Kjeldahl Moldsa Bacteria carbon nitrogen Penicillia Streptomyces Test material pHa (%) (%) (103 /g) (%) (106/g) (%) Soil Astoria 5.5 4.57 0.36 33 70 2.3 53 Edds 6.4 1.15 0.13 165 24 12.6 62 Klicker 6.7 3.30 0.27 65 17 2.3 60 Forest floor Douglas-fir 5.5 31.30 0.71 1,500 25 100.0 88 Mixed fir 5.5 37.90 0.77 3,560 68 15.8 94 Ponderosa pine 4.5 44.20 0.66 46 17 1.4 23 Methods of Bollen et al. (3).

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BOLLEN ET AL. : EFFECT OF CACODYLIC ACID AND MSMA

statistical analysis. However, the results from soil treated with among weeks of incubation for all test material and chemical either chemical were very similar. treatments (Tables 2 and 3). Data for forest floor material Analysis of variance established that the rate of CO 2 evolu- from each of the different vegetation types (9), and for the tion was significantly different (P < 0.01) among soils or for- different soils when combined showed that the rate of CO2 est floor either untreated or treated with various amounts of evolution was significantly different (P < 0.05) among concen- cacodylic acid or MSMA (Tables 2 and 3). This undoubtedly trations of As for forest floor material treated with either reflects the differences in the native complements of organisms herbicide and for soil treated with MSMA. Cacodylic acid had rather than a chemical-test material interaction since similar no effect (P > 0.05) on the rate of CO2 evolution from soil. effects are noted in controls. A more detailed analysis of these results revealed the fol- The rate of CO2 evolution declined markedly with time lowing relationships: during the incubation period. When data were combined, the 1. The rate of CO2 evolution declined with time in all rates of CO2 evolution were significantly different (P < 0.01) treatments. The decline is statistically significant (P <

Table 2. Evolution of carbon as CO 2 from 50g of soil treated with cacodylic acid or MSMA. Cacodylic acida MSMAa Week Week Arsenic 1 2 3 4 Total 1 2 3 4 Total Soil (ppm) (mg C/week) (mg C) (mg C/week) (mg C) Astoria 0 17 8 7 6 38 18 8 6 4 36 10 16 8 5 6 35 18 8 6 5 37 100 16 8 7 6 37 17 8 7 5 37 1,000 16 7 8 5 36 18 10 6 6 40 Edds 0 4 3 4 4 15 4 2 2 2 10 10 4 3 3 3 13 3 3 2 2 10 100 3 2 2 2 9 5 2 2 2 11 1,000 4 2 2 2 10 5 4 2 3 14 Klicker 0 8 4 3 4 19 8 3 2 3 16 10 8 4 3 3 18 9 4 3 3 19 100 8 4 3 3 18 8 6 3 3 20 1,000 7 3 2 3 15 10 5 3 3 21 aMean of three replications; specific statistical tests of hypothesis are in text and Figures 1 and 2.

Table 3. Evolution of carbon as CO2 from 50g forest floor material treated with cacodylic acid or MSMA. Cacodylic acida MSMAa Week Week Arsenic 1 2 3 4 1 2 3 Total 4 Total Forest floor (PPn) (mg C/week) (mg C) (mg C/week) (mg C) Douglas-fir 0 260 188 125 105 678 258 181 143 126 708 10 275 189 132 116 712 246 178 158 132 714 100 224 169 134 104 631 233 170 144 116 663 1,000 234 140 94 78 546 226 164 135 92 617 Mixed fir 0 355 201 184 129 869 300 194 156 148 798 10 360 194 164 134 852 292 176 157 155 780 100 325 186 169 127 807 292 184 158 115 749 1,000 328 189 136 105 758 283 186 131 121 721 Ponderosa pine 0 437 216 206 180 1,045 363 191 202 179 935 10 438 225 201 186 1,050 363 202 181 168 914 100 413 225 194 161 993 375 201 183 163 922 1,000 401 223 194 161 979 365 200 188 159 912 aMean of three replications, specific statistical tests of hypothesis are in text and Figures 1 and 2.

Volume 22, Issue 6 (November), 1974 559

WEED SCIENCE

400 1 0.01) and best described by a cubic equation in each case (Figure 1). The time of incubation — concentration Cacodylic Acid (Forest Floor) of arsenic interaction was not significant (P > 0.05) MSMA (Forest Floor) which means the decline in CO 2 evolution which oc- Cacodylic Acid (Soil) curred with time of incubation was not a function of the 3 300 12 presence or absence of arsenic. A MSMA (Soil) 0 The rate of CO2 evolution declined with increasing log C.) concentration of As in MSMA and cacodylic acid treated a) 0 forest floor material. The decline is linear with log concentration for MSMA and quadratic for cacodylic acid. o, 2002 8 MSMA in soil resulted in an increase in the rate of CO2 E 0 evolution which was linear with log concentration (Fig- 0 ure 2). 0 O The rates of carbon evolution were significantly differ- cr, ent (P <0.01) between any two soils and any two forest 1-0 100 4 E floor materials treated with a given herbicide.

0 Decomposition of organic matter and the release of nutri- U) ents is a particularly important function of microbes in the forest floor and soil. We used the total carbon evolved during 0 1 1 1 0 the 28-day test period (Tables 2 and 3) and the beginning 2 3 4 carbon content of the test materials (Table 1) as a basis for Weeks calculating the percent decomposition of organic matter for each treatment (Figures 3 and 4). Figure 1. Effect of time on evolution of carbon (mg/week) as CO 2 from MSMA and cacodylic acid treated forest floor material and soil. Com- Analysis of variance of single degrees of freedom were used bined data for forest floor material and soil treated with 10, 100, and to test for differences in organic matter decomposition among 1,000 ppms As as cacodylic acid and MSMA (three replications). The arsenic concentrations in a particular forest floor or soil rate of CO2 evolution was significantly affected by time (P < 0.01) and treated with MSMA or cacodylic acid. Increasing concentra- is best described by a cubic equation in each case. tions of both cacodylic acid and MSMA depressed organic mat-

7.0 a) a)

0 U)

E Cacodylic Acid (Forest Floor), Quadratic MSMA (Forest Floor), Linear "5 cn Cacodylic Acid (Soil), Not Significant MSMA (Soil), Linear

150 4.0 0 50 100 1000 Arsenic (ppm)

Figure 2. Effect of arsenic on mean evolution of carbon (mg/week) as CO 2 from MSMA and cacodylic acid treated forest floor material and soil. Combined data for forest floor material and soil 1, 2, 3, and 4 weeks after treatment with MSMA and cacodylic acid (three relications). The rate of CO, evolution was significantly affected by concentration of arsenic (P < 0.05) in all cases except cacodylic acid treated soil. 560 Volume 22, Issue 6 (November), 1974 BOLLEN ET AL.: EFFECT OF CACODYLIC ACID AND MSMA

ter decomposition in forest floor when arsenic reached 100 All microbes produce CO2 as they decompose organic com- ppm, except in ponderosa pine forest floor treated with pounds. Changes in the rate of CO 2 production reflect changes MSMA. In no case did 10 ppm As significantly affect organic in the general activity of microbial populations, particularly matter decomposition in forest floor. There was no effect of their role in organic matter decomposition and nutrient re- cacodylic acid on organic matter decomposition in Astoria or cycling. Our study showed the rate of carbon (as CO 2 ) evolu- Klicker soils, but there was a decline in the Edds soil when As tion declined rapidly with time in all treatments (including the reached 100 ppm. MSMA had no regular effect on Astoria or controls). This is usual in all organic matter decomposition Klicker soils, but 1000 ppm As did significantly increase or- studies. The more soluble organic components are rapidly at- ganic matter decomposition in Edds soil. tacked, leaving the more resistant materials which undergo slower and slower oxidation. Humus becomes the final residue and is most slowly decomposed. Differences in organic matter DISCUSSION content and extent of decomposition account for differences The small reduction in growth of bacteria in liquid culture at in CO2 evolution from different forest floors and soils. The 1,000 mg/L As indicates MSMA and cacodylic acid can in- concentration of As had a small but statistically-significant fluence soil microbial activity. The paper-disc assay with molds effect on the rate of carbon dioxide evolution from MSMA and bacteria, however, showed no effect even at concentra- and cacodylic acid treated forest floor material and MSMA tions of 10,000 mg/L As. This is attributed to the greater treated soil. The effect of arsenic concentration was less mobility of and opportunity for interaction between chemical marked than the effect of time. and organism in the liquid. The pure culture assays, in general, Organic matter decomposition in the F layer of the forest show MSMA and cacodylic acid are not particularly toxic to floor is a major importance in mineral nutrient release and common soil bacteria and molds of which our test organisms recycling. Organic content of the underlying mineral soil is are representative. Malone (17) showed sodium cacodylate much less and has already been extensively decomposed to temporarily increased numbers of soil bacteria and decreased humus. This is attacked only slowly by specific kinds of mi- numbers of molds after a nonselective control of meadow crobes, liberating nutrients continuously but in lesser amounts vegetation. There were, however, no long-term effects on than the large numbers of organisms in the forest floor. In our either microbial numbers or decomposition processes. study there appears to be little difference between the effects

Pseudotsugo Pseudotsuga Tsuga heterophylla menziesii Pinus ponderosa Tsuga heteropnyllo menziesii Pinus ponderosa Forest Floor end Forest Floor and Forest Floor end Forest Floor and Forest Floor and Forest Floor and Astoria Sod Edds nicker Soil Astoria Soil Edds Soil /Clicker Soil 5.00 5.00

a 4.00 4 00 C

IA0 0. 4 3.00 E 3.00 0 U / / 2.00 2.00 O U C 1.00 on 1.00 0

O o o 0 0 0 0 0 O 0 O 0 0 0 — oo O 0 O O 0 00 O o O O — Arsenic (ppm) Arsenic ( ppm ) Fizure 3. Decomposition of organic matter in forest floor 0 and soil FAure 4. Decomposition of organic matter in forest floor 0 and soil E. in 28 days at 28 C after addition of cacodylic acid. Within a par- al in 28 days at 28 C after addition of MSMA. Within a particular ticular forest floor or soil, bars with lower case letters in common were forest floor or soil, bars with lower case letters in common were not not significantly different (P > 0.05). significantly different (P > 0.05).

Volume 22, Issue 6 (November), 1974 561 WEED SCIENCE of MSMA and cacodylic acid on CO2 evolution from forest Alexander, M. 1961. Introduction to soil microbiology. John W. floor. The differences in response between chemicals in soil Wiley and Sons, New York, NY. 472 pp. Bollen, W.B., C.S. Chen, K.C. Lu, and R.F. Tarrant. 1967. Influ- will be largely masked by the predominant influence of forest ence of red alder on fertility of a forest soil: Microbial and floor. We feel, therefore, that the combined rates of organic chemical effects. Res. Bull. 12. Forest Res. Lab., School of matter decomposition in forest floor and soil in the field will Forestry. Oregon State University, Corvallis, OR. 61 pp. be influenced to approximately the same degree by either Bollen, W.B. and D.W. Glennie. 1961. Sawdust, bark and other chemical. wood wastes for soil conditioning and mulching. For. Prod. J. 11 : 38 -46. When organic matter is attacked by soil microbes, CO 2 and Bollen, W.B. and E. Wright. 1961. Microbes and nitrates in soils intermediate carbon compounds (organic acids, alcohols, etc.) from virgin and young-growth forests. Can. J. Microbiol. are produced. Oxidation of a substrate by a given organism is 7:785 -792. rarely complete. Even under fully-aerobic conditions only 60% Brown, A.W.A. 1951. Insect control by chemicals. John W. Wiley and Sons, New York, NY. 817 pp. to 80% of the carbon is converted to CO2 . Our tests did not Cooper, S.S. 1941. The mixed indicator bromcresol green-methyl determine if cacodylic acid or MSMA influences the pathway red for carbonates in water. Ind. Eng. Chem., Anal. Ed. of carbon metabolism. Based on CO2 evolution, both caco- 13:466 -470. dylic acid and MSMA altered the rate of organic matter de- Covey, R.P., Jr. 1969. The effects of certain herbicides and arsenic composition in forest floor. The greatest effects were at the on the growth of Phytophthora cactorum. Cir. 514. Wash. Agr. Exp. Sta. highest concentrations of As tested. Neither chemical had a Franklin, J.F. and C.T. Dymess. 1973. Natural vegetation of consistent effect on organic matter decomposition except in Oregon and Washington. USDA For. Serv. Gen. Tech. Rep. Edds soil where MSMA was stimulatory and cacodylic acid was PNW-8, 417 pp., illus. inhibitory when As levels were greater than 100 ppm. The Greaves, J.E. 1934. The arsenic content of soils. Soil Sci. magnitude of the effects of MSMA and cacodylic• acid on or- 38:355-362. Greaves, J.E. and E.G. Carter. 1924. Influence of sodium arsenite ganic matter decomposition in either forest floor or soil is on microflora of soil. Bot. Gaz. 77:63-72. small when As levels are less than 100 ppm. Some impact may Headden, W.P. 1910. Arsenical poisoning of apple trees. Bulls. 131 occur if arsenic levels exceed 1000 ppm in very small areas and 157. Colo. Agr. Exp. Sta. where chemical is spilled or water for washing hands and Jones, J.S. and M.B. Hatch. 1937. The significance of inorganic equipment is poured. spray residue accumulations in orchard soils. Soil Sci. 44:37-63. Kearney, P.C. and E.A. Woolson. 1971. Chemical distribution and In use of these herbicides for precommercial thinning pur- persistence of 4 C-cacodylic acid in soil. 162nd National Amer. poses, concentrations of arsenic greater than 10 ppm in forest Chem. Soc. Meeting. Pesticide Chem. Div. Abstract 28. Washing- floor and soil will occur infrequently and will be restricted to ton, DC. less than a square meter if careful handling and application Kearney, P.C. and E.A. Woolson. 1971. Microbial metabolism of 4 C-cacodylic acid. 162nd National Amer. Chem. Soc. Meeting. techniques are used (18). The impact of MSMA and cacodylic Pesticide Chem. Div. Abstract 29. Washington, DC. acid on the rate of organic matter decomposition will decrease Lamanna, C. and I.M. Shapiro. 1943. Sulfanilamide bacteriostasis with time as these compounds are metabolized to various in the presence of mercuric chloride and p-aminobenzoic acid. J. gaseous arsine derivatives or are complexed with iron, alumi- Bact. 45:385-394. Malone, C.R. 1971. Responses of soil micro-organisms to non- num, calcium, or other materials which reduce the phyto- selective vegetation control in a fescue meadow. Soil Biol. Bio- toxicity of organic and inorganic arsenic compounds (2, 14, chem. 3:127-131. 15, 19, 20). Norris, L.A. 1971. Studies of the safety of organic arsenical herbi- We conclude that cacodylic acid and MSMA do not serious- cides as precommercial thinning agents: A progress report. Pages ly affect forest soil microbial populations or their decomposi- 63-74 in Precommercial thinning of coastal and intermountain forests in the Pacific Northwest. Wash. State Univ., Pullman, tion of organic matter. Tests now in progress will determine WA. the effects of these herbicides on microbial nitrogen metabo- Von Endt, D.W., P.C. Kearney, and D.D. Kaufman. 1968. Degra- lism in the forest floor and soil environment. We feel that our dation of monosodium methanearsonic acid by soil micro-organ- tests will provide an adequate base for assessing the impact on isms. Agr. Food Chem. 16:17-20. microbial populations and soil fertility of arsenical silvicides Woolson, E.A., J.H. Axley, and P.C. Kearney. 1971. The chemis- used in precommercial thinning. try and phytotoxicity of arsenic in soils: I. Contaminated field soils. Soil Sci. Soc. Amer. Proc. 35:938-943. LITERATURE CITED Wright, E. and W.B. Bollen, 1961. Microflora of a Douglas-fir forest soil. Ecology 42: 825 -828. 1. Albert, W.B. and W.B. Paden. 1931. Arsenical poisoning of cotton 22. Zabel, R.A. and F.W. ONeal. 1957. The toxicity of arsenical com- soils. Science 73:622. pounds on micro-organisms. Tappi 40:911-914.

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