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Home , Nitrogen cycle, Sulfur cycle

Recycling and in Grass- Pastures

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AGRICULTURAL EXPERIMENT STATION OREGON STATE UNIVERSITY CORVALLIS STATION BULLETIN 610 JUNE 1972 Contents

Abstract 3 Introduction 3 The 3 The 8 Summary 11 LiteratureCited 12

AUTHORS: M. D. Dawson is a professor of soils science and W. S. McGuire is a professor of agronomy, Oregon State University. ACKNOWLEDGMENTS: The authors are indebted to Viroch Impithuksa for conducting the carbon-nitrogen-phosphorus-sulfur (C:N:P:S) analyses and to J. L. Young for invaluable assistance in writing the manuscript. Recycling Nitrogen and Sulfur in Grass-Clover Pastures

M. D. DAWSOTJ and W. S. McGuii

Abstract Indeed, the soil-- chain is a fascinating intra-system where the Improved grass-clover pastures uti-nitrogen and sulfur cycles have practi- lized under high stockingsystems cal significance. epitomize conservation management at The management practicescom- its best. Under intensive grazing andpared are (1) unimproved, indigenous in spite of nitrogen (N) or sulfur (S) grasses, (2) fertilized grass-clover cut losses through leaching, volatilization, for hay, and (3) fertilized grass-clover or sales of meat and wool from the intensively grazed. The purpose of this farm, good management permits sym- bulletin is to review certain features bioticfixationofnitrogen and re-of soil-plant-animal interrelationships cycling of N and S in amounts needed as they influence soil nitrogen and sul- for top production. In the comparisons fur cycles under different management of management practicesinvolvingpractices in grass-clover pastures. unimproved indigenous grasses with (1) fertilized grass-clover cut for hay and (2) fertilized grass-clover inten- The Nitrogen Cycle sivelygrazed,thisbulletin reviews In western Oregon, annual yields of certainfeaturesof soil-plant-animal6,000 pounds and 12,000 pounds of interrelationships as they influence soildry matter per acre are common on nitrogen and sulfur cycles. subterraneancloverandirrigated grass-whiteclover pastures,respec- tively. The 6,000 pounds of dry matter Introduction from healthy subterranean clover pas- Many acres of western Oregon landture should contain about 3 percent not ideally suited to cultivation cannitrogen. At least 180 pounds nitrogen produce much meat and wool by wayper acre would be needed to produce of intensively grazed pastures. High this 6,000 pounds of dry matter. Twice stocking rates on these improved grass-this amount of nitrogen would be clover pastures are a means by whichneeded to produce the 12,000 pounds man can economically provide goodof dry matter on irrigated grass-white quality, high livestock feed.clover pasture. But quality forage requires consider- The average total soil nitrogen con- able nitrogen, phosphorous, and tent under normal pastures (Table 1) in balance. Few crops have a higher demand for these nutrients than aTable 1.Average percentage total soil vigorously growing grass-clover pas- nutrients' ture. Percent It is of considerable economic and Nutrient environmental interest to examine the Total nitrogen 0.24 cyclical changes of the comparativelyTotal sulfur 0.02 mobile nutrientsnitrogen and sulfurTotal organic phosphorus 0.03 under variously managed pastures. 'From 12 Oregon grass-clover pastures. 3 N RETAINED BY STOCK VOLATILE STOCK GRAZED N A

GRASS CLOVER mdrgromd SYMBIOTIC N FOCER flnsference / N / N

NITRATE DEN1TRIFIED AND LEACHED

\I N FROM PLrr ROOTS SOIL ORGANIC N Figure 1.The nitrogen cycle on grazed grass-clover pasture. Adapted from T. W. Walker (12). is low, about 0.20 percent. In an acrein nodulated roots of the clover . furrow s]ice of soil (2 x 106 lbs.) that Numerousworkershavetriedto would amount to 4,000 pounds ofmeasure the amounts of nitrogen fixed nitrogen, but most of this is bound in by annually. Estimated net ad- thesoilin complex organic forms.ditions vary from nil (where the hay Grasses and clover obtain some of thisis removed) to 10 pounds nitrogen per soil-bound nitrogen as nitrate or am-acre (where the is sparse) to monium after the decay and minerali- 100 up to more than 400 pounds nitro- zation of soil . However, gen per acre (where effective legume there is evidence that only about 1.25is plentiful). The amount depends on percent (11)1 of the total soil nitrogen such factors as species of legume, a complex ingrass-clover pasturesis proven effective rhizobia strain, and a mineralized to plant-available forms host of environmental conditions (10). annually. This would mean at best 50 The amount of nitrogen fixed and pounds of nitrogen could be min-available, either for companion grass eralized from 4,000 pounds in an acreby undergroundtransferencefrom furrow slice annually, or a total deficitclover roots or for the clover plant it- of 130 pounds (180-50) of plant-avail- self, depends on effective nodulation able nitrogen per acre. which is only possible with proven ef- Figure1, adapted from Walker'sfective strains(Table 2). nitrogen cycle model (12), shows theThere is evidence that "native" strains nitrate mineralized from decaying soilof rhizobia are in many cases only organic matter is subject to uptake by partially effective in N fixation for the pasture, mainly by the grass com- clovers grown in Oregon. ponent. Also evident is an arrow indi- For subterranean clover, an aver- cating symbioticfixationof atmos-age of 150 pounds nitrogen per acre pheric nitrogen by the rhizobia livingis fixed symbiotically per year. That is more than enough to make up the 'Numbers in parentheses refer to Liter- 130-pound deficit of nitrogen previ- ature Cited, page 12. ously noted, except that, firstly, much 4 Table 2.Effect of inoculation with a othernutrientstoinsure optimum proven rhizobium strain on New Zealand symbiotic . Table 4 white c1over data illustrate the effect of applied sul- fur and on clover yield Inoculated Uninoculated and reflect the role these nutrients Avg. yield Avg. Avg. yield Avg. have upon thesymbioticnitrogen (dry plant (dry plant fixation efficiency. Such responses have matter) N matter) N been observed frequently in Oregon. lbs/A % lbs/A % Where grasses and clovers are growing 11,574 3.17 7,903 2.18 together, grasses utilize almost all the N available (11). Recent research showsthatsoil of this fixed nitrogen is not immedi-moisture stress may be as much a ately available to the companion grasslimiting factor on the ability of clover and, secondly,soil conditions often to symbiotically fix atmospheric nitro- preclude the clover's ability to fix nitro- gen as are deficiencies in plant nu- gen efficiently. Ineffectively nodulatedtrients. Kuo (6) found that nitrogen plants, due often to poor sowing tech-fixation by a legume is reduced as the riiques, produce clover which is lowsoil dries out (Table 5). Together with in plant nitrogen (Table 3). Data inleaching losses of during win- Table 3indicatethat conventional ter and volatile losses of nitrogen from inoculation was often ineffective. Tothe soil, such less-than-optimum condi- insure effective nodulation with thetions for nitrogen fixation increase the applied strain,it was necessary to practical difficulties of obtaining suffi- plant seeds with a lime-superphos-cient nitrogen for the yield of 6,000 plite mixture. pounds dry matter per acre in a grass- Once established, a legume must beclover pasture. supplied with adequate phosphorus, An ideal way to restore balance to sulfur,molybdenum,andpossibly the nitrogen cycle (Figure 1) is to use

Table 3.Percent effective nodulatjoof subclover and plant nitrogen content Percent effective nodulation four weeks after planting Average Washington plant Treatment Polk Co. Co. Coos Co. nitrogen (4 Uni noculated 14 2 3 1.29 Inoculated 14 2 44 Inoculat d and ''line- super mix' 72 96 79 3.54 Equal ,uixture of 20 percent superphosphate and lime.

Table 4.Mean subterranean clover yields and nitrogen content as influenced by applied sulfur and molybdenum Total Clover plant Treatment Diy matter nitrogen nitrogen lbs/A lbs/A J) 3,420 2.06 70 PS 5,220 2.78 145 PSMo 5,982 3.17 190

5 Table 5.Rates of nitrogen fixation in a legume as influenced by moisture stress at 50 degrees F' Soil water stress Nitrogen fixaon (bars) Soil status ( mg of N per day) 0.35 Field capacity 0.76 1.50 Moist 0.30 2.50 Moderately dr 0.23 'From unpublished M.S. thesis by T. Kuo (6).

Table 6.Nitrogen uptake from grass and grass-plus-clover pastures' Grass-clover pasture Management Grass alone Grass N Clover N Total lbs/A lbs/A lbs/A lbs/A Hay crop taken 50 188 346 534 Grazed 75 379 271 650 J. Melville and P. D. Sears, (7). the grazing animal. The data in Tableapparent when ryegrass was grown 6 (taken from New Zealand) illustrate in a field fertilized with superphos- the role of the grazing animal in thephate (300 lbs.of 20% superphos- nitrogen cycle. Where a hay crop isphate applied annually) that had been taken,underground transferenceof intensively grazed (4 sheep per acre) nitrogenfrom cloverhas providedfor the past 12 years. Furthermore, grass in association with clover withthe yield and nitrogen content of rye- 138 pounds more nitrogen per acregrass on thissoil were significantly than an all-grass pasture. This was a higher than of ryegrass grown on the small effect compared with the grazedsame soil which had virtually no graz- grass-clover pasture, where the grass ing during this period. The accumu- uptake of nitrogen amounted to 379lated soil organic nitrogen on the in- pounds per acre. tensively grazed grass-clover pasture A greater proportion of the nitrogen apparently is mineralized and cycled originally fixed by clover appears in at a rate sufficient to provide optimum the grass where urine is returned andnitrogen nutrition for the grass. Under serves as a nitrogen . Indeed, such conditions,the nitrogencycle at a stocking rate of three or four ewes appears balanced in such a way that per acre, dung and urine excreta would moisture and sufficient solar energy likely return an equivalent of at least alone drive the system near optimum. 150 pounds nitrogen per acre annually As has been vividly demonstrated by (12). A high stocking rate assists thePetersen and others(8), significant nitrogen recycling, and with cloverbenefits from excreta of grazing ani- apparently supplies the total nitrogenmals can be expected only under con- requirement for the expected yield ofdition.s of high stocking intensity over 6,000 pounds of dry matter per acrelong periods of time. in the pasture. Severalworkers(14,15)have Recent experiments using perennialrecognized consistent carbon, n I trogeil, ryegrass as an indicator plant supportsulfur, and phosphorus ratios in grass- this probability (Table 7). No yield land soils, as would be expected from response from applied nitrogen was the rather definite proportions of these 6 Table 7.Yield and nitrogen content of ryegrass grown on screened subterranean clover soils1 Management Yield Percent history Treatment (dry weight)plant nitrogen gramc Unfertilized, unutilized Check 1.86 2.66 PSK 2.74 1.85 NPSK 4.85 2.01 Fertilized, hay crop Check 3.25 2.14 PSK 3.52 2.14 NPSK 4.79 2.69 Fertilized, grazed 12 years Check 7.74 3.03 (4 ewes per acre) PSK 7.07 2.91 NPSK 7.60 3.07 'Soils had 12 years of different past management. From unpublished M.S. thesis by V. Im- pithuksa, OSU, 1971.

Table 8.Average carbon, nitrogen, phosphorus, sulfur ratios in topsoil from selected Oregon pastures Pasture Carbon Nitrogen Phosphorus Sulfur Unimproved 156 10 2.5 0.7 Fertilized pius grazed 142 10 2.8 1

elements in the soil organic matter.simultaneouslyprovidehigh-quality An increase or decrease of one oflivestock feed and a generous supply these elements is accompanied by aof nitrogen for recycling through the parallel increase or decrease in thesoil-plant-animal system. To produce others.Recent studieson Oregon6,000 pounds of dry matter, subter- pastures revealed a carbon-nitrogen-ranean clover will require at least 180 phosphorus-sulfur relationship (Tablepounds of nitrogen per acre annually. 8). The somewhat narower C:N:P:SWhile for one reason or the other, ratios obtained in the soils from fer-nitrogen symbiotic fixation might more tilized and grazed pasture suggest lessnearly approach 100 pounds nitrogen immobilization and more mineraliza- per acre, animal exereta at three to tion of nitrogen, phosphorus, and sul-four ewes per acre could well recycle fur. Such a situation would enhanceanother 150 pounds of nitrogen per efficient recycling of these plant nu-acre. Certainly some nitrogen is lost trients through the soil and reabsorp-by leaching, volatilization, and from tion by the pasture plants. High cor-meat or wool sales. Loss of N through relations were noted between soil nitro-removal in meat or wool probably does gen and total phosphorus as well asnot exceed 15 percent of the nitrogen between nitrogen and total soil sulfuringested by the animal, or about5 (Figure 2). This further illustrates thepounds nitrogen per acre. interdependence ofthesenutrients, Since mineralization of soil organic where the build-up of any one nutrientmatter (even in the absence of animal depends upon the supply of the others. excreta) could be estimated to equal 40 pounds nitrogen per acre, the nitro- Sumrnonj of Nitrogen Cycle gen economy for sustained high pro- High stocking rates on grass-clover ducinggrass-cloverpasturesinten- pastures fertilized with superphosphatesively grazed seems assured. Indeed, 7 05

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02

01

.2 .3 .4 .5 .6 Percent Total Nitrogen Figure 2.The relationship between soil total nitrogen and soil total sulfur. one might term the nitrogen cyclethe p]aiit protein yield that was ob- under intensive]y stocked grass-clover tained where sulfur was deficient in ptsturesi1s "econology," since it pre- the soil. serves and conserves a quality environ- In producing a 6,000-pound dry ment with a sound economic base forweight yield of subterranean clover providing quality livestock feed. containing an adequatetotalplant sulfur of 0.30 percent (4, 13), at least The Sulfur Cycle 18 pounds of sulfur per acre would be required. Furthermore, sulphate like Although sulfur is not fixed sym-nitrate is subject to leaching (Table 9). biotically by clover rhizobia or otherAssuming that cutting the clover pas- soil organisms asisnitrogen, thereture as hay removed 54 pounds of are striking similarities between thesulfur per acre from the 120 pounds nitrogen and sulfur cycles (Figure 3).of sulfur applied during a three-year Moreover, just as there is a close re-period (Table 9), only 53i pounds of lationship between available soil nitro- sulfur over the amount in the check gen and clover nitrogen content, theretreatment was recovered in the Steiwer isalso an intimate relationship be- profile. This would amount to a loss of tween available soil sulfur and plantabout 24 pounds of sulfur per acre nitrogen due to the essential role ofannually. sulfur in protein synthesis. Indeed, the The movement of sulfate into the 145 pounds of total p]ant N (Table 4) lower horizon of some western Oregon represents a pasture producing twicesubterranean clover bill soils following 8 Table 9.Net increase of sulphat& in two contrasting soil profiles Soil profile Soil depth Steiwer Nekia inches lbs. S/A lbs. S/A 0-6 1.5 8.5 6-12 1.5 11.5 12-18 2.0 14.0 18-30 0 1.0 'Extracted with 0.1 N KH.PO; 120 pounds of sulfur per acre were applied as over a three-year period.

winter rains will vary according to soiluptake and removal in hay or type (Table 9). The movement of soilor by leaching. The combined subter- SO4-S recently has been studied inranean clover sulfur uptake and soil Oregon. For instance, the Steiwer soilsulfate leaching loss could approach at retained little sulfate to a depth of 30least 40 pounds sulfur per acre an- inches, while the red Nekia soil re-nually. Available sulfur to balance the tained appreciable sulfate particularlyloss can come from four sources: (1) in the subsoil layers. Sulfate leachingthe atmosphere,(2) of from surface horizons and accumula-rocks and minerals, (3) mineralization tion as adsorbed SO4 in the lowerof soil organic matter, and (4) on-site horizons has been observed by otherexcreta from the grazing stock. Fer- workers (5). For a relatively shallow- tilizersulfur,of course,isanother rooted legume like subterranean clover,potential source. subsoilsulfateisoflittlevalueif Atmospheric returns of sulfur, either beyond the reach of feeder roots. direct or via rainfall, appear low in Thus far we have considered aspectswestern Oregon except for very local of the sulfur cycle (Figure 3) con-situations.Additions seldom exceed cerned with soil sulfur loss either byseven to eight pounds sulfur per acre

H2S LOSS S RETAINED BY STOCK

STOCK GRAZED A ATMOSPHERE

AS$ CLOVER URINE

SULFATE LEACHED SULFATE

S FROM PLANT S-MINERALS ROOTS WEATHERED SOIL ORGANIC S Figure 3.The sulfur cycle on grazed grass-clover pasture. Adapted from T. W. Walker (13). 9 annually, even from areas within onenitrogen annually should concurrently mile of the Pacific Ocean (3). Releaserelease four pounds of sulfur per acre. of to the soil from weatheringA sulfur deficit of 30 pounds per acre of sulfur-poor rocks also appears mini-annually would persist unless some mal. further sulfur additions were forth- Minerali2ation of soil organic mat-coming. This is consistent with results ter appears to be a primary source ofof numerous field experiments showing plant-available sulfur. Indeed, severalyield response from application of 20 workers (13) have suggested that into 40 pounds of sulfur per acre an- humid, temperate climates and givennually under hay management systems. reasonable soil drainage, most of the A familiar type of response to sulfur sulfur in the surface soil is contained applied to an ungrazed grass-clover in soil organic matter. Moreover, sul-pasture may be seen in Table 10. In fate availability to plants depends onboth years the pasture yield increased. the rate of soil organic matter min- The response to S was due to clover in eralization by (1, 9). the first year, yet by the third year it The amount of sulfate formed from soilwas due to grass. This demonstrates organic matter will depend on thethe influence of applied sulfur in stimu- organic sulfur content and factors af-lating clover to fix more N symbioti- fecting the activity of microbes. cally which, in turn, stimulated grass In particular, the N: S and possiblygrowth to the relative suppression of the N:P ratios of soil organic matterclover in a two-year period. Note that affect its rate of mineralization. Newthis occurred under ungrazed condi- Zealand studies(13) show an N:Stions. ratio for soil organic matter of about Anderson and Spencer(2)give 10:1, indicating that for every 10figures for subterranean clover of 1.71 pounds of NO3-nitrogen per acre min-and 0.058 percent nitrogen and sulfur eralized, one pound of SO4-sulfur perrespectively when no sulfur was ap- acre would be produced. For westernplied and 2.26 and 0.24 with sulfate Oregon soils, the N:S ratio of soil or-applied. Walker (13) reported an even ganic matter is higher than 10:1 ex-greater increase in clover nitrogen and cept in fertilized and grazed pasturessulfur content following a 200-pound (Table 8). The high correlation be-application of gypsum; nitrogen in- tween total soil N and S noted increased from 2.87 to 3.45 percent and Figure 2 further emphasizes the closesulfur from 0.17 to 0.34 percent. That relationship and constancy these nu-grasses compete strongly with clover trients have to one another. when mineral nitrogen is not limiting On the basis of a 10:1 N:S soil or-appears obvious from these data. What ganic matter ratio, a soil with 0.20 total is equally important is the grass com- nitrogen that released 40 pounds NO3-petition for available soil SO4, since

Table 10.Ryegrass.subterranean clover response to applied gypsumand resultant change iii species composition First year Third year Avg. yield Avg. yield Treatment dry matter Percent dry matter Percent (lbs. S/A) per acre clover per acre clover lbs. lbs. 0 3,675 30 2,813 10 10 4,590 30 3,825 50 20 5,524 70 4,583 30 40 6,976 70 6,278 30

10 for each 10 pounds of nitrogen incorpo- been annually top-dressed with super- rated in plant tissue there is a need forphosphate. However, in both locations about one pound of sulfur. A soil sul-one field had been intensively grazed fur deficit isfirst likely to be nutri-while the other had been hayed and tionally disadvantageous to clover andkept ungrazed. Ryegrass was planted contribute to a grass-dominant pasture. into each sieved soil and appropriately fertilized. For both locations, ryegrass Grazing and the Sulfur Cycle grown in soils from the "previously The grazing animal returns sulfur tofertilizedbutungrazedfield"re- the pasture by way of dung and urinesponded to applied sulfur, and yields just as it returns nitrogen. Walker con-were greater but there was no response cluded that 70 to 85 percent of theto sulfur where the soils had pre- sulfur ingested by stock is returned tosumably benefited from urine returns the pasture in urine (13) along withunder grazing. The percent plant sul- additional small amounts in the feces. fur confirms the increased sulfur up- Certainly, some sulfur of this excretatake fromsoilsof"previouslyin- is lost by volatilization, but most enters tensively grazed and previously top- the soil organic sulfur pooi or is reason- dressedgrass-clover pastures" com- ably quickly available for reabsorption pared with the "ungrazed pastures." by the grass-clover (Figure 3). The nitrogen content of the urine is Summary variable but reportedly is proportional to sulfur content at roughly a 5:1 Intensive grazing of improved pas- ratio (13). If, as has been suggested,tures under conditions such as in west- a urine patch contains a nitrogen con-ern Oregon, represents a system of centration equivalent to about 400farming where maximizing production pounds nitrogen per acre (12), it fol-can be synonomous with optimizing lows that such a patch also suppliesproduction. The high stocking rates of 80 pounds sulfur per acre. A stockinggrass-legume pastures elegantly illus- rate of three to four ewes per acretrate a soil-plant-animal interrelation- could therefore theoretically contributeship manifested through the nitrogen about 20 pounds sulfur per acre perand sulfur cycles. Soil fertility, pasture annum, a significant sulfur return thatyields, and animal production concur- is absent in a haying management pro- rentlyincreaseasimprovedgrass- gram of a grass-clover pasture. clover fields are intensively grazed. The effect of the grazing animal onProduction and conservation are sel- the reserve supply of soil sulfur afterdom such good bedfellows. Improved intense stocking is reflected in data grass-cloverpasturesutilizedunder from a recent greenhouse experimenthigh stocking systems epitomize con- (Table11).In each instance,the servation management at its best, sieved soil was removed from an old The success in producing quality subterranean clover pasture that hadpasture, especially improved hill grass-

Table 11. Yield and sulfur content of ryegrass grown in sieved surface soil (top 10 cm) from variously managed old subclover pastures' Location 1 Location 2 Ungrazed Grazed Ungrazed Grazed Applied nutrient(s) YieldSulfurYieldSulfurYieldSulfurYieldSulfur g/pot % g/pot % g/pot % g/pot N 2.61 .09 5.80 .27 3.76 .11 8.38 .23 NS 3.72 .41 5.03 .29 4.11 .32 7.59 .31 'From unpublished M.S. thesis by V. Impithutksa, OSU, 1971. 11 clover pastures, in western Oregon de- The clover plant plays a pivotal role pends primarily upon the efficiency of inthis system of intensive pasture the nitrogen and sulfur cycles. In pro- management. Furthermore, on many ducing 6,000 pounds of dry matter,soils it will require at least a decade about 180 pounds of nitrogen perof annual applications of superphos- acre and 18 pounds of sulfur per acrephate fertilizer and high stocking rates would be required. Under intensiveto reach the point where the cycling grazing, and in spite of N or S lossesof nitrogen and sulfur is sufficient to through leaching, volatilization, or salebe virtually self-sustaining. Evidence of meat and wool from the farm, goodsuggests that in many western Oregon management permits symbiotic fixa-situations, sulfur (not phosphate) be- tion of nitrogen and recycling of nitro- comes the first limiting nutrient for gen and sulfur in amounts needed forclover if the nitrogen and sulfur cycle top production. are to continue at peak efficiency.

Literature Cited Alexander, M. 1965. An Introduction lion of excreta by freely grazing cattle to Soil . John Wiley and and its effect on pasture fertility II. Sons, Inc. p. 372. Agron. J., 48:444. Anderson, A.J.,and D.Spencer. Starkey, R. L. 1950. Relations of mi- 1950. Sulphur in nitrogen croorganismstotransformationsof of and non-legumes. Aust. J. sulfur in soils. Soil Sci., 70:55. Sci. Res., 3:27. Vincent, J. M. 1965. Environmental Dawson, M. D. 1969. Sulfur on pas- factors in the fixation of nitrogen by ture legumes in Oregon. Proc. 12th the legume. Amer. Soc. of Agron., Ann. Pac. N. W. Fertilizer Confer- Agronomy No. 10, p. 386. ence. p. 150. Walker, T. W., H. D. Orchiston, and Jones, M. B. 1963. Effect of sulfur A. F. R. Adams. 1954. J. Brit. Grass- applied and date of harvest on yield, land Soc., 9:249. sulfate sulfur, and total sulfur uptake Walker, T. W. 1956. The nitrogen offiveannualgrasslandspecies. cycle in grassland soils. J. Sci. Food Agron. J., 55(3):251. Agric., 7. Harward, M. E., and H. M. Reisen- Walker, T. W. 1957. The sulphur hauer. 1966. Reactions and movement cycle in grassland soils. J. Brit. Grass- of inorganic soil sulphur. Soil Sci., land Soc., 12(1). 101(4) : 248. Walker, T. W., and A. F. R. Adams. Kuo, T. 1970. Soil physical conditions 1958. Studies on soil organic matter. and nitrogenfixationof . I. Accumulations of carbon, nitrogen, Unpublishedthesis,OregonState sulphur, and organic phosphorus in University Library. grassland soils. Soil Sci., 85:307. Melville, J., and P. D. Sears. 1953. Williams, C. H., and C. W. Donald. Pasture Growth and SoilFertility, 1957. Changes in organic matter and New ZealandJ.Sci.Tech.(A), pH in a Podzolic soil as influenced 35: Supp. 30-41. by subterranean clover and super- Petersen, II. C., W. W. Woodhouse, phosphate.Aust.J.Agric.Res., and H. L. Lucas. 1956. The distribu- 8(2) : 179.

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