Mesurement of ground electricl conductivities of different soil type nd their effect on growth rte of plnt Adeseye M. Arogunjo, Aderemi S. Adekol nd Kyode D. Adedyo Deprtment of Physics, Federl University of Technology, P. M. B. 704 Akure, Nigeri rogmuyi@yhoo.com, kbdedyo@yhoo.com Abstrct The purpose of this pper is to exmine the contribution of ground electricl conductivity on growth rte of plnts. Wenner rry method of mesuring ground conductivity hs been employed. Three types of soil smples were used for the study; lomy, clyey nd sndy soil. The results show tht lomy soil hs the highest men conductivity of 3.75x0-3 (m) - followed by clyey with vlue of.x0-3 (m) - nd sndy soil hs the lest men conductivity of 4.03x0-4 (m) -. Comprison is mde between the present results with those of Hck nd both hve good greement. The effect of this prmeter on growth rte of three vegetble crops selected for the study; Amrnthus Cruenthus, Amrnthus Tricolr nd Solnum Mcrocrpon were lso investigted. The results show tht lomy soil with highest electricl ground conductivity hs gretest tendency to support the growth rte of crops. Keywords: electricl conductivity, lomy, clyey, sndy, soil, pprent resistivity, growth rte, electrode On study leve to Deprtment of Physics nd Astronomy, Edwrds Accelertor Lbortory, Ohio University, Athens 4570 USA. Emil: remidekol003@yhoo.com
. ntroduction Soil is nturl mteril formed from vrible mixture of broken nd wethered minerl nd decying orgnic mtter, which covers the erth in thin lyer. Soil is of the utmost importnt to mn becuse it supports the growth of plnts, which therefore sustin humn existence (Ghildyl, 987). The processes of soil formtion hve been ttributed to the wethering process of rocks. Wethering is bsiclly combintion of disintegrtion nd synthesis. t tkes plce when rocks re first broken down into smller rocks nd eventully into individul minerls of which they re composed. The fctors, which determine the rte, type nd qulity of the soil formed in prticulr re, include the nture of the prent mterils, climte, topogrphy, orgnic influences or biotic fctors nd time. The combintion of these fctors determines the soil texture. The enormous vrition in electricl resistivity of different rocks nd minerls will invribly contribute to the reson why different types of soil will hve vrying electricl resistivity s well (Ndler nd Frenkel, 980) There re three bsic types of soil; these re sndy, clyey nd lomy soils. Sndy soil type is predominntly qurtz in size whose dimeter my vry from bout mm to 0.0 mm. t is generlly corse grined, loose, gritty nd well erted. t is well drined since percoltion is esy but cpillry is low due to lrge individul pores. Sndy soil hets up rpidly during the dy nd cools down quickly during the night. Since het destroys soil microorgnism, which hve gret role in humus ccumultion, nd there is lso high rte of leching, it is therefore expected tht it will not enhnce n increse in soil conductivity. Cly prticles hve lrge surfce re nd high power of wter holding cpcity. Soil rich in cly re generlly very sticky nd when they re wet, they re not esily permeble to wter. They contin very little ir due to the very nrrow pore spces, which
3 reduce dringe nd encourge cpillry uptke nd wter retention. For this reson, the higher the mount of cly in soil, the less the mount of wter vilble to plnts. Lomy soil hs good combintion of silt nd cly frctions. This is one of the best soils for crop production becuse it combines the good physicl qulities of silt with the good chemicl quntities of cly (Nyle, 990 nd Berri, 98) n this work, the Wenner rry method hs been used to mesure the soil electricl conductivity of the three soils. The effects of these prmeters on growth rte of plnt were lso exmined. The plnts selected for the study re Spinch green (Amrnthus Cruenthus), Spinch white (Amrnthus Tricolr) nd Egg - plnt (Solnum Mcrcurpon). The growth rte of ech of these vegetbles on the three smples of soil types ws crried out using liner mesurement of height of the plnt. These were chieved by tking mesurement of increse in height of the plnt everydy t constnt time (t) for two months.. Theory One of the most commonly used geophysicl techniques in the detection of electricl conductivity of soil is the electricl method. A number of such techniques hve been developed. The Wenner rry method, which hs good ccurcy (Prsnis, 986), hs been employed in this work. The purpose of the method is the determintion of the subsurfce resistivity contribution by conducting mesurements t the surfce of the erth. To chieve this, electric current is induced into the ground vi two electrodes nd the potentil difference, which is cused by the induced current, is mesured in two other electrodes. The mesured potentil gives n imge for the difficulty of the current flow through the subsurfce. This is n indiction of the electricl resistnce of the subsurfce.
4 Consider piece of solid conductor of length L nd cross sectionl re A, the reltionship between the resistivity, resistnce R, the mesured current, nd the potentil difference is given by (Wtkins, 98) L A L A R () Since resistivity is function of length nd cross sectionl re, for smll chnge in the conductor size therefore, A L () A typicl schemtic digrm of Wenner Arry Method is shown in Fig.. The potentil difference between electrodes C nd D for semi-infinite erth of uniform resistivity, is expressed by (Dobrin nd Svit, 988) K D C (3) where K is the geometric fctor defined s R R r r K (4) r = R = nd r = R = Figure : Schemtic digrm of Wenner Arry method A dc A C D B r r R R
5 The pprent resistivity cn then be computed from K (5) (6) Eqution (6) hs been obtined by substituting eqution (4) into eqution (5). t cn be observed tht the pprent resistivity is independent on the position of the electrode nd is not been ffected even though the potentil electrodes re interchnged. t is ssumed tht the pprent resistivity is uniform within the subsurfce (Dobrin nd Svit, 988); this vlue will then be equl to the true resistivity of the subsurfce structure of the soil. Therefore, the ground electricl conductivity cn be clculted from the reltion (7) 3. Mterils nd methods 3. Mesurement of ground electricl conductivity This study ws conducted t the Federl University of Technology, Akure Nigeri. During the field mesurements, four luminium probes (50 cm long), were used s electrodes. One dc ccumultor, one digitl voltmeter, one milli-mmeter, mllet, insulted cbles re some of the mteril used to investigte the resistivity of the erth.
6 Ground electricl conductivities of the three soils considered for the study were determined using the technique tht hs been described bove. The mesurements re for probe seprtion '', which vry between 0.05 m nd 0.50 m of 0.05 intervls. This distnce is one-third of the distnce between the two outer current electrodes. t therefore follows tht the distnce between the outer current electrodes would vry between 0.5 m nd.50 m. 3. Mesurements of growth rte of the plnts For plnts, soil is the fctor of mjor importnce, nd therefore the soil physicl chrcteristic my hve significnt contribution to its growth. The growth of plnt my be mesured in mny different wys. The prmeter chosen depends on the prticulr prt of plnt to be mesured. The different wys tht cn be used include: growth in height, growth in volume, growth in surfce re of the leves etc. n this work, growth in height method hs been employed. t is simple, speedy nd non-destructive. Results nd Discussion Dt from resistivity mesurement re customrily presented nd interpreted in the form of vlues of pprent resistivity. Apprent resistivity is defined s the resistivity of n electricl homogeneous nd isotropic hlf-spce tht would yield the mesured reltionship between the pplied current nd the potentil difference for prticulr rrngement nd spcing of electrodes. The pprent resistivities of lomy, clyey nd sndy hve been mesured using the Wenner rry method. The vrition of soil conductivity with probe seprtion for the three soils considered re presented in Fig. The conductivity of prticulr soil is not constnt despite the equl interspcing between the electrodes. For instnce, the highest conductivity of 5.00x0-4 (m) - ws observed t distnce of 45 cm while the lowest conductivity of 3.68x0-4 (m) - ws observed t distnce of 0 cm for
7 sndy soil. For cly soil, the conductivity rnges between 6.466x0-4 (m) - nd 4.974x0-3 (m) - t distnce of 5 cm nd 0 cm respectively. For lomy soil, the conductivity rnges between.764x0-3 (m) - nd 7.6x0-3 (m) - t distnce of 40 cm nd 5 cm respectively. The vrition of the conductivity is expected becuse it depends not only on the pprent resistivity, but lso on the nture of the topsoil. The greter soil porosity, the more esily electricity is conducted. ncresing concentrtion of electrolytes (slts) in soil wter will drmticlly increse soil electricl conductivity. Minerl soil contining high levels of orgnic mtter (humus) nd/or : cly minerls hve much higher bility to retin positively chrged ions (such s C, Mg, K, N, NH 4, or H) thn soil lcking these constituents (Jynes et l, 994; Willims 987). The presence of these ions in the moisturefilled soil pores will enhnce soil electricl conductivity in the sme wy tht slinity does (Rhodes nd Corwin 98). Soil electricl conductivity decreses slightly s temperture decreses towrd the freezing point of wter. Below freezing, soil pores become incresingly insulted from ech other nd overll soil electricl conductivity declines rpidly. All these fctors might hve contributed to the vrition in the electricl conductivity. The men ground electricl conductivity obtined is 3.75x0-3 (m) - for lomy soil,.x0-3 (m) - for clyey soil nd 4.03x0-4 (m) - for sndy soil. Obviously, this shows tht lomy hs the highest, followed by clyey nd then sndy soil. Tble shows tht the result of the present work compre very well with similr one by Hck (000).
8 Tble : Comprison of electricl conductivities (m) - of the present study with those of Hck (000) Lomy Clyey Sndy Present study 3.75x0-3.x0-3 4.03x0-4 Hck, 000.8x0-3 6.67x0-3.00x0-4 The overll pttern of growth for Spinch green (Amrnthus Cruenthus), Spinch white (Amrnthus Tricolr) nd Egg plnt (Solnum Mcrocrpon) on the three soil types re shown in Figs 3, 4 nd 5 respectively. As the seedlings emerge from the soil, there is period of ccelerting growth. This is sometimes referred to s period of exponentil growth phse. This is followed by liner growth phse nd then period when growth declines (Miller Rymond, 99 nd Berri, 98). The rte of growth is commonly expressed in terms of bsolute rte G. t is defined s the height of the plnt mesured over time intervl. t cn be evluted from the exponentil growth phse of the plnt nd is given by (Miller Rymond, 99 nd Tindl, 983) dh H H G (8) dt t t The results of the bsolute growth rte of the three crops considered in this study re shown in Tble. Tble : Absolute growth rte of the three crops considered. G (cm/dy) Lomy soil Clyey soil Sndy soil Amrnthus Cruenthus.000.773.333 Amrnthus Tricolr.50.9 0.79 Solnum Mcrocrpon.73.04 0.780 The bsolute growth rte is highest in lomy soil, followed by clyey soil nd then sndy soil. For instnce Amrnthus Cruenthus hs bsolute growth rte of cm/dy in lomy soil, it
9 decreses to.773 cm/dy in clyey nd further decreses to.333 cm/dy in sndy soil. The sme trend is observed for Amrnthus Tricolr nd Solnum Mcrocrpon. t therefore follow tht lomy soil with the highest men conductivity hs the greter tendency to support the growth rte of crops. t is the most conducting soil of the three soils under study. Conclusion n this work, electricl ground conductivities of lomy, clyey nd sndy soil hve been mesured. The vrition of conductivity with distnce ws lso investigted. The results show tht lomy soil hs the highest men conductivity of 3.75x0-3 (m) - followed by clyey soil with vlue of.x0-3 (m) - nd sndy soil hs the lest men conductivity of 4.03x0-4 (m) -. The effects of this prmeter on the growth rte of three vegetble crops selected for the study; Amrnthus Cruenthus, Amrnthus Tricolr nd Solnum Mcrocrpon hve been exmined. The results of the investigtion show tht lomy soil with the highest ground electricl conductivity hs the gretest tendency to support the growth rte of crops. The impliction of this is tht properly delineted ground electricl conductivity in the soil will help to determine res tht will support high yield of stple food crops. The insitu method s conducted in this work in the determintion of the effects of ground electricl conductivity on soil fertility is without the drwbck of size restricted chrcteriztion re nd long lbortory processes of soil nlysis. References Berri G. K., 98, Tropicl Plnt Science, Mcmilln Publisher ndi, pp. 43 53 Dobrin M. B., nd Svit, C. H., 988, ntroduction to Geophysicl Prospecting, McGrw Hill nterntionl, pp. 568 585 Ghilyl B. P., 987, Soil Physics, Wiley Estern Limited, New Delhi, pp. 30 33
0 Hck, R., 000 Geophysicl for slope stbility, Surveys in Geophysics., pp. 43-448 Jynes, D. B., Novl, J. M., Moormn, T. B., nd Cmbrdell, C. A., 994 Estimting herbicide prtition coefficients from electromgnetic induction mesurements, J. Environ. Qul., 4, pp. 3-35 Miller Rymond W., 99, An ntroduction to Soils nd plnt Growth, Prentice-Hll of ndi Privte Limited, pp. 63 64 Ndler, A., nd Frenkel, H., 980 Determintion of soil solution electricl conductivity from bulk soil electricl conductivity mesurements by the four-electrode method, Soil Sci. of Ame. J. 44, pp. 6 Nyle C. B., 990, The Nture nd Properties of Soils, Mcmilln Publishing Compny, New York, pp. 9 Prsnis D. S., 986, Principle of Applied Geophysics, Mcmilln Publishing Compny, New York, pp. 04 69 Rhodes, J. D., nd Corwin, D. L., 98 Determining soil electricl conductivity-depth reltions using soil conductivity meter, Soil Sci. Soc. Amer. J. 45, pp. 34-38 Tindl N. D., 983, egetble in the Tropics, Mcmilln Press, London, pp. 39 40, pp. 363 364 Wtkins J. S., 98, Electricl Properties of Soils nd Minerls, Mcmilln Press, London, pp. 83 89 Willims, B. G., nd Hoey, D., 987 The use of electromgnetic induction to detect the sptil vribility of the slt nd cly content of soils, Austl. J. Soil Res., 5, pp. 7 -
Ground electricl conductivity (mho-meter) 8.0E-03 7.0E-03 Lomy 6.0E-03 5.0E-03 Clyey 4.0E-03 3.0E-03.0E-03.0E-03 Sndy 0.0E+00 0.05 0. 0.5 0. 0.5 0.3 0.35 0.4 0.45 0.5 Probe seprtion, (m) Fig : rition of ground electricl conductivity with probe seprtion '' for for the three selected soil types
Height (cm) 35 30 Lomy 5 Clyey 0 Sndy 5 0 5 0 0 0 0 30 40 50 60 70 Time (dys) Fig 3: Growth pttern of Spinch green (Amrnthus Cruenthus)
Height (cm) 3 30 5 Lomy 0 Clyey 5 Sndy 0 5 0 0 0 0 30 40 50 60 70 Time (dys) Fig 4: Growth pttern of Spinch white (Amrnthus Tricolr)
Height (cm) 4 30 5 Lomy 0 Clyey 5 0 Sndy 5 0 0 0 0 30 40 50 60 70 Time (dys) Fig 5: Growth pttern of Egg plnt (Solnum Mcrocrpon)