POTENTIAL SUBSOIL UTILIZATION BY ROOTS by L. K. WIERSUM. Institute for Soil Fertility, Groningen, Netherlands INTRODUCTION

Transcription

1 Plnt nd Soil XXVII, no. 3 December 1967 PTENTIAL SUBSIL UTILIZATIN BY RTS by L. K. WIERSUM Institute for Soil Fertility, Groningen, Netherlnds INTRDUCTIN In griculture it is usully considered n dvntge for crop plnt to hve deep roots. Indeed, in mny cses the supply of wter under drought conditions my be sfegurded by the uptke of moisture from the subsoil. But the question lso rises whether the roots in the deeper lyers contribute to the minerl nutrition of the crop to ny worthwhile extent. In most soil types rooting in the deeper lyers is rther sprse nd the fertility in these regions is usully less thn tht of the topsoil. The uptke of nutrients from subsoil hs been demonstrted by number of experiments including those in which trcers hve been employed. The present experiments investigted the potentil feeder cpcities of roots in the deeper soil lyers. This problem becomes importnt in dry periods when prts of the root system locted in the upper soil lyers my become inctive. The possibility of ny interctions on the performnce of deep roots by roots feeding in the' topsoil ws not tken into ccount. For this purpose specil technique, relted to tht used by Millr 11, hd been devised (De Roo nd Wiersum 15 ), so tht the prt of the root system concerned with the uptke of nutrients ws mde to develop below predetermined levels. This technique lso mde it possible to obtin some informtion on the problem whether the ertion necessry for root ctivity in nutrient uptke ws higher or t the sme level s tht llowing growth. It is possible tht roots could still grow under conditionsof very poor ertion, which would prevent or restrict the uptkeof ions. 383 ^M i^d iv

2 384 L. K. WIERSUM EXPERIMENTAL PRCEDURES It hs been shown tht roots cn be trined down to predetermined depths in plstic tubes to llow soil contct of the emerging root system t vrible depths (De Roo nd Wiersum 15 ). For the mjor prt of the experiment the soil moisture conditions were comprble to those occurring in dry periods. There ws generlly n increse in moisture content towrds the bottom of the vessels, where stgnnt wter tble occurred. The ertion grdient ws inverse to the wter content nd reched so low level tht growth of roots into the wter tble ws impossible (Figs. 2, 3, nd 7). The oxygen diffusion rte dt given in Tble 3 nd Fig. 5 show tht this effect ws chieved. The clculted root performnce vlues for whet, brod ben nd Brssic re influenced by the fct tht, s the roots re situted t lower level, there is reltive increse in the mss of young bsorbing roots. Ech increse in depth implies more older roots being enclosed in the plstic tubes. For this reson the 1965 experiment with pottoes ws designed to eliminte this effect This ws chieved by plnting long-stemmed plnts, enclosed in the plstic tubes, t the desired levels. In ll cses complete root system ws in contct with the soil nd more frequent ertion mesurements were mde. The soil profiles in the vessels were homogeneous from top to bottom nd fertilizer ws pplied where necessry in 2-cm lyer t vrying depths, corresponding with tht of root entrnce into the soil. A schemticl drwing of the first experiment is depicted in Fig. 1. The min vessels consisted of either sbestos pipes, 1 cm high nd of 12 cm internl dimeter or of plstic pipes of the sme height nd bout 14 cm internl dimeter. The plstic pipes hd been cut lengthwise in hlves beforehnd nd could be lid open t the end of the experiment, fcilitting observtion of the root system. Twenty-five pipes were filled with soil t the desired density. Twenty of these vessels contined plnts grown in plstic tubes, 2\ cm dimeter, of 2, 4, 6, nd 8 cm length. In five of the vessels plnts were plnted out without plstic tubes nd with the full length of soil t their disposl. The vessels were plced in shllow bsin nd rndomized in ltin squre. The bsin ws filled with mud to which fresh orgnic mtter ws dded. Above the mud ws lyer of bout 2 cm wter which ensured tht the wter rising into the vessels ws nerobic. The stgnnt wter tble t bout 5 cm from the bottom of the vessels provided brrier to root growth nd no roots could escpe into the bsin. The different crops used in the experiment were germinted on moist snd nd the young seedlings were trnsplnted into the plstic tubes or 5-cm flowerpot. These were filled with loose substrte consisting of 1 prt grden soil, 1 prt vermiculite nd 1 prt pet mull. This substrte only contributed slightly to the nutrition of the plnts nd llowed very fst root growth downwrd. Wtering ws done by immersing the lower ends of the tubes in wter. Preliminry growth ws continued until roots were visible t the bottom end of ll 4-cm plstic tubes. By this time some of the 6-cm tubes were

3 PTENTIAL SUBSIL UTILIZATIN BY RTS 385 fertilized "ït wter lyer >Qy mud Fig. 1. Schemticl drwing of the experimentl setup. trversed by few roots. At this stge the plnts were trnsferred to their vessels. The plstic tubes with plnts were let down to their full length nd the surrounding soil filled in. The plnts t the moment of strting the experi- ' ment were of resonble uniformity nd, exept for those in the 8-cm tubes, were immeditely ble to enter the soil with new root growth. After plcement of the plnts in the vessels wter ws dded where necessry for the first few dys to ssist estblishment. Therefter no more wter ws given. The plnts hd to rely minly on wter rising by cpillrity from the bottom wter tble. As the experiment ws plced in the open ir under wire netting the plnts lso received some rinfll. None of the crops ws grown to ripeness, but since they were usully hrvested t stge where fruits were lredy present, they hd mostly ttined optimum size. During the experiment growth mesurements were regulrly mde. At the end of the experiment the bove-ground prts were cut off nd fresh nd dry weight ws determined. The root mss ws extrcted from the sbestos pipes

4 ^ L. K. WIERSUM by wshing, nd lso the roots in the plstic tubes were extrcted. Fresh nd dry weight determintions were mde. The plstic pipes were lid open to study root distribution. At hrvest the ertion conditions in set of vessels ws mesured electriclly with micro-electrode (Lemon nd Erickson 9 ) where the pltinum tip of the probe (Wiersum 19 ) ws replced by solid golden conus of bout 6 mm length. Uptke of N, P, nd K ws determined by chemicl nlysis. RESULTS Experiment with spring whet (Peko) The pipes were filled with lom soil of density of bout 1.29 (Tble 1). Fertiliztion of 2-cm lyer ws chieved by ddition of mixture of ion-exchnge resins sturted with N3, H2P4, K, Mg, nd NH4 (i 6 ml on 2 1 of soil). This ws done to prevent displcement of the dded nutrients, either by leching or cpillry rise. TABLE 1 Smple Lom soil Sndy pet soil Sndy soil PH (KCl) Humus % CC /o Prticles < 16 ix % Soil mixture in plstic tubes It took the seedlings 22 dys to let their roots grow down to the bottom of the 6-cm plstic tubes. After this period of initil growth the experiment ws plnted on My 3 nd lsted until August 2. The plnts in the 6-cm long tubes showed the best development, with the plnts in the 4-cm tubes second best (Tble 2). The lowest yield occurred in the 8-cm tubes. The fvourble growth in the 6-cm tubes is most probbly due to the fct tht fter plnting the new roots protruding from the tube immeditely enter soil with very fvourble moisture content. The plnts without tubes hve to strt in much drier soil nd lso to use more dry mtter in the formtion of n extensive root system.

5 PTENTIAL SUBSIL UTILIZATIN BY RTS 387 Net production ws clculted by subtrcting the plnt weight obtined in the control plstic tubes of equivlent lenght from tht of the yield obtined in the tubes in the soil-filled pipes. Free roots re those tht hve entered the soil in the min vessel. It is clerly evident tht the net sprout/free root rtio progressively increses s the roots hve been forced down to deeper level. We must conclude tht t these lower levels reltively smll mount of roots cri still feed plnt. Tht smll mss of minly young roots t depth below 8 cm still functions well is clerly demonstrted by the mount of nutrient bsorbed per unit root weight. It is bout ten times s high s tht clculted for root system occupying the whole soil column. This very high figure my lso be influenced by the reltive uncertinty of the net mount of N-, P- nd K-uptke, which hs been clculted by subtrction, nd the vlues obtined will be enlrged since complete root system will include inctive nd suberized roots. The observed fcts, however, clerly demonstrte tht these roots, which re situted in the deeper soil levels nd prtly ner the stgnnt wter tble re ble to bsorb nutrients s vigorously s roots in the other lyers of soil. Experiment with brod ben ('duiveboon') Here the pipes were filled with sndy pet soil ('veenkolonile grond') (Tble 1). The soil ws filled in with density of The experiment ws plnted on My 29, fter period of four weeks, during which time the seedlings were ble to get estblished in the plstic tubes nd in which time roots grew down to t lest / 4 cm. Six weeks lter, during flowering, the plnts were topped to obtin more even ripening. The experiment lsted until August 12. The best strt ws gin obtined with the 4- nd 6-cm plstic tubes, lthough ultimtely the plnts in full soil nd in the 2-cm tubes gve the highest yields. In Fig. 2 both the growth of the tops nd the development of the root system is depicted. It cn be seen tht the roots suddenly stop in their downwrd growth, especilly in the 6- nd 8-cm tubes, nd re close to the stgnnt wter tble. It is evident tht the roots leving the plstig tubes sty below the bottom end, nd tht only n occsionl root grows slightly upwrds. The dt obtined re compiled in Tble 2.

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7 PTENTIAL SUBSIL UTILIZATIN BY RTS 389 Fig. 2. A series of five plnts of the experiment with brod ben. All roots stop downwrd growth bove the stgnnt wtertble ( ). No. upwrd growth of roots. TABLE 3 Averge vlues of reltive oxygen diffusion rtes t different depths Depth in cm Whet... Brod ben. Brssic... Averge Ï8 The results confirm those obtined in the experiment with whet. The lowest net sprout/free root rtio is found with plnts with the norml root system. The deeper the mss of feeder roots is situted in the soil the lrger this rtio becomes, with declinefor the plnts with their free roots below 8 cm depth. Even t this depth nd under ertion conditions ner to the criticl point (Tble 3) the roots still behve well.

8 39 L. K. WIERSUM Experiment with Brssic npus vr. tbulri ('Snijmoes') For this experiment sndy soil ws used to fill the pipes (see Tble 1). The soil ws filled in with density of 1.35 nd no extr fertilizer ws pplied. The seedlings were grown for bout four weeks nd then plced in the pipes long with their tubes on My 24. The experiment lsted till July 8, when the plnts were hrvested. Plnts in the 2-cm tubes were the best in this cse, followed by those on the 4-cm tubes. The yields (Tble2) gin suggest tht the topsoil lyers my hve been dry for prt of the time. The behviour of the roots of these plnts, however, ws quite different from tht of the two other crops. Mny roots grew upwrds, towrds the surfce, fter leving the plstic tubes. The rooting ws not restricted to below the preset level. Also the roots of this species did not penetrte s ner to the groundwter level s did the other plnts, lthough the ertion ws better thnfor the bens. This fct long with the upwrd growth suggests strong preference for well erted lyers. It cn lso be seen (Fig. 3) tht the roots leving the 8-cm tube grew in zone djcent to tht where ertion prohibited further root growth. xygen supply must certinly hve been rther restricted t this depth for this species, which seems to hve high requirement. As result of this berrnt root behviour the results obtined in this experiment show slightly different trend. Deeper rooting enhnces the performnce of the roots, but the differences hve been levelled off. There is no cler cut optimum s the highest sprout/root rtio ws observed t 2 cm depth nd optiml N- nd P-uptke ws t 8 cm depth. Therefore with Brssic lso it ppers tht potentil root performnce insoil lyers t greter depths ws not mrkedly impired. As long s the roots were ble to grow they were lso bsorbing ions. The mount of nutrient uptke per unit root weight ws vrible, but t lest the deeper situted roots performed well. Experiment with pottoes Although the evidence obtined in the previously described experiments clerly points to the fct tht the deep situted root systems re ble to perform norml uptkeof nutrients, some nomlies

9 PTENTIAL SUBSIL UTILIZATIN BY RTS 391 Fig. 3. Prt of the Brssic plnts t the endof the experiment. Upwrd growth of the roots is clerly evident nd lso the greter distnce to the stgnnt wtertble ( ). still remined. This led us to devise new experiment in which the totl root system ws llowed to develop t different depths with long stemmed plnts, enclosed in plstic tubes, entering in the soil t predetermined depths. The root systems lwys consisted of old suberized roots nd of young ctive rootlets. Also more numerous ertion mesurements were performed. Experimentl procedure. The experiment ws performed with the potto cultivr Prefekt. Using cork borer, single eyes were removed from pregerminted seed pottoes with bout 1 ml flesh ttched. These smll pieces were given pretretment with gibberellic cid to force them to grow long, slender nd un~ brnched stems. They were then plnted in moist pet mull. After the stems hd ttined bout 3 cm length the young plnts were plced in smll flowerpots (dimeter 6 cm) filled with the sme soil s ws used in the experiments. Smll leves long the stem were cutoff nd plstic tube (dimeter 2.5 cm) of 6 cm length ws put round the stem. Under these conditions the plnts were further cultivted until the tips of the stems of most plnts hd reched the top of the tubes. At this stge the plnts with their enclosed stems nd rooted clod of soil were plnted in the vessels. These consisted of plstic pipes, dimeter 12.5 cm, filled with sndy mixture nd either 8, 6, or 4 cm long. After plnting

10 HJTTT 392 L. K. WIERSUM Fig. 4. Schemticl drwing of the tretments in the potto experiment. level of nerobic wter in the closed trough. some silicone rubber ws poured into the nrrow plstic tubes round the stem to form n ir sel t the bottom of the tube. The whole btch of 32 pipes ws plced in lrge trough filled with bout 5 cm of wter. The trough ws covered with lid, contining holes for the pipes. By letting strem of nitrogen gs flow underneth the lid the 'ground wter' ws kept s nerobic s possible. A digrm of the experiment is given in Fig. 4. The plnting depth ws, 2, 4, nd 6 cm in the 8 cm vessels, 2 nd 4 cm in the 6 cm vessels nd nd 2 cm in the 4 cm pipes. Every tretment ws replicted four times. The experiment ws in the open ir. As the plnts grew out of their tubes nd took on norml helthy growth the plnts were fixed to wire netting to prevent dmge by wind. After growth period of bout 1 weeks the plnts were hrvested, the pipes were lid open to study the root system nd ultimtely the tubers nd roots were wshed free of soil.

11 PTENTIAL SUBSIL UTILIZATIN BY RTS 393 A few young plnts were not plnted in the experiment but used s control for nlysis of the nutrient content t the strt of the experiment. After nlysis of sprout + tubers the net uptke during the experiment ws clculted by subtrcting the control vlue from the finl quntities. After hrvest the plnts were split into roots nd sprout + tubers. Fresh nd dry weight ws determined nd the N,- P-, K- nd C-content of sprout nd tubers ws nlysed. TABLE 4 ' Tretment A B C ': D E F G ; H Rooting ƒ Height depth ƒ of vessel /8 2/8 4/8 6/8 2/6 4/6 /4 2/4 Dt on yield, nutrient uptke nd root performnce of potto Dry weight g 29.6 ± ± ± ± ± ± ± ± 1.2 N- uptke mg P- uptke mg K- uptke mg Cuptke mg Averge mount of roots dry weight g Rtio sprout/ roots Results. The min dt re given in Tble 4. It is evident tht in nerly ll cses resonble growth hs occurred. nly tretments F nd H hve resulted in poor growth of the plnts, lso ccompnied by smller top/root rtio nd root mss. The top/root rtio vlues suggest tht the plnts with their totl root mss in the soil lyers with n intermedite to high moisture content hve the best root performnce. This confirms the previous results, lthough the generl rise in ctivity s the roots re situted t deeper levels is bsent here. This is prtly the result of the fct tht lwys the weight of the totl root system could be tken into ccount nd my prtly be influenced by the upwrd growth of someof the root systems. Tretment A (Tble 5) gin demonstrtes tht the roots in the top soil lyers my prtly hve been less ctive on ccount of low moisture content. Root performnce (net uptke per mg of dry roots) nd oxygen diffusion rte vlues t the depth of root entrnce into the soil re given in Tble 5. Here it cn beseen tht root performnce stys t high level even if the reltive.d.r.-vlues decrese to 3. Thus

12 394 L. K. WIERSUM TABLE 5 Tretment A /8 B 2/8 G /4 C 4/8 E 2/6 D 6/8 H 2/4 F 4/6 Rte of uptke in reltion to reltive.d.r.-redings.d.r. 6* 57 ± ± ± ± ± ± ± 1. Root performnce, mg/g roots N P K C Upwrd growth * Extrpolted vlue. 2 rem depth ' verge root depth 7 j 8. D.R.-vlues \ \ I Fig* 5. Grph of the oxygen diffusion rte mesurements t different depths nd. rooting depth ttined in the 8 cm vessels.

13 T~ PTENTIAL SUBSIL UTILIZATIN BY RTS 395 even roots below 6cm depth in the 8-cm vessels perform well. nly if the oxygen supply is still lower sudden decrese in root growth nd ctivity is clerly evident. In these tretments (F nd H) the oxygen supply of the root mss ws solow, tht if the roots hd not been plnted t tht level they would not hve penetrted into these lyers. Tht this is the cse cn be derived from the grph in Fig. 5, in which the.d.r. redings t different depths re plotted nd in which the ultimte root depth ttined is shown. This shows us tht roots re inhibited from further growth downwrd t vlue of 26. In the Tretments F nd H the roots lredy developed during the period of erly growth hve mnged to keep live nd hve hrdly mde new growth. It is no wonder tht under such conditions below the norml limiting vlue root performnce is impired, s is lso shown by the fct tht the roots hve not mnged to grow upwrds. This upwrd growth of prt of the root system, when relesed t greter depth, is indictive of serch for better erted lyers (Wiersum 2 ). This rther uncommon behviour underlines the fct tht ertion vlues between 3 nd 45 re reltively low for pottoes. DISCUSSIN In considering the quntity of nutrients the subsoil cn contribute to the crop number of fctors hve to be tken into ccount. The most importnt re root density, the nutrient sttus of the subsoil, nd its moisture content. The differences in these vribles my ccount for the lrge vrition in the published dt. With only very few exceptions ll informtion in the literture hs been obtined on undisturbed complete root systems. The extent to which the subsoil roots hve contributed to the nutrition of the plnts my hve been influenced by the bility of superficil roots to bsorb nutrients. If only prt of root system hs nutrients vilble certin compenstion in the form of incresed uptke my occur. But this phenomenon enhnces the importnceof subsoil roots if stress conditions occur in the superficil lyers. In cse fvourble conditions long with high chemicl fertility exist in the topsoil the contribution of the subsoil roots might be reltively reduced.

14 396 L. K. WIERSUM Fig. 6. Prt of the experiment with pottoes. The rubber stops re removed when tking.d.r.-redings. Fig. 7. Results obtined with pottoes. In some cses upwrd growth of roots nd stolons. S = depth of soil columns.

15 PTENTIAL SUBSIL UTILIZATIN BY RTS 397 Millr u showed tht the development of roots in the topsoil could be prevented by the use of glss cylinders of 2 cm length nd tht resonble uptke of nutrients could be obtined from zone below 4 cm depth. A review of the reserch on the uptke of nutrients from the subsoil including evidence tht phosphte could be bsorbed from depths to below 75 cm is given by Schütze 16. Goedewgen 7 lso gives some exmples. In more recent work the vilbility of rdioctive trcers hs mde it possible to clculte the contribution of seprte soil lyers. Both Murdock nd Engelbert 12 nd Shin nd Kshmnov 1 ' conclude tht P cn be bsorbed in substntil mounts from the subsoil, even from depths of bout 1 cm. Informtion on nutrient bsorption from the subsoil my lso be found in ppers by Clvert 2, Ellis et lß nd Beuchmp 1. The results obtined in experiments on subsoil tillge (DeRoo 14 ), liming of subsoil (Sluysmns et l. 18 ) nd informtion correlting growth nd depth of rooting (Henin nd Grs 8 ) or thickness of fvourble subsoil lyers (Ford 6 ) ll suggest extensive ctivity of roots in deeper lyers of the soil. Fr more interesting re the results of experiments where both uptke from nd the mount of roots in the deeper lyers of the soil hve been estblished. Fox nd Lipps 4 (196) cme to the conclusion tht where the topsoil becme dry 3 per cent of the roots of lflf t 2 to 4 cm depth bsorbed 62 per cent of the minerl uptke. Also Vn Lieshout 1 showed tht for corn nd ots the contribution in nutrition from thesoil t 6 to 8cm depth exceeded tht of the percentge of roots, viz 1.7 per cent roots contributed 9.2 per centof the uptkeof phosphte. Nkym nd Vn Bvel 13 lso noted tht P bsorption of 7 per cent of the totl could be ccomplished by 2 per cent root-mss t 6 cm depth. Fox nd Lipps 8 (1964) demonstrted tht 6 per cent of the totl root ctivity oflflf took plce t depths below 7feet s result of the more fvourble moisture conditions. The results obtined in the present experiments confirm the observtions mentioned bove. The results, especilly those obtined with brod ben, Brssic nd potto demonstrte tht roots growing on the fringe of lyers where lck of oxygen prevents further growth downwrd, still retin their cpcity for uptke of nutrients.

16 398 L. K. WIERSUM Thus the conclusion cn be mde tht the limiting oxygen requirement for growth nd bsorption of minerls re the sme. This mens tht ny live root in the subsoil hs the potentil bility to bsorb nutrients. The fct tht uptke per unit weight of roots usully rises long with the depth t which they occur cn be considered s the resultof two fctors. First, s the bsorbing roots re forced downwrds by mens of the plstic tubes n incresing prt of the min, thicker roots is not tken into ccount. The mount of free roots shows rising proportion of feeder roots s they re t deeper level. The root systems hving the whole depth of soil t their disposl lso tend to grow downwrds to the more moist lyers nd so prtly consist of older, thicker nd more suberised roots. Secondly, the moisture grdient in the soil column must be considered. Except for rinfll, the min supply of wter must hve occurred by cpillry rise. The upper lyers were rther dry during prt of the experiment. This must hve reduced the ctivity of the roots in these lyers. In the experiment with Brssic nd potto where the feeder roots grew upwrds gin, this effect cn be seen. Here the efficiency of thefree rootsshowsless vrition nd not such cler trend. The generl conclusion is tht the roots in the subsoil certinly re of potentil vlue in feeding the plnt provided tht plnt nutrients re vilble. Their reltive performnce my even rise to high vlues depending on the circumstnces in the topsoil. Their contribution to plnt nutrition will further depend on the fertility of the subsoil lyers, the moisture content of these lyers nd the mount of roots tht hve been ble to develop in these regions. SUMMARY By trining root systems through nrrow plstic tubes down to below predetermined levels the potentil cpbilities for uptke from deeper soil lyers ws investigted. In the experiment, crried out in sbestos pipes of 1 meter height, soil column with n increse in moisture content downwrds coupled with n inverse ertion grdient ws estblished. Yjptke per unit of roots in contct with soil ws higher in the deeper lyers. The increse in ctivity must prtly be ttributed to the method of clcul-

17 vu m PTENTIAL SUBSIL UTILIZATIN BY RTS 399 tion, but is in line with other observtions in the literture. Roots growing in soil lyers, where ertion is limited, still perform well, demonstrting tht growth nd uptke hve bout the sme oxygen requirements. Any root in the subsoil cn thus be considered s potentil bsorbing unit for nutrient uptke. ACKNWLEDGEMENTS The uthor wishes to express his indebtness to miss G. A. M. Svenije nd mr. G. Mesker for their contribution in crop nlysis nd mngement of the experiments. Mny thnks re due to Dr. D. Snow, The University, Reding, for the mny improvements in the English text. Received September 19, 1966 LITERATURE Beuchmp, E., Note on method of studying subsoils in the greenhouse by mens of lysimeter. Cn. J. Soil Sei. 43, (1963). Clvert, D. V., Absorption of nitrte by corn s relted to movement of nitrte nd wter in soil. Diss. Abstr. 23, (1962). Ellis, F. B., Newbould, P., nd Tylor, R., Uptke of nutrients from different depths in soil by plnts. Agr. Reserch Council Rdiobiol. Lb. Ann. Rept ,92-97(1963). Fox, R. L. nd Lipps, R. C, Distribution nd ctivity of roots in reltion to soil properties. Trns. 7th Int. Congr. Soil Sei., Mdison 196 III, (196). Fox, R. L. nd Lipps, R. C, A comprison of stble Strontium nd P 32 s trcers for estimting lflf root ctivity. Plnt nd Soil 2, (1964). Ford, H. W., Thickness of subsoil orgnic lyer in reltion to tree size nd root distribution of citrus. Proc. Am. Soc. Hort. Sei. 82, (1963). Goedewgen, M. A. J., Het Wortelstelsel der Lndbouwgewssen. Algemene Lndsdrukkerij, 's-grvenhge (1942). Henin, S. nd Grs, R., Vigueur des rbres fruitiers et épisseur de sol exploitée pr les rcines. Compt Rend. Acd. Sei. Pris 255, (1962). Lemon, E. R. nd Erickson, A. E., The mesurement of oxygen diffusion in the soil with pltinum micro-electrode. Soil Sei. Soc. Am. Proc. 16, (1952). Lieshout, J. W. vn, Invloed vn het bodemmilieu op ontwikkeling en ctiviteit vn het wortelstelsel. Diss. Wgeningen (Pudoc), 91 pp. (196). Millr, C. E. Studies of the removl of nutrients from subsoil by lflf. Soil Sei. 23, (1927). Murdock, J. T. nd Engelbert, L. E., The importnce of subsoil phosphorus to corn. Agron. Abstr. Nov. 1956, 34 (1956). Nkym F. S. nd Bvel, C. H. M. vn, Root ctivity distribution ptterns of sorghum nd soil moisture conditions. Agron. J. 55, (1963). Roo, H. C. de, Deep tillge nd root growth. Connecticut Agr. Expt. St. Bull. 644 (1961). Roo, H. C. de nd Wiersum, L. K., Root trining by plstic tubes. Agron. J. 55, (1963).

18 4 PTENTIAL SUBSIL UTILIZATIN BY RTS 16 Schütze, K. H., Ein Beitrg zur Frge der Ausnutzung der Untergrundnährstoffe. Diss. Fr. Wilh. Univ. Berlin (1939). 17 Shin, S. S. nd Kshmnov, V. M., The role plyed by perennil plnt roots which project into the subsoil. Biol. Abstr. (Plnt Sei.) 36, (1961). 18 Sluysmns, C.M. J., Wind, G. P., en Struys, L. C, Beklking vn de ondergrond. Lndbouwvoorl. 18, (1961). 19 Wiersum, L. K., Some experiences in soil ertion mesurements nd reltionships to depth of rooting. Neth. J. Agr. Sei. 8, (196). 2 Wiersum, L. K., Presumed erotropic growth of roots of certin species. Die Nturwissenschften 54, (1967).