Soil Moisture Conservation in Dryland Viticulture as affected by Con ventional and Minimum Tillage Practices *

Transcription

1 Sol Mosture Conservaton n Dryland Vtculture as affected by Con ventonal and Mnmum Tllage Practces * L. VAN UYSSTEEN and. W. WEBER Re peclvely from the OenologcaJ and Vtcultural Research nsttute, Prvate Bag X526, Stellenbosch 76 and the Department of Sol and Agrcultural Water Scence, Faculty of Agrculture, Unversty of SteJlenbosch, Stellenbosch 76 *Extract from a M.Sc. (Agrc.)-thes by the frst author, Stellenbosch, The effect of conventonal clean cultvaton, a straw mulch, chemcal weed control and a grass sward on the sol mosture regme n a dryland vneyard was nvestgated. The results demonstrate that both a straw mulch and chemcal weed control were very effectve n conservng wnter-stored sol mosture untl md- <).r even late summer to support the vnes durng the almost ranless growng season. Further, t appeared that a mere loosenng of' the sol surface perodcally by way of clean cultvaton cannot conserve sol mosture effectvely; t may, on the contrary, promote evaporaton n the top-sol layers, especally when done after ran. On the other hand, an undsturbed sol surface, as n the case of chemcal weed control, acts as a mulch n tself after the surface layer has dred out, thus reducng evaporaton. Even under condtons of frequent ranfall, as durng the frst part of 1976/77 season, the straw mulch cover treatment was stll superor to au other treatments n con ervng sol mosture. n many cases tllage operatons are performed manly the growng season, vz. September to March nclusve. out of mere tradton (Greenland, 1978). Unfortunately, t Compared to condtons n wne producng regons n the s not generally known that frequent tllage may cause Northern emsphere, e.g. France (growth season March damage to the sol rather than mprovng t as a medum to September), the Western Cape area thus receve at most for root growth (Barnes, Taylor, Throckmorton, Van den 18 mm as aganst the 375 mm of Montpeller and 439 Bergh & Carleton, 1971; Greenl.and, 1977). Even f done mm of Bordeaux (Saayman & Van Zyl, 1976). nfrequently wth the sole ntenton of keepng the sol By usng long term statstcs* a mean seasonal crop surface free from mosture consumng weeds, a slow but factor of,25 for vnes was determned (Van ZyJ 1975), ncessant breakdown of sol structure may result. These so that the mean seasonal (September to March) Classpractces may, n the long run, lead to the formaton of a A-pan evaporaton (Eo) of mm represents a consubsol tllage pan and the accompanyng pulverzaton of sumptve water use (Et) of 388 mm. Ths mples a the surface layer (Van uyssteen & Weber, 198a) wth an theoretcal sol mosture defct of 28 mm n the vneyards undesrable effect on water storage n the subsol layers. of ths area durng ths perod. Moreover, lght summer Many researchers concluded that clean cultvaton can- showers of less than 2 mm, contrbute very lttle towards not conserve sol mosture (Mohr, 197; Baeumer & replenshng the sol mosture store because of the preval- Bakermans, 1973; Unger & Phllps, 1973; Greenland, ng hgh temperatures, contnuous south-easterly wnds and 1977). On the contrary, plant resdues left as a mulch hgh atmospherc water vapour saturaton defcts (Van cover on the untlled sol surface, wll conserve sol most- Eedell, 197). A consderable amount of the 18 mm ure much more effectvely. For the Great Plans of the seasonal ranfall usually conssts of such lght showers. USA, Greb, Smka & Black (l97) reported that precp- Under dryjand condtons, all avalable wnter ranfall taton storage n the sol ncreased from 16%, wthout any should be stored n the sol for as long as possble to mulch, to 37% wth 6,72 ton/ha harvest resdues left as a brdge the dry seasons. Ths task s a major concern of sol mulch. At Bushland, Texas, USA, precptaton storage management n vneyards. Tllage practces hould be durng eleven months of fajlow after ltgated wheat, was appled whch would mnmze unproductve evaporaton 52% wth chemcal fallow treatment, and 26% wth a drectly from the sol surface, and ncrease productve mechancal (dsc tllage) treatment (Unger & Phllps, transpraton through the plant. 1973). n another experment under rrgaton condtons, Seen aganst the background of dwndlng water and precptaton storage durng fallow was 39% wth a her- land resources n the ard and sem-ard regons of the bcde treatment, as compared to an average of 2% for world, much more skll than was necessary prevously three tllage treatments, vz. tandem dsc, tandem dsc plus would be needed n future to ntegrate sol, clmatc and sweep, and sweep (Unger, Allen & Wese, 1971). Accord- other factors nto a management system whch would ng to McCalla & Army (1961), harvest resdues left on prove to be practcal for a gven stuaton (anks, 1978). the sol surface lowered sol temperatures because of ther owever, ths author ponted out that measures whch nsulatng effect, ncreased reflecton of radant energy effectvely decrease sol evaporaton have unfortunately and, consequently, reduced surface evaporaton. proved to be uneconomcal. Further research s needed to The ranfall dstrbuton pattern n the wne growng fnd management methods, such as " mnmum tjlage" or areas of the Western Cape s unfavourable for summer even "no-tlage" practces, that would modfy the sol crops, and summer droughts often occur. Ths regon has a clmate n the way desred (Allmaras, allauer, Nelson & long-term mean annual ranfall of about 6 mm, rangng Evans, 1977). from 465 mrn/a n the drer areas to 693 mm/a n the *Agrorneteorology Seclon, Sols and rrgaton Research nstrute, Prewetter areas. * Only 3% of the mean ranfall occurs durng tora. S. Afr. J. Enol. Vtc., Vol. 1. No

2 68 Sol Mosture Conservaton n Dryland Vtculture The effect of several tllage practces on the physcal propertes and on the water conservaton capabltes of a vneyard sol under dryland condtons was, therefore, studed. Van uyssteen & Weber (198a) have already reported on the changes n the physcal condtons of the sol brought about by such tllage practces, and ths paper reports the effect on sol mosture. MATERALS AND METODS The characterstcs of the expermental ste, sol and desgn, as well as partculars of the treatments appled have been descrbed by Van uyssteen and Weber (198a). nfltraton rate measurements were carred out usng a Double Rng nfltrometer. The followng meteorologcal data were regstered snce the commencement of ths study. () Ar temperature and relatve humdty by means of a thermohygrograph n a Stevenson screen. () Evaporaton from an Amercan Weather Bureau Standard Class-A-pan. () Ranfall. Mosture retenton curves were constructed from mosture content/tenson data as obtaned on undsturbed sol samples. These were taken at dfferent depths of the profle usng a sol core sampler whch holds two brass cylnders of 66,6 cm a nternal volume each. A flter paper dsc to keep the sol core n place was glued to the bottom of each cylnder. After ntal water saturaton of these cores, stepwse ar pressures startng at 2,5 kpa and endng wth 1 5 kpa (whch was regarded as permanent wltng pont) were appled by means of pressure plate and pressure membrane extractors. After attanng equlbrum at each pressure stage the mass of each sol core wth ts enclosng cylnder was determned. Fnally, the sol cores were oven-dred, and the water contents at each pressure nterval calculated as mass and volume percentages. n wnter feld water capactes were determned repeatedly on the dfferent plots. A rectangular earth basn havng a surface area of 3 m x 3 m was constructed, kept flled wth suffcent water n order to wet the profle down to 9 mm, and subsequently covered wth layers of straw and plastc sheets to prevent evaporaton. A set of fve mercury tensometers of dfferent lengths was nserted n the mddle of each basn. Gravmetrc sol mosture determnatons on sol samples taken n trplcate from the top layer were carred out at sx-hourly ntervals untl a constant water content was reached. n the case of the deeper strata, sol samples were taken daly around the tensometers down to 9 mm depth, usng a Vehmeyer tube, untl a constant water content at the depth of the respectve tensometer cup was reached. Tensometer readngs were taken smultaneously. Sol mosture depleton curves aganst tme were constructed by measurng the mosture content gravmetrcally every three weeks on all replcatons of each treatment durng the 1974/75 season, coverng the depth zones -3, 3-6 and 6-9 mm. The same procedure was followed durng the 1975/76 season at fortnghtly ntervals and for the depth zones -1, 1-3, 3-5 and 5-7 mm. The 7-9 mm depth zone was not consdered because profle studes revealed only a few roots deeper than 7 mm. Durng the 1976/77 season Bourdon-gauge tensometers were employed wth ther 5 mm-ong cups placed at depths of 1-15, 25-3, 55-6 and 75-8 mm. The tensometers were placed at or near a pont on each treatment plot where all the mportant sol propertes have been determned prevously (Van uyssteen & Weber, 198a). RESULTS AND DSCUSSON Meteorologcal observatons: The data for four growng seasons are summarzed n Table 1. Generally, about two-thrds of the ranfall of the growng seasons occurred durng the months September to November, the balance beng spread unevenly over the remanng months. The hghest monthly ranfall of 16,1 mm was recorded n November 1976, compared to a range of 22,2 to 36,9 mm durng other years. n accordance wth the hgh ranfall n 1976/77, the average quarterly maxmum ar temperatures for November, December and January 1976/77 were consderably lower (ca. 3 c) than those of the other seasons. The mean monthly maxmum relatve humdty was always hgh durng the nghts, resultng n mean seasonal maxmum relatve humdtes of hgher than 9% n all four seasons. The mean seasonal mnmum relatve humdtes vared between 4,7 and 49,4% for the four growng seasons. n accordance wth the hgh relatve humdtes, the seasonal Class-A-pan evaporaton was rather low, varyng between 1385 and 167 mm. Convertng these Class-A-pan evaporaton fgures to theoretcal sol mosture defcts by usng a mean seasonal crop factor for vnes of,25 (Van Zyl, 1975), and assumng that 1% of the seasonal ranfall nfltrates nto the sol, the followng values were obtaned. Growng season 1973/ / / /77 S. Afr. J. Enol. Vtc., Vol. 1. No Sol mosture defct (mm) 246,9 149,5 231,8 4,8 n realty the sol mosture defcts were hgher, snce lght showers of summer ran defntely do not reach the sol surface but are ntercepted by the vegetaton (Van Eeden, 197). t s evdent that 1976/77 was a favourable season as far as sol mosture storage from precptaton s concerned. Water nfltraton: The nfltraton rate may be consdered to be a parameter ntegratng varous sol physcal propertes of the same solum, provded that () t s determned durng the growng season, () suffcent water and tme are allowed to wet the sol unformly and completely down to requred depths, and () a large surface area s used n order to mnmze border effects. Snce ths experment was desgned to be a dryland feld tral, these condtons could not be met because of the rsk of water supply to the adjacent vnes. The determnatons had, therefore, to be carred out n early sprng, wth small volumes of water, usng a double rng nfltrometer and for no longer than 3 mnutes. Ths could, of course, only wet a very lmted surface area and a lmted depth but had the advantage that the measurements could be replcated several tmes per treatment. Fg. 1 demonstrates that the nfltraton rate was rapd on all treatment plots but that there were marked dfferences between treatments. nfltraton rate was the slowest on the herbcde (B4) plots on account of the crusts on the sol

3 Sol Mosture Conservaton n Dryland Vtculture 69 TABLE 1 Clmatc data of four vneyard growng seasons-netvoorbj, Stellenbosch Mean artemp. Mean Re. um. A-pan evaporaton Total Season Month Ranfall Max. Mn. Max. Mn. (mm) ("C) (C) (%) (%) mm/month mm/day 1973/74... Sept , 18,2 Oct ,1 23,8 Nov ,2 26, Dec ,6 26,7 Jan... 5,3 27,6 Feb.... 1,9 3,3 March... 1,4 27,9 Total/Mean... Sept.-March... 17,5 25,8 1974/75... Sept ,5 17,9 Oct ,8 2,1 Nov '.. 36,9 24,5 Dec ,5 26,7 Jan... 24,3 27, Feb ,5 3, March... 1,8 27,7 Total/Mean... Sept.-March ,3 24,8 1975/76... Sept , 21,2 Oct ,2 2,3 Nov ,3 23,4 Dec.... 5,8 26,7 Jan..., 3,3 Feb.... 8,2 27,8 March... 5,4 24,9 Total/Mean... Sept.-March ,9 24,9 1976/77... Sept ,4 18,6 Oct.... 2,3 21,9 Nov.,... 16,1 2, Dec ,4 24,9 Jan... 1,4 26,2 Feb.... 3,5 28,2 March... 17,2 26,8 Total/Mean.,... Sept.-March ,3 23,8 7, , 4,1 11, ,2 6,2 13, , 9,8 14, ,3 8,3 15, , 3,1 15, ,6 9,2 13, ,6 7,6 13,2 92,6 49, 1669,7 7,9 7, , 3,7 9, ,7 4,7, , 7,8 14, ,4 8,4 15, ,8 8,8 16, ,2 9,9 14, ,2 8,2,8 91,7 49, ,3 7,4 9, , 5,3 1, ,3 5,3, , 7,2 14, ,4 1,4 16, ,2 11,2 14, ,6 8,4 14, ,4 6,4 13,1 9,4 4,7 1 65,9 7,7 9, , 11, ,1 6,1 11, , 5,8 13, ,8 7,8 14, ,2 8,2 16, ,4 7,8 14, , 7, 13, 93,4 46,3 1384,5 6,5 surface but was stll adequate to prevent run-off of ran water under the prevalng precptaton rates, n spte of the % slope. Ths ndcates that the dry and dense sol crusts can easly be soaked and penetrated by ran fallng 3 mnutes or longer. The fastest nfltraton rate was measured on the permanent sward (B6) plots. Ths may be ascrbed to the occurrence of vertcal channels left by decayng roots of weeds, and the well-structured top sol layer consstng of water stable aggregates whch formed a large and mechancally hghly resstant macro pore volume (Van uyssteen & Weber, 198a). The nfltraton rate on the clean cultvaton (B5) plots was smlar to that on the sward plots. Because of the hghly compacted and nearly mpermeable tllage pan at 2-3 mm depth n the B5 plots (Van uyssteen & Weber, 198a), ths hgh nfltraton rate would not apply for deeper layers. Feld water capacty and sol mosture tenson: The feld water capactes (FWC) and the respectve sol mosture tenson (SMT) values of the top -1 and 1-2 mm layers, as well as ther means over the entre depth of all profles, are presented n Table 2. t appears from these results that dfferences n FWC between treatments were only small, vz. 3,4% between the hghest and the lowest value n the frst layer and 2,2% n the second. Although these dfferences are small, the FWC values show a clear trend n the -1 mm layers, n that the hgher values were obtaned on the mnmum tllage plots and lowest S. Afr. J. Enol. Vte., Vol. 1. No value on the conventonally tlled plot. Ths fndng may be ascrbed to the presence of water stable and mechancally resstant aggregates under mnmum tllage, compared to the vrtually total absence thereof under clean cultvaton. Therefore, the pore sze dstrbuton s stable n the former but appeared to be utterly unstable n the latter. TABLE 2 Feld water capactes (FWC) and ther sol mosture tensons (SMT) n the top sol layers of dfferently cultvated feld plots, as determned n stu Water contents at FWC Treatment Depth SMT (mm) Mass % Volume % (lcpa) Straw mulch -1 16,2 25,4 4, (B3) ,9 25,1 7,5 Mean: Entre profle 24,6 erbcde -1 16,2 24,8 4,3 (B4) ,9 22,9 4, Mean: Entre profle 23,5 Clean -1 17,1 22,7 5,7 cultvaton ,5 23,9 9, (B5) Mean: Entre profle 23,6 Sward -1 17,1 26,1 4, (B6) ,5 24,1 5,5 Mean: Entre profle 23,4

4 7 Sol Mosture Conservaton n Dryland Vtculture s / 1- / /!!! A A x x... ",--E) x- - --x!j.---ra z_ _ _ z B3 - Straw mulch B 4 - erbcdes B5 - Clean cultvaton B6 - Sward o TME (mn) FG. 1 Cumulatve water nfltraton wth tme on dfferently cultvated plots Furthermore, the sol mosture tenson at FWC n both layers of the clean cultvated plots was unproportonally hgher than that n the same layers, also at FWC, of the three mnmum treated plots. Ths observaton s n accordance wth the fndngs of Ehlers (1973) that, at the same mosture content, the sol mosture tenson of a noncultvated top sol layer s generally lower than that of a cultvated one. Therefore, the vnes n the straw mulch, herbcde and sward plots not only found more water absolutely but could also absorb the same amount of water wth less energy spent than those n the clean cultvated plots. Sol mosture fluctuaton and depleton observatons: Durng a wet season; Representatve graphs of sol mosture depleton and correspondng tenson fluctuatons wth tme n dfferent sol layers of the four treatments durng the wet 1976/77 season (341,3 mm ranfall) are presented n Fg. 2 and Fg. 3, whch are complementary. Fg. 2a and Fg. 2b show the gravmetrcally determned sol mosture depleton durng ths season for the 25-3 and 55-6 mm sol layers, respectvely, whereas Fg. 3b and Fg. 3c show the sol mosture tenson fluctuatons for the same layers. Addtonally, Fg. 3a and Fg. 3d show the tensometrcal values for the 1-15 mm and 75-8 mm sol layers, respectvely. Average values for FWC and PWP are represented by the horzontal lnes n Fg. 2, s. Afr. J. Enol. Vtc., Vol. 1. No whle values of both sol mosture "constants" are ndcated by means of arrows on the rght hand sde of the fgure. Apart from the sol mosture contents hgher than FWC at the onset of the observaton perod the curves for the 55-6 mm layer (Fg. 2b) show that the herbcde (B4) and straw mulch (B3) treatments had the hghest sol mosture content throughout the season. Perhaps on account of the exceedngly hgh transpraton coeffcents of the weeds (Van uyssteen, 1977), the sward (B6) treatment had the lowest sol mosture content. Although not so clear as at 55-6 mm depth, the same pattern seems to exst n the 25-3 mm sol layer (Fg. 2a). t can, therefore, be nferred that the mnmum tllage treatments B3 (straw mulch) and B4 (herbcde) conserved water from wnter and summer rans much longer than the B5 (clean cultvaton) and B6 (sward) treatments. Ths s affrmed by the correspondng sol mosture tenson graphs (Fg. 3b & Fg. 3c) whch show that the tensometrc curves of the straw mulch (B3) and herbcde (B4) treatments are at a consderably lower level than those of the clean cultvaton (B5) and the sward (B6) treatments for the greater part of the season. Ths effect was especally dstnct n the uppermost layer (1-15 mm), and was stll markedly apparent even n the deepest layer (75-8 mm), as s shown n Fg. 3a and Fg. 3d, respectvely. Moreover, the sol mosture content n the upper layers

5 Sol Mosture Conservaton n Dryland Vtculture a mm DEPT ACTUAL F.W.C. S (%) AV. F.W.C. 22,5% 2 16 AV. P.W.P. 11,44% b 28 ff 24 u 2 fl 16...: (')- )( A z.-.-. AV. P.W.P. B3 - Straw mulch B4 - erbcde B5 - Clean cultvaton B6 - Sward AV. F.W.C. - Average Feld Water Capacty AV. P. W. P. - Average Permanent Wltng Pont ACTUAL P.W.Pls () +- B4 (17,4) z. o DATE FG. 2 Sol mosture fluctuatons and depleton durng the relatvely "wet" 1976/77 growth season. S. Afr. J. Enol. Vtc., Vol. 1. No

6 72 Sol Mosture Conservaton n Dryland Vtculture a,.-,. '-' ff 48 a : 24 [f) 1-15 mm DEPT t-j---_"! T,f X"'x f! " ' (;),-,, \,', 4 _ T, _, '- " T-,', j' ",' ::,. ",)( ; -, 1 \,, "1! U, / /\! \,l :. :_1,:,: / /.\ / \, j \: *: :/ " G> A 1 -!,,',T ", ' : /_\ 1:,1 '; \ j" ", - G) _,' \t. / / / f lv f : :1,,, - "; )( 'A \, l (;) -, '<9-,*' \ /\ l' \ / 1 '/o....:j. 1 J, -/.r,--ot\, \ ",A.. \ " /()\x,,,... 7.t \ Jb /Q **,>'-<;> dp (;) -. / G) Gl b mm DEPT --- t1l a J:l E-< a E-< [f) 24...: [f) - B3 - Straw mulch x-- - -x B 4 - erbcde A----6 B5 - Z-.-.-z B6 - Clean cultvaton Sward DATE FG. 3 (a-b) Changes n sol mosture potental wth tme durng the "wet" 1976/77 season. s. Afr. J. Enol. Vte., Vol. 1. No

7 Sol Mosture Conservaton n Dryland Vtculture c nun DEPT --- : Eo-< Z W Eo-< 48.. ;:::...J d nun DEPT,-... : o - B3 - Straw mulch 6 ff Eo-< 48.. w J 24 x X B 4 - erbcde B5 - Clean cultvaton -.-.-z. 6 Z B - Sward o DATE FG. 3 (c-d) Changes n sol mosture potental wth tme durng the "wet" 1976/77 season. S. Afr. J. Enol. Vtc., Vol. 1. No

8 74 Sol Mosture Conservaton n Dryland Vtculture (1-15 and 25-3 mm) remaned hgh enough to prevent the tensometers from reachng the crtcal 85 kpa lmt, when they cease workng, only under the B3 treatment, whle these values were already reached n October n the B5 and B6 treatments. Up to the end of November 1976 the sol mosture tenson fluctuatons of the straw mulch (B3) and herbcde (B4) plots followed the same trend n the lower two sol layers (75-8 and 55-6 mm), wth the B3 curve a lttle lower than that of B4. Ths mght have been due to the fact that the effectve evaporatng sol surface of the straw mulch (B3) plots was vrtually nl, whle the dense surface crust of the herbcde (B4) plots served as a barrer to water vapour transfer from the sol to the atmosphere. owever, n the second and frst sol layers (15-3 and 1-15 mm), ncreasngly larger dfferences between these two treatments were recorded (Fg. 3b & Fg. 3c), wth the straw mulch (B3) curve runnng at a much lower level than the herbcde (B4) curve. The B3 treatment, therefore, appears to be more effcent than the B4 treatment. Ths may be ascrbed to the addtonal effect of surface straw, because complete weed control was attaned n both the B3 and B4 treatments. As a result of the better mosture regmes the vnes n the straw mulch and herbcde treatments grew exceptonally well compared to those n the clean cultvaton (B5) and sward (B6) treatments. owever, from December onwards, the trend n the sol mosture tenson was reversed. Fg. 3a, 3b and 3c show, contrary to all expectatons, that n the upper three layers (1-15, 25-3 and 55-6 mm) the tensometer curves of the clean cultvaton (B5) treatment were at the lowest level, thus ndcatng a hgher water content than under the mnmum treatments. Ths surprsng phenomenon may be due to the mosture utlzaton at a much faster rate by the vnes n the B3 (straw mulch) and B4 (herbcde) treatments n order to mature the much bgger crop that was eventually obtaned from these plots (Van uyssteen & Weber, 198b). Snce no roots extended below 7 mm depth n any treatment plot, sol mosture condtons remaned unchanged at the 75-8 mm measurement level from December onwards (Fg. 3d). Durng a dry season: The extent to whch the "total actually avalable mosture" (TAAM) of the entre sol profle was depleted under the dfferent treatments durng the dry 1975/76 growth season (18,9 mm ranfall as aganst 341,3 mm durng the wet 1976/77 season) s demonstrated n Fg. 4. The "actually avalable mosture" (AAM) of each layer down to 7 mm depth was calculated usng the followng equaton: (MC vol % - PWP vol %) x LT/OO = mm AAM where, MC = mosture content actually determned, PWP = permanent wltng pont L T = thckness of the ndvdual sol layer n mm, and AAM = actually avalable mosture n mm, and added up to gve the TAAM n the profle. n Fg. 4 ths stage (PWP) s ndcated by a horzontal lne. z_ _ -z B3 - Straw mulch B 4 - erbcdes B5 - Clean cultvaton B6 - Sward P. W. P. - Permanent Wltng Pont x _x DATE FG. 4 Changes n total plant avalable water durng the "dry" 1975/76 growng season. S. Afr. J. Enol. Vtc., Vol. 1. No

9 Sol Mosture Conservaton n Dryland Vtculture 75 t s nterestng to note that at any tme durng the dry 1975/76 season, except the fortnght from to 26 October, the TAAM content n each of the three mnmum tllage profles was hgher, and n the case of B3 much hgher, than n the clean cultvaton (B5) profle, and that the T AAM n the former outlasted that of the latter wth up to 42 days, as s ndcated by the vertcal dotted lnes n Fg. 4. On the other hand, the T AAM depleton curve (Fg. 4) of the B5 treatment shows a remarkable restoraton after the T AAM content fell far below PWP after 1 November, The same phenomenon was observed durng the wet 1976/77 season, albet over a much shorter perod of tme (Fg. 3a, 3b & 3c), and durng the 1974/75 season (results not publshed). No explanaton wll be attempted n ths paper. The longer conservaton of T AAM by the mnmum treatments B4 (herbcde) and, partcularly, B3 (straw mulch) was reflected n ths dry season, too, by the healther appearance and much stronger development of the above-ground parts, especally the much denser leaf canopy of the vnes (Van uyssteen & Weber, 198b); than was observed on the B5 (clean cultvaton) and B6 (sward) plots. The larger leaf canopy n the B3 and B4 treatments shadowed the sol surface optmally, and mght have contrbuted to the sol mosture conservng effect by keepng the sol temperatures lower than n the B5 and B6 treatment plots. SUMMARY AND CONCLUSON The effects of three mnmum tllage practces and one conventonal cultvaton method on sol mosture conservaton n a dryland vneyard under the clmatc condtons of the coastal regons of the Western Cape, were studed. The followng results are noteworthy: () The straw mulch treatment caused the sol mosture not only to outlast that of the other three treatments up to sx weeks, but also provded a supply of easly avalable water of suffcent volume to produce healther and stronger developed vnes wth much hgher yelds than were obtaned from the herbcde, clean cultvated and sward plots. () Vnes on the herbcde treated plots compared well wth those of the straw mulch treated plots. Sol mosture depleton curves determned gravmetrcally as well as tensometrcally yelded a smlar pattern n both dry and wet seasons, and ndcated that mechancal sol tllage wth mplements, has, at least as far as sol mosture conservaton and weed control s concerned, no effect equal to the applcaton of herbcde. () The clean cultvaton treatment produced less favourable results. At a very early and plantphysologcally most mportant growth stage, sol mosture was substantally depleted, whch resulted n weaker and less vgorous vnes than on the two mnmum treatment plots. The cultvaton wth mplements succeeded n keepng the vneyard free of water competng weeds but faled n ts commonly accepted man purpose, vz. to conserve sol mosture. Ths sol management method, therefore, appears to be redundant. (v) The benefts of permanent sward on sol physcal condtons were shown by Van uyssteen & Weber (l98a) to be the best, but the reverse was true as far as the effect of ths treatment on sol mosture conservaton s concerned. eavy transpraton losses caused by the sward resulted n the sol mosture store to be exhausted to such an extent that there was vrtually no avalable sol mosture left when the vnes needed water most urgently, vz. durng the swellng stage of the berres, about md-november. REFERENCES ALLMARAS, R. R., ALLAUER, E. A., NELSON, W. W. & EVANS, S. D., Surface energy balance and sol thermal property modfcaton by tllage nduced sol structure. Unv. Mnn. Agr. Exp. Sta., Tech. Bull. 36. BAEUMER, K. & BAKERMANS, W. A. P., Zero-tllage. Adv. Agron. 25, BARNES, K. K., TAYLOR,. M., TROCKMORTON, R.., V AN DEN BERG, G. E. & CARLETON, W. M. (Edts.), Compacton of agrcultural sols. Amer. Soc. Agrc. Engneers., St. Joseph, Mch. ELERS, W., Gesamtporenvolumen und Porengrssen Vertelung m unbearbeteten und bearbeteten Lssboden. Z.! Pflanzenern. u. Bodenkde 134, GREB, B. W., SMKA, D. E. & BLACK, A. L., 197. Water conservaton wth stubble mulch fallow. J. Sol Water Conservo 25, GREENLAND, D. J., Sol damage by ntensve arable cultvaton: temporary or permanent? Phl. Trans. R. Soc. Lond. B., 281, GREENLAND, D. J., The responsbltes of sol scence. Proc. 11th Congr. nternat. Soc. Sol Sc., Vol. 2, Unv. Alberta, Edmonton, Canada. June 19-27, ANKS, R. J., Challenges n future sol research related to clmate. Proc. 11th Congr. nternat. Soc. Sol Sc., Vol. 2, Unv. Alberta, Edmonton, Canada. June 19-27, McCALLA, T. M. & ARMY, T. J., Stubble mulch farmng. Adv. Agron. 13, MOR, P. J., 197. Verdampng, aftoop en erose. MsstoJ Veren. SA. (M.V.SA.) Joern. 1, SAA YMAN, D. & VAN ZYL, J. L., rrgaton of wne grapes n South Afrca. Paper presented 14th nternat. OV Congr., taly Updated Oct. 1976, OVR, Stellenbosch. UNGER, P. W., ALLEN, R. R. & WESE, A. F., Tllage and herbcdes for surface resdue mantenance, weed control and water conservaton. J. Sol Water Conserv. 26, UNGER, P. W., & PLLPS, R. E., Sol water evaporaton and storage. Proc. Natl. Conserv. Tllage Con! March 1973, Des Mones, owa. VAN EEDEN, F. 1., 197. rrgaton requrements of South Afrcan crops. Farmng S. Afr. 46(2), VAN UYSSTEEN, L., 'n Vergelykende ondersoek na de effektwtet van verskllende konvensonele en mnmum grondbewerkngspraktyke n de wngerdbou ten opsgte van grondvogbewarng en ander fsese eenskappe. M.Sc.-tess, Unv. Stellenbosch. VAN UYSSTEEN, L. & WEBER,. W., 198a. The effect of conventonal and mnmum tllage practces on some sol propertes n a dryland vneyard. S. Afr. J. Enol. Vtc. 1 (l), VAN UYSSTEEN, L. & WEBER,. W., 198b. The effect of selected mnmum and conventonal tllage practces n vneyard cultvaton on vne performance. S. Afr. J. Enol. Vtc. 1(2), VAN ZYL, 1. L., Wngerdbesproeng n de Stellenbosch-gebed bnne de raamwerk van de plaaslke grondwater-plant-atmosfeer-kontnuum. M.Sc.-tess, Unv. Stellenbosch. S. Afr. J. Enol. Vtc., Vol. 1. No