the effect of ridging on the soil water status of a waterlogged vineyard soil

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This article was downloaded by: [Millikin University, Staley Library] On: 16 October 2014, At: 08:48 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK South African Journal of Plant and Soil Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tjps20 The effect of ridging on the soil water status of a waterlogged vineyard soil P. A. Myburgh a & J. H. Moolman b a Viticultural and Oenological Research Institute (VORI) , Private Bag X5026, Stellenbosch , 7600 , Republic of South Africa b Department of Soil and Agricultural Water Science , University of Stellenbosch , Stellenbosch , 7600 , Republic of South Africa Published online: 16 Jan 2013. To cite this article: P. A. Myburgh & J. H. Moolman (1991) The effect of ridging on the soil water status of a waterlogged vineyard soil, South African Journal of Plant and Soil, 8:4, 184-188, DOI: 10.1080/02571862.1991.10634831 To link to this article: http://dx.doi.org/10.1080/02571862.1991.10634831 PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/terms-and-conditions

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Page 1: The effect of ridging on the soil water status of a waterlogged vineyard soil

This article was downloaded by: [Millikin University, Staley Library]On: 16 October 2014, At: 08:48Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office:Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

South African Journal of Plant and SoilPublication details, including instructions for authors and subscriptioninformation:http://www.tandfonline.com/loi/tjps20

The effect of ridging on the soil water status ofa waterlogged vineyard soilP. A. Myburgh a & J. H. Moolman ba Viticultural and Oenological Research Institute (VORI) , Private Bag X5026,Stellenbosch , 7600 , Republic of South Africab Department of Soil and Agricultural Water Science , University of Stellenbosch ,Stellenbosch , 7600 , Republic of South AfricaPublished online: 16 Jan 2013.

To cite this article: P. A. Myburgh & J. H. Moolman (1991) The effect of ridging on the soil waterstatus of a waterlogged vineyard soil, South African Journal of Plant and Soil, 8:4, 184-188, DOI:10.1080/02571862.1991.10634831

To link to this article: http://dx.doi.org/10.1080/02571862.1991.10634831

PLEASE SCROLL DOWN FOR ARTICLE

Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”)contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensorsmake no representations or warranties whatsoever as to the accuracy, completeness, or suitabilityfor any purpose of the Content. Any opinions and views expressed in this publication are the opinionsand views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy ofthe Content should not be relied upon and should be independently verified with primary sourcesof information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings,demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arisingdirectly or indirectly in connection with, in relation to or arising out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any substantialor systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, ordistribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use canbe found at http://www.tandfonline.com/page/terms-and-conditions

Page 2: The effect of ridging on the soil water status of a waterlogged vineyard soil

184 S.-Afr.Tydskr.PlantGrond 1991, 8(4)

The effect of ridging on the soil water status of a waterlogged vineyard soil

P .A. Myburgh* Viticultural and Oenological Research Institute (VORl), Private Bag X5026, Stellenbosch 7600, Republic of South Africa

J.H. Moolman Department of Soil and Agricultural Water Science, University of Stellenbosch, Stellenbosch 7600, Republic of South Africa

Accepted 12 July 1991

In a field trial conducted in a vineyard on a low-lying hydromorphic soil, it was established that ridging increased the depth from the soil surface to the water table. Ridging did not lower the level of the water table, but increased the soil depth above the water table by the same amount as the thickness of the additional top­soil layer. A good correlation was found between the depth to the water table and the water content of the overlying soil. The distribution of the soil water content with depth varied from a sigmoidal tendency to a virtually linear one as the depth to the water table increased. Ridges of varying width and height did not differ significantly in their ability to improve the internal drainage of the soil. However, during late summer, 600-mm high ridges lost more water through evaporation than 400-mm high ridges of the same width. The problem of higher evaporation caused by the increased soil surface of the higher ridges was overcome by applying supplementary irrigation. Unstable soil that landed on the ridges during the construction process resulted in unnecessary losses of irrigation water due to runoff.

In 'n veldproef in 'n wingerd op laagliggende -hidromorfe grond is gevind dat operd die watervlakdiepte vergroot het. Operd het nie die vlak van die watertafellaat daal nie, maar wei die diepte van grond bokant die watervlak met die dikte van die addisionele laag bogrond vergroot. 'n Goeie korrelasie tussen die diepte na die watertafel en die waterinhoud van die oorliggende grond is verkry. Die vorm van die grondwaterprofiele het vanaf 'n sigmo"idale neiging tot bykans reglynig gevarieer namate die diepte na die watervlak toeg~neem het. Operdwalle van verskillende hoogte en breedte het nie ten opsigte van die vermoe om interne dreinering te verbeter, verskil nie. Die 600-mm hoe operdwalle het egter gedurende die laaste fase van die groeiseisoen meer water deur verdamping as die 400-mm hoe walle met dieselfde breedte verloor. Die probleem van hoer verdamping as gevolg van die toename in oppervlak by die hoer operdwalle is met aanvullende besproeiing opgehef. Onstabiele ondergrond wat tydens konstruksie op die walle beland het, was die oorsaak van on no­dige besproeiingsverliese.

Keywords: Grapevine, ridging, soil water profile, waterlogging, water table

"To whom correspondence should be addressed

Introduction Ridging, a soil preparation method where the topsoil is heaped up to form continuous bands, can increase the effec­tive soil depth above a water table (Van Zyl, 1985). Owing to the transformation of the soil surface, ridging can be classified as a method of surface drainage (Anonymous, 1973; Hill, Scotney & Willey, 1977). According to Sweeney & Sisson (1988) the gravimetric soil water content at a depth of 300 mm was 3.5% lower on a ridged soil compared with an unridged control. In a grapefruit orchard in Israel, Steinhardt, Hausenberg, Klemmer & Shalhevet (1971) found that ridging increased the depth to the water table (DTWT) by 125 mm compared with a control. This difference only occurred during periods of extremely high rainfall and was ascribed to effective surface drainage. During the dry period the larger soil surface and thus higher evaporation led to

excessive drying of the topsoil. According to Camp (1982), this phenomenon resulted in a yield reduction of sugar-cane planted on wide ridges or so-called land beds.

Bomstein, Benoit, Scott, Hepler & Hedstrom (1984) stated that an increase in the DTWT normally favours plant growth and showed that an increase in the DTWT resulted in a linear increase in Medicago sativa (L.) production. HiJJel (1971) mentioned that a water table, if not too close to the root zone, can contribute to the water needs of agricul­tural crops. However, if the soil water is saline, it may

impede the growth of crops (lsraelsen & Hansen, 1967). The soil water content above a water table is strongly

influenced by the DTWT and any fluctuation of the latter, be it natural or man-made, will therefore affect the soil water profile. Various mathematical equations have I been developed to describe the soil water profile above a :water table. According to Hillel (1980), the accuracy of ,these functions depends on a number of assumptions which! limit the practical application of such models. However, it iis of interest to note that the soil water content does not dec~ease linearly but sigmoidally from the water table to th~ soil surface. The sigmoidal shape of the curve is the res~1t of simultaneous soil water losses by evapotranspiration and internal drainage following water application.

The aim of this study was to quantify the effect of ridges of varying width and height on the. soil water status in the root zone of grapevines above a fluctuating water table.

Materials and Methods A ridging trial was established during November 1985 on the VORl Experimental farm (Nietvoorbij) on a low-lying, hydromorphic soil of the Katspruit form (MacViear, Loxton, Lambrechts, Le Roux, De Villiers, Verster, "Merryweather, Van Rooyen & Harmse, 1977). Six treatments were applied and replicated five times in a randomized block design. On the control treatment (T 1) the soil was ploughed to a depth

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S.AfrJ.PlantSoil1991,8(4)

of 250 mm and the larger clods were broken up with a disc­harrow. A second unridged treatment (Tz), similar to Th was obtained by ripping the soil to a depth of 550 mm with the wiggle plough described by Van Huyssteen & Saayman (1980). The planting width was 1.5 m between vines and 3.0 m between adjacent rows for both TI and Tz• For the high double-row ridging treatment (T3), the topsoil was heaped up with an articulated grader to form a ridge 600 mm high. The ridges were trimmed by hand to obtain a 1.5-m wide flat crest to accommodate two vine rows. This manipulation increased the topsoil by 300 mm. The distance betwee.n the centres of two adjacent ridges was 5.0 m. Topsoil was scraped from the trough in such a way that no wheel traffic occurred on the loose soil of the ridge. The two vine roWS were planted 1.2 m apart on the ridges and indivi­dual vines 1.5 m apart in the row. A low double-row ridge treatment (T4) was obtained in a similar fashion, except that the topsoil was only heaped up to a total depth of 400 mm. To create a single-row ridge treatment (Ts), the topsoil was ploughed from the trough to form a bell-shaped ridge that was 400 mm high and 1.0 m wide at the base. Four passes on each side with a two-furrow mouldboard plough, drawn by a light crawler tractor, were necessary to obtain ridges of the desired dimensions. During this process traffic on the loose soil was inevitable. The single-row ridges were 3.0 m apart and therefore the same planting width as for treatments TI and T2 was used. ~

None of the above-mentioned treatments received supple­mentary irrigation. An additional double-row treatment (T6), similar to T3, was included to study the effect of supple­mentary irrigation on the soil water regime in ridges. Water was applied with 2800 32 L h-I microjets, while the irriga­tions were scheduled by tensiometry. The tensiometers were installed in the vine row at 300-mm and 600-mm depths on two replications of T6 •

Ridges were constructed during October 1984 and thy vines, Chenin blanc grafted onto 99 Richter, were planted in September 1985. Eight experimental vines were planted on each plot. Provision w~s made to eliminate overlapping treaUTIent effects by incorporating border rows and border vines. The experimental plots measured 12.0 x 10.8 m and 15.0 x 10.8 m, respectixely, for the single-row and double­row treatments, respectiyely. The soil water studies were performed during the 1987/88 growing season.

The depth to the wa~r table was monitored with piezo­meters, installed 300 mm from an experimental vine to a depth of 1.6 m. The actual depth was measured with a self­constructed apparatus (Figure 1). A measuring rod was Slowly pushed down the well until two terminals touched the surface of the water. Contact with water was clearly indicated by a deviation on a galvanometer. The depth to the water table was then read directly on a millimetre scale on the measuring rod. A theodolite was used to measure the difference in height between the points on the soil surface where the actual DTWT was determined and an imaginary reference plane parallel' to the original surface of the land. The DTWT from this reference plane was calculated by adding the difference in height between the measuring point at the soil surface and the reference plane to the actual DTWT as measured in the field.

Soil water content was determined by the neutron scatter

\ o 0

GALVANOMETER

ELECTRIC lEADS

DEPTH GAUGE

~ PVCTUBE r-- SOIL SURFACE

TERMINALS ~""-l!----

yqj-_~_ WATER

185

Figure 1 Apparatus for measuring the depth to the water table in a ridging trial at Nietvoorbij, Stellenbosch.

technique. The apparatus (NEA, Denmark) was calibrated in the field against volumetric soil water content derived from gravimetric soil water measurement (Myburgh, 1989). A 1.6-m aluminium access tube was installed on each plot on the vine row. The soil water content was measured over 300-mm increments to a depth of 1 500 mm.

During the first part of the growing season (10-09-87-11-11-87) both DTWT and soil water content were measur­ed weekly and, thereafter, every fortnight up to 03-02-88. On 25-11-87 cross-sectional profiles of the water content on a high double-row ridge (T3) and a control plot (T1) were determined gravimetrically. The soil samples were taken at points 300 mm apart on a straight line across the vine row, and at each point the 0-300, 300--600 and 600-900-mm depth layers were sampled.

All data were analysed by STATGRAPHICS and means were separated by Student's Q-test.

Results and Discussion The soil water studies were performed on all treatments except T2 • The latter treatment was merely included for observation purposes and will not be discussed. The actual DTWT reached a seasonal minimum on all treatments on 24-09-1987 (Figure 2). This was followed by a constant increase for the major part of the growing season. However, the rate of increase tended to diminish towards the end of the active growing period as the seasonal maximum DTWT was 'approached. The DTWT of the control (T1) was markedly lower in comparison with the ridged treatments for the entire season, but these differences were significant from 22-10-1987 to 22-12-1987 only. This phenomenon is indicative of the effectiveness of ridging as a method of surface drainage. Although ridge dimensions varied, no significant differences occurred in the DTWT between the ridged treaUTIents.

Ridging in itself did not lower the level of the water table, but increased the soil depth by the amount that the topsoil was heaped up. This is confirmed by the fact that when the

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186

1600 -

400 &--& T, 600·mmHIGHDOUBLf·ROWRIDGE

6~ -- - ---6 T4 40o.mm HIGH DOUBLE·ROW RIDGE

.--. T5 40Q·mm SINGLE-ROW RIDGE

0-- ----0 T6 IRRIGATED 600-mm DOUBLE·ROW RIDGE

~-1 ____ .1 _____ L ___ L---1 ___ '-____ . I

10/9 24/9 8/10 22/10 II/II 26/11 10/12 22/12 7/1 21/1 4/2

Date

Figure 2 Variation in the average depth to the water table as measured from the soil surface during the 1987/88 season. Bars represent LSD (P:or;;O.05).

DTWT, measured from the reference plane, is used; the differences between the control (Tl) and the ridged treat­ments (T3- T6) are no longer statistically significant (Figure 3). The lack of surface drainage might have been the reason for the lower DTWT on the unridged treatment (T1)

when measured from the reference plane. The influence of a fluctuating water table on the soil

water content of the overlying soil of a control plot is shown in Figure 4. The distribution of the soil water tended to be sigmoidal during early spring. However, the relationship of soil water versus depth tended to become more linear as the season progressed and the DTWT increased~ The straighten­ing of the curve may be due to a relative increase in the evapotranspirative losses of soil water from the 600--900-mm depth layer. The decrease in soil water content at the water table, as the latter receded, might have been brought about by varying soil layers, downward movement of the soil water and variations in the thickness of the capillary fringe (Hillel, 1980). The relatively low volumetric soil water content at saturation (c. 0.23 m3m-3

) can be explained

1800

1200

E .5.800

! 400

10/9 24/9 8/10 22/10

.--. T, CONTROL (UN RIDGED)

&--& T3 600·mm HIGH DOUBLE· ROW FUDGE

6-------6 T. 40o.mmHIGHDOUBLE.ROWRIDGE

.--a T5 40o.mmSINGLE·ROWRIDGE

0-- ----0 TI5 IRRIGATED 600·mm DOUBLE·ROW RIDGE

II/II 26/11 10/12 22/12 7/1 21/' 4/2

Date

Figure 3 Variation in average depth to the water table as calcu­lated from a reference plane parallel to the land during the 1987/ 88 season.

E .s .z:: a " Cl

S.-Afr.Tydskr.PlantGrond 1991, 8(4)

Soil water content (vol. %)

4 6 10 12 14 16 18 20 22 24

f'----'---'-'5 T, T3------.---.----T,.-----r-T~', ----.-,~, ~, I 300

T,

a

----------I T,

r----------600

J:~::::::::::::~:,: 1\,

f-~--------------- \ 1500 "-----------

Figure 4 Average soil water content above a falling water table as measured during the 1987/88 season for the unridged control. The horizontal broken lines indicate the depth to the water table as it increased from 29-10-87 (Tl) to 04-02-88 (Ts).

by the high bulk density (1.9 Mg m-3) and particle density

(2.6 Mg m-3) for this soil, as reported by Van Huyssteen

(1989). Regression analyses were carried out between volumetric

soil water content of the different depth intervals and the DTWT, as measured from 17-09-1987 to 04-02-1988, to quantify the effect of the water table on the overlying soil water content. With the exception of the 1 200--1 500-mm depth, a significant linear correlation exists between the soil water content of a specific depth increment and the DTWT (Table 1), with a decrease in soil water content as the DTWT increased. However, this effect decreased with increasing soil depth. When compared with the control (T1)

Tabel 1 Regression analyses for the relationship between volumetric soil water content (8v ) and depth to the water table (DTWT) as measured from 17-09-1987 to 04-02-1988 in a ridging trial at Nietvoorbij, Stellenbos~h.

I

Depth I

(mm) Treatment Regression analyses between 9. and DlWTI (n=12)

0-300 TI 9. = --0.1867 (DlWT) + 0.2913 I

(r = -0.93)· I

T3• T •• T, 9. = --0.2574 (DlWT) + 0.4361 (r = ~.90)·

T6 9. = --0.221 (DlWT) + 0.2635 (r = --0.73)·

300-600 T, 9. = --0.1882 (DlWT) + 0.3317 (r = -0.95)·

TJ • T4• T, 9. = --0.2996 (DlWT) + 0.3778 (r =:..o.9W

T6 9. = --0.1200 (DlWT) + 0.2773 (r = -0.90)·

600-900 TI 9. = --OJ'fJ67 (DlWT) + 0.2777 (r = -0.85)·

TJ • T4• T, 9. = --0.1835 (DlWT) + 0.3884 (r = -O.9W

T6 9. = --0.1650 (DlWT) + 0.3561 (r = -0.95)·

900-1200 TI 9. = --0.0405 (DlWT) + 0.2403 (r = -0.62)·

TJ • T4• T, 9. = -0.1472 (DlWT) + 0.3708 (r = -O.8W

T6 9. = --0.1129 (DlWT) + 0.3103 (r = -0.84)·

1200-1500 TI 9. = --0.0288 (DlWT) + 0.2456 (r = -0.52)

TJ • T4 • T, 9. = -0.0614 (DlWT) f 0.2840 (r = -0.75)·

T6 9. = --0.0432 (DlWT) + 0.2711 (r = -0.72)·

"Correlation significant at P.;;0.05

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S.Afr.J.PlantSoilI991,8(4)

and the irrigated ridged treatment (T6) the slopes for the non-irrigated ridged treatments (T3, T4 & Ts) showed. that the soil water content decreased more rapidly as the DTWT increased. The relative small slopes for the soil layers below 600 mm on the control treatment showed that, owing to the proximity of the water table, this zone had a high water content for the major part of the growing season. A sharp decrease in slope occurred on the ridged treatments below 1 200 mm, which indicated that the improved internal drain­age was confined to depths shallower than 1 200 mm.

The effect of the various treatments on the soil water content can be explained by considering the seasonal varia­tion of the 300-600-mm depth increment (Figure 5). Ridg­ing, irrespective of width and height, resulted in drier soil conditions during the first part of the growing season up to 22-12-1987. This tendency remained throughout the grow­ing season for T3 and Ts. The higher soil water content of the 400-mm high double-row ridge during the last stage of the season suggests a beneficial effect on soil water conser­vation of lower and wider ridges compared with higher and narrower ridges. In the case of T4 the increased exposed soil surface led to higher evaporation losses. The centre sections of the single-row ridges were not buffered against desicca­tion if compared with the wider double-row ridges where the outer soil layer offered some protection. However, from 22-12-87 until the end of the season the irrigated treatment showed no signs of excessive water losses, although it was ridged 600 mm high. For the first part of the season, up to 19-12-87, the relative lower soil water content on the ridged treatments occurred to a depth of 900 mm (data not shown). During the last part of the season this effect of ridging on the soil water content extended to a depth of 1 200 mm (Figure 6).

Excessive runoff occurred on three replications of the irrigated treatment, where unstable subsoil landed on the ridges during the construction process. This runoff made scheduling and calculation of water consumption virtually impossible. The negative; effect of the runoff on soil water replenishment was clearly indicated by tensiometer readings following an irrigation o~ 11-12-1987 (Figure 7). During the first part of the season the runoff problem was overcome by

I

25

c ! 8 15 . ; :a 10 OJ .---'" T, CONTROL (UNRIOGED)

'--A T3 600·mm HIGH DOUBlE·ROW RIDGE

f::,.- -- - -A T. 40Q·mm HIGH DOUBLE·ROW RIDGE

a --a T s 40Q'mm SINGLE·ROW RIDGE

0- - - --[) Te 'IRRIGATED 600·mm DOUBLE·ROW RIDGE

u/s 11/11 Dalum

Figure 5 Variation in average volumetric soil water content for the 300-600-mm soil layer as· measured during the 1987/88 season. I

25

.--. T, CONTROL (UNRIDGED)

'--A T 3 60o.mm HIGH DOUBLE·ROW RIDGE

t::.------A T. 40Q·mmHIGHDOUBlE·ROWRIDGE

a--a T5 40Q'mmSINGlE'ROWRIDGE

0- --- - [] Te IRRIGATED 600'mm DOUBlE·ROW RIDGE

187

Figure 6 Variation in average volumetric soil water content for the 900--12()()-mm soil layer as measured during the 1987/88 season.

applying the irrigation requirement for two consecutive days. As the water demand increased, more irrigation water had to be applied to wet the soil to a depth of at least 600 mm on the three replicates where runoff occurred. Consequently, the total supplementary irrigation water amounted to 193 mm on the runoff-prone plots compared with 138 mm on the replicates where no runoff occurred, al­though the actual soil water increase was more or less the same.

The effect of spatial orientation of ridges on the soil water distribution within a 600-mm double-row ridge and a con­trol plot is presented in Figure 8. It is clear that the soil water distribution across the ridge is asymmetrical. The drier soil conditions on the west-facing side can be ascribed to the fact that it was more exposed to global radiation during the afternoon. This may have caused increased evaporation.

80,-----------------~------------~

70

60

... ~ 50

'" i I 40

J 30

20

10

-~. ~ ...

- 0'

.--. 300 mm No runoff

0--0 600 mm No runoff

• - - - - --. 300 mm With runoff

0- - - - - 0 600 mm With runoff

0 ____ 0

___ --0

Time after Irrigation (days)

Figure 7 Tensiometer data showing the effect of loss of irriga­tion water due to runoff as measured during the period following an irrigation on 11-12-87.

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188 S.-Afr.Tydskr.PlantGrond 1991, 8(4)

West East West East

o

200 E .s 400 .c

a ~~~~~~~~~~~~~~~~~~~~~~~~~~il~~~ 25 600

000

E .s .e: a " o

400~~ 600smEE

o 400 000 1200 1600 2000 2400 2000 3200 o 400 000 1200

Horizontal distance (mm) Horizontal distance (mm)

D > 120/0 12-10% 10-0%

Figure 8 Cross-sections of the gravimetric soil water content in a 600-mm high double-row ridge ([3) and a control plot (T1) as measur­ed on 25-11-87.

Compared with the unridged soil, the water content of the centre section of the ridge did not differ drastically. However, the shoulders on both sides of the ridge dried out markedly and it can be speculated that this effect is likely to

increase during the later stages of the growing season.

Conclusions Ridging improves the internal drainage of soils where a high water table is present. However, this is not because of a lowering in the level of the water table, but because of an increase in the soil depth above the saturated zone by adding an additional soil layer. Sigmoidal to linear tendencies in soil water content profiles above the water table were found in this study. However, extrapolation of the data to similar situations will be difficult owing to the natural variation in soil texture and evaporation from the soil surface.

The increased exposed soil surface leads to higher evapo­ration rates which result in excessive soil water losses during the final stages of the growing season. Therefore, irrigation is recommended where soils are ridged. However, it is important that further studies be done to establish crop factors for viticulture on ridges.

Serious runoff of irrigation water can occur when structu­rally unstable subsoil is used for ridging. Further studies will have to be conducted to evaluate the effectiveness of, e.g. a straw mulch or chemical conditioning of the surface soil, to reduce runoff losses.

Acknowledgements The technical assistance of the Soil Science Staff of the VORl is gratefully acknowledged.

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BORNSTEIN, G.R.I., BENOIT, G.R., SCOTT, F.R., HEPLER,

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ISRAELSEN,O.W. & HANSEN, V.E., 1967. Irrigation

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ROUX, J., DE VILUERS, J.M., VERSTER, E., MERRY­WEATHER,F.R., VAN ROOYEN, T.H. & HARMSE, Hl.

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