SOIL PREPARATION

FOR SUSTAINABLE WINE AND TABLE GRAPE VINEYARDS

J.L. VAN ZYL & J.E. HOFFMAN

FOR SUSTAINABLE WINE AND TABLE GRAPE VINEYARDS

J.L. VAN ZYL & J.E. HOFFMAN

SOIL PREPARATION FOR SUSTAINABLE WINE AND TABLE GRAPE VINEYARDS COVER IMAGE: SHUTTERSTOCK All rights reserved Wine Industry Network of Expertise & Technology NPC (Winetech) VinPro Building Cecilia Street SUIDER-PAARL 7624 www.winetech.co.za AND South African Table Grape Industry (SATI) 1st Floor Paarl 7464 www.satgi.co.za The authors has made every effort to obtain permission for and acknowledge the use of copyrighted material. Please refer enquiries to the authors. No part of this book may be reproduced or transmitted in any form or by any electronic, photographic or mechanical means, including photocopying and recording on record, tape or laser disk, on microfilm, via the Internet, by e-mail, or by any other information storage and retrieval system, without prior written permission by the authors. First edition 2019 63 Main Street Southern Paarl

ISBN 978-0-620-84093-4 (print) ISBN 978-0-620-84097-2 (e-book) Set in Open Sans Regular 9pt/12

LAYOUT AND DESIGN by VR Graphics, www.vrgraphics.co.za PRINTING by Shumani Mills Communications, Tygerberg

2 |

We dedicate this book to our fellow soil scientists on whose shoulders we could stand to write meaningfully about soil profile modification for sustainable viticulture. J.L. van Zyl and J.E. Hoffman

SOIL PREPARATION | 3

FOREWORD

T he relationships between data, information, knowledge and wisdom are well-known. In this hierarchical model, data comes first, information is next, then knowledge follows and finally wisdom is on the top. The authors of this book are well equipped for taking all the steps in this hierarchy, and hence it is no surprise that their capabilities to translate research information into applicable technology is demonstrated throughout this book. One of the most important functions of soil is that it serves as a medium for root growth. However, such a blunt statement does not define the optimal conditions needed for a healthy and quality root system for sustaining commercial vineyards. The authors present excellent documentation, particularly in describing root impediments of a wide nature as well as management and amelioration practices needed to create favourable conditions for grapevine root growth. The nature of variation in soil types in the South African grapevine regions results in site-specific impediments and, therefore, necessitates unique and tailor-made soil preparation practices, which lead to a great deal of innovation for creating optimal rooting conditions.

Leopoldt van Huyssteen

4 | FOREWORD

FOREWORD

Throughout the book, the most important factors determining the specific soil preparation practices for grapevines, i.e. soil type, soil depth and the quality of the soil as rooting volume, are discussed. Root systems visually demonstrate what they experience in the soil. There is a direct relationship between aboveground growth of grapevines and the soil conditions its roots experience. Plants are excellent integrators of the conditions being experienced; they stand with their heads (aboveground growth) in the hot and dry climatic conditions and their feet (roots) in often poor-quality soil conditions and frequently too small soil volumes. Even though grapevines do not have a terminal growing tip, they will balance aboveground growth with rooting systems. However, if a vineyard is not being managed by taking this balance and the resulting bearing capacity into consideration, the vineyard will not be economically sustainable. An investment in creating a quality rooting volume ensures sustainable plant growth and farming.

The true value of this book lies in the crossing of the boundaries between research, applicable technology transfer and soil management practices. In this regard, the excellence in research, fine observations and the wisdom of the authors are clearly reflected in this book. Lastly, it is good for any scientific discipline to look back over time to reflect how certain soil preparation practices have evolved and what has been achieved with specific practices. However, only this will not take the science of soil preparation for optimal root growth much further if there is no looking ahead. The authors succeeded in looking forward and pointing out what is needed and what disciplines are needed for sustainably managing rooting conditions for grapevines. Soil Preparation for Sustainable Wine and Table Grape Vineyards is a timely publication in that it provides a multi-disciplinary approach with regard to soil preparation and root growth. The book is written in a style which will make it easy for other researchers, technology transfer experts and farmers to internalise the concepts being offered.

All of the above is illustrated by excellent photographs and

discussions of what roots experience in various soil types in widespread viticulture areas.

SOIL PREPARATION | 5

Crop protection, like any other industry, needs smart people. Companies tuned in to their workforce understand that people want to succeed and be good at what they do. Education is just as val- ued as practical experience to achieve the professional success and status which workers strive to achieve. The Villa Academy is committed to provide a new generation of skilled people in agriculture and offers applied studies in-house, allowing our industry to be equipped with the latest knowledge, scientific facts and experience in the field of crop protection. The Villa Academy was initially launched in 2012 to establish a training program for professionals employed in the crop protection industry and to provide skilled and qualified people for the industry at large. To this day, our aim remains the same – to bring a blend of high quality academic and hands- on experience to instil expertise and confidence in people in the Agricultural industry. Our focus is straightforward; we take your existing knowledge and improve on it, allowing you to gain direct access to advanced expertise vested in well- qualified and experienced lecturers. This will allow you to reach new VILLA ACADEMY EDUCATES ON CROP PROTECTION BEST PRACTICES

heights of knowledge, understanding, service delivery, decision-making and productivity. Classes are of one to three days duration and cover diverse topics, including: viticulture, disease management, weed management, insect management, agricultural economics, handling of complaints, adjuvants, application technology, wheat and barley culti­ vation, citrus cultivation, etc. The curriculum is characterised by strong emphasis on the problem-based method of learning in which students collaborate in groups to solve real-world examples of problems. The instructor acts as a facilitator to encourage self- direction and active engagement among

students to a greater extent than the passive, teacher-centric lecture model allows. Classes are intensive, and students are held to high standards. Assessment criteria in addition to in- class work includes written tests and a final project in which students create a production plan for a field crop pro- duced in their area by researching its significance, evaluating the suitability of climate and soil for production and applying key principles of plant produc- tion and protection. The crop chemicals business is rapidly evolving, and it has become highly technical. We’ve been through a decade of experiential and self-training, which just isn’t good enough for the environ-

ment we operate in. Knowledge is key. The Villa Academy strives to ensure that we have well equipped, well trained people in the agricultural industry. Apart from the courses we offer, we also actively participate in the publications of books such as this one to grow the knowledge of our industry even further.

Villa Academy – Grow your knowledge, reap success!

ACADEMY

Research, training and knowledge transfer

Winetech encourages the production of quality wines and other grape-based products through the application of environmentally friendly practices and the best technologies. Winetech supports training and education at all levels, including the development of resource poor and new entrant producers.

Winetech’s primary interest is to build a strong and healthy South African Wine Industry through co-operative CORE FUNCTIONS

RESEARCH

KNOWLEDGE TRANSFER

LEARNING & DEVELOPMENT The aim of Winetech’s Learning & Development Strategy is to develop the wine industry into an industry that is built on the capabilities of its people; is able to maximise its return on investment; and an industry of choice for new entrants.

Winetech focuses on generic research that benefits the entire South African Wine Industry.

Winetech ensures that the results obtained from industry funded research is timeously communicated in a clear and practical way in order for industry to implement new technologies.  

(participative) Research and Development initiatives.

Vinpro Building, Cecilia Street, Southern Paarl TEL. 021 276 0499 – FAX. 086 611 7817 – E-MAIL. andraga@winetech.co.za – www.winetech.co.za

RESEARCH & DEVELOPMENT FRAMEWORK

>>

>>

MISSION SATI delivers service excellence to create a progressive, equitable and sustainable South African table grape industry.

VISION South Africa is the preferred country of origin for table grape grapes and will provide every table grape producer with as wide a choice as possible of profitable markets.

TOOLS Technical and market access Communication and stakeholder engagement Information and knowledge management Transformation Human capacity development Technical support Finance and administration Research and development

VALUES Science-based Agility and Flexibility Transparency Outcome-driven

DRIVERS Market Access:

“Market Preparedness” “Market Accessibility”

<<

<<

VRGRAPHICS.CO.ZA_6691_0218633165

CREATING A PROGRESSIVE, EQUITABLE AND SUSTAINABLE TABLE GRAPE INDUSTRY

63 Main Street | Paarl 7646 Western Cape | South Africa

Tel: +27 21 863 0366 | Fax: + 27 21 863 0339 Email: info@satgi.co.za | Website: www.satgi.co.za

ACKNOWLEDGEMENTS The authors wish to express their sincere appreciation to the following persons and organisations for their help and support in publishing this book. – The South African Wine Industry through Winetech, as well as the South African Table Grape Industry for co-funding the writing of this book. – Villa Academy for financial support to enable the printing of hard copies of the book. – Dawid Saayman and Pieter Raath for reviewing the manuscript and their valuable suggestions to improve it. – Leopoldt van Huyssteen for writing the foreword, and fittingly so, because his pioneering research contributed significantly to this book. – The institutions and persons who supplied photographs; without them it would have been impossible to illustrate various concepts and practices. – Anel Andrag and Tarryn Wettergreen for their patience and help. – Lucinda Heyns for organising meetings and liaising with the different parties to keep to time lines. – Leandri van Heerden for the design and layout of the book.

10 | ACKNOWLEDGEMENTS

PHOTO: J. DE JAGER, VINPRO.

SOIL PREPARATION | 11

TABLE OF CONTENTS

1.

INTRODUCTION . .............................................................................. 14

2. SOIL IMPEDIMENTS TO ROOT PENETRATION ...................... 18 2.1 Soil compaction ....................................................................................... 19 2.2 Soil acidity ................................................................................................ 23 2.3 Soil stratification . .................................................................................... 24 2.4 Dense clay in the subsoil ........................................................................ 25 2.5 Hardpans . ................................................................................................ 26 2.5.1 Hard plinthite . ......................................................................................... 26 2.5.2 Carbonate hardpan ................................................................................ 27 2.5.3 Dorbank . .................................................................................................. 28 2.6 Waterlogging . .......................................................................................... 28 2.7 Weathered rock in the subsoil . ............................................................. 30 2.8 Coarse fragments in the soil .................................................................. 30 3. DETECTION OF ROOT IMPEDIMENTS ...................................... 34 3.1 Soil inspection and sampling ................................................................. 34 3.2 Soil physical parameters ........................................................................ 35 4. ROOT RESPONSE TO SOIL CONDITIONS ................................ 46 4.1 Root studies ............................................................................................. 49 4.2 Root pruning ............................................................................................ 54 5. SPECIAL SOIL PREPARATION STRUCTURES ............................. 60 5.1 Ridges ....................................................................................................... 60 5.1.1 Soils considered for ridging ................................................................... 60 5.1.2 Construction of ridges ............................................................................ 61 5.1.3 Grapevine performance on ridges ........................................................ 63 5.1.4 Vineyard practices adapted to ridging ................................................. 66 5.2 Terraces .................................................................................................... 69 5.3 Plant holes ............................................................................................... 76 6. SOIL DRAINAGE ................................................................................. 86 6.1 Subsurface water .................................................................................... 86 6.2 Scope and causes of waterlogging . ...................................................... 87 6.3 Surface drainage ..................................................................................... 88 6.4 Subsurface drainage ............................................................................... 89 6.4.1 Cut-off drains ........................................................................................... 90 6.4.2 Open ditches . .......................................................................................... 91

12 | TABLE OF CONTENTS

TABLE OF CONTENTS

6.4.3 Stone drains ............................................................................................. 92 6.4.4 Bore holes ................................................................................................ 93 6.4.5 Pipe drains ............................................................................................... 94 6.4.5.1 Drainage materials . ................................................................................ 94 6.4.5.2 Installation ............................................................................................... 97 6.4.5.3 Drain spacing ........................................................................................... 99 6.4.5.4 Pipe sizes and slopes ............................................................................ 100 6.4.5.5 Computer simulation models .............................................................. 100 6.4.5.6 Maintenance .......................................................................................... 101 6.5 Reclamation of saline soil .................................................................... 104 6.5.1 Characterisation of brack soils ............................................................ 104 6.5.1.1 Total soluble salts ................................................................................. 104 6.5.1.2 Sodium hazard ...................................................................................... 105 6.5.2 Reclamation measures ......................................................................... 106 7. CHOICE OF IMPLEMENT FOR SOIL PREPARATION . .......... 110 7.1 Conditions for effective implement action ........................................ 123 7.1.1 Soil water content ................................................................................. 123 7.1.2 Single versus double tillage ................................................................. 124 7.1.3 Direction of tillage ................................................................................. 125 7.1.4 Cutting width ......................................................................................... 125 7.2 Matching soil preparation with soil type ............................................ 127 7.2.1 Sandy soils ............................................................................................. 127 7.2.2 Stratified soils ........................................................................................ 127 7.2.3 Apedal and neocutanic red and yellow soils ..................................... 127 7.2.4 Duplex soils ............................................................................................ 128 7.2.5 Wet and dark-coloured soils ................................................................ 130 7.2.6 Shallow soils on rock ............................................................................ 131 8. APPLICATION OF AMELIORANTS DURING SOIL PREPARATION ......................................................................... 134 8.1 Lime ........................................................................................................ 134 8.2 Organic material . .................................................................................. 138 8.3 Phosphorous ......................................................................................... 139 8.4 Gypsum .................................................................................................. 139 9. RE-COMPACTION ............................................................................ 142

10.

ABOVEGROUND GRAPEVINE RESPONSE ............................... 150

11. BIBLIOGRAPHY ................................................................................. 158 ABBREVIATIONS, ACRONYMS AND SYMBOLS ...................................... 168

SOIL PREPARATION | 13

The modification of the soil profile before planting to remove impediments to root growth and provide an improved available rooting volume adequate for optimum grapevine performance is generally known as soil preparation. Various investigations in South Africa have addressed soil compaction, chemical restrictions to root penetration, root distribution, soil ameliorants such as lime, implements, soil types and many other related aspects. This body of knowledge is not only spread over many years and different generations of researchers and experts, but is also fragmented among many articles and journals. In short, a divide has developed between available knowledge and the end-users of the knowledge. Considering the determining role of soil preparation in the successful establishing of a vineyard and the huge costs involved, it is imperative that all the available information is compiled correctly in one place, hence the need for writing this book. In order to make this knowledge source as comprehensive as possible, relevant international information was integrated with the South African work. This will enable everybody to apply the best soil preparation practices for vineyard soils and eliminate confusion/pseudo-expertise around this important subject. INTRODUCTION

PHOTO: SHUTTERSTOCK.

14 | INTRODUCTION

CHAPTER 1

The majority of soils used for wine and table grape production in South Africa are notoriously shallow in their natural state, i.e. they are restricting root penetration due to various factors such as compaction, soil acidity and abrupt textural transitions. The results of such shallow soils are uneven and poor vineyard performance that eventually lead to unprofitable vineyards. Research has proved the relationship between available rooting volume and aboveground grapevine performance and consequently also the necessity for modifying the soil profile before planting. An optimum available rooting volume exists, exceeding which, the grapevine vegetative growth becomes excessive. Deep tillage before planting of a vineyard can therefore either be inadequate or overdone and wasteful. Soil preparation is not only necessary for establishing first generation vineyards but, due to re-compaction, is often also required for the replanting of existing vineyards. Soils can re-compact naturally with lapse of time, but can also be re-compacted through man-made actions e.g. by driving over deep- ploughed soil or by repeated mechanical tillage at the same depth. Man-made soil compaction in the form of traffic pans and plough pans occurs frequently in vineyards and orchards, and can in fact be found in most intensively mechanised crop production systems. Traffic pans occur mostly on sandy soils and are more common than plough pans which occur on clayey soils where tillage implements smear and compact soil just below the ploughing depth. Re-compaction can even have such unlikely causes as farmers allowing livestock to graze in their vineyards and orchards, without realising that severe compaction is caused by the treading of livestock (Mitchell & Berry, 2001). The worst compaction is by “pugging” i.e. plastic flow around animal hooves in wet soil (Singleton & Addison, 1999). Already in the first century after Christ, in his book De Re Rustica, the Roman agriculturist, Columella (Thayer, n.d.) also warned against re-compaction of loose soil when a new vineyard is planted. In a translation of his writings it is said: “For it is of no little importance that trenched ground be in a very loose state and, if possible, not violated even by a footprint; so that the earth, being evenly stirred, may give way generally to the roots of the young plant in whatever direction they creep out, not repelling their growth by its harshness….” Deep preparation of vineyard soils has a long history in South Africa. The well-known viticulturist, A.I. Perold (1926), had already discussed the pros and cons of “delving versus ploughing” and based his recommendations on soil type. He also recommended deep delving especially for table grapes. He did, however, differentiate between soils and mentioned that delving is not suitable for heavy clayey and loamy soils since such soils will quickly re-settle again after heavy rains. In the early parts of the 20th century, equipment for deep soil preparation was lacking, and farmers used hand delving to a depth of 60 cm and even 75 cm.

SOIL PREPARATION | 15

The use of dynamite to break up hardpans before planting of new vineyards and even between rows of existing vineyards was propagated in South Africa early in the 20th century (Quinah, 1912). The extent to which ‘dynamite-ploughing’ was actually applied in practice for the modification of vineyard soils has been lost in the mist of time. Theron (1930), however, reported the use of dynamite to break up shale soils in Europe, specifically in Portugal and on the Moselle slopes in Germany. He also reported that soil was prepared at that stage using big steam tractors and single furrow ploughs. The ploughs reached a depth of 30-90 cm and ploughing was done in summer when the soil was hard and dry. They then passed a disc harrow over the land in winter, when the soil was wet, in order to render the soil in a good condition for planting. The first contractor to provide a soil preparation service for grape farmers in South Africa started operating around 1930 (Figure 1.1).

FIGURE 1.1: Equipment of yester year: Soil preparation with a McCormick crawler tractor on the farm Hoogstede of Mr S.J. du Toit in 1930 (left) (Photo: J. Nolte) and plough used for deep soil preparation on the Delheim wine estate in the Western Cape pre-1961 (right) (Photo: J.E. Hoffman, Stellenbosch University). Deep soil preparation has, however, only been scientifically grounded with the visit of Schulte-Karring (1976) and publication of the results of soil preparation trials at Robertson and Stellenbosch (Claassen, Van Zyl & Kleynhans, 1973; Saayman & Van Huyssteen, 1980; Saayman, 1982). Since then, deep soil preparation on wine farms has become an essential practice. The aim is to remove all impediments to root systems in the soil down to a depth of 800-1 000 mm. Soil preparation also provides an ideal (and often the only) opportunity to rectify unfavourable soil chemical conditions such as high acidity and low phosphorous (P) content in the subsoil. Deep tillage, i.e. tillage to a depth of 80 cm and more, is not the only component of soil preparation by far. Wet soils have to be drained, an action that can only be done before planting a permanent crop. The necessity for using big machines, vehicle traffic, depth at which subsoil drains have to be installed,

16 | INTRODUCTION

CHAPTER 1

and special configuration of the drainage system, mean that such work must be completed before planting. Ridging, terracing of steep slopes, plant holes and even root pruning are further aspects of soil preparation that require special attention. Once one understands the principles underlying these practices, decision making becomes easier, e.g. when and how often root pruning should be done or whether mechanisation of planting fulfils the requirement for rapid root growth into the surrounding soil. Since soil preparation is primarily aimed at creating soil conditions that will allow root growth to sustain the desired grapevine performance, knowledge about optimum conditions for root growth and the relationship between roots and aerial growth is essential. The technique of soil preparation is to a large extent determined by soil type and the nature of the root impediments present. This can only be determined through inspection in a soil pit, combined with soil sampling and analyses. In addition to soil profile inspection, instruments such as the penetrometer exist that can be used to detect soil compaction and are suitable for application on farms. Considering the costs involved and the long-term nature of investment associated with establishing of vineyards, it will be worthwhile to employ the help of a soil expert. In the end it will be important to decide which kind of implement will be best suited to rectify a specific soil condition. Soil preparation can improve a soil, but if poor subsoil is e.g. ploughed to the surface, the end will be worse than the beginning. The South African wine and table grape industries are fortunate to have access to an array of implements and a pool of experience that can recommend the best horse for every course. Compiling all available knowledge together in book form has the additional benefit of identifying knowledge gaps as well as shortcomings in existing soil preparation practices. One such knowledge gap exists around the incorporation of organic matter into the soil during soil preparation. The prediction of the rate of natural re-compaction of different soils after loosening is also still lacking. The list of unanswered questions can be extended with issues around the practicality and necessity of removing old roots when vineyards are replanted, as well as the economic viability of some practices. In most cases where knowledge gaps exist, research has been done, but the investigations did not yield conclusive answers. Consequently research has to be expanded, repeated or done using different methods. Research on soil preparation is long-term in nature and the progress that has been made, is a tribute to researchers who were willing to dwell on the subject for many years.

SOIL PREPARATION | 17

SOIL IMPEDIMENTS TO ROOT PENETRATION The life span of vineyards is 20-25 years. Planting of a new vineyard is a long-term investment. Mistakes made during establishment, particularly regarding soil preparation, are in most instances impossible to correct once the vines have been planted. Soil preparation aims to modify the soil in several ways, namely to a) alleviate soil compaction, b) increase soil depth and improve poor soil physical structure, c) improve water storage, and d) allow deep chemical amelioration. The end goal of soil modification is, however, to remove root restrictions and provide a larger, more favourable soil volume for roots and subsequently better grapevine performance. Good root distribution, i.e . deep, even and dense root systems, is needed for healthy and high yielding grapevines that are also buffered against drought and deficient nutrient applications. Soil layers that impede root growth reduce the quantities of water and nutrients available to the plant. Furthermore, there is a balance between root size and aerial growth of grapevines. A restricted root system will consequently reduce aboveground growth.

2

PHOTO: J. DE JAGER, VINPRO.

18 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

Fortunately, various types of implements are available in South Africa to achieve all kinds of loosening and mixing of the soil. The high cost of these actions is, however, a limiting factor. Soil compaction is the main impediment to vineyard roots, but soil acidity, waterlogging, salinity, soil stratification, hardpans, subsoil clays and rock are also limiting factors in many localities. In fact, there are very few South African soils without root-impeding layers within 0.8 meter depth that can be planted to grapevines without deep soil preparation. 2.1 Soil compaction In its simplest form, soil compaction can be described as the densification of soil under unsaturated conditions (Bradford & Gupta, 1986). In its unpacked form, soil compaction is well-defined by Mitchell & Berry (2001) as “the detrimental modification of the pore structure when total porosity is so reduced that aeration, root penetration and drainage are restricted, bulk density is increased and hydraulic conductivity and permeability are reduced”. With increasing density, soil particles move closer together, interlock and their resistance increases against further deformation (Hillel, 1980). The visible result of this increase in the inherent strength of soil, is increased difficulty in tillage and also decreased root penetration. Root penetration is affected by both soil porosity and soil strength. Compaction particularly reduces the volume and continuity of large pores (Hillel, 1980). Roots are unable to reduce in diameter in order to penetrate rigid pores narrower than the diameter of their root caps (Wiersum, 1957; Taylor & Bruce, 1968; Cannell, 1977). Therefore, if roots attempt to grow through a compacted soil they must be able to open the pores by exerting a large enough pressure to overcome the mechanical strength of the soil. Although instances of deep root penetration and even the splitting of rocks by root growth are well-known, the aim in productive vineyards is always to get maximum root distribution using minimum root pressure. Compaction can occur in many soil texture classes, but well-sorted sandy loams and loamy fine sands with high fine sand fraction and low carbon content are especially susceptible to compaction (Bennie & Krynauw, 1985). It is however, not the presence of one particle size fraction alone, but particle size distribution that determines the compactibility of a soil. Moolman & Weber (1978) found that the effect of particle-size distribution was more important than that of fine sand alone. Moolman (1981) also established that high compactibility is linked to well-graded soils, i.e. soils that contain a wide range of particle sizes . Furthermore, the ease of compaction is also linked to the water content of the soil, i.e. when starting from a dry soil under the same physical impact, the soil density will increase until it reaches a maximum density at approximately 80 % level of saturation (Hillel, 2004). The practical implication

SOIL PREPARATION | 19

of this phenomenon is that soils should not be tilled or driven upon when they are too wet because this will cause higher compaction compared to drier soil. Wine and table grapes are grown on a wide diversity of soils as well as under varying management practices, and consequently all types of compaction occur in vineyard soils (Figure 2.1). Management of soil compaction therefore needs knowledge and awareness of how it happens, when it becomes harmful, how it affects root growth, and of course, how to rectify it.

CL1099

Surface crust

Wheel compaction Shallow traffic pan Deep traffic pan Loose soil Zoned loosening

15 30 45 60 75 90 105

Depth (cm)

Natural compaction

120

0

30

60 90 120

180 150 210 240 270

Width (cm)

FIGURE 2.1: Schematic illustration of the different types and positions of compaction generally found in vineyards (adapted from Van Huyssteen, 1989).

Soil compaction can occur naturally due to texture, wetness and the manner in which soils were formed in situ. Such naturally compact soils are the rule rather than the exception in the Western Cape of South Africa. In general, these vineyard soils are low in organic matter, contain non-swelling clays, receive large quantities of winter rain, have weak structure and are subjected to severe drying in summer, factors which cause serious natural compaction (Van Huyssteen, 1989). Natural soil compaction – indeed any kind of compaction – prohibits, or seriously limits, root penetration (Figure 2.2). Compacted soil will greatly benefit from soil preparation that removes the restricting layer(s) and allows roots to grow much deeper. Loosening the naturally compacted yellow B horizon of the Tukulu soil (Soil Classification Working Group, 1991*) would have changed it into a high potential medium for grape cultivation (Figure 2.2).

*The classification of all soils further on in this book, refers to the Soil Classification Working Group (1991).

20 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

FIGURE 2.2: Shallow grapevine rooting in a Tukulu soil due to natural compaction in the subsoil (Photo: ARC Infruitec-Nietvoorbij).

In addition to natural compaction, soil compaction can be caused by man- made mechanical forces of machinery wheels and tracks on the soil surface. Man-made compaction is, however, not only caused by vehicle traffic on the soil surface, but also by tillage tools operating below the surface. Depending on implement design, soil is simultaneously cut, compressed, sheared, lifted, displaced and mixed to a varying extent. Compaction, as a result of compression under tractor wheels and under implement shares and tines, occurs in vineyards as wheel tracks and traffic pans. Fortunately, because of the layout of a vineyard, tractors are always driven on the same tracks, i.e. vine rows enforce controlled traffic. Wheel compaction may already occur before planting of the vines after deep ploughing during levelling and other actions. The effect of a traffic pan on root distribution in a Wasbank soil is illustrated in Figure 2.3. Roots could not penetrate through the traffic pan to exploit the soft, loose plinthite layer underneath.

SOIL PREPARATION | 21

FIGURE 2.3: Shallow grapevine rooting in a Wasbank soil due to the presence of a traffic pan. The vineyard on this soil had to be uprooted because of poor performance (Photo: ARC Infruitec-Nietvoorbij). Impediment of grapevine root growth was successfully characterised by Van Huyssteen (1983) in terms of bulk density and penetrometer resistance. Penetrometer resistance, which is an indicator of soil strength, was measured by a constant-speed penetrometer. In a wide variety of soil types, a critical, albeit poorly defined, penetrometer resistance of 2 000-2 500 kPa was found above which root penetration of several crops other than grapevines are drastically impeded (Zimmerman & Kardos, 1961; Taylor & Gardner, 1963; Taylor & Burnett, 1964; Greacen et al., 1969; Bar-Yosef & Lambert, 1981). This critical penetrometer resistance can be reached either by increasing soil compaction or by decreasing water content of the soil, i.e. by drying of the soil (see “3.2 Soil physical parameters”). Several studies showed the relationship between indicators of soil compaction and grapevine root distribution. In a pot experiment with Chenin blanc/99R Van Huyssteen (1988) clearly found a gradual decrease in root penetration with increasing soil compaction (see “3.2 Soil physical parameters” for more information on the experiment). This result was obtained with several soil types.

22 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

2.2 Soil Acidity Soils having a pH KCl

below 5.5 are commonly found in the coastal areas of the Western Cape and even in the more arid Breede River and Olifants River areas (Saayman, 1981; Conradie, 1983). In the coastal region, these acid soils originate from certain types of parent rock, particularly granite, Malmesbury shale, Table Mountain sandstone and local colluvium, all of which are highly weathered due to age and high rainfall. Consequently the basic cations were leached and replaced by hydrogen ions (H + ) and aluminium ions (Al 3+ ). The latter element not only contributes to soil acidity, but it is also toxic to plant roots. Soil acidity can also be caused by high rainfall, continuous use of NH 4 + containing N fertilisers and even the application of large amounts of organic matter (Saayman, 1981). Normally the pH decreases with soil depth, i.e. it becomes more acid, which complicates the remediation of acidity. Soil acidity is determined by measuring its pH in either water (H 2 O) or potassium chloride (KCl). Most laboratories in South Africa use the KCl method, but it is useful to remember that pH KCl is roughly one pH-unit lower than pH H 2 O (Raath, 2016a). At low pH, aluminium (Al) in the clay becomes soluble and replaces cations such as calcium (Ca) and magnesium (Mg) on the clay complex. It also reacts with water to release more H + which leads to further acidification (Saayman, 1981). The optimum soil-pH (pH KCl ) for grapevines varies from 5.5-6.5 (Raath, 2016a). A pH KCl below 5.5 is not optimal for grapevines since root growth is inhibited and the availability of P and molybdenum (Mo) for uptake decreases. Acid soils are often also highly leached and contain low concentrations of nutrients such as nitrogen (N), Ca, Mg, and potassium (K). In addition, the solubility of aluminium (Al) increases with decreasing pH until Al ions become toxic and negatively affect root growth. Conradie (1983) demonstrated this impeding effect of soil acidity on the root growth of grapevine rootstocks. Interestingly, he mentioned that, in addition to Al toxicity, this result might have partly been due to an unfavourable physical structure of the acid soil. Soil amelioration with lime is the tested way to increase the pH, but lime has to be well-mixed with the soil, especially in the zone where the acidity problem occurs. Depending on the outcome of soil analysis, either calcitic or dolomitic lime must be used. Most important, however, is to remember that the alkalinisation effect of lime does not move downward in soil. Consequently lime application must be done during soil preparation, an opportunity when this ameliorant can be placed all through the entire soil profile, and especially into the subsoil (see also “8.1 Lime”).

SOIL PREPARATION | 23

2.3 Soil stratification Soil texture can significantly affect grapevine performance, but does not limit root distribution per se. A gradual increase in clay content with depth is not limiting to root growth. Abrupt (sharp) transitions between texturally different soil layers will, however, prevent root penetration. Such layering within soils, where layers differ profoundly in texture, and abrupt transitions between layers, are a characteristic of Dundee soils (Figure 2.4). In South Africa these soils are young and occur in many viticulture areas along rivers such as the Olifants, Orange and Breede Rivers; in other words they are alluvial in origin. The layering prevents a homogeneous root distribution and may even prevent roots from exploiting the full depth of these potentially deep soils.

FIGURE 2.4: Dundee soil showing stratification due to alluvial deposition of texturally different layers (Photo: J. de Jager, Vinpro).

Layered soils should be deep-tilled before planting to remove the layers and obtain good mixing. Depending on the lithological origin of the mother material, climate, and the environment, stratified alluvium can vary from well-drained to wet, acid to alkaline, and non-saline to saline and even sodic. Care should therefore be taken that any brackish and/or clayey soil layers are not brought to the surface (Saayman & Van Huyssteen, 1981a).

24 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

2.4 Dense clay in the subsoil Many soils in the vineyard areas of South Africa contain dense clay in the subsoil. These clays can be dry structured B horizons, wet poorly structured G horizons or even unspecified material. In the South African soil classification system (Soil Classification Working Group, 1991), the type of clay layer, together with the overlying horizon, will determine the soil form. Fourteen soil forms, including duplex soils ( e.g. Kroonstad and Estcourt), typically contain such root impeding clay layers (Figure 2.5). Soil preparation methods will differ among them depending on depth and thickness of the clay layer, wetness and other factors. The presence of a clay layer in itself does therefore not dictate a standard soil preparation technique.

FIGURE 2.5: Duplex soil (Estcourt form) with dense clay layer at 60 cm that impedes root growth and water infiltration (Photo: ARC Infruitec-Nietvoorbij).

SOIL PREPARATION | 25

An abrupt transition from the overlying horizon to the clay will cause free water to accumulate above the clay. The effective depth of soils with clay in the subsoil is directly limited by the clay and also indirectly by the recurrent waterlogging in the overlying more sandy layer. 2.5 Hardpans In South Africa three different types of hardpans that restrict root penetration, occur as described below. 2.5.1 Hard Plinthite Hard plinthite consists of iron (and sometimes also manganese) oxide. It normally forms under the influence of a fluctuating water table, but due to its resistance against erosion it is also found as relic material in old landscapes with no active water table. Genetically, hard plinthite is similar to soft plinthite, but the process of localisation and accumulation of iron-oxide have, however, continued much further to a stage where the material has hardened to such an extent that it cannot be cut, even when wet. Internally the structure of hard plinthite can vary from porous to massive and in most cases can be broken up to allow root penetration. In the South African classification system, hard plinthite is accepted as a B horizon, e.g. in Dresden, Wasbank (Figure 2.6) and Glencoe soil forms.

FIGURE 2.6: Hard plinthite in the subsoil of a Glencoe soil. Without deep loosening, this soil will be marginal for grape production (Photo: B. Oberholzer, Nulandis).

26 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

2.5.2 Carbonate hardpan Carbonate hardpans form in warm semi-arid areas from parent material that contains high Ca concentrations. Calcium carbonate forms a massive cemented layer (Figure 2.7), which is impenetrable to roots and has slow water permeability. According to the South African Soil Classification system, soil forms that have hardpan carbonate horizons in the subsoil are Immerpan, Prieska, Plooysburg, Coega, Askham and Gamoep.

FIGURE 2.7: Carbonate hardpan in the subsoil (Coega soil form), massive and impenetrable to grapevine roots (Photo: ARC Infruitec-Nietvoorbij).

SOIL PREPARATION | 27

2.5.3 Dorbank Dorbank (duripan in other classification systems) is reddish-brown massive to a stratified hardpan, cemented by silica (Figure 2.8) and is present in the subsoil of Garies, Oudtshoorn, Trawal and Knersvlakte soil forms. It is normally associated with environments that are more arid than areas where carbonate hardpans are found. Due to the dry areas where dorbank is formed, it can be saline and even sodic. Similar to the other hardpans, dorbank impedes root penetration and has low water permeability. Normally, by breaking up dorbank during soil preparation, a soil with a high potential for grapevines is created.

FIGURE 2.8: Dorbank, which is a serious impediment to root penetration, needs to be broken up before establishing a vineyard (Photo: ARC Infruitec-Nietvoorbij).

2.6 Waterlogging Free water can occur for shorter or longer periods at different depths in a soil profile. Sitting water tables are normally found in E horizons overlying lithocutanic, pedocutanic, prismacutanic and gley horizons. Fluctuating water tables are associated with uniformly textured soil material on concave landscapes, straight lower slopes and on flat highlands. Diagnostic soft or hard plinthic B horizons can develop at the transition between the upper boundary of the fluctuating water table and the overlying aerated soil. Waterlogging indicated by strengthening of grey colours, increases with depth below the plinthic layer. Free water can also accumulate in a variety of soil materials that

28 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

have low porosity and permeability (Figure 2.9). Grey colours, an indication of wetness, are consequently found in saprolite, G horizons and non-diagnostic gleyed loam or clay below non-hydromorphic diagnostic B horizons. Disadvantages that are associated with waterlogging include the following: i) Waterlogging limits the size of the root system and, surprisingly, water stress can be experienced during drought periods. ii) Wet soils are seriously compacted by tractor and implement traffic. iii) Tractors and implements get stuck in waterlogged soils and it is consequent- ly not possible to spray vineyards or till the soil on time. iv) Root diseases flourish under wet conditions. v) Due to the slow N-mineralisation under wet conditions, more N-fertiliser must be applied to wet soils than on well-drained soils to get the same plant response. vi) Toxic concentrations of iron, manganese, sulphides, nitrates, ethylene and organic acids are formed. vii) In warm dry climates, soils containing a water table can become saline due to the capillary rise of water and salt to the soil surface.

FIGURE 2.9: A wet Katspruit soil that is unsuitable for sustainable grape production and cannot be drained due to slow permeability. Ridging can be considered to improve the soil (Photo: ARC Infruitec-Nietvoorbij).

SOIL PREPARATION | 29

2.7 Weathered rock in the subsoil Rock, in varying degrees of weathering, restricts root, water and air penetration more than the overlying material. The extent of weathering and the original structure of the rock will, however, determine the degree of its impeding effect. Litho-cutanic B horizons (Figure 2.10) on folded fillites are normally open, loose and porous in contrast to horizontally-layered shale that is often extremely dense. The shallower, less weathered and harder the rock, the more negative its effect on root growth and development.

FIGURE 2.10: Weathered shale as part of the litho-cutanic B horizon of a shallow Glenrosa soil that will require deep ripping if it is to be considered for planting of grapevines (Photo: ARC Infruitec-Nietvoorbij). A further inhibiting factor associated with weathered rock is the release of soluble salts, especially in clayey sedimentary and metamorphic rock. Hard, weathered rock that has been loosened during soil preparation has a high permeability and free water moves freely through such a profile from higher to lower positions in the landscape. In such cases cut-off drains will be essential to remove excess water and salts from the land. 2.8 Coarse fragments in the soil The presence of coarse fragments in soil is not an impediment to root development, but justifies a short discussion because stony soils are so common in the Western Cape of South Africa and are often used for the growing of table grapes and wine grapes.

30 | SOIL IMPEDIMENTS TO ROOT PENETRATION

CHAPTER 2

Coarse fragments in the soil include fine gravel (2-25 mm), gravel (25-75 mm), stones (75-250 mm) and rocks (> 250 mm).The ability of a soil to retain and supply water and nutrients to plant roots are determined by the fine fraction (< 2 mm). The higher the coarse fraction in a soil, the lower its capacity will be to retain plant-available water and nutrients. Van Huyssteen (1984) also mentioned that soil tillage will be hampered when the volume of stones become more than 70 %. Stony/gravelly soils (Figure 2.11) display many positive properties. Growers refer to such soils as cooler. Furthermore, they are porous, well-aerated and often pose no impediment to root growth. In fact, grapevines often perform better on such soils because of their good physical properties, on condition, of course, that adequate water and nutrients are available (Figure 2.12). In practice that would mean light, frequent irrigations and an adaptation of fertilisation programmes. Coarse fragments positively affect soil with high contents of medium and fine sand as well as silt. Gravel and stones decrease the compactibility of these soils and limit the formation of surface crusts.

FIGURE 2.11: Stony Hutton soil with high potential for vineyards, on condition that it is deep-tilled before planting (Photo: J.E. Hoffman, Stellenbosch University).

SOIL PREPARATION | 31