JOHAN VAN ZYL and EDUARD HOFFMAN from the Soil Science Department, Stellenbosch University, authored a newly published book titled Soil preparation for peak and sustainable grapevine performance.
This is the first of a two-part series where they highlight aspects related to optimal preparation of soil for peak and sustainable grapevine performance.
SA is the leading country in the world regarding research and knowledge on soil preparation. This stems from the fact that the majority of soils used for wine and table grape production in SA are notoriously shallow (i.e. they re-strict root penetration). This results in uneven and poor vineyard performance that eventually leads to unprofitable vineyards. Many investigations in SA have addressed the reasons for poor grape-vine root development and methods to rectify this detrimental factor. This large body of knowledge is not only spread over different generations of researchers and experts, but also fragmented among many articles and journals. Consequently, SATI and Winetech jointly funded the writing of the book Soil Preparation for Sustainable wine and table grape vineyards on all aspects of soil preparation. The contents of this book, which was launched on 14 August 2019, extends far beyond deep tillage. Wet soils have to be drained, then there’s ridging, terracing of steep slopes, plant holes and even reclamation of brack soils, which are all additional aspects of soil preparation that can only be done before planting a permanent crop. 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 is essential. 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 above-ground growth. Starting with recommendations for “dynamite-ploughing” in 1912, all research on soil profile modification was reviewed in the new book and a synopsis made regarding soil conditions, root studies, grape-vine response and corrective measures. Fortunately, various types of implements are available in SA, to achieve all kinds of loosening and mixing of the soil.
Soil restrictions to root penetration
Natural soil compaction is the main cause of root restriction in the majority of vineyard soils in the Western Cape, but compaction can also be manmade through vehicle traffic and implement use (Figure 1). When soil becomes more com-pact, its strength increases and, particularly the volume and continuity of large pores decrease. Soil compaction makes tillage more difficult and causes poorer root penetration. Soil strength can be measured by penetrometers of various kinds and sophistication (Figure 2). Grapevine root penetration is drastically impeded by penetrometer resistances above 2000-2500 kPa, measured in soil near field capacity. Soil compaction is the main impediment to vineyard roots, but soil acidity, water logging, salinity, soil stratification, hardpans, subsoil clays and rock are also limiting factors in many localities. In fact, there are very few soils in SA without root-impeding layers within 0.8 meter depth that can be planted to grapevines without deep soil preparation. But, because one article won’t do justice to the comprehensive task of exploring all these factors, let’s focus on soil compaction and its alleviation.
Fig. 1: Schematic illustration of the different types and positions of compaction generally found in vineyards (adapted from Van Huyssteen, 1989).
Fig. 2: Depth of soil preparation assessed by a simple constant-speed penetrometer that registers soil strength mechanically (left) and a hand-operated constant-speed penetrometer equipped with load cell, monitor and data logger to display and capture soil strength with depth (right).
Root response to soil conditions
Grapevine root distribution is the most reliable, direct and accurate indicator of soil conditions. Unfavourable conditions in the soil, e.g. compaction and low pH, will be indicated by roots that cannot adequately penetrate such soil horizons, while roots which are well distributed laterally and vertically are proof of favourable soil properties (Figure 3). The root systems of grapevines in SA are, almost without exception, shaped by soil conditions and cultivation practices such as tillage and planting density, instead of the genetic traits of the rootstock. Therefore, assessment of grapevine root distribution in existing vineyards can give an excellent indication of which type of soil preparation would be necessary when vineyards have to be replanted. Similarly, root distribution will clearly show how effective soil preparation was before planting (Figure 3).
Fig.3: Good root distribution in a vineyard after effective deep soil preparation (left) and shallow grapevine rooting due to natural compaction in the subsoil of aTukulu soil that was poorly prepared before planting (right).
Ridging (also called mounding) is a soil preparation method for soils that cannot be prepared effectively through deep tillage, or for waterlogged soils that cannot be drained by subsurface drainage. Ridging entails the hilling up of the topsoil to form a continuous elevated ridge on which a crop can be planted. This practice creates a larger root volume for exploitation over impenetrable subsoil, or above water tables. In fact, ridging is a form of surface drainage and consequently ditches between the ridges must have a slope that allows outflow of water from a vineyard or an orchard. Construction can be done by using an excavator, an offset disc-harrow and even a grader, on condition that the wheels of the construction vehicle do not cause compaction under, or on the ridges. Ripping of the subsoil before ridging is beneficial and makes ridge construction easier. In field trials with grapevines, raised beds (ridges) improved internal drainage and soil aeration, but temperatures in the upper parts of ridges were higher than on un-ridged soil and cause increased water loss. Irrigation is therefore recommended when grapevines are planted on ridges. The dimensions of ridges are important. Soil temperatures and soil water depletion that are higher than normal result from single-row ridges with a soil surface to volume ratio of less than one, which, in turn, causes yield losses. Single-row ridges should be at least 500 mm wide at the crest with a base of at least 1 m, and lower than 400 mm. Double-row ridges that are 300-500 mm high with a flat or slightly concave crest, are recommended. Ditches must be 1.5 m wide to prevent vehicles from driving on the slanting sides of the ridge. In the Hex River Valley, table grapes are often planted on low ridges (ca. 20 cm high), popularly referred to as landscaping (Figure 4). Such landscaping will have no benefit on stony, sandy and other deep well-drained soils, but in a case study on a Westleigh soil, vines on ridges were visibly much more uniform and vigorous than grapevines on flat ground. Even though higher ridges would have been recommended on this wet Westleigh soil, raising the soil surface by 20 cm only had already induced a significant advantage regarding drainage, aeration, temperature and consequently in grape-vine growth. Vineyard practices such as pruning and harvesting are more difficult to perform on ridges, therefore ridging should be reserved for conditions where the soil dictates it. Mulching and herbicide weed control are recommended on ridges. Here, drip or micro-sprinkler irrigation should be used, instead of conventional sprinkler systems, which will cause too much run-off and erosion of the ridges.
Fig. 4: Well-constructed ridges (50 cm high) that were necessitated by dense clay in the subsoil of a table grape vineyard in the Hex River valley (left). Young table grapes planted in a single row on a low ridge, 20 cm high. The soil is a shallow Westleigh that is wet, has a low infiltration rate (in this case) and is hard-setting when dry (right).
Choice of implement for soil preparation
The correct choice of implement for soil preparation is determined by the soil type: soil that was effectively prepared; is loose to a depth of 800-1000 mm; has poor subsoil that was not brought to the surface, and absent unloosened banks between adjacent furrows; and the loose soil has a good structure (i.e. no large clods occur that cannot be exploited by roots). Rippers are useful to break up hard layers without mixing the soil and so is the wing plough, an adaptation to the ripper (Figure 5A). Although the wing plough loosens a larger soil volume than the ripper, neither of these ploughs can mix ameliorants with the soil. Delve ploughs are available in various adaptations, each designed for a specific goal. The standard delve plough is used for mixing of Dundee and Oakleaf soils, but has the disadvantage of bringing poor subsoil to the surface (5B). By changing the size and shape of the mould board, a shift-delve plough can be made (5C). This plough allows topsoil to flow over the mould board and mix with the subsoil behind the board. Soil layers will mainly be displaced sideways.
The finger delve plough loosens the soil thoroughly with little upward displacement of the subsoil (5D). Soil preparation using excavators, the so-called “handjiedol”, is also employed in SA. Soils having thick “dorbank” (hard-pan) layers as well as soils containing lime “heuweltjies” are occasionally prepared through blade delving. This entails mov-ing the topsoil aside and breaking up the subsoil with a ripper, followed by moving and mixing the soil using the tractor blade.