What time of the year, with which implement and to what depth should soil preparation take place
|Has soil preparation not become a routine procedure like so many others in the vineyard, inter alia fertiliser applications Do we still pause to query the type of soil and its characteristics
Most important of all is to understand the reasons for soil preparation. The emphasis should be on alleviating soil compaction, i.e. to increase porous space and reduce bulk density. Texture is without any doubt the most important soil characteristic, seeing that many other soil characteristics derive from it. Texture provides the building blocks for structure and any action that destroys structure should be banished.
Once we have decided on soil preparation, be it for chemical or physical reasons, what are the procedures to be followed The following reasons and procedures for soil preparation should persuade you to use common sense when thinking and making decisions about your soil.
Reasons for soil preparation
Western Cape soils are inherently dense/compact, or acidic, especially in the subsoil. The reasons being that the age of the parent material is much older than for example in countries such as Australia and America. Malmesbury shale is between 55-1 000 million years (Ma) old, Cape granite between 500-600 Ma and Table Mountain sandstone between 300-400 Ma old. By contrast the Bordeaux reddish and yellowish brown foothill soils of France are probably 50 Ma and alluvial gravels less than 2Ma old (D.Saayman, 2006 personal communication).
There is also a high degree of weathering of the parent material, accompanied by the loss of basic cations that have been replaced by nitrogen and that release toxic aluminium. Loam minerals, mainly kaolin, illite and sesquioxides, with low exchange capacity of 3-5 cmol(+)/kg-1, are formed, in contrast with the young and alluvial soils’ exchange capacity of 10-25 cmol(+)/kg-1.
In general the limited depth and poor physical structure of most grapevine soils require intensive soil preparation. Restrictive layers are broken up to depths of 800 – 1000 mm to make large soil volumes available for root growth and give the soil an improved buffer ability against the influence of unfavourable climatic and nutritional conditions.
In his extensive soil preparation studies Van Huyssteen (1988) regularly emphasised the advantages of preparing the soil to the required depth. By so doing the soil is able to integrate climatic effects such as water retention, drainage and soil temperature. Effective soil preparation with the appropriate implements is able to create a homogeneous root medium, which promotes uniform root distribution throughout soil depth. As a result manipulations above the ground, such as pruning, trellising, plant width and crop load, can be flexible. The general recommendation is to loosen grapevine soil with subsoil problems to a depth of at least 800mm so that the soil appears homogeneous.
Recent international articles emphasize the fact that grapevine root functioning through poor soil physical qualities seriously hampers the nutritional uptake of the grapevine (Cass, 2006). In general significant uptake of water takes place between field capacity and wilting point, but certain unfavourable factors such as high soil strength, high water content and poor aeration may reduce the spectrum of water availability.
Soil penetration resistance with values of 2 Mpa is generally accepted to be the absolute limit for grapevine root penetration. Phosphorus and potassium uptakes in particular are sensitive to the degradation of soil physical characteristics due to their dependency on short distance diffusion and the necessity for relatively high soil moisture content.
Where massive compaction in the subsoil occurs in soil types such as Glenrosa, Oakleaf, Hutton and Clovelly, these soils will react well to soil preparation with regard to root growth, vigour and production. A less obvious, but very real problem regarding these soils is excessive acidity, especially in the subsoil. With finger mix delve plough (Fig. 1) as used in the accompanying figures 1&2, the ideal mixture of soil above and below the surface is obtained.
Procedures for soil preparation:
To implement once-off, correct preparation for the perennial grapevine, these procedures should be followed:
1. Soil analyses:
Chemical adjustments, for example the application of the right amount of lime to adjust the soil’s pH, as well as the phosphorus status, go hand in hand with soil preparation. The accompanying fig.4 clearly shows the importance of adjusting soil pH in order to enable the root development of rootstocks to function optimally. In most instances soil that had been limed to a pH of 6 significantly increased the root mass of rootstocks.
2. Soil profile inspection:
It is necessary to make profile pits for soil classification so that one can judge which cultivation method is required. Nowadays there is excellent collaboration between earthmovers and soil scientists, the latter often being requested, with the approval of the producer, to first view or classify the soil. Neat profile pits (Fig.5) up to a depth of 1500mm and deeper offer very good decision making ability.
3. Implement selection:
The next step is to decide whether preparation should take place with a bulldozer/caterpillar (Fig. 6). Tractors are no longer suitable for deep soil preparation, seeing that their soil surface contact is insufficient, there is not sufficient power available on the soil and if used, slipping wheels could add to soil compaction. Caterpillars are the only source of power that can be used successfully. Caterpillars displace more power to the soil with less compaction due to the larger soil surface contact area of the caterpillar tyres.
4. Type of mix:
Currently the use of digger loaders, known as “spit” (delve), is definitely an option to consider. It is very important to use large machines and the technique should be correct. If layers occur due to, inter alia, drastic differences in texture (duplex soils), a digger loader should be used with caution, since it mixes layers, and one would prefer finger mix delve plough, which keeps layers in position.
It is necessary to determine the furrow width (width between the plough-shares) of a ripper well in advance, mainly due to cost implications (Table 1) and the necessity thereof. If you want to avoid large lenses of undisturbed soil between the furrow widths, this will have to be a careful decision. With a cross action the lenses can usually be reduced. If cultivation takes place to a depth of 900mm, the furrow should be , i.e. 600mm.
5. Period of soil preparation
The period of soil preparation is of the utmost importance. In excessively dry soil large clods will break, especially on the surface, which creates an undesirable situation when the soil is trenched. For example, a ball of soil (Fig 7) should be able to be rolled and then fall apart with the slightest of pressure for the soil climate to be suitable for cultivation. Climate and lime requirement will then determine whether the soil should be cross-delved or alternatively require a single action. Wetting of the soil for 4 hours with sprinkler irrigation, applying 25 mm water, and allowing for 48 – 72 hours’ drying out, will limit the formation of large clods.
6. Cost implications:
The motivation for soil preparation is and remains a long term input, initially rather expensive, but the long term objective is that the grapevine will benefit from this action for approximately 20 years. Table 1 gives us an indication of the possible difference in costs of various options.
The following should be kept in mind at all times:
- Excessive tillage can result in destruction of the structure and the soil, when dry, will have been tilled to a powdery matter.
- Excessively wide furrow widths result in poor mixing with large untouched soil banks/lenses.
- Cultivation row should be selected correctly with a view to subsoil drainage.
- The soil climate should be correct; too dry will form large clods; too wet will smear the subsoil.
- Regular evaluation of soil preparation is essential. That means making regular profiles (keep trencher handy) so that progress and quality may be judged by the experts.
Cass, A., 2006. Poor soil physical properties can hinder nutrient uptake in vines. The Australian & New Zealand Grapegrower & Winemaker pp 19-22.
Conradie, W.J., 1988. Effect of soil acidity on grapevine root growth and the role of roots as a source of nutrient reserves. Technical Communication no. 215. Viticultural and Oenological Research Institute, Stellenbosch. Republic of South Afirka. Department of Agriculture and Water Supply.
Van Huyssteen, L., 1988. Soil preparation and grapevine root distribution – A qualitative and quantitative assessment. Technical Communication no. 215. Viticultural and Oenological Research Institute, Stellenbosch. Republic of South Africa. Department of Agriculture and Water Supply.