FERTILISER GUIDELINES FOR THE WINE INDUSTRY

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Villa Academy educates on crop protection best practices

built careers both in academia and in the field, a combination they leverage in diverse classrooms. Part of the Villa Academy’s uniqueness, is that students at different levels of experience are often grouped in the same classroom. At first that would seem like a challenge for anybody working in an academic environment. But when you have a mixture of people with 20 or 30 years of experience in the field with no tertiary education and then a graduate fresh out of university with zero experience, and they sit in the same classroom, they all come out better people, wanting to know more and realising that they actually need to learn more. Classes are of one to three days duration and cover diverse topics, including: basic toxicology, crop protection reference material, product labels, disease management, plant growth regulators, weed management, biotechnology, rodent control, insect management, biologicals, handling of complaints, adjuvants, application technology, viticulture, wheat and barley cultivation, fly control, etc.

Crop protection, like any other industry, needs smart people. Companies tuned in to their workforce understand people want to succeed and be good at what they do. Education is just as valued as practical experience as a way to achieve the professional success and status which workers strive to achieve. The Villa Academy identified a need to bring young people into the industry, and obviously bringing in a new generation of salespeople necessitates someone to train them. Previously there has been a lot of discussion in the industry about how it needs to be done, but not much action – it got to a point where Villa Crop Protection decided it should take the lead. Founded in 2011, the Villa Academy occupies two campuses in Cape Town and Johannesburg. A mix of entrylevel and seniorlevel professionals benefit not only from attending classes but also from taking the opportunity to network. A blend of academic and handson experience is a theme underlying the Villa Academy’s philosophy. Approximately 30 lecturers have

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 environment we operate in. The position of agrochemical sales professionals is such that if they make a mistake, the consequences can wreak havoc not only on customer relations, but also on local economies. Furthermore, because their customers often have agriculture degrees themselves, agents need the appropriate knowledge to be able to deal with customers and to be adequately equipped to talk with them on the same level. The crop chemical industry in South Africa is small. If everyone could understand and appreciate the bigpicture benefits, we could all be winners. In order to also ensure that students, growers and technical advisors in local agriculture do have access to handbooks and relevant literature the Villa Academy embarked on a program to sponsor the compilation and publishing of handbooks earmarked for local agriculture.

The curriculum is characterised by strong emphasis on the problembased method of learning in which students collaborate in groups to solve realworld examples of problems. The lecturer acts as a facilitator to encourage self direction and active engagement among students to a greater extent than the passive, teachercentric lecture model allows. Classes are intensive and students are held to high standards. In just the threehour “Introduction to the Crop Protection Industry” class, students are expected to be able to demonstrate their understanding of key concepts including history of the industry, past and future industry trends, market segmentations, the role of Chinese and Indian suppliers of generic products, classification of suppliers and products, product lifecycle and postpatent technology, seeds and GM technology. Assessment criteria in addition to inclass work includes written tests and a final project in which students create a production plan for a field crop produced in their area by researching its significance, evaluating the suitability of climate and soil for production and applying key principles of plant production and protection.

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Contents

1 2 3

Introduction

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Soil and leaf samples. Pieter Raath

Interpretation of soil analyses. Pieter Raath

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• Different methods of analysis

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Chemical correction during soil preparation.

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Braham Oberholzer • Lime and gypsum

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Maintenance fertilisation of macro- and micro-elements.

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Dawid Saayman • Role • Functions • Determining a fertilisation programme • Timing of fertilisation

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Types of fertiliser and application practices. Bennie Diedericks

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• Inorganic – granular/water soluble/liquid • Organic – manure/compost/cellar waste • Cover crops

• Foliar nutrition • Biostimulants

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Relationship between fertilisation/nutrient status of a vineyard and grape composition/wine quality/ wine sensory characteristics. Francois Viljoen

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Fertilisation of rootstock mother blocks and nursery soils. Dawid Saayman

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5

Introduction

A fertilisation guideline manual for vineyards was published in 1994 (Conradie, 1994). This manual provided a solid foundation for the industry, but inevitably after 20 years new information about fertilisation of vineyards has become available locally and internationally. The need therefore arose to revise the vineyard fertilisation manual of 1994 in order to incorporate new research results that have been released or views that have changed. Diverse recommendations within the wine industry, especially between various advising companies, which are based on diffe rent methods of analysis, norms and principles have emerged, prompting Winetech to justify this revision. This revision aims to bring about credible decision-making tools for producers and advisors that are in the best interest of the producer and are especially based on the findings of fertilisation trials. Many questions exist regarding the fertilisation of wine grapes and in particular the specific nutrient element requirements, which the 1994 vineyard fertilisation manual currently does not provide answers for. It is important that all advisors in the SA wine industry shall make fertilisation recommendations to producers based on the same scientific assumptions/facts. It is also important that everyone in the industry is brought up to date with the latest research information (locally and abroad), as well as practical knowledge and experience. This document can also serve as training material (module for vineyard nutrition) for all students in soil science and viticulture.

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2 Soil and leaf samples

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COLLECTION OF SOIL SAMPLES

The purpose of soil sampling and soil analysis is: • to classify soils in order to suggest practices for fertiliser and lime applica tion; • to predict the probability of a profitable reaction to application of fertiliser; • to evaluate soil fertility; • to determine which specific soil conditions could be improved by applying soil amendments; and • to detect and correct imbalances in nutrient concentrations. The most important advantage of soil analysis is probably the ability to moni tor changes in soil fertility in order to apply corrective action before nutritional stress occurs. Soil analysis is essential before soil preparation to ensure that physical and chemical defects can be corrected during preparation. Regular analysis is required in existing vineyards to ensure that optimal conditions for growth are maintained. SAMPLING FOR SOIL PREPARATION: Profile pits are necessary to evaluate the physical/morphological characteristics and to determine the borders of management units. For smaller commercial areas, sampling should be done on at least a 50 m X 50 m grid. If the initial soil examination reveals that large soil differences occur between points, more pits should be dug to determine where the soil transitions are. Before com mencement of sampling, the site should be divided into its various cultivation units. These units are areas which will require similar management based on soil form, depth and the incidence of coarse fragments. Separate samples are collected from each of these cultivation/management units and samples from the different profile sampling points within the same unit may be mixed in order to obtain representative samples for that management unit. The depth/ thickness of the different horizons/layers, as well as an estimation of the coarse fraction must also be noted. With soil preparation for vineyards it is desirable to collect samples from the topsoil and subsoil separately. It is also often desirable to sample underlying clay material separately, since salts can move upwards to the overlying soil layers during dry periods. The depth of sampling must be indicated. A sample of 1 kg is sufficient.

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EXISTING VINEYARDS: After preparation, samples are collected per management unit. To ensure that the samples are representative of the unit, it is once again constituted from subsamples which are taken from different places in the unit. If the soil has been ridged, the samples should be taken from the ridge only. The first sam pling ought to be done shortly after preparation to determine whether optimal conditions were created during preparation. For maintenance fertilisation, soil samples must be taken at least every three years, except in cases of very sandy or stony soils where leaching can reach serious proportions. Samples should then be collected every two years. These samples are taken in the vine row. Because soil composition can vary drasti cally over short distances, long term trends can be best determined if samples are collected more or less at the same places. Vines should be marked for this purpose to ensure that samples are collected more or less at the same place. Weak patches in the vineyard should be sampled separately. Because the cultivation action often disturbs the transition between soil lay ers, samples should be collected from fixed depths. Samples should always be taken at the same depths in order to establish a history of results, which can be correlated with fertilisation application. If the person who does the rec ommendations does not have records of previous analyses, more than one layer must be sampled. In order to determine the effect of especially lime and phosphate fertiliser which have not yet reacted completely, it is preferable to collect samples at depths of 0-150 mm and 150-450/500 mm. Considering that few roots are found deeper, deeper samples are not necessary. If the soil is shallower than 450-500 mm, sampling should be limited to above the restricting layer. The presence of underlying clay in the sample can affect results to such an extent that they are meaningless. A sample of 1 kg is sufficient. STONEY SOIL: If the soil is very stony and the stones are so large that it cannot be sampled, an estimate of the incidence of stones must be made, i.e. indicate the volume occupied by stones. This information can have an appreciable effect on the fertiliser recommendation and must accompany the soil samples to the labo ratory. A stone correction is used in the estimation of gypsum, lime, potassium and phosphate fertiliser requirements.

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HANDLING OF THE SAMPLES: Place the composite sample in a clean plastic bag. Label the bag with name and the sample identity. If several composite samples were taken, label each one differently and keep a record of the areas where each sample was taken. Only one form needs to be filled out for each group of samples. The more complete the information provided, the better the ultimate laboratory recom mendation will be.

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LEAF SAMPLING

The aim of leaf analysis is to relate the mineral content of the plant to its physical appearance, growth rate and yield or quality of the harvest. The in terpretation of results depends on the assumption that a significant biological relationship exists between the elemental content of the vine and its growth and/or production. This technique requires sampling at the correct phenological stage (time of sampling) according to a specific protocol. Leaf analysis can serve as a diagnostic tool for wine grapes, but has the shortcoming that in practice it is often influenced by factors such as scion and rootstock cultivar, cultivation practices, cultivation area, seasonal climate, diseases and soil type. A general norm that makes provision for all conditions, therefore, is inevitably very wide-ranging. Leaf analysis can consequently not be used as the only norm for establishing a fertilisation programme, but should be seen as supplementary to soil analyses. Leaf analysis can be useful, however, in case studies. In such cases, leaves are sampled in a specific way from the “sick” vines, in addition to a similar sample from adjacent, nonaffected vines. The leaves from the healthy/better vines then serve as direct control and the time of sampling and all other vari ables are less critical or not applicable. No norm is used, but rather the relative differences between the two samples. The purpose of leaf analysis can therefore be summarised as follows: • To serve as an aid to evaluate the supplying capacity of the soil for nutri tional elements; • To evaluate the effect of treatments on the nutritional status of the vine; • To evaluate the relationship between the soil nutrient status and the plant reaction as an aid to estimate fertiliser requirement; • To diagnose suspected deficiencies of nutritional elements.

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TIME OF SAMPLING: Leaf samples should be collected annually at the same physiological growth stage. In this regard flowering, fruit set or veraison can be used. Leaf compo sition changes very rapidly during flowering, to such an extent that it is some times difficult to interpret figures obtained at this stage. On the other hand, the older leaves by the time of veraison are often already very dilapidated which makes it difficult to collect a representative sample, but for certain elements like potassium (K) for example, veraison remains a good time for sampling. The most practical compromise that is reasonably satisfying for most elements, is to collect samples during the fruit set period. Fruit set is understood to be the period stretching from the end of flowering until the pea berry stage (berries with a diameter of approximately 5 mm). This period lasts at least 3 weeks and for most cultivars in the Western Cape, the last week in November will usually fall within this time frame. LEAF BLADE OR PETIOLE? For vineyards, which have differing nutritional status, the petiole normally indicates greater differences than the leaf blade. On the other hand, the com position of the petiole also differs to a greater extent within the same vineyard than is the case for the leaf blade. Furthermore, the boron status is better reflected by the leaf blade than the petiole. As a point of departure it is thus recommended that the petiole is sampled for analysis at fruit set. In certain cases, however, it will also be necessary to analyse the leaf blade. Norms for the elemental contents of leaf blades and petioles are indicated in Chapter 5.

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FIGURE 1: Leaf sampling.

SAMPLING PROTOCOL: The way in which vines are sampled for analysis has a major effect on the re sults obtained. As shown in Figure 1, leaves opposite the bunch, or if removed, the leaf on a bearing shoot between nodes 3 and 5 are sampled. Thirty leaf blades or petioles, immediately separated at sampling, are sufficient. Either one can be analysed although, as already discussed, petiole analyses are generally regarded as more accurate. Sampling time should be at fruit set or veraison. The phenological stage (e.g. set or veraison) of sampling must be indicated. Leaves should not be sampled during the hottest part of the day since it can influence leaf composition. It is preferable to collect samples in the morning. Samples are placed in a clean plastic or paper bag and should be kept cool until delivered to the laboratory. The samples must not be frozen under any circumstances.

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3 Interpretation of soil analysis reports for vineyards

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INTRODUCTION Soil sampling for analysis is regularly done by viticul turists and wine grape producers. Interpretation of the chemical results is often complicated by the variety of extraction methods and ways in which results are expressed. In this chapter the acknowledged analytical methods and the expression of the results thereof are discussed, as well as how to interpret them. Other typical analysis methods and ways in which analytical methods are expressed are also discussed and com pared with the accepted South African norms. Soil analysis reports in South Africa typically contain the following information and analytical results: TEXTURE Soil texture determines the water holding capacity of a soil and the extent to which cations are bound to the soil (negatively charged clay particles). The rate at which nutrients are leached from the root zone is there fore largely dictated by the soil texture. Furthermore, potassium (K) and phosphor (P) norms in particular are influenced by soil texture, making it essential to distinguish between sandy, loamy and clayey soils. Not all laboratories report soil texture by default – in which case the texture is established by the “finger method” and the soil classified as sandy, loamy or clayey. Most laboratories will only report texture when specifically requested, upon which a full textural anal ysis is carried out and the exact percentages of sand, silt and clay then specified.

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SOIL pH The pH of soil is determined in either potassium chloride (KCl) or water (H 2 O). Most laboratories in the Western Cape use the KCl method, while water pH is mostly reported in European and American laboratories. Although the difference is not always constant, soil pH KCl is roughly one pH unit lower than pH water . The reason for this is that the K + ions in the solution displace the H + on the clay lattices (the exchangeable H + ), which is then measured along with the active H + -ions in the soil solution. A solution with a pH KCl below 5.5 (pH water < 6.5) is regarded as suboptimal for grapevines. The lower the pH, the more acidic the soil, e.g. there is a higher concentration of active hydrogen ions (H + ). The more acidic the soil, the higher the solubility of aluminium (Al 3+ ); until it reaches a toxic concentration that negatively affects root growth. Grapevines underperform in acidic soils due to poor root functioning, leading to reduced water and nutrient uptake as well as possible pathogen and nematode infection. Lime should therefore be applied to make a correction. Various methods for determining the lime requirement have been developed. The well-known Eksteen method has proven to be reliable for South African soil conditions and vineyards (Eksteen, 1969). Calculation of the lime requirement is fully discussed in Chapter 4. The optimal soil pH (pH KCl ) for grapevines varies between 5.5 and 6.5. Below this range root growth is increasingly impeded and both phosphorus (P) and molybdenum (Mo) becomes gradually less available for uptake. Acid soils also often are highly leached and depleted of nutrients, e.g. nitrogen (N), potassium (K), calcium (Ca) and magnesium (Mg). Above this pH range, both P and the other micro-nutrients (except Mo) become less available for plant uptake, as indicated in Figure 1. This is due to immobilisation when P reacts with Ca and micronutrients with hydroxides and carbonates. To prevent nutrient deficien cies on soils with high pHs, regular P fertilisation and annual foliar applications of micro-nutrients is required.

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NITROGEN PHOSPHORUS POTASSIUM SULPHUR CALCIUM MAGNESIUM

IRON MANGANESE BORON COPPER AND ZINC

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MOLYBDENUM

3.0 3.5 4.0 4.5 5.0 5.5 6.0

6.5 7.0 7.5 8.0 8.5 9.0

Optimal pH KCL

FIGURE 2. Impact of soil pH on availability of nutrients.

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PLANT-AVAILABLE NUTRIENTS In South Africa plant-available nutrients are normally extracted by means of two extractants – mostly ammonium acetate (NH 4 Ac), but some laboratories also use Mehlich III. Although laboratories will often refer to “exchangeable” nutrients, their figures normally indicate “plantavailable” or “extractable” nutrients, which include “soluble” as well as “exchangeable” nutrients. In practical terms this means that the nutrients that could be leached out with water (soluble), is determined together with those that are retained on the clay lattice (exchangeable). As indicated below, it is especially important to keep the above-mentioned in mind for saline soils. RESISTANCE A saturated paste extract of the soil is prepared using distilled water, and its resistance to allow the flow of an electrical current through it is measured. The unit in which resistance is expressed is “ohm”, and it is reciprocal to electrical conductivity (mS/m). Salts, e.g. potassium, sodium and chloride, conduct electricity and therefore reduce the resistance of the soil solution. The lower the resistance measurement the larger the quantities of salts in the soil, i.e. the more saline the soil. A resistance below 300 ohm indicates that an excessive amount of salts are present in the soil – to the level that vine performance is negatively affected. If the resistance is 200 ohm and less, the soil is classified as saline. The lower the resistance, the larger the negative impact on the vine will be. Different salt fractions are encountered in soils. Both the exchangeable sodium percentage (ESP), i.e. the percentage constituted by Na, as a fraction of the total amount of exchangeable cations (Svalue), and the specific resistance serve as criteria for classifying the type of soil salinity.

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If the conductivity of a saturated soil extract exceeds 400 mS m -1 and the ESP is less than 15%, the soil is classified as saline. In the case of soil with an ESP > 15 %, containing free gypsum or lime, it is classified as a “salinesodic soil”. In both these cases the salts may simply be washed out using good quality irrigation water – providing that free gypsum is present, otherwise the soil colloids will disperse making the soil impermeable for water. In cases where gypsum is not present in the soil, the leaching water should be saturated with gypsum before it comes into contact with the soil. PHOSPHORUS (P) In soil analysis reports phosphorus (P) is usually indicated in mg/kg. The opti mal plant-available concentration depends on the soil texture and soil pH. It is therefore important that laboratories also report the soil texture. Depending on the extraction method being used, the norms for optimal P-concentration also differ because the extractants differ in pH and aggressiveness by which the P is extracted. A comparative list of norms are supplied in Table 1, indicating the applicable values for the most commonly used extractants. Due to the fact that P extracted with Bray I, Bray II and Mehlich III reduces as the soil pH increases, distinction needs to be made between the norms used for soils of different pH. Furthermore, the P that is required to raise the concentration to the minimum level might not necessarily be reflected when the soil is extracted at high pH. For soil with a pH regime that is regarded as optimal for grapevine production (e.g. pH KCl 5.5 to 6.0), Bray II and Mehlich III extraction provides similar values and reflects most accurately the available concentration of P in the soil. Bray I extractions is consistently lower and for vineyard soils the accuracy thereof with regard to P availability has not yet been confirmed.

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TABLE 1. Minimum soil phosphorus concentrations required for grapevines grown in different soil pH regimes, as applicable for different extractants # .

Extractant Olsen Citric acid Mehlich III Bray I Bray II mg/kg

Soil pH KCl

Soil texture class

Sandy Loamy Clayey Sandy Loamy Clayey Sandy Loamy Clayey Sandy Loamy Clayey

– – – – – –

25 30 35 25 30 35 25 30 35 25 30 35

25 30 35 20 25 30 20 25 30 18 21 25

20 25 30 15 20 25 10 12 15 10 12 15

20 25 30 20 25 30 20 25 30 15 18 21

<5.0

5.0-6.0

3

10 12 15 10 12 15

6.0-7.0

>7.0

Sandy (0-6% clay) / Loamy (6-15% clay) / Clayey (>15% clay) # Data on the relative extractability of P with different extractants were supplied by C.P. Beyers, Nitrophoska. Bray I: 0.025M HCl + 0.3M NH4F Bray II: 0.1M HCl + 0.3M NH4F Mehlich III: 0.2M acetic acid (CH 3 COOH) + 0.25M NH 4 NO 3 + 0.015M NH 4 F + 0.13M HNO 3 + 0.001M EDTA Olsen: 0.5M NaHCO 3 Citric acid: 0.05M citric acid (C 6 H 8 O 7 ) Mostly, a simplified approach is followed by laboratories serving the wine industry, namely when the soil pH KCl < 7.0, either a citric acid, Bray I, Bray II or Mehlich III extraction is conducted and similar norms are used (Table 1). For soil with a pH KCl ≥ 7.0, an Olsen extraction is often done, and the norms in Table 1 are used. The logic is that an Olsen extraction is less aggressive and has a higher pH, theoretically reflecting the lower rate of P release in the root zone at higher soil pH conditions. Olsen, however, has been shown to extract only 5-7% of citric acid extractable P. In reality, this means that the total P in the soil can become excessively high (e.g. if extracted with Bray I or Bray II), while the Olsen P remains below the norm.

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Although soils with high pH were traditionally extracted using the Olsen ex tractant, the pH of many soils have currently decreased to pH KCl < 7.0, which means that P from the labile pool increases in availability. If a Bray I or Bray II extraction is now done, excessively high P-concentrations are often obtained. It is therefore suggested that Bray I extractions are done on soils with pH KCl > 7.0, and the Bray I norms in Table 1 are used to calculate P-requirements. If analysis is done using one of the other extractants, the provided norms can be used with reasonable reliability – but the use of Olsen extractions should be avoided. Depending on the clay content of the soil, the P-content should be augmented to the specific norm. For soil preparation the average P-content is determined to 600 mm soil depth. To increase the P-content by 1 mg/kg in the soil for 300 mm depth, 4.5 kg P should be applied, therefore 9 kg P per ha for 600 mm depth. On high pH soils (pH KCl > 7) it may be an option to adjust the recom mended figure downwards and increase the annual maintenance fertilisation volumes. For production vineyards the P-content is calculated to a soil depth of 300 mm only, e.g. 4,5 kg P per ha must be applied for every 1 mg/kg with which the concentration needs to be raised. In the case of high pH soils, where P is easily retained, the annual fertilisation requirement must be calculated and split over three instalments throughout the season. During the harvest 0,7 kg P is removed for each ton of grapes pro duced and maintenance fertilisation should be calculated accordingly, except where soil analyses indicate that the P-content is optimal or above the norm. It is important to avoid excessive applications of P, since this may limit potassi um uptake. Phosphate contents of more than 50 mg/kg in sandy soils, 60 mg/ kg in loamy soils and 70 mg/kg in clayey soils, can be problematic in any pH regime. The stone and gravel volume must therefore always be used in the calculation of the P requirement, to prevent over-fertilisation with P.

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POTASSIUM (K)

As far as grapevine potassium (K) nutrition is concerned, soil texture also plays an important role in the interpretation of soil analyses. Firstly, K is leached very quickly out of sandy soil, and secondly, clay minerals can play an important role in K-binding. It is not recommended that K is applied on sandy soils during soil preparation – it could easily leach out on such soils. A broad norm that may be set for K-nutrition on sandy soils, is that annual maintenance fertilisation of 3 kg K per ton of production is applied.

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As already stated, in South Africa the plant-available K is mostly extracted using ammonium acetate (NH 4 Ac), however some laboratories also use Mehlich III. Similar results are obtained between the two extractants for soil with pH KCl < 6.0, where Mehlich III values are approximately 0.9 x NH 4 Ac (Nathan et al ., 2005). For soils with a higher pH (e.g. pH KCl > 6.0), Mehlich III extracts less K than NH 4 Ac (Sawyer, undated). On heavier soils (loamy & clayey soils), the general norms indicated in Table 2 may be used as guidelines for maximum K-values (Conradie 1994). These norms are linked to the differences in clay mineralogical types occurring in the various regions, and are more or less representative of K-contents which constitute 4% of the total interchangeable cations. TABLE 2. Maximum and excessive norms for potassium concentration in soil, as determined using ammonium acetate, to ensure optimal grapevine performance without affecting wine quality (Conradie, 1994).

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Breede River

Olifants River

Region

Coastal

Karoo Orange River

Maximum norm

70

80

100

100

120

mg/kg

Excessive concentration

105

120

150

150

180

Adjustment of K-concentration during soil preparation is only required in excep tional circumstances. Where deficiencies do occur or are expected in heavy soils, the average K-requirement is determined to a soil depth of 600 mm. For soil with a K-concentration below the norms mentioned above, K-fertilisation should be applied. In the case of production vineyards the K-content is only determined to a soil depth of 300 mm. The requirement per hectare is 4,5 kg of K to increase the K-content in the soil by 1 mg/kg over 300 mm in depth. During soil preparation (to a soil depth of 600 mm), 9 kg K per ha should therefore be applied for each 1 mg/kg increase required in the soil. Since excessive K-contents in the soil may cause problems with colour and pH in wine, over-fertilisation should be avoided.

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CALCIUM (Ca) AND MAGNESIUM (Mg) Both calcium (Ca) and magnesium (Mg) are essential nutrients, required for optimal grapevine performance. Generally they are so abundant in soils that have a pH within the optimal range that fertilisation with these nutrients is not required. Through proper lime applications, to raise the soil pH and R-value to 10, sufficient Ca and Mg are applied to the soil to satisfy the nutritional re quirements of the grapevine (Eksteen, 1969). Nevertheless, some sandy soils with an excessive volume of stones, might be deficient in Ca and Mg even though the soil pH is optimal. Likewise, when the wrong lime (e.g. calcitic lime) is used in Mgdeficient soil, applications of Mg might be required. Plant-available Ca and Mg can also be extracted using NH 4 Ac or Mehlich III. Similar results are obtained between the two extractants for soil with pH KCl < 6.0 (Nathan, 2005). The following general norms may be used as guidelines for minimum Ca and Mg concentrations. The Ca:Mg ratio must preferably also not exceed a value of 6.

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TABLE 3. Minimum norms for calcium and magnesium concentration in soil, as deter mined using ammonium acetate, to ensure optimal grapevine performance.

Sandy soil

Clayey soil

Nutrient

mg/kg

cmol(+)/kg

mg/kg

cmol(+)/kg

Calcium

360

1.80 0.30

500 120

2.50 1.00

Magnesium

40

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MICRO-ELEMENTS (B, Mn, Zn, Cu)

Soil analysis reports for vineyards usually indicate zinc (Zn), manganese (Mn), boron (B) and copper (Cu) contents in mg/kg. Zinc, Mn and Cu are extracted with either EDTA, HCl or DTPA, while B is extracted with hot water. As soil pH increases the extraction efficiency of HCl reduces dramatically, making it un suitable for use on soils with a pH KCl > 5.0, and even then it does not extract Mn adequately (Table 4). Extraction levels with DTPA appear to be similar to EDTA.

TABLE 4. Comparison of EDTA and HCl as extractants for micro-nutrients in soils of different pH values (Lambrechts, unpublished).

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Cu (mg/kg)

Cu (mg/kg)

Cu (mg/kg)

Soil-pH KCl

EDTA HCl

EDTA HCl

EDTA HCl

5.0 6.0 7.0

0.36 0.28 0.21

0.45 0.17 0.07

0.64 0.51 0.41

0.80 0.23 0.07

67 34 17

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5

1.4

Micro-elements are required by the vine in small quantities and the availability thereof is directly dependent on the pH of the soil solution. Where the pH is high, manganese (Mn) and zinc (Zn) are inaccessible to the plant since these elements do not remain in the solution. Sometimes these elements may therefore be present in the soil, but not accessible to the plant. Consequently soil analysis is not a reliable means of determining the availability of micro-el ements. To ascertain whether the metals are excessive or insufficient, leaf analyses should be done. In soils with low pH, boron (B) and Zn deficiencies can also be expected. On the other hand, Mn in low pH soil may be so solu ble that it could become toxic to the vine. Lime applications will solve these problems by making B and Zn more available to the plant and Mn less soluble (Van Schoor, et al ., 2001). In Table 5 the norms for optimal micro-nutrient concentrations in soil are sup plied. In cases where the concentrations of the nutrients in the soil are below the norms, deficiencies may occur. In such cases the vineyard should be monitored visually for symptoms of deficiencies. If there is still any doubt, leaf analyses should be done. In the past, Cudeficiencies very rarely occurred in vineyards, due to the use of fungicides that contain Cu. With the reduction in use of these products, attention should also be given to this micro-nutrient.

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TABLE 5. Minimum micro-nutrient concentrations in soils with pH KC values of between 5.0 to 6.5.

B

Mn 2.0

Zn

Cu 0.5

0.3

0.5

If deficiencies of micronutrients do occur, foliar nutrition is a simple solution. Table 6 can be used as a guideline to determine whether the status of the various micro-elements are satisfactory. TABLE 6: Micro-element concentrations in petioles, at fruit set, for assessing the nutrient status of vineyards (mg/kg). Micro-elements Deficiencies Sufficient High to excessive Fe *NA 30-180 *NA Cu <3 5-10 25-50 Zn <15 25-150 *NA Mn <20 30-60 >300 B <25 30-70 >100 Mo *NA 0.2-0.4 *NA

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*NA – not available.

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The basic cation saturation ratio (BCSR) concept compared to the sufficiency level of available nutrients (SLAN). As guideline upon which soil analyses are interpreted and fertilisation is applied, the basic cation saturation ratio (BCSR) concept (better known as the Albrecht system) is based on an assumption that plants will only grow optimally if there is a balanced ratio of cations (Ca 2+ , Mg 2+ and K + ) for each soil according to its cation exchange capacity (CEC). Fertilisation is therefore done according to the soil’s needs, not the plant’s. In a review article, Kopittke & Menzies (2007) trace the BCSR concept back to the late 1800’s and found that since its origin, no research data has been able to prove the existence of any “ideal” basic cation saturation ratio. Instead, they found that promotion of the BCSR concept resulted (and will result) in inefficient use of re sources and fertilisers. Research by various scientists has shown that the SLAN (sufficiency level of available nutrients) concept, where a minimum concentration of available nutrients in the soil is required for optimal plant nutrition, also applies to vines. Although the “ideal” soil for grapevines can vary dramatically from region to region and between different soil types, its composition is based on a minimum level of nutrients that is re quired in the soil to supply the vine with its nutritional requirements. Nutrient element balancing is used in some cases to evaluate soil. This technique uses, for example, a Ca:Mg ratio of approximately 6 as indication as to whether calcitic or dolomitic lime should be used. For vineyard soils the ideal saturation percentage of exchang able cations is Ca 80%, Mg 15% and K 4%, which brings about a Ca:Mg:K ratio of about 20:3.75:1. In practice, however, it is not necessary to aspire to this “ideal” ratio for grapevines.

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4 Chemical correction of soils during soil preparation

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INTRODUCTION

The characteristics of South African soils is such that in most cases vines should not be planted without thorough soil preparation. During soil prepara tion a single opportunity is presented to correct the nutrient status of the soil, especially the subsoil. In this chapter emphasis is placed on the application of lime and gypsum in particular, while phosphate is discussed in Chapter 3 and also in Chapter 5.

APPLICATION OF LIMING MATERIALS:

Liming is done to neutralise excessive soil acidity. In acidic soils an excess hydrogen ions is present, which as such is not detrimental for plants, but does give rise to high aluminium contents in particular, which is toxic for root growth and therefore could be detrimental for nutrient uptake. CALCULATION OF LIME REQUIREMENT In the past reference was always made to the lime requirement of a soil, i.e. maintaining a certain pH value for the specific soil, but currently sufficient sci entific proof exists to rather refer to the lime requirement of a specific crop in terms of the acid saturation approach. It is thus not enough to merely know the pH value, because this is not an indication of the amount of ameliorant required for correction. An accurate determination of the lime requirement can only be made if soil analyses indicate the exchangeable acidity (H + ). The Eksteen method has to date been found to be the best for determining the lime requirement for agronomic, fruit and vineyard soils in the Western Cape. According to this method the so-called R-value is used, which indicates the Ca+Mg/H ratio in soil and has an exponential relationship with the pH values of the soil. To obtain a desired pH KCl of 5,5, a R-value of 10 is required. A R-value of 5 will only correct the pH to 5, which is acceptable for most sandy soils. By making use of the exchangeable acidity (H) and the exchangeable calcium (Ca) and magnesium (Mg) values of a soil, expressed in cmol/kg, the lime requirement for grapevines can thus be calculated using the Eksteen formula.

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Where the Ca:Mg ratio is ≥6, the amount of lime required for every 300 mm soil layer is calculated as follows: tons lime ha -1 = [(H x 10) – (Ca + Mg)] x 0.727

If the Ca:Mg ratio is <6, the formula is: tons lime ha -1 = [(H x 10) – (Ca x 1,25)] x 0.727

Distinction must be made between the two kinds of lime that may be applied, namely calcitic lime (CaCO 3 ) and dolomitic lime (CaMg(CO 3 ) 2 ). Dolomitic lime must only be applied if the Ca:Mg ratio is higher than 6 and/or if the Mg content of soils indicate minimum values as indicated in Table 3. Calcitic lime is used when the Ca:Mg ratio is 6 or lower. Since lime is not easily soluble in water it moves very slowly in the soil. Con sequently it should be applied during soil preparation so that it can be worked into and mixed with the soil to a depth of 1 000 mm. If the pH is lower than 5.5 in existing vineyards, the lime requirement should only be calculated to a soil depth of 300 mm. Grapevines are lime tolerant and grow well in soils containing free lime, with pHs KCl around 7.2. Excessive lime application therefore should normally not be harmful. Soils with high stone fractions can however be seen as an exception, and in such cases it is important to take the stone fraction into consideration.

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The calculated lime requirement therefore should be adjusted downwards, using the stone volume percentage in the following formula:

Lime requirement x [1 – (stone %/100)]

In soils with high organic material contents, a low pH is largely caused by hydrogen ions associated with the organic material, and to a lesser degree by hydrogen ions created by high concentrations of aluminium. The exchange able H, determined at a pH of 7, therefore should be adjusted for soils with C contents of >1%. The adjusted H can then be used in the Eksteen formula.

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Conradie (1994) proposed the following empirical adjustments: 0-1 % organic material: no adjustment 1-2 % organic material: apply 80 % of the calculated requirement 2-3 % organic material: apply 60 % of the calculated requirement 3-4 % organic material: apply 40 % of the calculated requirement

Adjustments made according to this approach generally result in realistic recommendations. Later research by Smuts (2001), however, led to the proposal of an amended formula:

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H – [(7 – field pH KCl ) x %C x 0.202]

At this stage, it would seem however, that recommendations made according to these two approaches are comparable with each other. Further research is currently being conducted. CHOOSING THE TYPE OF LIME: As calcium and magnesium constitute an integral part of a grapevine’s nutri ent requirements, it is critically important that the vine should not experience deficiencies of these two elements. Liming before planting is undoubtedly the best way to prevent calcium and magnesium deficiencies. Many lime products are available, like inter alia agricultural lime, dolomitic lime, slaked and unslaked lime. Gypsum is not a lime product, as it is a neutral salt and does not contribute to an increase in soil pH. Gypsum is used as a source of Ca and/or S, especially on saline soils to displace sodium (Na). In order to obtain the desired result in the soil as well as the desired reaction of the grapevine thereto, it is important that the neutralisation ability and re activity of the available lime products, as well as the source of the products,

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is known beforehand to ensure that the required product is delivered from the correct source. Lime reacts with the soil to increase the hydroxyl (OH - ) ions in the soil solution, and therefore reduce the soil acidification (H + ) and achieve an increase in soil pH. The most common product is agricultural lime or calcitic lime – CaCO 3 , which contains about 40% Ca and consequently its application will also increase the calcium levels in the soil. If the magnesium levels in the soil are < 40 mg/kg, dolomitic lime – CaMg(CO 3 ) 2 , is recommended. This product contains approximately 12% magnesium and 20% calcium. When lime is ordered, it is important to take the distance between the lime source and the producer’s farm into consideration as transport costs can be very high. It is equally important, however, to also take note of the neutralisation ability of lime products. In order to compare products, the neutralisation ability of pure CaCO 3 is defined as 100%. The neutralisation value (CCE = calcium carbonate equivalent values) of calcium oxide (CaO) and calcium hydroxide [Ca(OH) 2 ], for example, is considerably higher than that of calcium carbonate (CaCO 3 ). The oxides and hydroxides of magnesium also have considerably higher neutralisation values than magnesium carbonate. It is therefore possible that the CCE of an agricultural lime can be more than 100%. The CCE values of different wellknown products are indicated in Table 7:

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TABLE 7: Different lime products, their composition and neutralisation abilities.

Chemical composition in pure form

Neutralisation ability (CCE)

PRODUCT

Calcitic lime/ agricultural lime

CaCO 3 ( 40% Ca)

50-85

CaMg(CO 3 ) 2 (20% Ca and 12% Mg)

95-110

Dolomitic lime

Ca (OH) 3 (54% Ca)

120-135

Slaked lime

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THE NEUTRALISATION VALUE OF LIMING PRODUCTS The reactivity of all lime types is furthermore also dependent on the fineness thereof, so that the largest possible surface can come into contact with the soil and soil water in order to neutralise soil acidity. The reactivity of microfine agri cultural lime (90% finer than 0,25 mm), therefore is significantly higher than that of agricultural lime that merely conforms to the minimum legal requirements for “ordinary” agricultural lime (50% finer than 0,25 mm).

THE TIMELY AND CORRECT APPLICATION OF LIME ON THE LAND

Lime should be broadcasted on the surface at least six months before planting and immediately incorporated into the soil, so that the pH correction can be achieved before the grapevine extends its roots into the soil. EFFICIENT MIXING OF LIME WITH THE SOIL If the soil is too moist when preparation takes place, it forms a ‘paste’ and the lime will not be thoroughly mixed with the soil particles. If the soil is too dry, large clods form and good mixing is also not obtained. The water content of the soil should be between wilting point and lower plastic limit (where a stable sausage can be rolled between the palms of your hands). THE EFFECTIVE REACTION OF THE LIME IN THE SOIL, IN ORDER TO OBTAIN THE DESIRED RESULT If soils were left fallow (i.e. soils that have not been cultivated for long periods of time) and where the pH is very low, it is recommended that lime be applied in instalments over at least two seasons. The last requirement is very important and that is to determine if the soil is fit for planting immediately after liming, or whether planting should be postponed so that the soil reaction can bring the pH nearer to the optimum of >5,5. There is a general perception that the more calcium in compound form is put into the soil, the higher the pH or soil reaction will increase. However, it is not only Ca compounds, but also magnesium, potassium and sodium compounds that have an impact on the soil’s pH levels. Excess potassium, also known as potash (K), and sodium may have a greater effect on soil pH than calcium and magnesium.

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APPLICATION OF GYPSUM

THE OBJECTIVE OF AGRICULTURAL GYPSUM APPLICATION The objective with the application of agricultural gypsum is primarily to recover saline soils. Dissolved salts from salt-rich rocks like Malmesbury shales that accumulate in soils create problems in vineyard soils, especially because the soil becomes too compact, decreasing water infiltration. High salt levels in irrigation water can also compress the soil over time and this can be corrected with gypsum. In saline soils the exchangeable sodium as percentage of the total cation exchange capacity (CEC) is too high and the sodium adsorbtion ratio (SAR) of the saturated extract is also too high. Another use of gypsum is also for the improvement of the soil structure. If the infiltration ability of a soil is low, gypsum improves the water relations and air permeability. The electrolyte concentration of irrigation water is low – irrigation of fresh water together with the leaching in of gypsum makes it possible to irrigate without the soil being compacted. Application of agricultural gypsum does not increase the pH of the soil and therefore should not be used for this purpose. It can however increase the exchangeable calcium levels. HOW MUCH GYPSUM IS REQUIRED? To determine the required amount of gypsum that should be applied per hectare to saline soil, the Na (expressed in cmol/kg) may be multiplied by 3,4 to determine how many tons of gypsum per hectare are required for each 300 mm of soil depth. Some analysis reports indicate sodium in mg/kg or parts per million (ppm). In such cases the Na (mg/kg or ppm) should be divided by 230, to obtain the amount of Na in cmol/kg. Since watersoluble Na can simply be washed out through effective drainage, gypsum is only required to displace the exchangeable Na, i.e. that which has been adsorbed onto the soil particles. Laboratory analyses usually indicate total Na, i.e. soluble as well as exchangeable Na, which means that in practice too much gypsum is often recommended. It is therefore advisable to apply no more than 10 tons of gypsum at any one time. After a year, soil analyses should in stead be performed again to determine whether additional gypsum is required.

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For soil preparation the gypsum requirement should be determined to a soil depth of 900 mm. Only 50% of the gypsum should then be worked into the soil. The rest must be broadcasted across the surface to assist infiltration and leaching. The calcium ion content of the rain/irrigation water is increased so that it can displace the sodium and prevent the soil from dispersing. Note that a maximum of no more than 10 tons/ha should be applied in total. As far as existing vineyards are concerned, the gypsum cannot be placed in the subsoil, so it serves no purpose to calculate the gypsum requirement deeper than 300 mm. Once again a maximum of 10 tons per hectare per annum may be applied, but only 5 tons at a time to prevent too much K and Mg from being displaced. Reconsider the situation after application of the first 5 tons. Preferably gypsum should be applied before the rainy season or before irri gation, to ensure that the gypsum is washed into the soil and the Na leached. The amount of agricultural gypsum that needs to be applied to correct nutrient imbalances, usually latent S deficiencies, is approximately 1 ton per hectare. As gypsum has a very low solubility, large amounts will never be harmful. FINE AND COARSE GYPSUM The reactivity of agricultural gypsum is determined by the purity and fineness thereof – the finer the gypsum, the quicker it will dissolve. In soil with low permeability, fine gypsum that can dissolve quickly is required. In soil with a high permeability (sandy soil), on the other hand, fine gypsum may leach out before soil correction and nutrition is effectively achieved. A coarser gypsum is therefore recommended for such soils. Agricultural gypsum is a 90% to 95% calcium sulphate product and contains on average about 22% calcium and 17.5% sulphur on a dry weight basis. The water content varies from 23% to 30%, while in the case of gypsum as by-product of double superphosphate preparation it contains from 0,4% to 1% phosphorus, e.g. phosphogypsum. WHERE DOES AGRICULTURAL GYPSUM ORIGINATE FROM? Natural gypsum comes from Yzerfontein, Kolkies River and Vanrhynsdorp, and phosphogypsum from Chloorkop, Potchefstroom, Phalaborwa and Pho keng. Both types are available at SA Lime and Gypsum. Phosphogypsum is the by-product of the wet-acid production of phosphoric acid from phosphate rock. It is primarily hydrated calcium sulphate (CaSO42H 2 O) with impurities of phosphorus (P), fluorine, (F) silicon (Si) and aluminium (AI), as well as traces of various trace elements and heavy metals.

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