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Guidelines for monitoring soil fertility, plant nutrient status and compost quality in organic wine grape production systems

Pieter Raath & Chris Schutte, ARC Infruitec-Nietvoorbij, Stellenbosch

Organic production systems differ radically from conventional ones. Compared to conventional producers, it is often thought that organic farmers are not scientific in their approach to crop production and vine nutrition. The fact is that any wine grape producer who wants to produce wine grapes of good quality needs to be aware of the nutritional status of his soil and vines and adapt his fertilisation practices accordingly.

Producers who want to convert to organic farming, or who are already farming organically, often lack knowledge regarding soil management and plant nutrition. They realise that they need to improve soil fertility, but often do not know how to evaluate the efficacy of their fertilisation practices. Generally, they do not have the expertise available to help them detect nutritional imbalances and to prevent such imbalances from occurring as a result of their cultivation practices.

This article provides practical guidelines to wine producers (both organic and conventional) that make use of organic fertilisation practices, regarding responsible monitoring of their soil fertility and plant nutrient status. In addition to this, information regarding compost analyses and the interpretation thereof, is given.

Soil

Guidelines regarding the monitoring of soil fertility are given below.

SOIL SAMPLING

Time of sampling

  • Sample soil before converting to organic production and thereafter once every year during the first three to four years. After this period the effect of added organic material on the soil's fertility should have stabilised and the frequency of analyses can be reduced to once every second or third year.
  • Sample soils every year at the same time, preferably in early August, before any fertilisation/compost is applied.
How to sample

  • For the first three years, samples should be taken from three different soil depths, i.e. at 0-15 cm, 15-30 cm and 30-60 cm. From the fourth year sampling depths can be changed to 0-30 cm and 30-60 cm only. Where samples are taken prior to establishment of a new vineyard, the 60 to 90 cm depth should also be sampled. This is to determine whether there are any chemical restrictions in the deeper soil layers that need to be corrected before planting.
  • Sample soil with an auger or spade over the whole depth. Put the soil in a plastic bag which is clearly marked, indicating the:
    (i) block name or number,
    (ii) depth of the sample and
    (iii) farm's name.
  • The sample should be taken on the berm (bankie), midway between two plants, after having removed any surface debris.
  • If the soil is homogenous within a specific block/land unit, combine soil from three or four locations within the block. If different soil types or textures occur within the same block/land unit, soil samples from each soil/texture type should be taken and clearly marked under different names. Soils should also be sampled separately where vines vary in vigour or growth within the same block/land unit.
  • The sample should weigh at least one kilogram in total. Rock and stone fragments form part of the soil and should not be removed.
SOIL ANALYSES

What to analyse the soil for

  • For a good indication of the soil's nutrient status, it should be analysed for:
    (i) Macro-nutrients - P, K, Ca, Mg
    (ii) Micro-nutrients - Mn, Zn, B, Cu
  • An indication of the soil's nitrogen supplying capacity and organic matter content can be obtained from the C:N ratio through analyses for:
    (i) Total nitrogen (N)
    (ii) Organic carbon content (C)
  • The soil's acidity and salinity should also be determined, respectively, through:
    (i) pH determination of the soil (if below 5,5 a lime requirement should be done), and
    (ii) the determination of the soil's electrical resistance (ohm) or conductivity (mS/m), as well as its Na content.
  • The soil's ability to hold water and nutrients like N and K, as well as its potential to compact, is largely determined by its texture. An indication of the soil's texture, i.e. the percentage sand, loam and clay, is therefore very helpful.
  • PLEASE NOTE: If soil analyses during the first year indicate more than sufficient K (only on clayey soils), P, Ca, Mg, micro-element contents and high pH and resistance, there is no need to analyse the soil for these elements annually. Every three years would suffice. The soil's total N and organic C contents, however, have to be determined until it stabilises. On sandy soils, maintenance applications of K might be required.

Interpretation of soil analyses

  • Recommendations regarding the nutrient status of the soil should be made by someone experienced in the nutrient requirements of vines.
  • The following information is required to interpret the analytical results and to formulate recommendations:
    (i) Whether the soil originates from an existing vineyard or whether recommendations are needed for soil preparation.
    (ii) The age of the vines.
    (iii) Cultivar.
    (iv) Vigour of vines (i.e. weak, normal or vigorous).
    (v) Plant spacing (in metres).
    (vi) Estimated yield for the next season.
    (vii) Type of mulch and cover crop used.
    (viii) Fertilisation programme followed during the past season.
  • When interpreting soil analyses, various factors like texture and mineralogy of the soil, etc., need to be taken into account, making it a fairly complex process. Some very basic guidelines for the interpretation of soil analyses are, however, given in Table 1.

Grapevines

Guidelines to determine and monitor vine nutritional status are given below.

LEAF SAMPLING

When to sample

  • Leaf samples should be taken before converting to organic production and thereafter once every year, at the same time, during the first three to four years after conversion. Leaf samples should then only be taken every second to third year, or when nutrient deficiencies are suspected.
  • Sample leaves every year at the same time, either during fruit set or veraison.

How to sample

  • Take leaf sample from positions opposite the developing bunches.
  • Separate the leaf blade from the petiole immediately after sampling to minimise translocation of nutrients.
  • Sample at random from at least 10 vines, scattered throughout the block.
  • The ideal is to analyse both the blades and petioles, but to save on analytical costs, analysis of the petioles is sufficient.
  • The total sample should consist of at least 40 petioles/leaf blades.
  • The leaves should be placed in a perforated plastic bag.
  • If the samples need to be stored or are transported over large distances, they must be kept in a fridge at about 00C or transported in a cool box to keep them fresh.
  • If unavoidable delays occur in getting the samples to the laboratory, it is best to dry the leaves/petioles in the sun before transportation.

LEAF ANALYSES

What to analyse the leaves/petioles for

  • As an indication of the vine's nutrient status, its leaves/petioles should be analysed for: (i) Macro-nutrients - N, P, K, Ca, Mg, Na
    (ii) Micro-nutrients - Mn, Zn, B, Cu
Interpretation of leaf analyses

  • Recommendations regarding the nutrient status of the leaf samples should be made by someone with experience on the nutrient requirements of vines.

  • The following information is required to interpret the analytical results and to formulate recommendations: (i) Age of the vines.
    (ii) Vigour of vines (i.e. weak, normal or vigorous).
    (iii) Cultivar.
    (iv) Fertilisation programme followed in the past season.
  • Some basic guidelines for interpreting leaf/petiole analyses are given in Table 2 (the norms are for samples taken at fruit set).

Compost and compost teas

SAMPLING

How to sample compost

  • Take samples from various positions of the compost heap/batch in order to obtain a representative sample of the whole heap/batch. If the compost is still in a heap, take the samples from at least 20 cm deep.
  • Place the sample in a plastic bag and keep it in a cool place, out of direct sunlight, until it can be delivered to the laboratory.

How to sample compost tea

  • Stir the tea and allow a bit to run out of the tank before taking a sample. The sample should be put in a clean and watertight plastic or glass container and should be at least 300 ml. Keep the sampled tea out of direct sunlight.

COMPOST/COMPOST TEA ANALYSES

What to analyse compost/compost tea for

  • As an indication of its nutrient status, it should be analysed for:
    (i) Macro-nutrients - total P and K
    (ii) Micro-nutrients - Mn, Zn, B, Cu, Fe
  • As an indication of the nitrogen supply potential, it should be analysed for:
    (i) Total nitrogen (N)
    (ii) Organic carbon content (C)
    (iii) C:N ratio
  • As an indication of the soluble nutrient content in the product, i.e. the nutrients that will immediately be available for uptake by the plants, the following analyses must be done:
    (i) NH4+ and NO3- for nitrogen
    (ii) K, as a water-soluble extract
    (iii) P, as a citric acid extract
  • To detect possible contamination with salt, sodium or heavy metals, the product can be analysed for:
    (i) Na and Cl
    (ii) Cd and Pb
Interpretation of compost/compost tea analyses

  • The nutrient contents of the compost or compost tea will determine how much will have to be applied.
  • The ratio of nutrients in the compost/compost tea determines whether additional nutrients need to be applied from other sources to correct nutrient imbalances.
  • The Na, Cl and heavy metal contents help to determine whether it is suitable for use (or not) on a specific soil.
  • Guidelines on the minimum and maximum levels of nutrients and heavy metals in compost are given in Table 3.
  • Compost tea analysis should be interpreted in terms of nutrient contents and the ratios thereof. The nutrient contents will determine how much of the tea needs to be applied, while the ratio of the nutrients will determine whether additional nutrient applications are needed (to prevent the development of an imbalance of nutrients in the soil), and whether the tea can be used at all. Factors that need to be taken into account are the following:
    (i) Soil with more than 10% clay will bind K, and cause a build-up of potassium if it is applied in significant amounts in the form of compost tea.
    (ii) In soil with more than 10% clay, the N content of the tea should always be equal to, or higher than, the K levels since a build up of K will occur in these soils causing problems with nutrient imbalance and wine quality.
    (iii) From the known nutritional requirements of the grapevines, calculate the amount of compost tea that needs to be applied to provide the macro-nutrients that need to be applied and then determine how many kg of micro-nutrients and heavy metals will also be applied. Use Table 4 to determine the maximum amounts of micro-nutrients/heavy metals that can be applied to the soil per year without having a detrimental effect.

Conclusion

With proper monitoring practices, the organic producer should be able to determine the trends that occur in his vineyard soils with regard to fertility and also the vines' nutritional status. The development of nutritional disorders is therefore prevented, while optimal vine nutrition can be ensured, resulting in healthier plants and better quality grapes.

Calculations regarding the amount of organic fertiliser, compost or compost tea that need to be applied can be done by the authors. Advice regarding the need for specific nutrients from organic sources can also be given.

References

1) CONRADIE, W.J., 1994. Wingerdbemesting. Handelinge van die werksessie oor wingerdbemesting, gehou te Nietvoorbij op 30 September 1994. LNR-Nietvoorbij Instituut vir Wingerd- en Wynkunde, Stellenbosch.

2) SGS, 2000. SGS Organic Production Standard. SGS South Africa (Pty) Ltd., Stellenbosch, 70pp.

3) VAN SCHOOR, L., CONRADIE, W.J. & RAATH, P.J., 2000. Riglyne vir die interpretasie van grondontledingsverslae vir wingerd. Wynboer 14, 96-99.

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