The use of compost in viticulture can, as in other agricultural/horticultural applications result in a wide range of positive effects. However, there is also scope for potentially detrimental effects.

The use of compost replenishes soil humus, which is reduced particularly in cultivated soils; in Germany for example at a rate of approximately 4 t/ha per year. Long-term compost use has been shown to increase organic matter levels and it is assumed that compost dressings of 8 – 10 t dry matter (dm) are sufficient to maintain or increase soil organic matter levels.

Compost contains all macro- and micronutrients essential for plant growth. However, not all nutrients are readily available in mineral forms for plant uptake. Considerable amounts of nitrogen and phosphorus are organically bound in the compost and are released only once the organic matter is mineralised through microbial activity. The level of readily available mineral nitrogen contained in compost and the degree of nitrogen release due to the mineralisation process following compost application are of particular interest.

The nutrient budget in Table 1 shows that a compost application of approximately 10 t dm/ha (20 m³/ha) should be sufficient to meet the demand of grapevines, except for nitrogen. However, the apparent lack of nitrogen in the budget is alleviated through airborne nitrogen deposits (30 – 50 kg/ha per year in Germany), through mineralisation of soil humus reserves or through leguminous cover crops.

Most, or a high proportion of phosphorus, potassium, magnesium and calcium found in recycled organics compost is available to plants immediately or becomes plant-available over time. Approximately 20 % of phosphorus in compost react like P in mineral fertilisers and are immediately available for plant uptake while the remainder is more strongly bound and will become available later. Virtually all potassium supplied with compost can be used immediately by plants.

Table 1: Availability and supply of nutrients contained in 10 t dm/ha (20 m³/ha) of an average bio-waste compost in comparison to the nutrient demand of grape vines

The situation is more complex with nitrogen of which only a small proportion is directly available to plants initially and the remainder being mineralised and released only over time (3 – 4 years). As a rule of thumb it is generally assumed that approximately 5 % of the total amount of nitrogen found in recycled organics compost is present in a mineral form and hence directly plant available and that annually approximately 10 % of the total nitrogen is mineralised over the next few years. It is estimated that in total approximately 40 % of all nitrogen contained in compost at the time of application will become available to plants.

Several research projects focused on this aspect but found inconsistent results. Horticultural trials established that the use of immature compost provided relatively little additional nitrogen, also during the second year after application while the use of mature compost delivered a flush of soil nitrate, which decreased over time. This effect of using mature compost was confirmed by one experiment in vineyards while two others showed that even the use of mature compost provided little additional nitrogen for plant uptake (Figure 1).

In order to reconcile conflicting research results and to solve many open questions related to nitrogen availability and the mineralisation of organic matter, which is important both from a plant nutritional and environmental point of view, a 10 year long-term, co-operative research project was established in Germany. It aims to provide a better understanding of the long-term dynamics of mineralisation and nitrogen supply potential of compost.

However, most available data relate to temperate climatic conditions in Europe and it has to be expected that nitrogen dynamics associated with compost use are quite different in climatically different wine growing regions. An assessment of nitrogen availability from composted chicken manure and slaughterhouse waste in tropical conditions showed that compost is not necessarily a slow release fertiliser. Surprisingly, according to plant growth results, composted chicken manure provided more nitrogen than urea during the first seven weeks of the trial and generated a flush of growth which peaked after nine weeks simultaneously with that of urea fertilised plants and at almost the same level (Figure 2). Considerably more research is warranted to examine the effects of using compost in various climatic conditions.

Nierstein

Biebelsheim

Note: Organic fertiliser applied annually to provide 50 kg N/ha

Figure 1: Effect of recycled organics compost and organic fertiliser on soil nitrate levels
(0 – 60 cm) in two vineyard soils over a three year period

Figure 2: Effect of inorganic and organic nutrient sources (2 g N/12 lt. pot from each source) on plant growth (grass) in tropical conditions

In many experiments it was shown that compost use could substantially improve soil physical, chemical and biological properties, which are often important factors in determining its fertility status. The improvement of these soil properties results often in indirect benefits such as reduced erosion, ease of cultivation or a reduced disease incidence.

Compost use showed inconsistent effects on grape yields, depending on the type of compost used, the vineyard soil and the control it was compared against. A 3-year trial in an organic production system started to show beneficial long-term effects of compost use in the last year of the experiment.

The use of compost as mulch resulted in substantial yield increases in some Australian trials. An observed three-fold yield increase was primarily due to increased survival of bunch numbers in very dry growing conditions (Figure 3). Additional nutrient supply through the use of mulch was not looked at.

Figure 3: Effect of 7.5 cm mulch layer (50 cm wide) on grape yield and bunch survival in 18 month-old vines (conventional production)

According to the available literature, compost use on grapevine makes relatively little difference to the quality of the must or wine generated from these grapes.

Potential negative effects

Particularly nitrogen and phosphorus have the potential of causing detrimental environmental effects if compost is used inappropriately. Generally compost does not have high nutrient densities and only a limited amount of the total nutrients contained in compost is immediately available. However, if large quantities of compost are used (mulching) or if compost is applied to soils with high organic matter levels, nitrate leaching can occur. This is a potential problem particularly in viticulture since grapes have relatively little nutrient requirements and, as a survey in Germany has shown, many vineyard soils are already very well supplied with phosphorus. In Germany the agricultural/horticultural use of compost is limited to a maximum of 10 t dm/ha per year (30 t dm/ha every three years) by way of federal legislation.

Depending on the quality of processed input material, compost may contain considerable heavy metal contents. Growers need to check for compliance with their certifying bodies. It was shown however, that sources other than compost can also contribute significantly to the heavy metal load received by a vineyard. Grapevines take up very little heavy metals and very little is deposited in the grapes. Any potential residues are filtered out in wine production, which is why heavy metals do not pose a problem for wine drinkers. However, high levels of heavy metals can have detrimental effects on plant growth and microbial activity. Heavy metals are, therefore, in a vineyard situation more of concern with regard to the long-term protection and stewardship of the soil.

Most of the obtained research data and information on the use of compost in viticulture originates from Europe, specifically Germany. A wide range of positive effects can be attributed to the use of compost, some of which were also shown in vineyard trials. Compost provides essential plant nutrients but their release over time seems unpredictable. This is why research efforts in Europe now focus on this aspect which is important both from a plant nutritional as well as environmental point of view. Research in other regions has focused on the use of compost to redress the most pressing local problems, for example water shortage. However, future research into the use of compost should also investigate aspects such as nitrogen mineralisation from compost in warmer climatic conditions and the release of nitrogen and phosphorus from compost to assess its nutritional value over time and its potential detrimental effects if used inappropriately.

It has to be realised that both the soil and the compost represent biological systems whose interaction depends on a range of factors, many of which are not as well understood as previously thought. Compost use tends to show its full potential only after prolonged use. Therefore it is encouraging to see that several new research projects, which assess the effects of compost use in viticulture, are long-term, running for 5 – 10 years. The only drawback is that the results will not be available for quite while to come and we have to wait for a long time to implement them.

Due to limited space references are not included but can be obtained from the author either separately or as part of the full report.