Insufficient leaf surface to ensure adequate post-harvest fertilisation uptake.
The storage of carbohydrate reserves mainly takes place in the trunk and root cortex, after which it is used by means of enzyme activity for predominantly new root growth. Ample root reserves play a big role in the bud break of the next year, as well as bunch ripening at the end of the season. One tonne of grapes removes approximately 3 – 4 kg of nitrogen, 0.7 kg phosphate and 3 kg potassium per season. In cases where carbon and soil microbe activity are sufficient and therefore able to break down the shoots and leaves quickly enough, these amounts can however differ.
There are two main root growth stages during the plant’s growing season. The first occurs at the beginning of the growing season right after budding, which is the most important time for nutrient uptake. It is also important to understand that from budding to flowering the grapevine mostly depends on its reserves established during the previous growing season’s post-harvest fertilisation. Vine leaves contribute to production until the leaf on the primary shoot reaches 50% of its mature size, and 75% of its mature size when the leaf is located on a branch shoot. The second root growth stage commences during, or right after the harvest period, depending on the ripening period of the cultivar.
In the case of early cultivars, root growth takes place approximately two weeks after the harvest period, but for late cultivars it can start during the harvest already. During the post-harvest period, a grapevine takes up approximately 38% of its nitrogen requirement, 23% of its phosphate requirement and 16% of its potassium requirement. This can vary depending on the area, cultivar and rootstock combinations, but it does emphasise the importance of good post-harvest fertilisation. If sufficient post-harvest irrigation or rain is not available to wash in the fertiliser, it is important to establish cover crops and fertilise them well to ensure that the organic material can release nutrients at a later stage when broken down in the soil.
The accumulation of reserves late in the season is notably impacted by stress factors such as drought, waterlogging, high crop load and poor cultivation practices such as disease control. The quality and length of the growing period after harvest play a major role here, therefore unnecessary leaf fall due to diseases or mechanical harvesting should be prevented at all costs. If vines have few or no active leaves, the post-harvest fertilisation should rather be applied in the following season as extra nutrition.
The uptake of macro-nutrients over the season can be seen in Figures 1, 2 and 3. Three macro-elements will be discussed separately, in terms of the vine’s post-harvest requirement when maintenance fertilisation is applied.
FIGURE 1. Seasonal uptake of nitrogen by Chenin blanc/99R in sand culture.
FIGURE 2. Seasonal uptake of potassium by Chenin blanc/99R in sand culture.
FIGURE 3. Seasonal uptake of phosphorus by Chenin blanc/99R in sand culture.
Nitrogen
Nitrogen recommendations are mainly calculated according to the vigour required. The wine goal is also a good indication of the desired growth, with moderate growth recommended for quality. In the case of mass bearers, more growth will be recommended. The post-harvest nitrogen fertilisation should comprise about one-third of the total season’s fertilisation to ensure enough reserves for the next season’s bud break.
Keep in mind however that poor growth is not always attributable to insufficient nitrogen applications alone, as the following factors can also play a significant role:
- Waterlogging.
- Dehydration.
- Salinisation.
- Nutrient deficiencies.
- Aluminium toxicity in low pH soils.
- Nematodes.
- Phylloxera.
- Root pruning at the wrong time.
- Competition from weeds for water and nutrients, especially in the case of couch grass and other perennial crops.
Table 1 can be used as a guideline to determine the vigour of vineyards for the most recent season, which can then be used to determine the nitrogen requirement for the next season.
TABLE 1
To calculate the amount of nitrogen required for post-harvest fertilisation, the following formula can be used:
3 kg x 38% = 1.14 kg N per tonne of grapes produced. We can thus apply 1.14 kg x 30 tonnes per ha = 34.2 kg of clean nitrogen. Be sure however, to take the past season’s vigour, as described in Table 1, into consideration to determine the actual amount of N to be applied as maintenance fertilisation.
Potassium
Potassium plays an important role in several of the plant’s metabolic processes and thus can be considered the most important metal cation. Potassium is taken up from budding to harvest, mainly because it is required by the grapes during this period (Figure 2). It is one of the most mobile elements in the vine and is important for the regulation of osmotic pressures and subsequently water management of the vine. It also plays a role in the transport and condensation of carbohydrates (reserves) and is therefore particularly important for carbohydrate-rich plants like grapevines. Although potassium is translocated from the leaves to the grapes, there is still a peak uptake immediately after harvest, which constitutes about 15% of the annual requirement. Potassium reacts with organic acids in grapes, especially tartaric acid, to form salts during ripening, which is why excessive K-fertilisation can lead to higher pH’s in must. It is not the only or primary cause of high pH’s in must, however.
We can therefore use the following formula as guideline to determine the post-harvest potassium fertilisation:
3 kg x 15% = 0.45kg K required per tonne of grapes produced. For 30 tonnes per ha, 13.5 kg clean potassium is applied as post-harvest maintenance fertilisation.
Phosphate
Although phosphate is an essential macro-nutrient, it is required in very small amounts by the grapevine. The grapevine is also able to utilise less accessible sources of P due to specific root properties and mycorrhiza associations in the soil and root environment. Soils with sufficient living organisms can thus help the vine to utilise phosphate sources. The long growing season also allows the vine enough time to absorb adequate amounts of phosphate. Consequently, deficiency symptoms are rarely observed in the field despite the generally low P contents of South African vineyard soils. Phosphate is bound so quickly by clay in the soil, though, that little movement takes place. The phosphate concentration in the soil solution is pH dependent, with the highest concentrations found between pH 3.5 – 6.0. In Figure 3, a peak uptake is nevertheless noticeable after harvest when the most phosphate is absorbed.
We can therefore use the following formula as guideline to determine the post-harvest phosphate fertilisation requirement:
0.7 kg x 23% = 0.11 kg P required per tonne of grapes produced. For 30 tonnes per ha, 3.3 kg phosphate can thus be applied as post-harvest fertilisation. In most cases, the requirement is so small that it can be omitted from the post-harvest programme. The fact that this element is not very mobile, also ensures that available phosphate isn’t leached out of the soil and will still be available post-harvest for uptake by the plant.
Disease control
The leaves of the grapevine should be protected at all costs. Ensure that harvest machine settings are correct to minimise leaf damage. In cases where disease pressure was high during the season, it is critically important to spray immediately after harvest to protect the vine’s leaves. Wettable sulphur and copper should provide sufficient efficacy and are also the most economic options. To achieve adequate nutrient uptake, it is paramount that leaf transpiration should still take place, while photosynthesis will ensure that enough carbohydrate reserves can be stored. Photosynthesis also ensures proper cane ripening.
Healthy leaves are vital to ensure reserve accumulation – disease control post-harvest will be necessary in this case.
Weed control
Weed control can be applied on the berms to limit competition for water and nutrients when the post-harvest fertilisation is applied. The control of perennial weeds, especially couch grass, can be very beneficial at this stage, as the herbicide is translocated to the roots.
Cover crops
If soil moisture is sufficient, a seedbed can be prepared in advance. Also make sure that good quality seed is obtained, free from weeds like ryegrass. Cover crops play a major role in improving the soil carbon content, which in turn can improve the storage of reserves in the soil. The roots in the soil also increase aggregate formation. This means that a better soil structure is formed which enhances water infiltration, aeration and root penetration. Naturally, the plant is then better able to accumulate reserves and ensure that more elements are stored and captured in the soil.
Soil compaction
In the case of compacted soils, the post-harvest strategy should be adjusted to first alleviate the compaction before fertiliser is applied. Root pruning should be avoided at all times and only considered as a last resort if soil compaction cannot be uplifted with any other methods. Alternative options to uplift compaction, are cover crops and chemical correction of the Ca-Mg ratio, for example. In many cases, compaction occurs due to a chemical imbalance in the soil. A mechanical action will only uplift the compaction temporarily, after which the soil will become compacted again.
Conclusion
Post-harvest fertilisation, together with the accumulation and storage of sufficient reserves, is critically important to ensure optimal bud break and subsequently a good harvest in the next season. It is thus crucial to keep the grapevine and soil as healthy and optimal as possible to enable maximum build-up of reserves post harvest. It can also be beneficial to divide the maintenance fertilisation into two instalments to prevent nutrients from being washed out and also allow the plant sufficient time for uptake. This strategy will also limit the stimulation of regrowth.
For more information, contact Klaas Coetzee at klaas@vinpro.co.za.
Â
Click here to get your copy of WineLand Magazine.
