Once a crop has been harvested, it enters a state of rest prior to the resumption of its biological cycle. Before it flowers, the plant starts to build up reserves of substances which it will use to help with the budding and flowering in the next biological cycle. That is why there is a step that is essential but commonly neglected: post-harvest fertilising.

The post-harvest period for a crop runs from the end of the harvesting of the fruit to the rest period for the trees. This rest is characterised by the start of changes to the colours of the leaves, particularly in deciduous species, which can vary greatly between species and is a sign of the start of their resting phase.

For example, in a vineyard it can last between 30 and 45 days, depending on the location. In contrast, in a cherry orchard it can last between 100 and 120 days. As a result, the quantity of nutrients to apply in the pre- and post-harvest periods must reflect the length of this stage.

Let us tell you everything you need to know in this new post from Herogra Especiales. 

 Why bother with post-harvest fertilising?

The main reason for doing this is to provide the nutrients the crop will need when it begins its new biological cycle, guaranteeing satisfactory production. However, providing nutrition is not the only reason, there is also a phenological reason, as this period, both prior to the rest period and during the rest period itself, is when the roots are most active.

We understand fruit trees as a balance: on the one side you have the above-ground part and on the other side you have the roots. The balance changes throughout the year, favouring the growth of one part or another by diverting more resources to it.

For this reason, in the state prior to rest in which either the leaves have fallen (in the case of deciduous fruit trees such as grapevines), or their above-ground part is at rest (in the case of evergreen fruit trees, like citrus trees), the roots are active so they can take in the nutrients needed for the next biological cycle.

Picture 1: Outline of the root-above ground process Source: (Campo y negocios, 2021)

Post-harvest fertilising application methods

Post-harvest fertilising can be carried out in different ways:

• Fertigation: providing the nutrients necessary through irrigation, taking advantage of the roots being more active after the harvest.
• Foliar option: this must be performed before the leaves fall or the trees enter into rest since the leaves must be in good condition to ensure the best possible absorption of nutrients.

Foliar fertilisation can be used as a supplement to fertigation, as corrective applications when there are symptoms of nutritional deficiencies or when we want to improve the nutritional reserves of the tree to make sure it comes out of its winter rest in good condition. At that time, we take advantage of the fact that the fruit has been harvested but the tree has still not entered its rest period to perform this type of application. This is common practice for many fruit trees.

In foliar applications of macro and micronutrients, nutrients that move through the phloem are provided, so that they can translocate and form part of the plant’s reserves.

Important nutrients

In the post-harvest period it is important to have the right levels of macro and micronutrients:

MACRONUTRIENTS

• Nitrogen: this is important in the winter rest to ensure that the tree is in good condition for the coming season. Between 20% and 30% of the total nitrogen needs are built up during this period.

For the other two macronutrients, phosphorus and potassium, around 10-15% of the total needs come from this period. All the nutrients are essential for the correct development of the tree and any excess or shortage of any of them will lead to crop problems in the following season.

• Phosphorus: this helps both with the growth of the root and the flowering and fruiting, since it supports the process for photosynthesis, transporting of nutrients, synthesis of proteins, etc.
• Potassium: this is involved in the photosynthesis process, guaranteeing cellular elongation and therefore the growth of the crop’s tissues. It plays a key role in the ability of the plant to withstand stress.

MICRONUTRIENTS

The most important micronutrients in the post-harvest period are boron and molybdenum, which are important for boosting flowering, setting and development of the fruit, ensuring a good harvest.

• Boron: is recommended for the post-harvest since its application at that time is more effective than in spring in relation to the concentration in the flower structures.
• Zinc: is hugely important for growth and also plays a part in the germination of the pollen, so it is very important in the setting and also in the synthesis of Auxins, the plant hormones that affect the elongation of the pollen tube and the setting.

Post-harvest fertilising in deciduous species

These species will shed their leaves after harvest and before the resting stage. Therefore, fertilising should be carried out in this period between the harvest and the falling of the leaves.

One of the most important deciduous species is the grapevine. At the beginning of spring, grapevines need their reserves to bud and start growing buds and leaves. These reserves consist of carbohydrates such as sugars and starch that have been built up in the plant’s permanent structure: branches, trunk and roots, with the greatest proportion being found in the latter. These reserves are only possible thanks to post-harvest fertilising, and foliar applications are recommended.

Post-harvest fertilising in evergreen species

Olive trees and citrus trees, such as the orange tree, do not shed their leaves prior to the rest phase, instead entering this phase directly after a period of time.

In olive trees, potassium is a key nutrient in the post-harvest stage due to the high consumption of this when forming the fruit. This has a direct impact on:

• Oil yield and fruit size.
• Problems with its root absorption in Andalusian olive trees on limestone and clay soils.

As part of its cycle, once the campaign in the months of January and February has ended, the potassium levels in the leaf increase considerably until the fruit sets. From that point they begin to decrease, a process that will last until the harvesting of the olives. The larger the harvest the more pronounced this reduction will be.

In addition to potassium, we must take into account other macronutrients such as phosphorus and nitrogen and certain micronutrients that are essential if we want to restore the nutritional balance of the olive tree and ensure the next harvest, in terms of both the quantity and quality of the fruit.