Balanced mineral nutrition plays a relevant role in an adequate vegetative development of trees, both in the formation stage and in the fruit production cycle.

In trees in production, this adequate vegetative development must be translated into fruit that is sufficiently balanced to allow a post-harvest life in optimal conditions. The quality of the fruit, is produced in the orchard! Since once in the packinghouse, fruit quality is either maintained or eventually deteriorated. 

In the current production conditions, an integral approach to the orchard is required, interpreting the greatest number of productive variables to allow us to adjust the nutritional programs for its use in a rational way and adjusted to each productive reality, considering the multiple factors that are at play in this species.

Focusing nutrition in a program concept is of the utmost importance, because this way, each season we can ask for a better productive performance from the orchard, to the extent that we incorporate all the nutrients that are necessary according to the demand of the different components of the tree. Fruit nutrition has to be a tailor-made suit, because each orchard has its own particularities, “one size does not fit all”.

Among the main considerations for the establishment of an adequate nutritional program are aspects such as: variety, rootstock, planting density, tree age, production history, yield estimation for the cycle in which fertilization is being planned, nutrient recycling, irrigation systems (fertigation), soil drainage limitations, irrigation need/irrigation frequency (which could condition the partitions to be considered in the fertigation program), soil analysis, foliar analysis, fruit analysis and quantification of the vegetative expression of the orchard (vigor index). 

In the same concept of relevant aspects for the establishment of the nutritional program, it is important to get used to building a history of nutritional analysis for the different quarters, in terms of soil, leaves and fruit, which will allow us to optimize nutritional decisions. 

Bases to take into account in nutritional analyses

In the case of soil analysis, it is ideal to update the information every 3 years, and the maximum validity should be 5 years, given that current production systems are much more intensive and highly extractive; otherwise we are looking at information that may no longer represent the current reality of the soil. 

The ideal time for soil sampling should be at the beginning of winter: July-August, in orchards in full production, to be the starting point of the new spring fertilization cycle. The perfect procedure does not exist, but it is important to have a methodology that allows us to make comparisons across seasons and to analyze the values in light of the management that was carried out during that period (hence the relevance of setting a sampling time). 

Some special situations may require monitoring soil nutrient levels throughout the season, for example: July-August; December-January; March-April, in order to understand some processes that may be occurring at the soil level and that are related to the productive behavior we are observing in the trees (e.g., late mineralization, modifications in pH, E.C., etc.). 

An important case can be found in the southern zone of Chile, where pH in many cases is the protagonist in terms of levels below sufficiency. This generates a very large gap between CIC and SB, due to the large participation of H+ and Al+.

Therefore, it is always important to compare the cations (Ca, Mg, K and Na) with the SB and not with the CEC for an effective comparison.

Regarding foliar analysis, the idea is to collect them around 110-125 DFFB, so that this information will be useful in improving the summer post-harvest foliar program and in planning the next production cycle. The timing of harvesting refers to the fact that nutrients must be stable in the plant for it to be an effective reading. 

In terms of dates, it should be sufficient to be able to take foliar samples after December 25. 

On the other hand, the purpose of the fruit analysis at harvest is to measure the quality of the nutritional plan that has been implemented, both via soil and foliar, between the pre-flowering period and harvest, and thus optimize the management of the next productive season. The information generated allows for the association of all that was implemented at the orchard level with the potential of the fruit in storage, according to the balance that was achieved.

Vigor index: The nutritional program’s “gearbox” 

An indicator that has allowed us to continue optimizing the nutrient supply is the determination of the vigor index at the level of each block and according to the main varieties. This indicator has been very useful in segregating the vegetative growth behavior of different quarters and/or orchards and thus establishing a fertilization plan that is more closely tailored to the needs of each condition.

Keep in mind that the correct definition of “vigor” is the vegetative development within the growing season, recognized in practice as the evolution of the annual shoot. Not to be confused with the size of the trees, since it is possible to recognize trees of large size, but not very vigorous because the annual development is deficient, and vice versa.

For the vigor index, a scale has been established that fluctuates between 1 and 5, with an optimum value of 3.5. As that this indicator begins to decrease and approaches values of 1, it means that we are in the presence of weakened trees, probably in serious problems in terms of development and growth recovery. At the other extreme, as the index begins to increase until it reaches 5, it means very vigorous trees, with consequences in terms of low fertility, inadequate distribution of nutrients in the different organs, low luminosity (excess shade) and probably an association with phytosanitary problems such as some wood fungi and bacteria (Table 4).

The vigor index is related to the quality of the fruit wood and the tree’s capacity for renewal, since to the extent that we have fruit wood with sufficient vigor, it means that we will have fruit with optimum caliber; otherwise, trees with weak fruiting material result in fruit with deficient caliber and that arrive at harvest with serious difficulties in achieving adequate parameters; even recent evaluations indicate that it would also be associated with poor post-harvest performance.

The idea is that the vigor index is evaluated at various times during the season, for example: fruit set – jacket fall, veraison – pre-harvest, even during pruning, it can be rechecked for the following cycle, since it allows us to observe the vigor of the fruit wood and the quality of the renewal material. 

Assessments during the fruit growth cycle allow us to react with certain modifications in the nutritional program, in situations where the vigor of the tree is being affected by the high nutrient demand of the fruit (as happened in some quarters during the previous season) or at the other extreme in the presence of low loads to increase the vigor of the tree. 

If it is not possible to make all the corrections in the pre-harvest cycle, it will support the nutritional decisions of the post-harvest summer program that will be implemented immediately after harvest and after the establishment of the nutritional balance. 

The idea of this indicator is to be the link between what is planned and the expression we see in the trees, allowing fine tuning during the season, so as not to end the cycle with weakened or very vigorous orchards, without having had any signal.

In the search of generating this information more accurately and with less human performance, we have been working in recent seasons with the use of images (mainly through the use of drones), both orthophotos (RGB), spectral thermal images and the use of indicators such as NDVI (Normalized Difference Vegetation Index), for the interpretation of biomass (misnamed vigor) permanently and even recognizing the evolution in the season. These works are being carried out with good preliminary results in order to be able to reach a more concrete management in the short term.

The work that is carried out permanently every season in the field together with the technical team at the orchards (teamwork), will allow an adequate establishment of the nitrogen requirement for the productive cycle. Since, based on the combination of vigor index, rootstock and yield estimation, the contribution of Nitrogen units per hectare is calculated. 

As an example, for a Lapins/Colt combination, with a vigor index of 3.5; with an effective extraction factor of 3.5 N units/ton fruit produced and a yield estimate of 15 ton/ha, meaning that it has a requirement of 53 units of N/ha.

The way in which these units could be supplied is mainly via Calcium Nitrate, with the idea of starting the fertilization cycle to the soil around pre-bloom/white bud in a partialized way (start loading the system). 

The above could undergo some changes depending on the need for irrigation or the availability of water for early irrigation. This could adjust the number of partial irrigations to be used in the fertilization program. In very heavy soils, fertilization irrigation (“technical irrigation”) is not advisable and, in this case, the nutritional program should be adjusted to a lower number of partial irrigation (the tree is an integration of all productive factors, otherwise, the nutrient supply will be absolutely inefficient). In orchards in the country’s south, given the lower water demand, in many cases it is necessary to opt for short fertilization irrigation, otherwise it would not be possible to fertilize almost until December.

Another relevant element is the contribution of potassium (K). In terms of requirement, consider extractions around 7.5 – 12 K units/ton fruit produced, which varies according to rootstock, varietal requirements, balance of the element in the sum of bases, important changes in the estimation of yields advanced in the growth cycle, among other considerations.

Under this concept of an integral program, calcium intake has become more and more relevant, based on the fact that it is a fundamental element for a sufficiently balanced fruit. From the perspective of inputs, the first of them could come from Calcium Nitrate (15.5% N + 18.9% Ca), which is implemented early in the season, even before bloom in areas where soil temperature and irrigation allow it; then comes the use of floable Calcium, there are countless options on the market, most with comparable concentrations in terms of Calcium input in terms of comparative weight/weight. At similar Ca w/w concentrations (e.g. 25% Ca = 35% CaO), select the best options in terms of solubility and cost (US$/kg). Regarding the contribution of Ca units, via flocculent calcium, the idea is to consider values around 25 Ca units/ha. However, formulations with a higher concentration of carriers such as carboxylic acids, vegetable extracts and even amino acids can make a lower concentration of flocculent Ca more efficient in its use with good results.

With the idea of Calcium as a “luxury” consumption element, its incorporation is complemented with the foliar program, which starts in the flower and continues until a little before harvest, so that the fruit always develops with a good supply of Calcium (under the slogan “more is more”). In addition, it is important to maintain a regular application frequency, and this is addressed in the planning of the application calendar at orchard level (orchard staff work to ensure compliance with the plan). 

It is very important to implement at an orchard level what was structured as a fertilization plan, trying to adjust the optimum incorporation times, if we want to achieve a positive response. It is worth mentioning the words of one grower: “the secret is to really implement what was planned and that it does not remain only on paper”.

Nowadays, a nutritional program with a complete supply of nutrients via soil and foliar could represent between 25 – 30% of the cost of the phytosanitary program, hence the relevance of a correct selection of the sources to be used. In the case of foliar lines, it is very important to select those that allow us to carry out mixtures with absolute safety.

Under the concept of fertilization program, we must not lose sight of the fact that everything we have previously mentioned is also accompanied by most of the foliar elements, such as: Potassium, Magnesium, Phosphorus, Boron, Zinc, Amino Acids, Biostimulants, Multiminerals and Bioelicitors, according to the requirements of each orchard’s condition.

The implementation of the most appropriate management for each orchard requires significant team work; both by the internal staff and the professionals who provide technical support to the fruit projects, because today more than ever a comprehensive view of the tree is required to meet productive demands.