Cutting-edge fertilization

The effects of climatic instability, the reduction of soil organic matter levels and increasingly competitive market conditions require the adaptation of fertilization practices by using advanced tools capable of managing and reducing the spatial variability of our vineyards

Variable rate distribution performed using a prescription maps

by Marco Sozzi

The succession of productive seasons characterized by fluctuating climate, such as long-lasting periods of drought interrupted by abundant rainfall, has important repercussions on the nutrition of the vine, due to the influence that these events have on the water and nitrogen cycle. Likewise, such climatic events can have important effects on the vegetative-productive balance and therefore on the ripening process of the grapes. In particular, temperatures above the seasonal average can determine an excessive increase in the sugar content of grapes and a rapid decrease in titratable acidity. Excessive rainfall can cause water stagnation and leaching of soil nutrients. Moreover, the specific defense needs of the vineyard impose several entries into the field, which can deplete the soil structure, further reducing root absorption capacity.

Unilateral distribution diagram of a fertilizer spreader

In this context the need emerges to find the best technological solutions for the management of vineyard fertilization. On the one hand, through the use of slow-release fertilizers able to ensure the nutritional needs of the vine for extended periods, reducing losses due to runoff and nitrification, and on the other hand by using variable rate fertilizer distribution systems able to distribute the right amount of fertilizer according to the nutritional needs of each single area of the vineyard (or even of each single plant). In recent years, many manufacturers of agricultural machinery have developed variable rate fertilizer spreaders for the vineyard, mainly adapting solutions developed for the open field.

From a constructive point of view, centrifugal fertilizer spreaders or spreaders equipped with rotating distributors have been proposed for variable rate applications in vineyards. In both cases it is possible to adjust the distributed dose: by acting on the opening light of the hopper as far as centrifugal spreaders are concerned, or by modifying the rotation speed of the distributor on fertilizer spreaders equipped with rotating distributors. These solutions require adequate calibration in order to determine the quantity of product distributed per unit of time depending on the variations in the hopper opening or in the rotation speed of the rotary distributor.

Evaluation test for the flow coefficient variation

The most advanced variable rate systems are equipped with load cells capable of detecting the weight of fertilizer loaded into the hopper. These cells are usually installed in four orthogonal points between the hopper and the frame, in order to adjust the variations due to the inclination or acceleration of the machine. The presence of load cells also allows the spreader to perform an automatic calibration of the amount of product distributed (although it is advisable to perform manual calibration in order to avoid distribution errors during calibration in the field). On fertiliser spreaders without load cells, the inclination is adjusted through the use of inertial sensors (IMU), which may also be present on fertiliser spreaders with load cells. For some years now, manufacturers of agricultural machinery have started to equip their spreaders with electronic control units compliant with the ISOBUS standard (ISO11783), ensuring interoperability with different monitors and simplifying implement setup and steering operations. Recently, a number of fertilizer spreaders have appeared on the market, capable of independently distributing the product on the right and left sides, ensuring specific distribution for each individual plant. This solution has been made possible thanks to the installation of separate hoppers and distributors for each of the two sides. The amount of fertilizer to be distributed must be properly weighted, taking into account several agronomic aspects. Fertilization must be able to ensure the restitution of nutrients removed or lost during the previous year in order to preserve previous fertility levels. To do this, it is necessary to take into consideration not only the productive level of the vineyard, but also the losses due to leaching and the state of the soil. Such reasoning, although widely used, are characterized by a high degree of approximation and moreover they do not take into consideration the spatial variability naturally present in vineyards, which is manifested by many fertility gradients.

Spreader and sensor mounted on the tractor used for simplified variable rate fertilization

[su_box title=”A simplified approach” box_color=”#43634b”]The previously presented on-the-go approach is being tested in an experiment carried out by the TeSAF Department of the University of Padua and the CREA-Viticulture of Conegliano (TV). This experimentation involves a vineyard characterized by a considerable spatial variability that affects both the vegetative vigor and the characteristics of the harvested grapes. In this case the variable rate fertilization has the ultimate goal of reducing this variability. During the 2020 and 2021 seasons, spring fertilization was carried out using the Trimble Greenseeker sensor to automatically control a variable rate fertilizer spreader (Kuhn MDS 12.1 Q). In order to define the connection between the NDVI values read by the sensor and the amount of fertilizer to be distributed in each point of the vineyard, ten sampling points were identified in which the amount of fertilizer to be distributed was quantified according to the development characteristics of the vine and at the same time the vegetative vigor (NDVI index) was measured. Using these measurements, the relationship used by the fertilizer spreader for distribution was then defined. Although the experimentation is still in progress, it is possible to summarize some preliminary results. In terms of the effects on vegetation, the variable rate fertilization interventions ensured a higher percentage growth of the more fertilized plants (located in areas of the vineyard with low vegetative vigor) compared to plants fertilized with a limited amount of product (in order to limit vegetative growth). In terms of variability, these fertilization interventions allowed to reduce the coefficient of variation of the vigor index by 40%. Although the system currently requires a monitor equipped with a satellite positioning system (GNSS), this is not essential for this type of application. In any case, the system used for this experimentation has a significant cost, but considering the current system of incentives for this type of solutions (Agricolture 4.0, Sabatini and possible PSR) it could be an excellent option for farms with high surface area planted with vine.[/su_box]

In order to take these aspects into account some sensors can be used that can determine the state of vegetative vigor (eg NDVI) of plants in a rapid and non-destructive way. These sensors are able to return vigor maps which, properly integrated with the data of production level and soil fertility detected in a few points, can be used to define prescription maps to be used with modern variable rate fertilizer spreaders.

In order to simplify this methodology (which requires several passes through the vineyard in order to define a prescription map) a simplified approach (on-the-go) can be used, which involves the prior definition of a relationship between the values read by the sensors and the amount of fertilizer to be distributed. This report, loaded into the monitor used for the management of the fertilizer spreader, will then be used to automatically distribute the fertilizer in the vineyard, eliminating the need to make a double pass with the spectral sensor.

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