The truffle is an ectomycorrhizal fungus, which is hypogeous and edible, living in symbiosis with the roots of certain species of trees and shrubs, such as the holm oak and common oak, among others. Basically, the tree supplies the fungus with the sugars required for its development and growth and, in exchange, the truffle helps the roots to better absorb and synthesise the mineral elements.
It has been more than 60 years since “the truffle hunt” took an unexpected turn, evolving from being an activity that is half-way between a hobby and eking a living out of the forest, in deprived areas, to become a more profitable operation, with a significant socio-economic incentive providing hope for certain geographical regions in Spain, where advanced depopulation went hand in hand with the reduced success of the agricultural and industrial sectors.
Gudar-Javalambre is one of these Spanish regions that has benefitted from this ‘cultural metamorphosis’, which has transformed the driving force of its economy, leaving behind the concept of subsistence farming to become the reference epicentre for growth and investment in the national and European truffle-growing sector.
Situated in the foothills of the Iberian mountain range, in the province of Teruel, with a mountainous environment that encourages thermal fluctuations throughout the year and, therefore, conditions that are favourable for the development of winter truffles, Gudar-Javalambre has learned how to use its weather feature, i.e. a marked seasonal variability and harsh extremes (very cold winters and mild summers), with well-aerated soils, a slightly alkaline pH (7.5 – 8.5) and sandy loam and plenty of sunshine, promoting something that, in agriculture, was more difficult… the profitable farming of the fields through intensive cropping, i.e. truffle-growing.
A third positive factor, its rainfall rate, is also a determining influence, or at least it was, to the tune of 500 to 800 mm of annual rainfall, which the truffle needs for its growth and ripening. The availability of water, on the other hand, is of primary importance for this crop, above all in the summer, when its requirements play a deciding role in its development.
The Tuber melanosporum, known as the black winter truffle or Perigord truffle, is the most used in the kitchen on account of its excellence and penetrating aroma.
It represents around 95% of truffle production in the Gudar-Javalambre region and it is often found growing next to the evergreen or holm oak (Quercus ilex), although there is an interesting development insofar as it is also found growing next to the common or English oak (Quercus robur), Portuguese oak (Quercus faginea), kermes oak (Quercus coccifera), common hazel (Corylus avellana) and other species of trees and shrubs.
The rest of the production includes varieties, which are less importance and less common, such as Tuber aestivum, also known as the black summer truffle or the truffle of Saint Jean, with a milder aroma, taste and intensity than the abovementioned, and Tuber brumale, known as the Muscat truffle, is of inferior quality and size to those mentioned above; both types having a lower culinary and commercial value.
The increasing demand for black truffles, which is well above the production capacity of truffles in any zone, has resulted in the emergence of certified mycorrhization nurseries, with truffle fungi inoculated into oak seedlings (the main species accommodating this culture in the region) to satisfy a strong domestic demand for the crop, as well as the export requirement, which represents another chapter in the socio-economic transformation of the region. Of course, increased support through the establishment of truffle research and experimental centres provides solid backing, thus confirming the sector’s growth, strength and future.
Truffle farming is a relatively new agroforestry technique, so there is not sufficiently detailed information available on the advantages of the management and use of irrigation to allow us to plan the depths of water applied throughout its cycle, unlike other crops, where there is a direct relationship between the supply of water and crop production; and we must remember that in this system, the oak trees are nourished in order to receive the mycorrhizal fungi around their roots.
For this reason, the concept of applying irrigation water has been created pseudo-empirically, via irrigation sequences, of 6-7 hours in some cases, with a bimonthly frequency (between 25-35 mm/irrigation), where the main criterion is to compensate for the lack of rainfall, which at other times has been able to cover presumed requirements that are yet to be defined.
It is obvious that a healthy tree, well-nourished and with the correct mycorrhizal association, has a greater potential for truffle production, so, without going into greater detail for documentary reasons, this is the goal to be pursued under the different conditions experienced by each producer.
Therefore, implementing the irrigation systems over the last 25-30 years has played a decisive role, allowing the crop to become established and considerably increasing the production figures because, bearing in mind the reduction in annual rainfall, the irrigation systems represent an essential support tool in the whole truffle-farming sector.
According to the statement made by Eladio Salvador Rendón, one of the pioneers of modern truffle farming and one of the most experienced in the region, “for truffle-farming, the uniformity of distribution of the water applications is particularly important. In the past, the rains were more constant, and the summers were not as hot as they are today, so the fungus developed naturally; over these last few years, there have been prolonged periods of drought and, thanks to irrigation, we have been able to provide the crop with the water that it needs. As a result of the progress in irrigation technology, we have been able to adjust the system so that the precipitations that fall naturally meet the actual water requirements, thus ensuring a very even production.
The crop’s water requirements invlove maintaining a constant level of moisture throughout the year, even though it is certain that during the 3-4 summer months it is essential to have more abundant applications of water to achieve a better quality.
To achieve uniformity in the production and quality, then irrigation must be applied constantly, with a need to define the moisture contents of each type of soil so as to be able to manage the amount of water applied to the crop.
Based on this experience, the concept of a well-designed irrigation system for truffle farming will guarantee the survival of the plants in the initial stages and will represent a solid basis for the crop to obtain the desired size and uniformity of the truffles.
For this reason, in order to try and define technically the different times when the varying amounts of water can be applied, it is necessary to provide the irrigation system with a versatility that enables substantial changes to be made to the wetted surface of the “scorch” (volume of soil with mycorrhizal roots and with a potential for production, the surface normally showing a lack of vegetation), by slightly modifying some of the components, such as using the double-stage rotor incorporated into NaanDan Jain’s Aquasmart 2002 and Aquamaster 2005 micro-sprinklers.
The double-stage aspect of these emitters allows us to achieve the correct hydraulic dimensions of the system from the beginning, in a definitive manner, by applying the irrigation water in a focused way when the young plant is at its weakest and, of course, with far shorter irrigation sequences. When the extension of the rooting system gives the plant a certain self-sufficiency (more or less 2-3 years of life), the rotor of the micro-sprinkler will be modified to achieve the definitive wetted diameter, encouraging even more the rooting system to extend horizontally.
On the other hand, knowing and being able to control certain parameters of the “scorch” will allow the producer to anticipate and optimise the depth of the water applied; with its nutritive effects on the oak tree; and through preventative actions that could mitigate, for example, the temperatures that could be harmful to the development and growth of the fungus. At this stage, the AGTech application with Root Sense sensors, which determine the water potential and temperature of the soil, allows decisions to be made that are complementary to the planning of the irrigation, thus guaranteeing success in the monitoring and control of the crop’s development and growing conditions.
By way of an example, excessive heating of the soil continuing at nightfall because of the high air temperatures that have built up during the day, leads to a fall in the production of truffles, thus giving an extra significance to any measurement and control actions that may be taken within the context of the development of the fungus.
In this respect, Root Sense provides data on changes in the soil temperature and, by configuring an alert, it allows for immediate action to be taken to reduce the temperature by triggering irrigation, the device also being capable of measuring and determining the soil water potential at a considered point.
It is obvious that the experience and wisdom drawn from the human factor, unquestionable pillars of support for this crop, have contributed and continue to contribute towards the socio-economic changes and growth of the sector, in the same way as water and irrigation management supplement the natural decline in rainfall and its respective interaction with the soil, representing as a whole a combination of elements that are pivotal in the efficient development of Modern Truffle-farming.
Acknowledgements. We would like to thank Eladio Salvador Rendón, truffle-farmer and nursery grower, from the region of Gudar-Javalambre, for his help and wisdom, which has enabled us to gain an in-depth knowledge of the truffle crop, for the preparation and publishing of this article.