In this first blog post we will discuss one of the four main cultivation techniques used in truffle growing: watering.
As all truffle growers know, a summer without water is equivalent to a bad truffle season. Scientists have also demonstrated this through temporal production data over the 20th century(Baragatti et al 2019). Watering is a practice that was already used in the 19th century as in Auguste Rousseau’s plantation in the Vaucluse(Le Tacon, 2016). More recently, at the end of the 1970s, scientists from the INRA in Nancy were the first to demonstrate experimentally that well-managed watering with plaster probes – we will come back to these probes later – significantly increased production (17 to 40 times; Le Tacon et al., 1982). However, since these early experiments, watering has remained very empirical.
In 2016, under the impetus of the French Federation of Truffle Growers, INRAE and CTIFL, the national experimentation project CULTURTRUF was born, funded for 6 years by FranceAgriMer. CULTURTRUF aimedto study the effect of cultivation techniques on the biological cycle of the truffle and the water regime of truffle fields. Among several objectives, the main one was to provide truffle growers with a simple decision support tool to manage watering in truffle fields in a reasoned way. To do this, INRAE scientists, including Claude Murat, decided to measure the water potential(pF), which corresponds to the force needed to extract water from the soil. This force indicates whether or not water is available in the soil. The pF index taken into account was on the one hand the pF of 2.5 corresponding to the maximum water retention of the soil, and on the other hand the pF of 4.2 reached when the soil has dried and water is no longer available in the soil. For more details on this concept, see Our concept, our innovation.
Observations in the first part of the CULTURTRUF project between 2016 and 2019 suggested a watering threshold for Tuber melanosporum at pF 4, corresponding to the temporary wilting point(Bach and Murat). This watering threshold was then validated experimentally on several sites showing that for this species, watering when the pF reaches a value of 4 allows to significantly increase the production, and even to save a production in drought conditions, compared to trees not watered(Bach and Murat). A factor of times 4 to times 6 was then observed.
Concerning the quantity of water brought, it was estimated, for a very dry year, a contribution of1 to 3 cubic meters of water, per tree. The quantity of water brought for a watering should be of 20 to 30 mm at least.
Another interesting result of this project was to highlight that in addition to the already well identified summer water stress, the stresses observed in spring also have a negative impact on truffle production.
In conclusion, for Tuber melanosporum, it is now advisable to monitor the soil pF at 10-12 cm depth and to water when the pF reaches 4. The use of probes to measure the pF should therefore become widespread, but beware of the reading range of the probes, because it must be in a range up to pF 4.
However, many probes on the market do not allow this measurement, or allow it but at very unaffordable costs (example of TEROS21 probes associated with the ZL6 recorder – average basket 1500 euros).
This is whyINRAE developed a first prototype tool between 2018 and 2019, marketed today by WETRUF and since 2020, the pF Tracer One associated with its plaster probes, allowing to read the pF between 2.5 and 4.5, and at a lower cost.
The study of the experimental data is still in progress within the project CULTURTRUFto determine the watering thresholds for other truffle species but also to determine the impact of the implementation of ancillary devices (shading, mulching), to reduce soil temperature and maintain soil moisture.
In particular, we can ask whether the watering threshold of pF 4 is also valid for T. aestivum var uncinatum?
We will see this in a future publication…