Farmers are increasing the use of fertilizers in chestnut, the only cash crop produced in
the mountainous areas of northeastern Portugal. This calls for more studies to guide them towards
a more ecological intensification. The effects of three controlled-release fertilizers, two that release
nutrients over three months (BoskGrow 20:05:20_3m, Exactyon 18:05:13_3m) and one over six months
(Exactyon 18:05:12_6m), and an organic amendment authorized for organic farming (Humix 12:03:05)
were compared with an untreated control during a three-year field trial (2019–2021). BoskGrow
20:05:20_3m, Exactyon 18:05:13_3m and Humix 12:03:05 gave significantly higher nut yields (90.6
to 97.0 kg tree−1, average 2019 + 2021) than Exactyon 18:05:12_6m (66.3 kg tree−1) and the control
(69.5 kg tree−1). Leaf concentrations of nitrogen and potassium tended to be higher in the BoskGrow
20:05:20_3m and Exactyon 18:05:13_3m treatments, and they were stated as the most important causes
in the establishment of the two productive groups. Humix 12:03:05, although less concentrated in
nutrients, led to a chestnut yield at the level of the most productive treatments, possibly due to the
multiple positive effects of organic matter on soil and plants. Under the conditions of this experiment,
where rainfall is low in the summer, fertilizers whose nutrient release takes a long time, such as
Exactyon 18:05:12_6m, seem not to be a good fertilization option due to reduced nutrient uptake and
increased levels of soil inorganic nitrogen at the end of the growing season. Humix 12:03:05 emerged
as a possible solution for organic producers.
