Hi Jorun,
Thanks for your thoughts.
My sickly olive is near the edge of a terrace, with a steep slope below. So it is not at much risk of water-logging. The fields down below are a different matter (see photo, 19dec18), but the olive trees down there are showing none of the symptoms of my sickly one.
However, in the autumn when the rains began (intermittently), I was still irrigating and this olive is on the same run as my citrus trees' irrigation. So it might have been getting some water. But not enough to stress the roots, IMO.
Reading-up on mineral deficiencies in olive, specifically
Joe Connell's presentation 'Olive Mineral Nutrition'
http://ceglenn.ucdavis.edu/files/90442.pdf:
- the most common deficiencies are in N, K and Bo
(in California at least, deficiencies in Zn, Ca, Mg, P, Cu, Fe, Mn, Mo are ~unknown)
- my tree is not showing classic symptoms of deficiency of K or Bo
- my tree might be showing signs of N deficiency
(small and yellowish leaves, poor shoot growth, sporadic bloom, poor fruit set)
This paper gives more detail on symptoms, some of which hint at a boron deficiency, viz.
"- Chlorosis (yellowing) and death of the growing points.
" - Trees suffering from boron deficiency appear chlorotic from a distance and delay entering the vegetative stage."
[https://www.haifa-group.com/ro/olives/crop-guide-olive-trees-nutrition]
But my tree's leaves are yellowish all over; they don't show the classic brown tip and yellow band between the brown and green; nor the 'monkey-face' fruits.
I note with boron, that there is a risk of toxicity due to over-fertilization.
//Mike