There are actually two slightly different ways to interpret the phrase "fertile ash". Both are valid, and you'd need to look at the context of the phrase to determine exactly which is meant (this is why I asked where you came across the term).
Sense 1: "fertile" in the sense of "helping fertility"
The Oxford English Dictionary, under the entry for ‘fertile adj.’, gives:
Causing or tending to promote fertility.
The first citation for this sense of the word is from 1597. Other dictionaries contain similar definitions: the American Heritage Dictionary has "Rich in material needed to sustain plant growth"; dictionary.com has "conducive to productiveness"; and Webster's New World College Dictionary has "causing or helping fertility".
Since, as you acknowledge, ash can make soil more fertile, the collocation "fertile ash" seems perfectly valid. If by "not valid according to the Oxford dictionary" you mean that that specific combination of words isn't listed, you're using a very odd definition of "valid": there are a huge number of adjectives which can reasonably be applied to any noun, and you can't expect every possible combination to be listed in a dictionary.
Sense 2: "fertile" in the sense of "producing vegetation"
Gordon Stanger's answer provides a good explanation of how the term "fertile ash" works under the interpretation of "producing vegetation" in itself, rather than as an admixture: while newly deposited ash is unlikely to support plant growth on its own, a relatively small amount of weathering makes it an excellent substrate for plant colonization. For instance, Parnell and Foster (2012) write:
The physical attributes of volcanic ash allow good drainage, but also high retention of plant-available water, which would have been essential to the earliest plants occupying moist lowland areas. It also provides excellent ‘tilth’, the physical condition related to fitness as a seed-bed. The combination of water and nutrient availability would have made ashes a favourable setting for the seeding of primitive plants.
Of course, at some point in the weathering process it becomes more of an "ash soil" than an "ash" -- as with many categories in Earth Science, the boundary is a little fuzzy. But given the speed with which ash deposits are colonized, I think it's fair to say that they're fertile (in the sense of "producing vegetation") even while still in the "ash" category.
Usage as a scientific term
You can find the term "fertile ash" used in many peer reviewed scientific publications -- for example, May et al. (1985), Eckmeier et al. (2007), Sullivan (1982), and Utkin (2013). Thus, it's a scientific term in the sense of a term which can be (and is) used in scientific writing.
It's not entirely clear to me what you mean by a "scientific term". I agree with Gordon Stanger that it's not a "scientific term", in the sense that you won't find a specific entry for it in a scientific reference work -- it's just an adjective applied to "ash", not a specific, extensively studied topic. Nor is it one of those terms which has a different meaning in a specific scientific context (like "catastrophic" or "domain"). When used in science writing, it could certainly be called a scientific term, but its meaning is the same as when used in everyday language.
Conclusion
"Fertile ash" is not a misnomer; it has at least two reasonable interpretations, in terms of dictionary definitions of its constituent words; and it is a "scientific term" in the sense of a term that can be used in scientific publications.
Responses to comments
can you include a screenshot of the definition?
I'm unsure how this improves the answer, but am happy to oblige:
(If you meant that I should screenshot the entire entry, I'm afraid that's not practical -- it's hundreds of words long and thousands of pixels tall, and would add enormous bulk to this answer without providing any relevant additional information!)
it's not listed on dictionary.cambridge.org for example nor is it listed on the oxforddictionaries link in my question
Many dictionary publishers don't make their most complete dictionaries freely available online. As far as I know, all the dictionaries available on CUP's site are for learners of English, and thus aren't particularly extensive. OUP's free offering is is (I think) based on their 1-volume Oxford Dictionary of English, rather than the full Oxford English Dictionary. They're certainly good starting points, but don't expect them to be exhaustive.
References
Eckmeier, E., Rösch, M., Ehrmann, O., Schmidt, M. W., Schier, W., & Gerlach, R. (2007). Conversion of biomass to charcoal and the carbon mass balance from a slash-and-burn experiment in a temperate deciduous forest. The Holocene, 17(4), 539-542.
May, P. H., Anderson, A. B., Balick, M. J., & Frazão, J. M. F. (1985). Subsistence benefits from the babassu palm (Orbignya martiana). Economic Botany, 39(2), 113-129.
Parnell, J., & Foster, S. (2012). Ordovician ash geochemistry and the establishment of land plants. Geochemical transactions, 13(1), 7.
Sullivan, A. P. (1982). Mogollon Agrarian Ecology: An Appraisal and a New Model. The Kiva, 1-15.
Utkin, V. P. (2013). Shear structural paragenesis and its role in continental rifting of the East Asian margin. Russian Journal of Pacific Geology, 7(3), 167-188.
