I'm developing an ecology for a portion of my world that's geologically active and I remembered hearing that volcanic ash in soil makes it more fertile to agriculture.
After extensive googling I've found surprisingly little information about the subject, and nothing convincing. Is this just a science "fact" that gets spread around without actually being completely true? If it is true, or partly true, what is the chemical process at work, and under what conditions is it the most effective?
As always: It Depends.
Assuming enough water and sunshine, crop growth rate boils down to the concept of limiting nutrients. These may be: nitrogen (via ammonia or nitrates), phosphorous (via phosphates), potassium, and sometimes others. In typical continental settings (i.e. subduction related vulcanism), lavas may be enriched in potassium and phosphorous but not nitrogen. So there will be some effect from rock composition.
Perhaps more important, there will be a refresh effect. If we have regular ash falls, this will 'top up' the soil as compared to normal soils, replacing nutrients. This is a similar effect to flood plains.
So, to answer, it tends to be true that volcanic eruptions increase fertility, but the most effective kind are from explosive ash-fall vucanism you get in regions like Italy or the US Cascades. The kind of flood-plain basalts you get in Iceland probably don't help.
Plants follow the law of the minimum. There's a ratio at which nutrients occur in plant matter. If it can't get those amounts, it will be limited by the one nutrient that falls below the ratio. Let's say a plant wants to grow 123 grams of biomass. It needs 106g of carbon, 16g of nitrogen, and 1 g of Phosphorous (this is called the Redfield ratio, and it occurs mostly in phytoplankton). OK, let's say it has just enough carbon and Phosphorous, but what happens when it only has 15 grams of nitrogen? It still has to maintain the ratio, so it winds up using only 99g carbon and 0.93g Phosphorous, even though it has more than enough carbon and phosphorous.
Sometimes, nutrients come in different forms, and only some of these forms are easily accessible to plants. Certain nutrients cycle through these forms in different ways. Inaccessible nitrogen, for instance, can be fixated by certain kinds of bacteria to become accessible once again. Phosphorous on the other hand is much more likely to go from accessible to inaccessible, but accessible forms can be supplied by volcanic soils. I don't mean to say it's just Phosphorous causing growth after an eruption (it depends strongly on what gets released, as well as what limits the plant prior to eruption) but this is the sort of thing that can go on.
I disagree with the statement that flood-plain basalts don't make fertile soils. The most fertile soils in southeast Australia are on the Victorian volcanic plains. The soils of the Deccan Plateau of India are considered very fertile. There are very fertile soils formed on the Columbia River basalts of eastern Washington state and Oregon as far as the Willamette valley. In addition to the soils formed in place, weathered basalt can contribute to the fertility of soils formed on transported sediments, e.g. the silty soil of the Palouse region in eastern Washington state.
Lava flows may take longer to break down because ash fall has higher surface area but the flow tops are often rough and fractured, enhancing the weathering rate. Cinder cones and other 'mild' pyroclastic eruptions are also a feature of basaltic volcanism that contribute to faster weathering.
