Charcoal cannot replace the need for adding mineral nutrients.
I am an unabashed charcoal enthusiast. Used properly, adding charcoal to soil improves biomass production and soil health. Sometimes dramatically when soil productivity is low. Certainly part of the effect is increased nitrogen use efficiency: less N lost to nitrification and leaching. Charcoal also tends to be associated with higher post harvest soil levels of P and K for reasons that are not entirely clear. Perhaps this effect also is due to increased efficiency.
Most TP enthusiasts, myself included, are convinced that the most mysterious effects from adding charcoal relate to soil biology, more than they relate to direct physical and chemical effects, although those realms play important roles also. And, in keeping with my previous post, it seems clear to me that increased energy efficiency is a critical bit here. Plants and microbes are growing more biomass with less effort for reasons that can't be entirely explained by traditional nutrient-based perspectives. Yes, the charcoal adds potassium, yes it raises soil pH, yes it increases soil water and nutrient holding capacity. But the results speak to more, much more.
The behavior of charcoal amended soil seems to defy the limits of the soil-biology system understood by traditional science. However, it would be entirely foolish to think that simple soil nutritional requirements are not still in play. Nutrient deficiencies limit living systems. Charcoal may promote efficiencies that help stretch the budget in regards to those limits, but in the end, the most limiting nutrient before adding charcoal is probably still going to be the most limiting nutrient after adding charcoal.
What got me thinking about this was consulting soil scientist Doug Edmeades’ posts on soil organic matter. The first, Carbon farming: take-off or rip-off, explored how carbon sequestration efforts can cut both ways. The second, Soil Organic Matter Matters, hits on the most-limiting-nutrient.
Pasture plants need 16 nutrients. Without all 16 the clover will disappear, the pasture will be N deficient, the quality grasses will fail, pasture production would collapse followed by a need to cut back the stocking rate and, given sufficient years, a farm would be back to native pastures and bush. In the process soil carbon levels would decline.
Collapsed pasture production is no idle threat. We know that the collapse of legumes in pasture systems in Europe and in the eastern US helped motivate the expansion of the western US. Against that historical backdrop, Benjamin Franklin famously demonstrated sulfur deficiency when he added gypsum to alfalfa to form the words "This has been plastered". Doug Edmeades mentions this because soil carbon sequestration enthusiasts seem to have temporarily lost track of these limits. The same caution applies to charcoal.
There is great potential for increasing productivity through judicious use of charcoal. However, TP enthusiasts must not lose sight of the fact that charcoal cannot replace the need for adding mineral nutrients.