When I was formulating my first "palm special" fertilizer (a 15-5-10) I used composted sewage sludge as a additive. At the time I had several reasons for choosing this ingredient. For one, it is a relatively inexpensive "bulking agent" that can be used to reduce the N ratio of other nitrogen sources. Also, being an ecologist, I thought it seemed environmentally prudent to use compost products.
At the University of Florida, Ft. Lauderdale Research Center we planted palms in blocks where we would test various palm products. Imagine the disappointment of this recently hired Assistant Professor when I found an inverse relationship between my wonderful "palm special" and the actual growth of the palms. As I raised the rate of this fertilizer in the palm plots the palms looked yellow and stunted. Clearly something was amiss.
At this same time I was interested in "old nursery tale" that recommended throwing a handful of superphosphate in the planting hole when transplanting landscape plants. We trialed this theory and again found an inverse relationship - the more phosphorus (P) you added to the soil the worse the palm looked.
In both of these trials the palms developed stunting of the new growth. Some of the palms with higher P levels actually died. Putting two and two together I deduced phosphorus must have some bizarre interaction with palm nutrition. This turned out to be true, but was not the reason my first fertilizer formulation failed so miserably.
It wasn't long after this that we started experimenting with the addition of compost in potting soils. The results were mixed, but we definitely saw a stunting in many of the container grown palms. On other plants like Ficus benjamina and Schefflera the addition of compost seemed to have little affect.
By now we were pretty sure that we had found a link between induced manganese deficiency and compost products. For that reason I reformulated some of the early "palm specials" without compost and, sure enough, the palms grew much better and didn't develop Mn deficiencies.
The question is why does this happen with many palms, but not other landscape and container plants? I don't think we have proven what I feel is the strongest hypothesis, but it seems to make sense why composts "tie-up" Mn.
To understand this better you need to have some understanding of "cation exchange capacity" in soils. Cations can range from a 1+ to 3+ in soils. These cations provide a site where the nutrients in fertilizers can attach to so that they don't immediately leach out of the soil. Clay soils have a high CEC while in sandy soils it is low or almost nonexistent. To increase the CEC of sandy soils we often add organic matter (like peat) since as this breaks down it adds CEC to the soil. It may be that composted sludges create an almost "super CEC" that holds onto certain nutrients (particularly Mn) so strongly that they are unavailable to certain plants.
We know that palms seem to have special macro- and micronutrient needs. Perhaps they are less efficient at taking up certain nutrients. Or maybe these palms evolved in tropical soils where the Mn rates are high to begin with. Not all palms show Mn deficiencies as quickly as Queen, Coconut, and Date Palms.
So, am I against the use of composts as a source of plant nutrients? Absolutely not - but the interaction of these products with Mn availability needs to be considered with certain palms. If you are using composts or manures and are happy with the results, go ahead and continue. If you are formulating a palm fertilizer I would re-read the sign on the door by Laurel & Hardy.
