The question specifically references 'organic' fertilizer. I don't think urea, 8-3-9, or calcium nitrate are NOP approved fertilizers. The short response is that tree tree trimmer mulch is the best organic fertilizer for the OP's area that I know of. The TLDR; version is below :-).
I'm a firm adherent to the philosophy of "go big or go home," and after roughly a decade of applying 8 - 12 inches of tree trimmer mulch every two years to the entirety of my ~10,000 sq foot backyard, I've drawn some conclusions. To give you an idea of how much mulch that is, it's over 1,000 cubic yards, and if it were applied all at once, it would cover my backyard to a depth of around 4 feet. After decomposition, it has raised the soil of my backyard nearly a full foot.
It took many years for the mulch to become a net contributor to soil nutrient levels (it's generally not until the 3rd mulch that it starts to show positive results), but the difference has been overwhelmingly positive. Many of my trees went from chlorotic to healthy with dark green leaves. Even my carambola, which would not fully green up under any fertilizer regimen, is now permanently dark green with zero fertilizer application. Pretty much anything that gets planted in the backyard now grows like a weed. In contrast, trees planted in unmulched areas struggle to grow and present with obvious nutrient deficiencies.
As far as soil pH goes, my soil went from a pH in the high 7's to a pH in the high 6's. That's a full point pH drop, which is significant considering that the pH scale is logarithmic. Fully composted organic material tends to want to neutralize pH, so both an acid and a basic soil would want to go neutral (pH 7.0) with enough organic material. Organic material, having a high CEC, has a very high buffer capacity, meaning that it's hard to move the pH away from neutral -- a good thing in the case of calcium rich South FL soil. When I asked Dr Crane what to do with the adjacent lot (which was unmulched and devoid of topsoil) to make it amenable to growing fruit trees, his recommendation was "green manure" (mulch), as it would neutralize the pH and provide nutrients to a dead soil.
Aside from the fact that mulch is a great slow release contributor of nutrients, it also builds a very healthy soil, abundant with microorganisms and soil builders (eg, worms and millipedes). My soil is teaming with life, and worm castings are everywhere.
Notwithstanding the above, there are some drawbacks to heavy mulching that I've observed over the years. For one, organic matter can tie up some minor elements -- in my case copper and manganese. Fortunately, the only tree which shows manganese deficiency is the jaboticaba, and I'm currently working on a good way to correct this. The copper deficiency is actually a blessing in disguise, as it allows me to spray (OMRI-listed) copper fungicides (which are indispensable for growers not within a couple miles of the coast in South FL) without fear of causing copper toxicity.
The other drawback is that nitrogen levels stay high, which is not ideal for mango trees. The two problems with this are high growth rate and high incidence of soft nose and jelly seed. However, based on an article I found in FSHS, application of gypsum may be able to correct the latter problem while preserving the beneficial effect of larger harvest that higher nitrogen permits (
http://fshs.org/proceedings-o/1962-vol-75/364-371%20(YOUNG).pdf). I'm going to be experimenting with gypsum application for next year's harvest.
I know that the mulch vs no mulch is a highly debated topic, and I know that some growers refrain from using it (eg, Ms Wenzel), but I think the positives outweigh the negatives, unless you already have an ideal soil. I can't say that it's perfect, but neither is relying on fertilizers to provide nutrients in a sandy / deficient soil; that's pretty much hydroponic cultivation :-). And, if one isn' t exclusively growing mangoes -- which tend to be the only species here which reacts adversely to nitrogen input -- then mulch is a no-brainer.
Many granulars have urea as one of the nitrogen sources.
A very prominent local mango grower is not so keen on utilizing a lot of mulch as it leads to an abundance of organic matter that over time, can alter the soil's ph which can be detrimental for the mango tree's absorption of needed minor elements.
Topic: Mango Growers.....Best Organic Slow Release for Florida? (Read 4337 times)