The best soil for kumquats is well-drained loamy soil. Kumquat trees, like their citrus cousins, don't enjoy wet feet. They can rot easily if the drainage is not good, especially when grown in pots. You can tell by looking at the yellowing leaves or the crops they put out.
Soil for Kumquat Trees in Pots
If you're growing kumquat trees in containers, there are some good potting soil options. Here is one from EBStone:
This citrus & palm potting soil is 100% natural & organic. You can use it as a ready-made mix for your plants or as a soil amendment for your native soil. The mix has pumice, sand & redwood bark, which are all great for drainage. There's also peat moss to help with the acidity & moisture retention. This one is available in a big bag at local nurseries or hardware stores.
Another good option is Kellogg Palm, Cactus & Citrus All Purpose Mix. You can use the mix for kumquat trees in containers as well as those in ground. It ships in a big 1 cu. ft bag at a super cheap price on Home Depot.
To make sure the drainage is good in your pots, drill some drainage holes in your containers. You can also place some rocks or netting at the bottom to help the water flow out more easily.
Soil for Kumquat Trees in Ground
If the native soil in your property does not provide enough nutrients for the plants, you may need some kumquat fertilizer. You can make one if you like.
There are some potting mixes that you should not use for in-ground kumquats. Check if they have vermiculite in the mix. This type of rock holds the water a bit more than the perlite. Which may not be ideal for in-ground conditions because it can lead to root rot.
Be sure to spread some mulch around your kumquat on hot summer days to keep them cool. If it's winter & the chance of frost damage is high, you may need to dig the plants up & pot them inside the house.