What it says on the tin. Because it's so annoying to shift click it all into fields and pots.
We could use buckets to carry it, with like 10kg per bucket. Right click to use all to fertilize or max to fill a pot or prepare a field. Shift right click to use 1kg at a time.
A super neat feature to go with this would be wheelbarrows. Holds 100kg of humus, while pushing you can use the humus on the fields as if you had it on your cursor. This would save a TON of clicks.
I guess the compost bins would still need a small inventory to hold the earthworms they produce. Humus should simply be represented with a bar.