When placing a farm plot, it starts off at 100 Porosity. Over time it drops down to 0. Using the hoe brings it up by a couple, like 1.5 or 2 or some such.
Certain plants require a certain porosity. One plant may require a minimum of 30, others may require a minimum of 50.
So if you place a planter and leave it alone for a day and come back, it will be down to 0 and you will need to use the hoe on it before planting anything.
After harvesting your plant, you will need to use the hoe on it to bring the porosity up to whatever you need.