I grow my vegetables in containers and have three Meyer lemon trees grown from seeds. What is the best recipe for potting soil? Best recipe to remedy used soil? And is there an N-P-K guide for each kind of plant category (citrus, herbs, leafy veggies, etc)? Thank you!
Re: potting soil recipes, you can use any bagged potting soil, but I prefer not to use anything with "moisture retentive" additives. Then I like to add a quantity of perlite, 1/2 - 1/4 the volume of potting soil. Please understand that there is not one "best;" every gardener finds their own favorite. The most important thing always is to make sure the soil drains well. Here is an article for you on growing lemon trees: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm
container gardening: https://www.gardeningknowhow.com/edible/vegetables/vgen/quick-guide-to-container-vegetable-gardening.htm
and some basic soil information: https://www.gardeningknowhow.com/edible/vegetables/vgen/vegetable-garden-soil.htm
Re: fixing used soil, I'd recommend adding a variety of organic improvements, plus perlite for drainage. You can look for specific fertilization guides for each kind of vegetable, but generally speaking, a good well-drained soil, plenty of light, sufficient water without drowning roots, and a balanced fertilizer used with a light hand (plants make their own food remember - fertilizer is a helper, and can easily be overdone) is the basic formula for success.