https://www.proflowers.com/blog/homemade-plant-food

The link talks about plant food, but I really like the image within the page that outlines how to identify a few nutrient deficiencies.

Then, the page continues to discuss nutrient deficiencies: nitrogen, phosphorus, potassium, magnesium & calcium. This is interesting & helpful. It can be made with items most of us already have in our pantries.