Help my tomato plants!

Hey fellow permies,

My indoor tomato starts are showing some discoloration on the bottom leaves. They are yellow with a brown spot. They were started in March.

Any ideas?

From the pictures it seems the biggest issue is that your light levels are too low. Notice how 'leggy' the plants are, with long lengths of stem between leaf internodes? This is from low light levels.
Without adequate light, it is hard to say if there is a fertility issue. Light and water are more primary needs for plants than food.