Plants that vine and grow very fast need more nitrogen early, but that needs to be stopped as flowers develop so over lush greenery is avoided. At the midpoint, more P and K are useful. There is a fertilizer called calcium nitrate that has one of the most immediately useful forms of calcium, which is also very important for building plant cells. You dissolve it in water and apply.
If the leaves are yellowing at the tops of the branches and stalks this means the plants have a lot of food. If the yellowing starts in the oldest leaves, it could be that they are thirsty. If the whole plant goes yellow it may need food.
Thank you @Dirtmechanic and Odif. I have iron coming..should have picked up some Geritol to give them!---and it just dawned on me that I really have missed something I my potting mixture from all these years. I did not give them Any plant food in the pots beside castings. Goodness I wait until I get space to go crazy with, then I act crazy...geez!!!
Geritol for iron ? Do they still make that stuff ? Just find some rusty nails and throw them into a container of water and let sit a while ! Water will turn a nice rusty red ! Dilute before feeding the plants !
Could you send a foto? Depending on the deficiency---there could be different patterns of yellowing. Are the yellow leaves on the bottom of your plants , in the middle or on top?
@Sjoerd...this is the pot before everything started growing.. you can see it beginning and I didn't stop it then and it was much worst..almost no green as they grew. I haven't taken too many pics since, but it is doing much better. I watered with epsom salt/water, waited 2 days, then added granulated veggie fertilizer and worm castings (watered that in well). It stopped yellowing one day after the epsom salt and new leaves show no sign of deficiency at this point. Time to water with worm tea again though.
What a mystery this is, Hummer. When I look at the container, i see that not all of the plants have yellowing leaves. That is encouraging. The yellowing courgette leaves do make me think that they are the young leaves that have finished their job...The newer growth seems to be green. That one courgette seems too small to be blooming though. I hope that the plants do not have too rich a soil. We will have to look again in a week or so to see if there are any definitive signs present. I am scratching my head on this one.