Vegetable crop symptoms can come suddenly.

My soil has lots of Calcium (Ca) upthrust from the sea bed. Nitrogen (N) fertilization with urea/ammonia augments Calcium (Ca) uptake. This makes roots absorption of Potassium (K) & Magnesium(Mg) potentially less.

In the Ca rich soil environment (naturally occurring or excessive additive induced) the high pH of urea's ammonia needs monitoring. This form of N, in abundance, contributes to an initial rise in the local soil pH from.

With transient pH elevation less Iron (Fe) & Zinc (Zn) are able to be utilized & confusing symptoms of deficiency can appear. To correct a percieved lack of what plant should be doing compensatory over fertilization is common.

If the urea/ammonia fertigation levels get up to 200 ppm there is an actual outflow of K ions. The way to avoid the ensuing under developed lateral/feeder/root hairs & resultant stunted plant growth is to keep sufficient K applied at the same time as urea/ammonia.

Urea/ammonia starts to volatilize when the soil moisture increases & is exposed to rising temperature. The decreased ammonia concentration (& soil bacteria nitrification) are cross linked to chemical reactions which is how the pH drops & acidifies a growing medium.

Many, with high pH nutrient brews, use urea based MG in fertigation solutions as the water (hydrolysis) can bring enough H+ ions out to edge the liquid pH downward. The ratio guideline would be 1 tablespoon of MG for each 20 gallons of fertilizer mix. Of course, some may get residual urea/ammonia in the growing medium.

In containers urea/ammonia operate in a closed eco-system. Basicly 5-10% urea/ammonia will be safe to use, because it will not drive pH excessively; 15% is considered an upper limit & at 20% hidden problems await.

Low pH favors excessive uptake of Fe & Zn, but stymies N. This is where chasing leaf chlorosis (yellow) by automatically adding more N drives in more Fe & Zn (which are meant to be trace minerals), plus accumulates excess N risking plasmolysis.

Forum sages' advice has been consitent: fertilize weakly to avoid pitfalls later in vegetable growing season. The distinction between soil pH & liquid fertilizer pH detailed elsewhere should be kept in mind. This was written since many use what they consider "balanced" brands of fertilizers that are formulated with urea N in container growing.