There is such a thing as using too much fertilizer.

You imply using guano on vegetable plants in late July/early August (mid-summer). Vegetable plants should be blooming and setting/forming edible portions by then and lots more fertilizer could promote foliage growth at the expense of the edibles.

If the plants were already growing well by mid-July, even watering is more important. If they seemed slow to grow, a little more fertilizer, regardless of type, can help.

Guano is a component of some common, bagged fertilizers.

What is an example of your vegetables that were mostly foliage?

Guano - bat, seabird or seal - is an excellent organic fertilizer, often included with blended organic fertilizer mixes but also used as-is. In addtion to the typical major plant nutrients, guano contains a lot of trace elements and micronutrients that are not always present in other ferts.

Since most organic fertilizers take time to breakdown and the nutrients become available to the plants, applying it early in the season is better than later. Applied in moderation/package instructions, it is pretty darn hard to "over do" with organic fertilizers. And guano - as are a lot of organics - is not exactly cheap. 1-2# for 100 sq. ft is all that is necessary.

Before taking the time and expense to add fertilizers, perhaps unnecessarily, you might want to consider having a soil test done. That will tell you if you have any deficiences and how to correct or if you should be looking at something else altogther.

Peppers are the big one (I don't bother with tomatoes, I don't have the right yard, and there are zillions out there for the same 4-week period). Would like better yields on all my veggies, including spinach and letttuce.

Well, spinach and lettuce do not fit into the foliage/fruit ratio mentioned above as they are all foliage and no fruit. It can take 2 or 3 years of soil improvement to get conditions right for growing large leaf crops, unless you are dumping a foot of compost on top of the ground or in a raised bed and planting in that. If you are growing at ground level in yard soil, planting a cover crop in late summer and working it into the soil in late winter can help over the long run, as well as adding various fertilizers in early spring.

If you are trying bell peppers, those require an excellent, rich soil and even watering and perfect weather conditions to get store-sized fruits. I see very few casual gardens here that generate full-size peppers. Bell peppers set fruit in a fairly narrow range of temperatures and some years here just don't cut it at bloom time.

It is actually easier to grow hot peppers, and that includes many hybrids that have the appearance of a jalapeno but have been bred to be very mild. Pimento-type peppers can also do well in the same conditions bell peppers struggle in.

The vegetable forum likely has more frequent advice.

I say it depends on the type of guano you are using . Bat guano more times than not are higher in nitrogen and hardly any phos or potash such as 10-0-0. The guano I chose is the peruvian seabird 12-11-2 and as you can see it would be high in nitro and phos. If you use this in containers pay close attention to the measure amounts. Mine came in pellets so I use a coffee grinder to make it into a flour for easier measuring . This stuff is strong so if it says 1 tsp wouldnt hurt to go light on that as more is not better . I usually mix it well into the medium before adding a plant, then water well.

Using a non-liquid organic fertilizer - like bird/bat guano - in any container planting is not a very effective or efficient practice. Even ground up and added to the planting mix will not help especially as organics require the digestion and assimilation of soil organisms to release their nutrient load and these creatures are just not present in any significant numbers in a soil-less container or potting mix.

You would be far better off using a liquid organic (immediately water soluble) product, of which there are very few, or a standard synthetic fertilizer, either slow release, granular or liquid.

Proper soil structure takes care of many problems that are falsely diagnosed as lack of fertilizer.

Mike