Hello, Ginny! Thank you for your question to Ask Extension/ Ask a Master Gardener. (We have several names! )

Thank you for the many details that you gave regarding the issue that you are having with your zucchini plants. It was very helpful in zeroing in on the problem. Your question asked if you should have done anything differently to prepare your soil. The short answer is that, in clay soil it is helpful to work in organic amendments, such as compost and pine fines, which will loosen the clay and improve drainage. Here is some additional information about soil ammendment:

https://extension.missouri.edu/news/clay-soil-a-mixed-blessing-but-easily-corrected-1815

https://durham.ces.ncsu.edu/2020/04/making-the-most-of-your-gardens-soil/

But hopefully it is not over for your zucchini garden for this season! You might still get some good zucchini this year! I hope the following will help you.

The good news is that your zucchini plants are probably not dying, but are just distressed. (Without a picture I can’t be sure, but that is my guess.) The best news is that the issue you are seeing is common and is usually not caused by disease. Disease would be much harder to control, could infect the soil and could be spread from plant to plant.

It is most probable that you are seeing “Blossom End Rot” (BER) which is common in tomatoes, squash, peppers and watermelon. It is classified as a physiological disorder, not a pest or pathogen. It is caused by a lack of calcium available to developing fruit. Zucchini is technically a fruit. However, the solution is not adding calcium to the soil, as most soil has sufficient calcium. Four environmental conditions are most likely to cause or contribute to BER. (Recommendations for solutions are later in the response, so don’t give up!)

The first is under or over watering. When these plants get too dry they are unable to transport the calcium needed by the plant and BER results. Likewise, overwatering of plants, or poorly draining soil can cause root damage, impacting the plants’ ability to transport nutrients. If your clay soil does not drain well this can cause issues, especially after heavy rainfall or if you have over-watered. If root damage is too extensive, or if root rot has set in, the plant might die.

The second is an excess of nitrogen in the soil, known as nitrogen toxicity. Your excellent description makes me think that this might be your problem. Soybeans are known to be a nitrogen-fixing crop, meaning that they add nitrogen to the soil. Although all plants need nitrogen, too much can be problematic for some, such as your zucchini. This is from North Carolina State University:

“Over applying fertilizers that are high in nitrogen is another common cause of blossom end rot. Nitrogen promotes rapid, dark green, leafy growth. Plants that are growing very quickly often cannot move enough calcium into fruits to support proper development, leading to blossom end rot. To avoid blossom end rot, be careful not to overdo it with high nitrogen fertilizers.”
https://pender.ces.ncsu.edu/2020/03/why-are-my-squash-rotting/

The third condition that could be causing your problem is low soil pH. If the pH is less than 5.5, nutrients can be locked in the soil and not available to the growing plant.

And the fourth is extreme heat or unseasonable cold. Obviously, there is nothing you can do about that!

Solutions:

Water

Your plants need about an inch of water per week. Using a rain gauge will help you know if you need to supplement water. Nothing fancy is needed. I use an old measuring cylinder, but you can find lots of ideas online, or purchase a rain gauge. When you water, be sure to water the soil, not the leaves. In other words, don’t water from overhead; try to avoid getting the leaves wet, because minerals and chemicals in tap water can result in damage to leaves which is an invitation to pests and pathogens. If your clay soil does not drain well you might need to consider a raised bed gardening or tilling and amending your garden in late fall or spring. There are many options for raised beds, some of which do not require building a wall. I would be happy to send more information if you request.

Nitrogen in Soil

The only way to know if there is an excess of nitrogen in you soil, is through a soil test. Although you can use a home test available at garden centers or online, I would highly recommend that you send a soil sample to a lab for testing. (We do not receive any compensation from soil testing.) It is inexpensive and fairly easy; the Michigan State University lab charges $12 for a basic test and an additional $20 for a nitrogen level test. Directions for a soil test can be found here: https://ohioline.osu.edu/factsheet/hyg-1132

More detailed directions can be found here: https://hamilton.osu.edu/sites/hamilton/files/imce/Soil%20Testing%20for%20Ohio%20Lawns,%20Landscapes,%20Fruit%20Crops,%20and%20Vegetable%20Gardens%20_%20Ohioline.pdf

You noted that the fertilizer you used at planting was a 12-12-12, which a higher nitrogen ratio than recommended for vegetable gardens. This from Texas A&M University about choosing ratio in fertilizers:

“Most gardeners should use a complete fertilizer with twice as much phosphorus as nitrogen or potassium. An example would be 10-20-10 or 12-24-12. These fertilizers usually are easy to find. Some soils contain enough potassium for good plant growth and don’t need more. But since a slight excess of potassium will not injure plants, it is usually best to use a complete fertilizer. Do not use lawn fertilizers on gardens. They contain too much nitrogen, and many have chemicals for lawn weed control that can injure or kill vegetable plants.”

So … What should you do if a soil test reveals that you have an excess of nitrogen? You can apply a layer of sawdust or wood mulch which will take up some of the nitrogen. You can also plant some nitrogen-loving plants in your garden. Green leafy plants, like kale or rhubarb are some examples. And, of course, stop adding nitrogen fertilizers until you can verify through observation or soil test that your nitrogen levels are no longer toxic to your zucchini. Additionally, proper drainage in your garden will help keep your nutrients balanced.

Soil pH

If your soil test reveals an acidic pH balance you can add amendments to increase the pH level, that is, to make it more alkaline. Please contact us if you need more information.

Recommendations:

You still have plenty of time in this growing season to get some lovely zucchini! It is possible that the nitrogen levels are stabilizing in your garden,  or that you can improve the drainage and/or water in your garden. Your plants should continue to flower; zucchini can go from blossom to harvest in four to eight days!

Other tips - Immediately remove any “fruit” or blossoms showing signs of rot. Remove yellowed leaves and prune the plant if needed. Practice good garden hygiene: Remove any deadfall from around your plants. Likewise remove dead or diseased leaves or branches. Clean pruners or scissors before moving on to another plant. (I use rubbing alcohol; some MGV friends use antiseptic wipes.) Keep an eye out for pests and contact Ask a Master Gardener/Ask Extension if you need advice about pest management.

If you have any additional questions, would like more information, or need clarification on this response, please feel free to reply to this thread and the question will come to me. I’ve included some additional links to research based articles below. When researching topics for your garden, adding site:edu will help you find reliable information.

Good luck with your zucchini and happy gardening!

https://extension.unl.edu/statewide/dodge/blossom-end-rot-of-vegetables/

https://pender.ces.ncsu.edu/2020/03/why-are-my-squash-rotting/

http://vegetablemdonline.ppath.cornell.edu/factsheets/Tomato_BlossRt.htm