zucchini blossom end rot brown end on zucchini squash
Blossom end rot can be super frustrating, especially because the fruit was ripening up and looking good before the end turned brown and mushy

So What Causes Blossom End Rot Anyway?

Blossom end rot is caused by a calcium deficiency.

You might assume that the solution is to add some eggshells + magnesium to your soil to counteract this, but not so fast.

Your soil may be lacking in calcium, but it's also important to understand that calcium is water soluble. It's transported by water, so a lack of water may lead to a lack of calcium.

It might be a good idea to add some egg shells, gypsum, or lime to your garden, but make sure you are addressing any potential water issues above that. Remember that lime makes soil more acidic, so understand your soil pH before deciding to add lime.

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If you garden in the desert like I do the hot, dry season often takes a toll on our plants in unexpected ways. One of these is with calcium transport.

We can sometimes get summer squash through the entire summer in Tucson, but it's reliant on a few factors. Only a very nutrient rich soil will help zucchini and summer squash survive the 115F temperatures and sub-10% rH. Shade cloth may help, though I find that adequately watered. Summer squash in rich soil doesn't even need shade cloth.

On the other hand, if you've got poor quality soil, with a lower organic matter content its ability to retain moisture will be severely reduced.

This, in turn, can lead to moisture problems during the dry season.

Moisture problems can then lead to calcium problems, which can then lead to blossom and rot.

The chain is complete, low quality soil in harsh conditions might be the culprit.

Always be sure to trace each symptom all the way back to the root cause, as adding calcium will not improve the moisture retention properties of depleted soil.

And remember that you should always add magnesium anytime you add calcium, because Cal + Mag go together.

How Does Soil pH Affect Calcium Transport?

How Do You Treat Blossom End Rot?

It might be worth adding some calcium to your soil, if your squash are suffering from blossom and rot.



That being said, this may not necessarily fix the root cause of the issue if it's water or pH related.

If blossom and rot shows up during the driest time of year, you may have a water issue.

Assuming you can afford to increase your watering, that may be a solution.

If you live in a desert like I do, and have to conserve water, soil improvement may be the way to go. Higher quality soil retains more moisture.

Additionally, it's worth noting that adding lots of organic material as mulch to your garden will help lock in moisture, and slow down the drying out of your soil.

Mulch may be the cure to your calcium deficiency that's presenting as blossom end rot.

Test your soil pH to determine whether it's in the proper range for calcium absorption.

Above all, improve your soil and issues like this will naturally be mitigated. Great soil and a thriving microbiome are the cornerstones of regenerative agriculture. Many issues will self-correct when your soil is dialed in right.