daninthedirt

nitrogen leaching in wicked heat

daninthedirt (USDA 8a, HZ10, Cent TX, Sunset z30)
August 31, 2019
last modified: August 31, 2019

I’ve learned something satisfying this August. For the last few years, I’ve been perplexed and frustrated by a melon bed (cantaloupe, honeydew, Armenian cukes) of mine that produces fantastically in May, June, and July, and then just quits in August. Now, my growing season goes through November so, um, something is wrong here. August is when the wicked heat (100F+ temp) sets in, so I had figured it was just heat stress, and maybe the plants were just getting “tired out”. Leaves were no longer dark green, and no flowers, so no new fruit. No insect pests. No disease.

In previous years, I’d just rip them out. This year, I did something different. I lathered on a lot of ammonium nitrate. I figured it would either kill them, or … ? What happened was stunning. The plants almost immediately started producing loads of new dark green leaves, and soon flowers were appearing. Of course, the flowers come off of new foliage, so if you don’t have new foliage, you don’t have flowers or fruit, and nitrates push new foliage. The whole bed is now quickly reviving, even though the heat is still on.

I think the explanation for all this is simple. It’s the wicked heat. Nope, not heat stress on the plants. These plants love heat. It’s leaching. In early summer I was watering about half an inch a week. Once the 100F+ temps hit, I was watering almost three times more. Soil nitrates, which are the primary source of soil nitrogen, are soluble, and I’m pretty sure when the intense heat hit I was just washing them out. Phosphorous and potassium don’t wash out as readily, especially in my alkaline soil, nor do trace elements. My spring soil always tests high for those anyway. I was doing occasional and spotty treatments with MG, but that wasn’t enough. I dug loads of compost in at the beginning of the season, in February, and as that degrades in the soil it very gradually provides some nitrogen. But when August came around, I was probably flushing it out faster than it was being created and the compost had probably largely degraded by then anyway. Nitrate leaching is an important concern in commercial farming, though when heat isn’t wicked, the threat is mostly from high rainfall.

Seems to me the main lesson here for southern gardeners who are looking for a long, productive growing season, and who garden in wicked summer heat, is to pump up the soil nitrogen when you start watering a lot because that heat is bearing down. Lawn fertilizer! 20-0-0 or higher. A quarter the cost of MG. I’m guessing the other lesson is that compost, as a weak and slow nutrient, may not be an effective nitrogen supplement if you have a very long growing season and lots of irrigation.

Comments (4)

  • hidesertca

    That's good to know. Mine also slow way down in August compared to June and July. I'll have to try 5- 1- 1 fish emulsion once a week and see if that helps.

  • itsmce (zone 6b, Kansas)

    Interesting. Thanks for the thorough explanation.

  • daninthedirt (USDA 8a, HZ10, Cent TX, Sunset z30)

    I should add that an obvious question is - how much? Too much nitrogen will cause nitrogen burn. Here's what I did. I assumed that the dose of lawn fertilizer that was good for the lawn would be OK for veggies. For 20-0-0, you're supposed to apply 5 lbs over 5 weeks on a 1000 square foot lawn. 1 lb is 3 cups. If you do the math, that works out to about half a cup a week for my 150 square foot bed. I guess if you're watering a lot, and suffering a lot of leaching, you might want to use a little more.

  • CA Kate z9

    Interesting idea, Dan. We too have a long, HOT growing season and everything stops until it cools down. I'm going to try the nitrogen trick and see if it works for me too.

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