Thoughts? Explanation? Similar experiences?

Zeuspaul

Well water also seems better than public water.

I have had too much rain this year though.

There is also going to be a different range of photosynthesis happening on a cloudy, rainy, cooler day than a blazing hot day.

Many veggies shut down on a hot summer day and do better with more indirect light, but that will depend on particular plant and your climate details.

It isn't the rain, but the stress relief that the rain brings with it.

Rain is also better for what it does not have in it. Hard water can inhibit nutrient uptake, and municipal water can contain chlorine, which is poison to a plant. Acid rain is not the problem it once was, due to EPA regulation.

It is also worth mentioning that rainfall is often a good soaking, which reaches deep into the root zone. In contrast, many who use spray irrigation do so inadequately, or moisten only the top few inches of soil. It is pretty hard to match 1" of rainfall using a hand sprayer.

Free chlorine doesn't stay in water (which is why it isn't used for disinfecting tap water these days). What is used is chloramine. Now chloramine has been tested, and shows no direct effect on plants, but it is true that it could interfere with bacteria that are helpful to plant growth (as well as bacteria that aren't!) That's not an effect that you're likely to see right after watering. If you're brewing beer or wine, however, I'd suggest staying away from chloramine-treated water. It takes several weeks for chloramine to beak down in soils.

Yes, acid rain isn't a "problem" anymore. Because the rain that falls isn't as acidic as it used to be. But it is acidic. You want sulfur from acid rain? It comes from sulfuric acid. There are far better ways to get sulfur into your garden.

Sorry, but rainfall as a helpful replacement for tap water is largely mythology.

So I have to guess that if one lives in a climate where it is very dry (I don't), ones crops might perk up more after a rain than after watering with a hose.

Snippet

AAES research indicates that rainwater may contribute to the natural N available to crops, but is not the sole source.

Two forms of N received by the soil in rainwater are ammonia, an atmospheric gas that dissolves in rainwater, and nitrate, a compound that is produced during lightning strikes. Each year more thunderstorms occur in the Gulf Coast region than in other parts of Alabama, so greater nitrate-N deposition in that part of the state would be expected. Upon entering soil systems, nitrate and ammonia dissolved in rainwater are available for uptake by crops.

So precipitation supplies just a small amount of soil nitrogen compared to good biological sources.

http://www.extension.umn.edu/distribution/cropsystems/dc3770.html

It is true that municipalities add lime to tap water to make it slightly alkaline. I think that's to reduce corrosion in their pipes.

Be aware that highly alkaline water is easily neutralized if there are no alkaline minerals in it. So high pH isn't necessarily bad. This is poorly understood. It's "alkaline" versus "alkalinity". Two different things. See, for example

http://extension.umass.edu/floriculture/fact-sheets/water-quality-ph-and-alkalinity

So if you have high pH but low alkalinity tap water, the soil will neutralize it pretty easily. If you have pH=9.2 tap water, I'd get it tested for alkalinity before you worry too much about using it on your garden.

Don't get me wrong. Rain is great. It's just not magic.

I'll take a different approach.

Assume rainfall only needs to make up for a daily nitrogen defiiciency to witness a greening effect. Assume a 25 percent nitrogen deficiency would be noticable. Then if rainfall can supply 25 percent of the plant's daily needs one would see a greening effect.

If the nitrogen fertilzer requirement for vegetables is 100 lbs per acre per year and we need to supply 25 percent of that we need 25 lbs of nitrogen per year per acre. That is about .07 lbs per acre per day.

Using 5 lbs nitrogen per 78 inches of rain we get about .06 lbs nitrogen per acre per day assuming a one inch rainfall.

Zeuspaul

I think a good timely rainfall just does a good job of thorough watering that has a lasting effect. In warmer climates, it can also have a cooling effect on the otherwise stressed plants.

So, I would not bank on it, except what my rain gauge tells me.

Some things really depend on your climate. We rarely get a soaking rain. But, when it rains in my arid climate, the plants are less stressed as there is moisture in the air. My tap water comes from an area with a lot of limestone and for me, rainwater is precious because if there is enough, it helps to leach the built up minerals from my tap water. One of my long range projects will be to work on getting gutters on the house so I can collect some of that precious, soft rain water.

Jim

The most popular reason is that lightning turns atmospheric nitrogen gas into a form of it, nitrous oxide, that’s beneficial for plants. So when nitrogen-enriched rain falls from the skies, it gives plants a boost.

My hypothesis is based on information covered in a book I'm writing on electro-horticulture (& plant electrophysiology) - that an abundance of free-floating ions from looming electrical storms creates a charge imbalance on plant cell walls. This imbalance will lead to a wide array of physiological changes within the plants ranging from increased growth rate to increased pigmentation and photosynthesis abilities. Charge imbalances induce "action potentials" within electrically active cells in plants affecting metabolism, structure, hormones, and gene expression.

...except for this week, when we had 3.5 inches of rain in one day! The plants did not appreciate being drowned.