Why is rain so much better than watering with rainwater?

This simple question keeps me occupied since a friend told me there is magic in rain, because it makes his plants grow so much better than watering with rainwater that he caught. Magic doesn't suffice as an explanation as there always is a scientific explanation. Another perma-culture friend observed the same but had no real explanation either...
We are in the middle of a hot dry summer, i have quite some rainwater to my disposal, i catch it from my roof and my neighbors roof and i pump it into my pond, i scoup it from there, warm organically enriched water from a small pond full of life and nutrients from fish that live in there. I water scarcely, only plants that do badly, that have lost leaves, not every day though. I prefer to give them quite a bit in two tours around the garden so the water seeps in to the deeper layers. And then leave them to it depending on the temperature / wind / sun hours. Most perennials and shrubs and trees hold out quite well and get the odd splash of water. Trees especially have deep roots, and i am hesitant to give them something, forcing them to root deeper.
Plants survive, but rarely thrive when it's so hot and dry and i turn to watering a lot. When it rains however, i noticed plants suddenly shock back to life and produce new foliage.
Why oh why wise friends on the permies forums is that so? Is it even so? Or is it just me?
Is my watering even doing something, or am i imagining i am keeping the plants alive?

There's these things people call 'micro clusters'. My limited understanding is that it is basically a relatively weak electrical bond between water molecules. It's supposedly related to surface tension and supposedly builds up as water sits in mass. Running over rapids reduces it, supposedly flowform and other water structuring tech reduces it, schaubergers vortexes presumably reduced it, and rain water falling from the sky is already lacking these masses. The concept is that these electrically bonded masses of water stick together, are harder for microbiology/roots to access, have a tendency to slide deep into the soil profile all together, and are generally just less accessible to flora than water that is less bonded to itself. that's one theory that seems scientific.
there's also the possibility that the electric activity in the atmosphere that tends to accompany rain storms has some positive effect on plant growth.
Or maybe it's just that the faerie folk like it best and their dancing foot steps are the only fertilizer any of us truly need

Rainwater has varying amounts of dissolved nitrates, which enter the rain via the interaction of lightening and the nitrogen in our atmosphere.

Plants love nitrates.

I'm guessing that storing rainwater in a tank or other vessel creates some degree of ion/cation exchange that changes/removes the nitrates.

you sure that it's not just the amount of water?  

Apologies for the nasty metric units but it's super-simple in metric... if you have, let's say, 1 hectare of land and just 1.6mm* of rain falls, the total amount of water that landed on your 1Ha is 16,000 litres.  That's over 4230 US gallons.  I would take bets that most don't apply that much water, unless you've got a darn good supply and a nice irrigation system.

* 1/16" which ain't much rain, over about 2½ acres.

Putting it rather more simply:  your garden is probably a darn sight bigger then your roof.  (Thanks to Burra for that insight )

Austin, the amount is much bigger for sure than the amount i am watering, but i water straight at the trunk into the rootzone, quite some more then what the rain manages to do in a night of raining 15mm. Somehow it profits more of rainfall, puzzling...
F Agricola, that makes sense, although the water i personally pour onto my plants comes from a pond with fish. Although plants are abundant and taking up nitrogen, they won't have devoured all of it.
Micro clusters, that's interesting, i had no idea that exists.

My theory formed recently since my daughter read in a book plants have 15 senses more then we do. Which triggered my imagination that plants somehow sense the weather changing to rain and as they know there is going to be water, they get ready for it and once it's there the bacteria who hide in their shell like cocoon structures pop out, and the plant exhales it's sugary root exudates feeding the microbes into a population explosion. The predators come to feed on the microbes leaving nitrogen rich waste that the plant uses to grow new shoots.

The second half of this explanation is fact, the first i made up, because my daughter just told me she has not finished reading this book yet and that she doesn't know what these senses are. But as rain is so important for plants, i am pretty sure plants sensing rain coming and plan accordingly must have had an immense evolutionary advantage over others they would have to be everywhere.

It makes sense as well when men comes along watering plants, they survive nicely but wait with exudates until their senses tell them to do it. Instead of the monkey with the watering can. They can't sense us in that way.

But i much prefer the dancy elf feet explanation!

I suspect dissolved oxygen might also be a factor. I can't find any scientific reference to back this up (other than something from 1917) but my observation is that higher the oxygen level in irrigation water, better the soil health and so healthier plants. Rainwater probably has higher dissolved oxygen levels than stored rain water.

After it rains, it evaporates. Evaporation is good and bad. It is not good because up to 75% of rain evaporates in June here in Istanbul, 30-50% in spring time and 10-25% in winter time (I know this from my rain water collection system). That's why it is best when it snows. Soil gets saturated when it snows but not when it rains. That is one of the reasons why we build swales., to give some time for rain water to soak in, limit run off and also evaporation (this is usually ignored but very important for hot climates).
More importantly, evaporation is good because plants probably "take a day off" from transpiration. Evaporation of 75% of summer rain will take a lot of heat off the ground and plants. Transpiration is expensive in energy-wise. Instead it can spend this reserved energy to grow.

We always talk about soil biota. There is also bacteria in the air, leaf surface and such. They also need water.

You can trick plants into thinking as if it rained. When it is cooler (like 10-11pm) water the plants generously and also sprinkle all over and around them. You will observe a similar reaction as it actually rained. Don't do it when it is warm or will get warmer, plants may get shocked.

I think if you took your captured rain water and figured out a way to water that amount over an extended period like the rain does, you would have the same results. I think it's the period of time they are receiving water that makes the difference.

I share Austin's belief. Especially watering by hand. Rain will cover every square inch. Hand watering will not. My guess is the water starts wicking away from the plant to the drier areas. With rain, there is water front, back, left, and right of plant. Only real option it has is to soak in.

I've noticed and wondered about the same thing.  I noticed that when I watered in Texas and Oklahoma it didn't seem to have any effect at all.  Even a brief shower would perk the plants up more than my watering.  Perhaps part of it is the dissolved nitrates and oxygen in the rainwater.  Perhaps another part of it is that when I am having to water plants to keep them alive it is HOT.  When the rains come, the temperature drops considerably and the sun isn't beating down on the plants.  Perhaps the combination of cooler temps, cooler water, and relief from the beating sun has something to do with the difference.

When we just started out at our place we put a lot of time in irrigation, using the water from our dams. As far as we could see it didn't help all that much at all. Rain however does... So we can also confirm there is a difference. But our explanation seems to be different.

Our soils are pretty alkaline, so when rainwater comes in contact with it, it dissolves the calcium and other elements creating alkaline water in our dams and ponds. When we then water with it we seem to increase the alkalinity of the soils around the plants. When it rains however the opposite seems to happen, the rain then actually flushes some of the salts and calcium deeper down into the ground, helping the soil life to create a better balance. At least we think this is what's happening here.

Rene Nijstad wrote:When we just started out at our place we put a lot of time in irrigation, using the water from our dams. As far as we could see it didn't help all that much at all. Rain however does... So we can also confirm there is a difference. But our explanation seems to be different.

Our soils are pretty alkaline, so when rainwater comes in contact with it, it dissolves the calcium and other elements creating alkaline water in our dams and ponds. When we then water with it we seem to increase the alkalinity of the soils around the plants. When it rains however the opposite seems to happen, the rain then actually flushes some of the salts and calcium deeper down into the ground, helping the soil life to create a better balance. At least we think this is what's happening here.

Interesting theory.  I know that house plants do better with stored rainwater than they do with our tap water.  But then, since we don't like drinking our tap water without it going through our Berkey filter first, I'm sure the plants don't find it palatable either.

What I find interesting is the near-consensus that rain watering is in fact more effective than various other irrigation methods, even though theories about why this seems to be so are all over the map.

I certainly agree that a nice soaking rain has an amazing regenerative effect on my garden beyond anything that I can accomplish by watering, no matter how much water I apply!

Most of my garden is large containers and tall raised beds, so when I patiently stand there with a watering wand attached to my garden hose, I can definitely apply as many inches of irrigation water as any conceivable rain event.  (I usually don't, though, as this would be excessive and would wash fertility out of my container soils.)  

It is my belief that the rural-water-department water is less than ideal for direct application; it has a bit of chlorine and a lot of nitrates (agricultural runoff; it makes algae blooms if let stand in containers).  I water deeply with it on very hot days to "rescue" thermally stressed plants and it's better than not doing so; but an actual rainstorm has much better effects on plant growth.  

When the weather is cool enough that my plants aren't in danger of cooking in their containers, I don't water from the hose.  Instead, I maintain several barrels of "live" water that I refill from the hose; these have snails, goldfish, and aquatic plants living in them.  I spot-water my container plants by hand as needed with a pitcher from these barrels.  The plants are a lot happier than they are when they get the garden-hose wand treatment, but still nowhere near as happy as when they get real honest rain.

Dan, I've noticed that explanations are all over the map as well and i've thought up another one to add! The role mycorrhizal fungi play. They attach to plant roots and even stretch into the rootcells, significantly expanding the rootzone. They are basically everywhere in the soil acting as a giant sponge sucking up all vapor and transporting it toward the plant in exchange for sugar. It could explain why even a brief shower pirks up so many plants as well.

The role mycorrhizal fungi play. They attach to plant roots and even stretch into the rootcells, significantly expanding the rootzone. They are basically everywhere in the soil acting as a giant sponge sucking up all vapor and transporting it toward the plant in exchange for sugar. It could explain why even a brief shower pirks up so many plants as well.

That was going to be my reply but even though the other thoughts seem spread all over the map that is just the point: The sum of these effects is probably greater than any one of them alone.

Because I have studied the effect of changing the electron energy level in water because I use magnets in my therapy work for this purpose, I have come across studies showing that irigation water passed between strong magnets becomes more readily available to soil life. Running water through long lines tends to strip this electron energy from the water.
Water tends to condense in the form of small hexagons sort of like spaghettios  but in its less energetic state it is a long chain like spaghetti. Speghttios are much easier for cells to suck in that spaghetti.

Hugo Morvan wrote:...
My theory formed recently since my daughter read in a book plants have 15 senses more then we do. ...

This is a great discussion! Hugo, I would love to know the name of that book, even if it's in French. It sounds like a great summer read!