Perhaps it's the nitrogen in the rain water.

Zeuspaul

I agree with Zeuspaul, Rain always does better than watering for me.

Well water also seems better than public water.

I have had too much rain this year though.

Certainly rainwater tends to be higher quality than municipal water.

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.

Around here, in HZ10, the answer is pretty simple. In midsummer, with temps well over 100F, the plants just slow down because of heat stress. I can water the bj out of them, and they still slow down. When it rains, it's cooler, and the sun isn't as bright. Heat stressed plants are a lot happier and grow faster.

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

P.S. In certain parts of the country, I'd be hesitant to think of rainwater as healthier (for people or plants) than groundwater. Rain washes the air, and the air in many places is quite dirty. I believe that on the east coast, the pH of rainwater can be quite low (that's called "acid rain"). Now, not so much in the LA basin, as the alkaline dust there tends to neutralize the acid. Even rainwater in unpolluted areas is naturally slightly acidic (pH~5.5). It is simplistic to suppose that, because it comes out of the sky, rainwater is better than groundwater.

Sometimes I do hope for a bit of acidic rain, for the sulfur in it. And for those with really high ph soil it would be appreciated too.

I agree with those who have mentioned nitrogen in rainfall, as a source of vigor... especially from a good thunderstorm.

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.

It is possible that some air is dissolved in rain water that include nitrogen and oxygen. Also, I have heard, when there is lightening during a rain , some hydrogen peroxide is created that also is mixed with the rain water. It is the extra oxygen atom in H2O2 that is said to be beneficial to plants as well. That is why some gardeners foliar feed their plants with H2O2 solution. I, personally, do not know much of this stuff has been substantiated.

Nitrogen that rain picks up in the atmosphere (which is N2) isn't plant-usable nitrogen. That's what legumes do. It's called "nitrogen fixing". They convert the former into the latter, which are nitrogen compounds. If you want to get nitrogen from rain, then rain on your plants with pee. So nitrogenation from regular rainfall is simply incorrect. Well, the nitrogen you get from acid rain has some nitric acid, which is fixed, but the acidity sure isn't helpful. Plant beans!

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.

Frankly I always thought it was just because rain provides a great deal more water than does my watering. I mean it soaks all the ground everywhere at once. When I water only very small areas (comparitively) are getting water at any one time. Water is what moves the nutrients around in the soil.

My crops always do better with rain water. We don't live close to a big city or have any factories within a hundred miles or more. Everything gets watered better and deeper when we have a good rain

It occurs to me that the only reason why crops might care about whether the soil ten yards away has been watered is the humidity. That is, rainfall increases local humidity, because large areas of ground are evaporating surface water. Humidity is a function of the moistness of the soil over a large area, and not just the moistness of the soil in the garden itself.

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.

The Alabama Agricultural Research Station measured available nitrogen in rainwater.

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.

Here is a link that might be useful: AAES research

Yes, both the nitrogen (which is not molecular nitrogen, but molecules created in lightning) and the lack of minerals help. In my case, my soil has a pH=7.6. Tap water has a pH of 9.2. I guarantee that rain water is far better than tap. The only counter to this tap water is a lot of organic matter.

"The amount of N 2 fixed by legumes into usable N can be substantial, with a potential for several hundred lbN/acre/year to be fixed in an alfalfa crop. Any portion of a legume crop, that is left after harvest, including roots and nodules, supplies N to the soil system. When the plant material is decomposed, N is released. Several non-symbiotic organisms exist that fix N, but N additions from these organisms are quite low (1 - 5 lb/acre/year). In addition small amounts of N are added to soil from precipitation . The amount of N supplied from precipitation averages 5 - 10 lb/acre/year in Minnesota."

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.

Nitrogen introduction due to lightening, is just a hypothesis. But nitrogen fixing by the roots of legumes, alfalfa, Fenugreek, ... has been established and has been know for a long time. This is separate from turning over certain cover crops(, fenugreek, oats, clover, ..) aka green manure.

The fact that rainfall has some fixed nitrogen is interesting, but let's do the math. 5 lbsN/acre/year means about 0.1 lbN/acre/rainfall. My 10x20 foot plot is 0.005 acre, so that means 0.3 g/plot/rainfall. That's about a tenth of a teaspoon of N in my whole plot per rainfall. I'm hard pressed to believe that's going make my plants perky. Now, others may believe in homeopathic gardening but frankly, I don't. I should note that urine is 10 gN/liter, so I'd probably get as much N if a squirrel peed on my garden plot. At least I get something out of those troublesome squirrels!

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

I am not really worried, it is the only water I have. But with this water last year, a drought year from March to August, I had the most pitiful greens since I started gardening. The fruiting plants were early, but average. I note that 5-10 lbs in Minnesota -> 10-20 in Michigan ->20-40 on the Atlantic coast (and 1-2 in Nevada). There is something to be said about having the N already pre-mixed with water (corn growers do that these days) and in a form suitable for absorption. There is also the signal the plants get from decreasing pressure, making them pull water faster from the ground. The hose will not do that.

let's do the math

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

Just like for humans, absorption is key. Spinach has a lot more iron per calorie or per pound than steak, but spinach iron is 1% absorbed, and steak's iron is 22% absorbed. I am not taking sides because I understand that, if my pH is 7.6 and pH=4 rain starts falling, good things will happen regardless of nitrogen in the rain water, so I really can not be conclusive. All I am saying is that N amount may not be everything.

Just like for humans, absorption is key. Spinach has a lot more iron per calorie or per pound than steak, but spinach iron is 1% absorbed, and steak's iron is 22% absorbed. I am not taking sides because I understand that, if my pH is 7.6 and pH=4 rain starts falling, good things will happen regardless of nitrogen in the rain water, so I really can not be conclusive. All I am saying is that N amount may not be everything.

FWIW, 18 lbN in 1 acre 78 inches deep in water has 1 ppm N, according to my calculations. (Please verify.) So that's the rainwater concentration, according to this source. The EPA limit for nitrate in local tap water is 10 ppm. Many locales have tap water with 1 ppm nitrate or higher, especially central and north-central U.S. The grass is greener in western NY and Michigan! But it's not an urban phenomenon. Some of it is fertilizer runoff in groundwater. So it's possible that where you live, you put more nitrogen in your garden with tap water irrigation than you do with rainwater irrigation.

Here is a link that might be useful: nitrogen and groundwater

I think that a lot of rain water effects are just perceptions. As daninthedirt analyzed Alabama official study, it should not have any measurable effect on my garden, in terms of adding nitrogen to it..

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.

Well, I can tell you my veggies always look better after a rain. They are definitely greener if for no other reason than the dust has been washed off.

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.

While it is true that legumes can fix nitrogen, the benefit can be overrated. Common beans, for example, are rather poor at fixing nitrogen. And the nitrogen is not necessarily available at the time it is needed. See the link below for details.

Jim

Here is a link that might be useful: Nitrogen FIxation By Legumes

I'm quite late to this discussion... Yet I would like to add-in my own 2 cents:

The most popular reason is that lightning turns atmospheric nitrogen gas into a form of it, nitrous oxide, thatâÂÂs 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.

Interesting thread.
It almost sounds like the guy's on "The Big Bang Theory" disscusing a topic.
"Electrophysiology" I like that hypothesis.
That could explain why it seems like the gass and weeds grow faster and bigger next to my "hot wire"
It must also make the grass taste better cause the cows and horses eat that grass along the fence line first.
Mabey I'll run a hot wire around my pepper plants this year.

Maybe I missed it but no one mentioned that tap water, at least around here, is full of chloramine. I mean we can smell it when we turn on the tap. Granted, Omaha has some of the worst drinking water in the country. But one of my professors did mention the nitrogen in rain fall. May be other things in the rain as well. I mean, storms also create a lot of ozone. WHat else could be coming down with that rain?

What's rain???? Nancy

You want to see rain? Trade me houses.

Ditto to what derek, the original poster, said. My plants shoot up rapidly after a good rain. It always amazes me. I don't, nor do I feel a need to, understand the science behind it. :)

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