Why Plants Grow Better After Rain, and What It Tells You About Your Irrigation Water
Every grower and superintendent has seen it. A good rain comes through, and within a day or two the turf is greener, the trees perk up, and the whole property looks better than it has in weeks. You didn't change your fertilizer or your program. The rain did something your irrigation system hasn't been doing.
Then, just as reliably, the flush fades. A week later things look tired again, and sometimes worse than before the storm.
That surge and crash isn't random. It shows you exactly what your irrigation water is missing and what it's leaving behind in your soil.
What rain has that irrigation water doesn't
Rainwater and irrigation water are both "water," but they aren't the same drink.
Rain is nearly free of salts. Well, canal and effluent water carry dissolved minerals: calcium, magnesium, sodium, chloride and, above all, bicarbonate. Rain carries almost none. For a few days, your plants drink water that isn't adding to the salt load in the root zone.
Rain is full of oxygen. Rain falls through the air and arrives saturated with dissolved oxygen. That oxygen reaches roots and soil life that often sit in tight, low-oxygen soil. It also helps soil microbes convert nitrogen into nitrate, the form plants take up most easily.
Rain brings a little nitrogen of its own. Rain carries small amounts of nitrogen from the atmosphere. Combined with the oxygen, the result is a quick, visible flush of green growth.
Rain soaks evenly and deeply. A slow, steady rain wets the whole profile evenly, without the dry spots and puddles irrigation often leaves.
Water is hydration, not nutrition. — Todd Eden, CEO, HCT, LLC
What's dissolved in the water decides what the plant can actually drink. After a rain, that drink is about as clean as it gets.
Why the green-up doesn't last
If rain is so good, why does the improvement fade?
The plant runs out of nitrate. Nitrate is one of the most soluble things in the soil, so plants use it up fast. Once it's gone, roots take up the next most soluble thing available, and in many soils that's sodium. The plant goes from a clean, nitrogen-rich drink to a salty one, and growth stalls. (Learn more about sodium and chloride in irrigation water.)
Rain doesn't fix what's locked in the soil. Rain moves the most soluble salts, sodium in particular. It doesn't dissolve what has built up for years: calcium and magnesium cemented into carbonate scale, fertilizer bound in forms the plant can't use, and sodium and chloride held in a hardpan or confining layer. When the rain stops, all of that is still there.
You go back to the same irrigation water. Once the sprinklers or drip come back on, the root zone again gets bicarbonate, hardness, sodium and bacteria, every cycle. The conditions that were holding your crop or turf back return with the first irrigation.
The bicarbonate problem: hard water turns into cement
Bicarbonate is one of the most common and most overlooked irrigation water problems in the West and South.
While water is moving, bicarbonate, calcium and magnesium stay dissolved and cause no harm. When that water evaporates on the soil surface, in the root zone or at the emitter, they bond together and form calcium and magnesium carbonate scale. Todd calls it cementation:
Hard water is fertilizer, until evaporated to dryness. Then it is cementation. — Todd Eden, CEO, HCT, LLC
That cement shows up as:
White crust on soil, emitters and sprinkler heads
Water that puddles or runs off instead of soaking in
Iron, zinc or manganese deficiency even after fertilizing
Soil that seals, cracks and gets harder every season
Rain never adds to that load. Your irrigation water adds to it every time it runs. (Learn more about bicarbonates in irrigation water.)
Why "just water more" makes it worse
When turf or crops struggle with salts, the usual response is to put on more water to flush them down. It feels like making your own rain. It isn't the same.
Extra irrigation water brings more bicarbonate, sodium and hardness with it.
Saturated soil loses its oxygen. Soils go anaerobic, which favors the bacteria behind black layer, sour odors and slime.
Flushing moves only the most soluble salts. Scale, bound fertilizer and confining layers stay where they are.
You pay for the water, the pumping energy and the fertilizer that washes past the roots.
HCT's rule is simple: water to water, not water to flush. Irrigate based on soil moisture in the root zone. Leaching works best once water can actually move through the profile, and that means dealing with what's sealing it first. (See Infiltration: Getting Water Down.)
Don't forget the biology
Irrigation water also carries bacteria, and bacteria build biofilm, a slimy layer that coats soil particles, clogs drip lines and well screens, and shields scale and nutrients from treatment. In low-oxygen soil, sulfate-reducing bacteria create black layer and hydrogen sulfide gas (the rotten-egg smell).
Part of why plants respond to rain is that rain briefly sends oxygen into those soils. Healthy soil biology needs that oxygen every day, not just after a storm. The catch is that you can't tell your water's bacteria load by looking at it. We've tested pea-green lake water with no bacteria problem and crystal-clear water loaded with bacteria. The only way to know is to test.
A real-world example: firm after the rain, finally
At Wildhorse Golf Club in Texas, eight years and several thousand feet of French drains hadn't solved wet, soft fairways. Once the irrigation water was treated, within 90 days the fairways held up to carts and mowers after rain. The treated areas drained better than the areas with added drainage, drain-water salinity dropped to the lowest levels on record, and salt build-up stopped. (Read the full story.)
More drainage didn't solve it. Changing how the water behaved in the soil did.
How to make your irrigation water behave more like rain
You can't make it rain on schedule. You can change what your irrigation water does once it reaches the soil.
Test your water and soil the way your plants experience them. Most soil tests use lab water, a 1:1 ratio and a soak of under 24 hours. HCT's Legacy Analytical Testing uses your own irrigation water, at 4 parts water to 1 part soil, soaked for 72 hours. It shows what your plants can actually drink with your water, and it includes a bacteria test that standard panels leave out.
Keep minerals usable instead of just lowering pH. Sulfuric acid lowers pH, but it doesn't remove calcium, magnesium or sodium. It also adds sulfate, which forms new scale and feeds black-layer bacteria. WaterSOLV™ Curative is a sequestering chemistry dosed at a few gallons per million gallons of water. It keeps minerals in an ionic, re-dissolvable form, so what dries in the soil goes back into solution with the next irrigation instead of hardening into scale.
Put oxygen back, every day. WaterSOLV™ BC is an inhibited, catalyzed hydrogen peroxide solution. It breaks down biofilm and harmful bacterial waste and breaks down to oxygen and water. That's continuous oxygen in the root zone instead of a short burst after each storm.
Harvest the fertilizer you already own. Years of fertilizer and hard water leave most soils holding large reserves in forms plants can't use. Treated water helps release them. (See How Much Fertilizer Is Already in Your Soil?)
Water to the root zone, not to flush. As soils open up, most sites can water less. Across HCT programs, water use commonly drops by about 15%.
Frequently asked questions
Why is rainwater better for plants than tap or well water?
Rain is nearly free of dissolved salts, rich in dissolved oxygen and carries a small amount of nitrogen. Most well, canal and municipal water carries bicarbonate, hardness, sodium and bacteria that build up in the soil over time.
Why do plants turn so green after a thunderstorm?
Rain delivers oxygen and a little nitrogen and helps soil microbes convert nitrogen to nitrate, which plants absorb quickly. The result is a fast, visible flush of growth.
Why does my turf or crop decline a week after rain?
The nitrate runs out, roots go back to taking up sodium and other salts, and irrigation water starts adding bicarbonate and hardness again. Rain doesn't dissolve the scale, bound nutrients or confining layers already in the soil.
Can I just water more to flush salts out?
Extra water moves only the most soluble salts, adds more bicarbonate and sodium, and can leave soil saturated and low on oxygen. Fix what's sealing the soil first, then water to the root zone.
How do I know what's in my irrigation water?
Test it, including bacteria. HCT's Legacy Analytical Testing looks at your water, its bacteria and how your soil behaves with your own water, then turns the results into a written prescription.
Ready to find out what your water is really doing? Order Legacy Analytical Testing, or contact an authorized HCT dealer to get started.
Results vary by site, water quality and management practices.
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