Where do you hide 20,000 gallons of water?

December 19, 2013 12:00 AM

Todd root core What do National Beer Day (April 7) and National Tortilla Chip Day (my personal favorite, Feb. 24) have in common? Neither would be possible without soil. Good thing Dec. 5 was World Soils Day!

I was recently reading an article in celebration of World Soils Day highlighting the numerous and awesome benefits of organic matter for healthy soils. Having spent the better part of the last five years thinking deeply about soil organic matter, I wasn’t surprised to read many of these amazing benefits. But I was a bit surprised to read this:

For each 1% increase in soil organic matter, soil can store an additional 20,000 gallons of water.”

That’s an impressive statistic! According to the USGS, the average person uses 80 to 100 gallons of water per day in the US.  This means the extra water stored from increasing organic matter just 1% would last most of us up to 9 months for all the water we use in our daily activities.  I can imagine what a game changer that would be for water thirsty crops, particularly in drought years like much of the central US has seen the past two summers.

But what does that statistic really mean? My first reaction to that impressive number was to ask how much soil is needed for that kind of water storage? An acre? A hectare? Thinking about it, it must be based on a soil volume; but how deep?  A foot? A meter? Is it as true for a Montana barley field as it is for an Iowa corn field?

Digging into it (bad soil pun, sorry...), I started seeing that or similar estimates popping up in other places—all without reference to a data source. To my nerdy satisfaction, several sources did provide an estimate of area and depth, but these estimates seemed pretty variable. For example, one source reports increasing organic matter 1% stores 22,000 gallons of water per acre to a depth of 30 inches. Another reports that raising organic matter 1% leads to storing 16,500 more gallons of water an acre to a depth of one foot—an almost 90% higher increase per inch of soil compared to the previous estimate!

Rather than take estimates like the “1% soil organic matter stores 20,000 gallons of water” at face value, I think it’s important to consider where such estimates come from, so we can better understand what they mean and how to use them. So I asked: why were these estimates so different? Surely there’s a solid body of scientific literature these estimate are derived from to be so prevalent in the both popular media and technical reports. My curiosity was piqued, and I had to get to the R-horizon (soil speak for “bottom”) of this.

After extensive searching, I found several reports that cite a paper published in 1994 in the Journal of Soil and Water Conservation by Berman Hudson. However, in looking at this article, I found that the author never derives any estimates of gallons of water storage from increased organic matter. In true scientific fashion, the paper contains tables with means and estimates of coefficients of variation.  Even this soil scientist found it hard to visualize what the results would mean for storing water in the dark crumbly soil I love to see in my garden.

But a key point is made clear in the final figure of Hudson’s paper: soil textures (the amounts of sand, silt, and clay that make up a soil) have a huge impact on the change in water holding capacity of a soil with increased organic matter.  Because of the big differences in the surface area of soil particles, large particles like sand grains really can’t hold a candle to the really small soil particles like silt and clays in terms of holding on to water. And this affects how increases in organic matter can change water-holding capacity, too. Boost organic matter 1% in a sandy soil, for instance, and water holding capacity increases. But that same increase in a silty loam soil will store even more water.

How much water, you ask?  This report from the Northern Colorado Water Conservancy District (using data from Hudson’s regressions) shows that increases in plant available water were almost 70% higher in the silty loam compared to the sandy soil for the same change in organic matter.

There is no arguing that boosting organic matter in our soils is a key part of healthy living soils that produces all manner of benefits for us.It’s also apparent that, upon closer inspection, the level of benefit we get for that boost is not universal: the benefit depends on the texture of the soil being considered, which differs from place to place. I never did get an answer to the question of where those estimates that were so different came from, and—despite their huge implications—it doesn’t seem like there is an extensive body of scientific literature supporting them. But at least now we have a pretty good idea why they were so different.

In sum, it doesn’t matter if you are an Iowa corn farmer, a Montana barley grower, or just someone like me who enjoys a washing down a pile of tortilla chips with a hoppy brew to celebrate World Soil Day, soil organic matter is something we all depend upon. And if we have more hot, dry summers like the past two, it’s something everyone will depend on even more—no matter where you live. Still, it would be nice to have more scientifically based data to work from.  

Dr. Todd Ontl completed his dissertation, titled Soil carbon cycling and storage of bioenergy cropping systems across a heterogeneous agroecosystem in the LESEM Lab in November 2013. He’s now a post-doctoral fellow with Michigan Technological University and the US Forest Service Northern Research Station in snowy and beautiful Houghton, Michigan. Todd loves soil.

Comment 1 

20,000 gal 
Submitted by John on Fri, 07/10/2015 - 22:14

Hi Todd, fascinating piece. Exactly what brought me to your post. In the time since you wrote this, have you come any closer to some satisfying #s for water absorption and holding capacity as SOM increases at various depths and types of soil?

Comment 2 

Soil organic matter and water 
Submitted by Todd on Thu, 07/30/2015 - 09:07

Hi John,
Thanks for your comment, John. I actually haven't seen anything new in the scientific literature since this was posted. However, I have seen another blog post very similar to this one that takes on this very issue. Lara Bryant's post on the Natural Resources Defense Council Staff Blog dated May 27, 2015 (http://switchboard.nrdc.org/blogs/lbryant/organic_matter.html) asks the exact same question of where this number comes from. While her answer is much the same as mine, she does consult Dr. Michelle Wander--a soils expert at the University of Illinois--who provides some insights into the math behind this estimate. The posting also cites the 1994 Hudson paper, so either that post was inspired by my original blog post, or there really isn't much else out there in the scientific literature about soil organic matter impacts on water storage.

But the "20,000 gallons for every 1% increase in SOM" statistic continues to pop up. I recently took the USDA Natural Resources Conservation Service's online "Soil Health Quiz" (http://www.proprofs.com/quiz-school/story.php?title=mte1mduwoaih7n&fb_re...) in order to test my knowledge of soil health. I was relieved to see I passed with flying colors, and one reason was the True/ False question: "Each one percent increase in soil organic matter could increase water holding capacity by 20,000 - 25,000 gallons per acre. Horray! They provide a real estimate, and a range of the effect, presumably due to the impacts of soil texture. But to my disappointment, they list their source as a
Kansas State University e-Update that does not cite anything in the primary literature, or further explain the importance of soil texture, let alone other soil properties such as soil bulk density. I was pleased to see Lara Bryant's post does cover these topics.

So, as you can see, the importance of SOM for soil functioning--especially water holding capacity--is getting just as much press these days. And for good reason, 2015 is the International Year of Soils. It's great to see both the importance of soils, and the importance of how our actions managing our soil resource, impact critical issues like water availability for crops and humans alike.

Comment 3 

Organic matter by soil type and % O.M.
Submitted by Paul Salon on Fri, 11/13/2015 - 21:24

I was re-searching for the information about how much increasing soil O.M. by 1% could increase available soil water holding capacity and then found it in a power point of mine. I also found it when searching on some other power points on line which was probably where I found it originally. It related to the original article you cited from Hudson then two researchers from ARS "figured" out how to create this table below. I do not know how they came up with the table but you could ask them. I just thought I could multiply the inches of available water holding capacity by 17,154 gallons per acre inch and related that per foot or at least the top foot. (excuse the formatting of the table if it does not transfer). So in a silt loam going from 2 to 3% there is an increase of 0.5 inches or 13,577 gallons. Again I did not figure out how they developed the Table from the original article but maybe there is some hard science behind it after all it was on the internet. Which is the point of your original post.

Soil Organic Matter and Available Water Capacity
Inches of Water/One Foot of Soil
Percent SOM.. Sand..... Silt Loam...Silty Clay Loam
1.................. 1.0...... 1.9........... 1.4
2.................. 1.4...... 2.4........... 1.8
3................... 1.7...... 2.9 .......... 2.2
4................... 2.1...... 3.5 ......... 2.6
5 ................... 2.5......4.0 ......... 3.0
Berman Hudson
Journal Soil and Water Conservation 49(2) 189-194 March – April 1994
Summarized by:
Dr. Mark Liebig, ARS, Mandan, ND
Hal Weiser, Soil Scientist, NRCS, Bismarck, ND

Comment 4 

sea levels 
Submitted by Jace McCown on Fri, 11/04/2016 - 14:17

I just ran some numbers on this, on the low end of 20,000 gal, increasing the SOM of all the worlds agricultural lands (12.137 billion acres) means sequestering 242.7 trillion gallons of water, that's equates to a 1/10" drop in sea levels at 29 quadrillion gallons per 1' sea level change (based on current surface area of 140 million square miles). Wow.

Comment 5 

Sea level 
Submitted by Frode Haugsgjerd on Sat, 06/24/2017 - 07:45

Probably hard to model, but on top of that you can add the colling effects of sequestered carbon, cooler soil and increased local rainfall.