The connection between soil organic matter and soil water

The connection between soil organic matter and soil water

Anna Cates, University of Minnesota Soil Health Specialist

*This article was originally published by Minnesota Crop News on March 24, 2020 and shared here with permission.

One benefit of increasing soil organic matter is to store more water in your soil. Why does this happen? Because soil organic matter creates pores in a range of sizes. Exactly how much more water is stored due to soil organic matter will depend on soil texture, though.

Why Are Soil Aggregates Important?

Soil organic matter is a busy mix of materials- fragments of last year’s stalks and roots, earthworm casts, and living microbes and invertebrates, to name just a few. These materials are broken down by physical and biological processes. For example, freezing and thawing causes plant residue to lose its structure. Tiny dissolved molecules flow deep into the soil with rainwater. Hungry invertebrates, fungi, and bacteria consume complex living and dead organic material and excrete nutrients they don’t need in a smaller, simpler form. These small organic molecules can stick to clay surfaces. Clay surfaces covered with organic material grow like snowballs, and soil aggregates are formed. 

Soil aggregates are critical for holding water in the soil for two reasons. First, a well-aggregated soil has large pores between aggregates to let water enter the soil profile. Second, small pores within aggregates hold water tightly enough to keep it around, but loosely enough for plant roots to take it up. It’s critical that soil both let water flow through, and hold water for later. If your soil doesn’t let water infiltrate, you have ponding, runoff and soil loss, and lower plant water supply. If your soil doesn’t hold water, plants suffer from drought.

SOM fig. 1
It’s critical that soil both let water flow through, and hold water for later.

So, soil organic matter is critical for forming aggregates, and aggregates are critical for holding water. Because of that link, there is definitely a positive relationship between organic matter and water-holding capacity. How much water-holding capacity increases depends on your soil type.

Animal manures are one option for increasing soil organic matter and soil health. Animal manures can produce immediate benefits for improved soil aggregation and reduced soil runoff and erosion.  Manure use over multiple years can produce long term benefits by increasing total soil organic matter. Animal manures value for improving soil quality and yield were discussed previously in Poultry Litter’s Agronomic and Natural Resource Benefits and Does Manure Benefit Crop Productivity?

Soil Water vs Plant Available Water

We’re mostly interested in the soil water as it relates to plant-available water. Plant-available water capacity is water held by soil against the pull of gravity (i.e., it doesn’t wash through) but not too tightly for plants to draw it in. You see a bigger bump in plant-available water capacity when you increase organic matter in coarse-textured soils than finer loams or clays. This is because coarse soils naturally have larger pores between particles and really need the organic matter to develop small pores. Fine-textured soils already have small pores and aggregate more easily, so there are diminishing returns on increased organic matter. More soil organic matter means more soil pores and lower bulk density. Some of those pores are large, which is great for infiltration, but won’t increase plant-available water capacity.

You can calculate how much more available water holding capacity you might get from increasing organic matter, but the number varies with soil type. For example, a recent compilation of studies found that available water capacity in medium-textured soil increased by 1.03% with every 1% OM increase (Minasny & McBratney 2017). If you’re starting with available water capacity of 22% (moderate for a silt loam according to NRCS), adding 1% OM would bring you up to 23.03% available water capacity (Table 1). 

How Much More Water Is Available?

You can estimate how many gallons that adds up to in a 1ft depth of soil. Increasing OM by 1% increases available water capacity by about 3,400 gallons per acre in that medium-textured soil, on top of an estimated existing 71,000 gallons available water capacity. 3,400 gallons is about a 1/9 inch rainfall or irrigation event. That’s 3,400 gallons in the soil, instead of lost as runoff. That water prevents drought stress and holds soluble nutrients, like nitrate, that plants will be able to access. Notice that while available water capacity increases by about 3,500 gallons in both a loamy sand and a silt loam, for the loamy sand that 3,500 represents 1/10 of its new available water capacity- a much more striking increase!

3,500 gallons is just an estimate. What’s important is that increasing organic matter fundamentally changes the soil structure. We can’t push soil from a loamy sand to a clay loam. But management focused on protecting soil structure and building soil organic matter, like reduced tillage and continuous living cover, can build organic matter and improve soil function.

Table 1. Estimates of available water capacity (AWC) increases with soil organic matter (OM) increases, 0-12” soil samples.
Soil texture*AWC increase per 1% OM increase (%)**AWC increase per 1% OM increase (gal.)Initial AWC (gal.)AWC after 1% OM increase (gal.)
Loamy sand (0.5-3%) 1.13 3,666 32,583 36,249
Silt loam (3+% OM) 1.04 3,383 71,682 75,075
Clay loam (3+% OM) 0.82 2,665 55,391 58,056

*Average initial AWC by soil texture from NRCS data:

**Average increase in AWC  per 1% OM for coarse, medium and fine soils based on Minasny and McBratney 2017, converted from increase per 1% OC using van Bemmelen Factor (OM% = C% / 1.724) 

This article was reviewed by Rick Koelsch and Leslie Johnson

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