The link between soil organic matter and soil water

By Anna Cates, State Soil Health Specialist

One of the benefits of increasing soil organic matter is storing more water in your soil. Why does this happen? Because the organic matter in the soil creates pores of different sizes. However, the exact amount of water stored due to soil organic matter will depend on the texture of the soil.

Soil organic matter is a loaded mix of materials – fragments of last year’s stems and roots, earthworm casts, and living microbes and invertebrates, to name a few. These materials are broken down by physical and biological processes. For example, freezing and thawing cause plant residues to lose their structure. Tiny dissolved molecules flow deep into the ground with rainwater. Hungry invertebrates, fungi, and bacteria consume complex living and dead organic matter 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 matter grow like snowballs and soil aggregates are formed.

How Soil Aggregates Affect Soil Water

Soil aggregates are essential for retaining water in the soil for two reasons. First, a well-aggregated soil has large pores between aggregates to allow water to enter the soil profile. Second, small pores in the aggregates hold water firmly enough to hold it, but loose enough for plant roots to absorb it. It is essential that the soil allows water to drain away and retains water for later. If your soil does not allow water to infiltrate, you will have puddles, runoff and soil loss, and a decreased water supply for plants. If your soil does not retain water, plants suffer from drought.

Thus, soil organic matter is essential for forming aggregates, and aggregates are essential for retaining water. Because of this link, there is definitely a positive relationship between organic matter and water-holding capacity. The increase in water holding capacity depends on your soil type.

Water capacity available to the plant

We are primarily interested in soil water as it relates to the water available to plants. The water capacity available to plants is the water held by the soil against the pull of gravity (i.e. it does not pass through) but not so tight that plants can l aspire. organic matter in coarse-textured soils than finer silts or clays. This is because coarse soils naturally have larger pores between particles and really need organic matter to develop small pores. Fine-textured soils already have small pores and aggregate more easily, so there are diminishing returns on increasing organic matter. More soil organic matter means more soil pores and lower bulk density. Some of these pores are large, which is great for infiltration, but will not increase the water capacity available to plants.

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

Table 1. Estimates of available water capacity (AWC) increase with increasing soil organic matter (OM), soil samples 0 to 12 inches.
Soil Texture* Increase in AWC by
1% increase in MO
Increase in AWC by
1% increase in MO
Initial AWC
AWC after
1% increase in MO
Loamy sand (0.5-3%) 1.13 3,666 32,583 36,249
Silty loam (3+% MO) 1.04 3,383 71,682 75,075
Clay loam (3+% MO) 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 from Minasny and McBratney, 2017, converted from increase per 1% CO using the van Bemmelen factor ( OM% = C% x 1.724)

You can estimate how many gallons that adds to a 1 foot soil depth. Increasing MO by 1% increases AWC by approximately 3,400 gallons per acre for this medium-textured soil, in addition to an existing available water capacity estimated at 71,000 gallons. 3,400 gallons is approximately one 1/9 inch rainfall or irrigation event. That’s 3,400 gallons in the ground, instead of being lost to runoff. This water prevents water stress and contains soluble nutrients, such as nitrate, that plants can access. Note that while the available water capacity increases by about 3,500 gallons in a loamy sand and loamy loam, for loamy sand, that 3,500 is one tenth of its new available water capacity – a much more dramatic increase!

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

This article was corrected on April 2, 2020. The original version overestimated gallons gained in available water holding capacity with increasing organic matter. Thanks to the eagle-eyed readers who pointed out the discrepancies.

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