Soil Water Storage

In most of the world, crop yields are limited by timely rain or irrigation. The truth is that rainfall only promotes plant growth after it soaks into the soil. Water that is taken below the surface quickly has less of a chance to become runoff and is less exposed to evaporation. 

Maximizing the amount of water available for plant growth could be one of the most profitable improvements available to a producer. The following information is exurbs taken from an article written by Stewart B Wuest, Soil Scientist, USDA-ARS Soil and Water Conservation Research, Pendleton, OR, pertaining to how soil takes in and stores water.

This article explains what determines the fate of water that arrives on the soil surface and how our soil management strategies can improve water storage and reduce runoff and evaporation.

Principle 1: Water moves in surface films - Gravity is only one of the forces controlling water movement, and it is not the most powerful. Water clings to soil particle surfaces. It spreads out and creates water films that connect particles that touch, creating a network.

Water does tend to move downward after the water films become thick enough that their surface tension is no longer sufficient to resist the pull of gravity. When the water comes to a gap where there is no surface-to-surface contact, water starts to accumulate and form drips hanging into the void. 

The water backs up until the drops are large enough to fall or bridge the gap. This slows water movement and can nearly saturate the soil above the gap. This situation is common on finely tilled soils during the first several rainfall events.

In a naturally consolidated loam soil (either never tilled or several years since being tilled), water can move downward fast enough to prevent most surface water runoff.

Water movement downward is a chain reaction. Water entering the soil at the surface thickens the water films there, and that reduces tension, so water flows down, thickening the water layers below. The chain reaction is like turning on a faucet with a hose attached. Water introduced on one end of the hose moves all the water a little farther down the hose, so this can happen quickly.

The most important concept here is that voids do not help increase water flow under normal circumstances. Except at the soil surface, water is not moving in the voids (roots holes, cracks, and inter-aggregate spaces) because it is under tension, and for water to enter an air space, it needs to be forced there under pressure. Excessive soil compaction is bad, of course, and it is important for a soil to be well aerated to allow gas exchange for plant root growth, but increasing voids through tillage does not improve water infiltration (or not for very long, as we shall see). Tillage decreases the number of contact points between soil particles, and this interrupts water films and slows the advance of water.

Principle 2: Getting past the surface - On many agricultural soils, the limit on the maximum possible rate of water infiltration is determined by the top inch at the soil surface. The force of raindrops, or exposure to drying or freezing, liberates sand, silt, and clay size particles, and they become incorporated in the moving water. 

When a drop of water finds an opening in the soil surface, the water moves downward, carrying the soil particles along until they get trapped somewhere. Therefore, small soil particles move wherever water is moving, and they stop when the water slows or stops. The soil particles then settle into a dense crust that, over the first few rain events of the season, seals the soil surface and plugs any available cracks and holes. The process accelerates as more water accumulates, more sediment settles, and the soil surface becomes more saturated under even small rainfall events.

This is how classic sheet and rill erosion happens. When dry, we see the soil sediment as crusts—fragile but dense if undisturbed. When wet, they are amazingly watertight. Even rough-plowed soil will form these seals because soil that melts off the clods settles in the lowest points, gradually causing small ponds that collect more sediment and increase in size with every event.

Soil crusts slow water infiltration and generate runoff, but they also promote evaporation. In semi-arid regions, it is not unusual for rain to be followed soon after by dry, sunny, or windy weather. Soil crusts are densely packed because the soil particles were settled together when they dropped out of the accumulated water. 

In this tight layer, there is a lot of water being held by capillary forces, and that water is exposed to the sun and wind. The water films are relatively thick, so evaporation on the exposed surface of the crust draws water up and out of the entire thickness of the crust. 

Most or all of the water from a small rainfall event can be lost immediately if it is held at the surface in a crust. Soil that shines in the sun after a rain or during a thaw is evidence of a saturated surface layer where water is suspended—unable to infiltrate and subject to high rates of evaporation.

How to avoid sealed soil - How do we prevent or reduce soil sealing? There are two mechanisms that seem to work independently to prevent crusts and maintain high water infiltration rates. One is to develop and maintain a high soil organic matter content at the soil surface. No matter how you measure it, more organic matter at the soil - atmosphere interface helps to strengthen soil aggregates and prevents soil particles from becoming entrained with moving water. Crusts will not form if soil particles are not mobile. Raindrops are more likely to penetrate right where they fall, and if they cannot and instead accumulate and move downhill, they are less likely to bring soil sealing sediments to the low spots in the furrow or into cracks and holes.

High organic matter content works to improve water infiltration even in plowed and thoroughly tilled soils. We know this because long-term experiments where manure or legume residue has been applied demonstrate much greater infiltration rates than where only synthetic fertilizers have been applied even though all treatments had been intensively tilled (Fig. 1). 

Generally, however, the only practical way to increase soil organic matter at the soil surface is to reduce or eliminate tillage. This keeps crop residues on or near the surface instead of burying them and reduces mixing of the top inch of soil with deeper soil that has lower organic matter content. It is a matter of concentrating organic matter at the surface and keeping it there, and since this is where the soil seal happens, it is very effective at improving water infiltration.

The other way to prevent crusts is to maintain surface residue cover. A little is good, more is better. Surface residues protect soil aggregates from raindrop impact and other environments factors.

Given the above, it is easy to understand why minimum tillage and no-till systems have proven to be good at reducing or practically eliminating runoff. If a management system has very low soil disturbance, soil organic matter accumulates at the surface, and generally the same practices leave large amounts of crop residue covering the soil. Even relatively small reductions in tillage can promote significant soil organic matter stratification, and they generally leave more surface residue. Every reduction in tillage and every increase in surface residue produces measurable improvements in maximum water infiltration capacity.

For more information on this topic, you can find the entire article by searching “Soil Water Storage Stewart Wuest.”