Field Capacity Is Not Saturation
Land Profile, Inc., Yakima, Washington
Published in Soil Profiles
Vol. 5 No. 2 Spring 1995
FIELD capacity and saturation involve two different water contents. In medium textured soils, field capacity is about 50% of pore volume. By definition, saturated soil has soil water filling 100% of soil pore volume. Soil moisture at field capacity may meet a limited colloquial definition of saturation, but my observations are that field capacity soil moisture will not induce anaerobic conditions. Although we who identify jurisdictional wetland conditions have been strongly directed to include all of the capillary fringe in deducing depth to saturated soil conditions, my observations with the dipyridyl indicator are that only the first few inches of the capillary fringe are sufficiently restrictive of oxygen transfer to induce anaerobic soil conditions.
The above statement has some history to it. I was out with another wetland professional [T. J. Stetz, Army Corps] a couple of weeks ago and we were discussing the definition of saturated soil conditions. I regret to say that I mistakenly agreed with him that when soil scientists describe soil as "saturated", we can mean "above field capacity". We went on to deduce that since saturation induces anaerobic conditions, soil moisture anywhere above field capacity induces anaerobic conditions. I knew this was somehow wrong at the time, but we were so balled up in figuring out our delineation that we didn't get back to this subject. Anyway, I now thoroughly regret ever thinking of saturation as anything other than chock full of water. I've put a lot of thought into this and I am here to tell you that it takes a full load of water to induce redoximorphic features: up until the soil pore space is almost full of water, oxygen replenishment can meet microbial demand. The math used to compare oxygen transfer to soil demand for oxygen is pretty straight forward. It points to a soil moisture content near maximum as needed to induce anaerobic conditions. I used a formula passed on to me by a soil scientist at North Carolina State University. It is soil-texture and moisture-specific. I routinely use it to calculate oxygen demand treatment capacity for wastewater sprayfields. The formula addresses soil pore volume limitations on oxygen diffusion, based on a work authored by Francis Clark, an USDA-ARS microbiologist. It assumes ideal temperatures promoting maximum microbial activity and root respiration. It goes into a lot of detail as to what occurs at the root surface.
The Clark article is of additional interest to soil scientists who work with wetlands in that it projects the length of time of soil saturation needed to induce low oxygen soil conditions: 96 hours to cause crop damage in the flood irrigated example given by Clark.
[1]
T = D - R
Where, T= kg-O2 ha-1dy-1 available to meet total oxygen demand (TOD)
Where, R= kg-O2 ha-1dy-1 demanded by root respiration, microbial activity
[2]
D = K A
Where, D= kg-O2 ha-1dy-1 of oxygen diffusion rate in soil
K= 1740 kg-O2 ha-1dy-1 oxygen diffusion rate in air
A= cm3/cm3 of air filled soil pore volume at field capacity
[3]
A = V - S - W
Where, V= Total volume, 1 cm3cm-3
S= Soil particle volume, cm3cm-3
W= Water volume, cm3cm-3
[4]
S = Sd/Pd
Where, Sd= Density of soil mass, 1.25.
Pd= Density of soil particles, 2.63.
[5]
W = WgSd
Where, Wg= gravimetric percent of water held within soil.
Inputting various soil moisture values yields the following results:
For anaerobic conditions to be induced at field capacity, soil respiration-based oxygen demand ( R ) in the above example would have to increase from 161 kg O2 ha-1dy-1 by another 715 to 776. This increase could arguably be due to a higher carbon content in wetland soils promoting respiration. I would expect a wetland soil to have a higher O2 demand than an adjacent upland soil, but certainly by no more than 200 kg O2 ha-1dy-1. An increase of 200 kg O2 ha-1dy-1 would imply to me a tremendous amount of rotting vegetation, a condition not sustainable under naturally occurring circumstances.
Increasing respiration from 161 kg O2 ha-1dy-1 by another 200 to 361 produces a calculated oxygen equilibrium moisture content of 92% of saturation (177% of field capacity; W:S:A = 49:47:4). Even at a high rate of respiration, anaerobic conditions would not be expected until well above field capacity.
I do not mean to oversimplify an exceedingly complex process: formulas can never do full justice to the interplay of factors in soils. There may be all sorts of factors (tortuosity, microsaturation, thin horizons of compaction or cementation) that serve to further decrease oxygen diffusion in soil. These complications will certainly increase with depth. Serving to reduce the effect of these complications is the fact that hydric soil formation is largely a surface soil and root zone phenomenon.
My observations support the applicability of the formula to field soil conditions. I have spent years trying to get a handle on "anaerobic soil failure" of wastewater sprayfields. I find no evidence in the literature or in my experience that would lead me to believe that we should expect anaerobic soil conditions to be induced by maintaining soil moisture near field capacity soil moisture. Thousands of acres of drip irrigated grapevines and hopvines in Yakima would be in trouble if prolonged soil moisture above field capacity could induce anaerobic soil conditions.