Water Retention Difference

C. Water Retention Difference

Also called Available Water Holding Capacity (AWHC), Water Holding Capacity (WHC), or Available Water Capacity (AWC)

The amount of water that a soil can hold between –33 kPa (–1/3 bar) and –1500 kPa (–15 bars) soil-water tension within the zone accessible to roots is the water retention difference of the soil. The water retention difference of the whole soil is calculated by estimating the amount of water each horizon can hold, determining which horizons are sufficiently accessible to plant roots to be significant sources of water, and summing the water retention differences of the accessible layers.

Water retention difference is commonly expressed in cm water/cm soil. Classes of water retention differences (Table 9) are based on the amount of water retention difference in the upper 1.5 m of soil, or above a root-limiting layer, such as a lithic or paralithic contact.

A number of factors are used to determine the water retention difference of individual horizons. These include texture, clay mineralogy, soil structure, volume of coarse fragments, organic matter content, and bulk density. For the contest, only texture and volume of coarse fragments will be used to estimate the water retention differences of individual horizons above 1.5 m.

Estimated water retention in relation to texture is provided in Table 10. If the instructions for a pit require judging a profile that is less than 1.5 m deep, then assume the last horizon extends to a depth of 1.5 m unless it is directly underlain by or contains a lithic or paralithic contact.

Contestants are to assume that plant roots are sufficiently restricted by these contacts that no water is available below the contact. Coarse fragments are considered to have negligible (assume zero) moisture retention so estimates must reflect the coarse fragment content (i.e., subtract the percentage of coarse fragment volume from the whole-soil volume). Table 11 is a sample calculation of water retention difference.

Table 9. The five classes for water retention difference.
Water Retention Class	Water retention
Very Low:	< 7.50 cm
Low:	7.50–14.99 cm
Medium:	15.00–22.49 cm
High:	22.50–29.99 cm
Very High:	> 30 cm

Table 10. Estimated relationships among water retention difference coefficients and soil textural class.
cm H₂O/cm soil	Textural Class of Soil Horizon
0.20	silt, silt loam, silty clay loam, loam, clay loam, and very fine sandy loam
0.15	sandy loam, fine sandy loam, sandy clay loam, sandy clay, clay, and silty clay
0.10	coarse sandy loam, loamy fine sand, loamy very fine sand, and loamy sand
0.05	loamy coarse sand and all sands

Table 11. Example calculation for water retention difference.
Horizon	Depth	Textural Class	Coarse Fragments (%)	Water Retention (cm)
Ap	0–12	ls	0	(12cm)(0.10) =	1.2
Bt1	12–28	sc	0	(16cm)(0.15) =	2.4
Bt2	28–54	scl	0	(26cm)(0.15) =	3.9
2Bt3	54–105	l	5	(51cm)(0.20)(0.95) =	9.69
2Bt4	105–132	grl	20	(27cm)(0.20)(0.80) =	4.32
2R	132+	-	0	(18cm)(0.00)(0.00) =	0.00
				Total water retention difference =	21.51
				=	MEDIUM