Soil Quality Assessment Strategies for Evaluating Soil Degradation in Northern Ethiopia

Table 3

Soil quality indicators, scoring curve, and threshold and baseline limits used for evaluating eight LUSMS in northern Ethiopia.

Indicator	Scoring curve	Threshold		Baseline^c	Optimum^d	Slope at baseline	Source of limits
		Lower^a	Upper^b
Sand (%)	Optimum	0	60	L, 30	36	0.440	Natural ecosystem, best managed soils
				U, 50
Silt (%)	More is better	0	38	19	—	0.249	Best managed soil, natural ecosystem
Clay (%)	More is better	0	31	15	—	0.268	Natural ecosystem
SAS (%)	More is better	10	60	30	—	0.296	Harris et al. [17]; natural system
BD (g cm^−3)	Less is better	1	2.2	1.5	1.2	−0.323	Harris et al. [17]; natural ecosystem
A-horizon depth (cm)	More is better	0	20	10	—	0.318	Natural ecosystem
MWHC (%)	More is better	20	58	30	—	0.198	Gregory et al. [41]; natural system
OC^e (%)	More is better	1	6.5	3.5	—	1.046	Kay and Angers [42]; natural ecosystem
CEC (cmolc kg^−1)	More is better	6	46	20	—	0.245	Natural ecosystem, best managed soil
TN (%)	More is better	0.05	0.54	0.30	—	25.408	Natural ecosystem,
TP (mg kg^−1)	More is better	200	1316	650	—	0.005	Natural ecosystem
Pav (mg kg^−1)	More is better	5	29	15	—	0.433	Mausbach and Seybold [43]; natural ecosystem
Zn (mg kg^−1)	Optimum	2	20	L, 10	14	0.855	Mausbach and Seybold [43]; natural system
				U, 18
Fe (mg kg^−1)	Optimum	10	50	L, 20	26	0.577	Harris et al. [17]; natural system
				U, 40
Earth worm count; per m^2	More is better	0	11	5	—	0.470	Natural ecosystem

aSoils at or below the threshold values are prone to structural destabilization, erosion, and low productivity; so the scoring value is 0.
^bSoils at or beyond this values no further increase in productivity or decrease in erosion rate are achieved the upper threshold; values at and above this level thus receive a score of 1.0.
^cValues receive a score of 0.5 and are generally regarded as the minimum target values.
^dThe value is given a score of 1.0 if the desired relationship is bell-shaped.
^eAccording to Kay and Angers [42], irrespective of soil type if SOC contents are below 1%, it may not be possible to obtain potential yields.
L: lower; U: upper; SAS: soil aggregate stability; BD: bulk density; MWC: maximum water holding capacity; OC: organic carbon; CEC: cation exchangeable capacity; TN: total nitrogen; TP: total phosphorous; Pav: available phosphorous; Zn: available Zinc; Fe: available iron; —: implies not applicable.