Soil strength assessment using threshold probability approach on soils from three agro-ecological zones in Eritrea

Soils in many agro-ecological zones in Eritrea, a country in sub-Saharan Africa, are low in productivity due to erosion, low organic matter and poor soil management. Recently, mechanised farming has been intensively practiced to increase agricultural food production. However, the use of heavy machinery and vehicles can cause soil compaction that reduces soil productivity and crop yield. A study was conducted to determine the in-situ soil compaction and the probability of exceeding a root restricting threshold penetration resistance (PR) value of 2 MPa in three agro-ecological zones that varied in soil type, topography and climate. Soil cone penetration resistance measurements were taken up to a depth of 600 mm in 30 m by 30 m field plots with three replicates located in the three agro-ecological zones. Simultaneously, core samples were collected to determine soil moisture and bulk density from surface and subsurface soil layers. In all the soils, moisture contents during soil penetration resistance measurements were nearly the same as the field capacity soil moisture content (−10 kPa soil moisture suction). Penetration resistance values measured from 0 to 600 mm were averaged in depth increments of 100 mm. For clay loam (Adiguadad) and loam (Sheeb) soils, the penetration resistance values up to a soil depth class of 0–300 mm were statistically insignificant and categorised as minimum mechanical impedance to root growth (<1.0 MPa). In the sandy loam soils (Western lowland soil) the penetration resistance reached 2 MPa at 250 mm depth implying strong mechanical impedance to roots. Logistic regression analysis indicated that the probability of exceeding 2 MPa was significantly affected by soil moisture, bulk density and soil type. The probability increased with bulk density and decreased with soil moisture for clay loam, loam and sandy loam soils.