The unconsolidated material immediate the surface of the earth serves as natural medium for the growing plants. Soil depth defines the root space and the volume of soil from where the plants fulfil their water and nutrient demands.

Shallow soil (depth<45 cm) formed on schist cultivated with olives in the island of Lesvos (photo by C. Kosmas)

Soil water-storage capacity and effective rooting depth are mainly related to the soil depth. Soil degradation due to soil erosion is a serious threat to the soil quality and productivity in hilly areas. The effects of soil erosion on productivity depend largely on the thickness and quality of the topsoil and on the nature of the subsoil. Productivity of deep soils with thick topsoil and excellent subsoil properties may be virtually unaffected by erosion. However, most hilly soils are shallow or have some undesirable properties in the subsoil such as petrocalcic horizon, or bedrock that adversely affects yields. In either cases, productivity will decrease as the topsoil gets thinner and undesirable subsoil is mixed into the topsoil by tillage.

Soil formed on Tertiary and Quaternary consolidated formations usually have a restricted soil depth or the thickness of the soil above the bedrock or the limiting subsurface layer is small reducing the rooting depth and rainfed vegetation can not be supported under hot and dry climatic conditions leading to desertification.

Semi-arid landscapes by definition are water-limited and therefore are potentially sensitive to environmental change and to plant growth. The water available for plants to grow depends on the climatic conditions (rainfall, evapotranspiration) and the soil water storage capacity. The water storage capacity of a soil is defined by the water holding capacity (WHC) of each soil layer and it is related to soil texture, soil depth, amount of rock fragments, parent material, etc Any degrading process that limits the rootable soil space depletes its nutrients and water supply capacity and reduces bioactivity.

Given certain physical soil characteristics and underlying parent material, two soil depths, very important for land protection from desertification can be distinguished: the critical and the crucial soil depth. The critical depth can be defined as the soil depth in which plant cover achieves values above 40% (see figure). Extensive studies conducted under semi-arid climatic conditions in hilly areas of Greece with soils formed on various parent materials have shown a critical depth of 25-30 cm. On soil less than that depth the recovery of the natural perennial vegetation is very low and the erosional processes may be very active resulting in further degradation and desertification of the land. When a hilly landscape of marginal capability is cultivated, agriculture should be abandoned before the soil reaches the critical depth.

Relation of percentage vegetation cover of Sarcopoterium sp and soil depth measured in areas with soils formed in pyroclastics magmatic glomerates) and schist-marble (Kosmas et al., 2000).

While the critical depth is a limit to cultivation, the crucial depth can be defined as a lesser soil depth on which perennial vegetation can no longer be supported, and the whole soil is rapidly washed away by wind or water erosion. This is an irreversible process. Crucial depth is affected by the type of parent material in which soil is formed. Soils formed on pyroclastics are the most sensitive in their capability, with a crucial soil depth of 10 cm below which vegetation can not be supported. Soils formed on schist-marble metamorphic rocks have a higher capability to support perennial vegetation under the same climatic conditions, with crucial soil depth of around only 4-5 cm.

Soil degradation due to soil erosion is a serious threat to the soil quality and productivity. The effects of soil erosion on productivity depend largely on the thickness and quality of the topsoil and on the nature of the subsoil. Productivity of deep soils with thick topsoil and excellent subsoil properties may be virtually unaffected by erosion. However, most hilly soils in Mediterranean are shallow or have some undesirable properties in the subsoil such as petrocalcic horizon, or bedrock that adversely affects yields. In either cases, productivity will decrease as the topsoil gets thinner and undesirable subsoil is mixed into the Ap-horizon by tillage, or as water-storage capacity and effective rooting depth are decreased. Studies conducted in hilly areas of Greece cultivated with cereals showed a significant positive correlation between topsoil depth and wheat production (see figure). This relation relationship was largely influenced by slope position on the landscape.

Relationship of wheat biomass production and soil depth measured in hilly areas of Greece (Kosmas et al., 2001)

• very shallow (<15 cm)
• shallow (15-30 cm)
• moderately deep (30-60 cm)
• deep (>60 cm)
  

Kosmas C., Gerontidis St., Marathianou M. 2000. The effect of land use change on soil properties and vegetation establishment under various lithological formations in the Lesvos island. Catena, 40:51-68.

Tsara, M., Gerontidis, S., Marathianou, M., & C. Kosmas, 2001. The long-term effect of tillage on soil displacement of hilly areas used for growing wheat in Greece. Soil Use and Management, Vol. 17, pp. 113-120.

Kosmas, C., Gerontidis, St., Marathianou, M., Detsis, V., and Zafiriou, Th. 2001. The effect of tillage erosion on soil properties and cereal biomass production. Soil & Tillage Research J. 58:31-44.

Agricultural University of Athens, Laboratory of Soils and Agricultural Chemistry, Iera Odos 75, Athens 11855, Greece

Dr Constantinos Kosmas
email: lsos2kok@aua.gr