Water management in irrigated rice - Rice Knowledge Bank ...

Ψ Air–100 MPaDemandLeafXylemStemRootStomatal cavityΨ Leafг StemΨ Rootг Leaf–1 MPa–0.4 MPaWater phase Vapor phaseEndodermisг RootSupplyWater flowSoil particleΨ Soil–0.02–0.1 MPaΨroot – Ψsoil Ψleaf – Ψroot Ψair – Ψleaf= =гroot гstem гleafFig. 2.2 Schematic overview of water flow and water potentials in the soil-plant-atmosphere continuum. Adapted fromLövenstein et al (1992).Table 2.2. Some typical soil water tension levels in relation to the growth of upland crops (e.g., wheat, maize, cotton).Name pF value ExplanationSaturation –∞ All soil pores are filled with waterField capacity 2 Soil water content that is considered optimum for upland cropsPermanent wilting point 4.2 Soil water content at which most upland crops cannot extractwater from the soil any more and show permanent wiltingAir dryness 7 No free water in soil pores any moreavailability, soil microbial population, soil organicmatter buildup/decomposition, and occurrence ofsoil pests and diseases (Chapter 3.6). Figure 2.3illustrates soil water tensions measured in an aerobicsoil where rice was grown under nonfloodedconditions like an upland crop.Each soil has a specific relationship betweenthe tension of the water and the amount of water:the lower the amount, the higher the tension. Thus,a small amount of water in the soil and a hightension both reflect a condition of relative “waterscarcity” and drought. The relationship betweensoil water tension and soil water content is calledthe “soil water retention curve” or the “pF curve.”The shape of the pF curve depends on soil type,especially on its texture (mixture of clay, silt, andsand particles), bulk density (the weight of a soilover its volume), mineralogy, and organic mattercontent. Examples are given in Figure 2.4 for atypical clay soil and a typical sand soil. There arefour important points on the pF curve for uplandcrops: saturation = pF –∞ (H = 0 cm), field capacity= pF 2, permanent wilting point = pF 4.2, andair dryness = pF 7 (see Table 2.2). The amount ofwater at each of these four tensions is different foreach soil type. When rice fields are flooded, thepF curve is not needed as soil water contents arealways at saturation. However, when a rice soil13