SOIL: The farmer’s most important asset Part 10: Soil water (ii

how quickly the soil will drain following Water heavy rains or irrigation. The higher the saturated hydraulic conductivity of Soil column the soil, the quicker excess water will flow through the profile. Any factor that influences the size and arrangement of the soil pores, will influence the saturated hydraulic conductivity of the soil. In sandy soils, the pores are larger and the resistance to water flow is relatively small, and for that reason the water will flow rapidly through the profile (FIGURE Flow rate = Saturated 3). When sandy soil is compacted, for hidraulic flow example by vehicles in a road or via plough compaction, the pores are reduced in size and the flow of the water becomes slower (for example standing water on a farm road after a rain FIGURE 2: Schematic representation of shower). In a clayey soil the pores are saturated flow through a soil column. smaller (although far greater in numTypes of water movement in the ber) with the resistance which is now soil relatively high and again the water will The hydraulic conductivity of a soil flow more slowly through the profile. is its ability of water to move under Interestingly, is that the flow through the influence of the driving forces or a pore with a 1 mm diameter is equal the soil’s ability to allow water to flow to the flow through 10 000 pores of through it. The hydraulic conductivity 0.1 mm. This simply emphasises that (or flow) takes place through water- the flow rate is mainly determined by filled pores and along films of water. the number of larger pores in the soil. The hydraulic conductivity is for that Unsaturated flow reason dependent on the moisture As soon as a soil has been excessively content of the soil. The greater the wetted after heavy rain or irrigation water content of the soil, the greater and allowed to drain to the drained the hydraulic conductivity and vice upper limit, hydraulic conductivity due versa. It therefore means that the to gravity is very low and the matrix water flow is highest directly after potential is the dominant driving force. wetting and thereafter it gradually Movement under saturated conditions decreases as the soil dries out. The is rapid and mainly downwards with forces that determine the flow rate are little lateral movement. Under condigravity and the matrix potential. tions of unsaturated flow, the water flow is much slower and the moveSaturated hydraulic ment can be upwards, downwards or conduction ability laterally. Matrix potential and moisture The highest hydraulic conductivity of content differences are now the main a soil will be at saturation. A layer of driving forces and therefore unsaturatwater on top of the soil surface will ed flow becomes increasingly slower increase the flow due to a positive as the soil dries out. matrix or pressure potential. This is The unsaturated flow is of particular illustrated in FIGURE 2 as the rate at importance to the soil user, as it is the which water flows through the profile, main method causing water moveand is measured at the bottom of ment from the wetter parts, further the profile. The saturated hydraulic away from the plant root, to the drier conductivity of a soil is an indication of parts surrounding the root. In a sandy loam soil, the flow under hours unsaturated conditions is between 1 and 2 mm per day for a soil which is close to hours the lower limit. It is a rate that will supply hour hours enough water to the plant roots. As the soil gets drier, the flow rate is reduced to a point where water hours movement is so slow that the water does FIGURE 3: Schematic representation of water movement in (a) sandy and (b) clayey soils. a b not reach the plant root quick enough and the plant begins to wilt. In the case of

clayey soils, this point will be reached quicker than in sandy soils and drought symptoms are more quickly visible in the clayey soils. On the drier side of the spectrum the picture is turned around and the flow in the clayey soils is quicker than in sandy soils and clayey soil can keep plants alive longer than sand (although under constraint).

Movement of water vapour

Soil pores that are not filled with water are filled with air. Normally the humidity of the soil is about 98% (high vapour pressure). When the soil close to the surface is dry, the humidity there can decrease due to evaporation (lower vapour pressure) and water vapour will move from the higher vapour pressure area (soil) to the lower vapour pressure area (atmosphere). This movement of water vapour in soil is of minimal significance in agriculture. The quantity of water in vapour form is about 15 litres/ha in the upper 15 cm of the soil layer, compared with 375 000 litres/ha water in the liquid form at the lower limit of plant available water.

Summary

Water movement in soil is of extraordinary importance to the farmer, as it is the method that determines the provision of water to plants. Various soil types react differently where water retention and hydraulic conductivity are concerned. With the planting of crops in various soils, the farmer must keep the behaviour of the soil in mind, as it can have a significant impact on the sustainability of the farming enterprise.

In the next article in this series, the behaviour of water in soil will be discussed further.

References

Bennie, ATP. 1981. Soil Science 354. Soil and Water Management. Unpublished class notes for GKD354. University of the Free State, Bloemfontein. Bennie, ATP. 1985. SA Coop, Vol. 5, Nr. 1 Brady, NC. 1990. The nature and properties of soils. 10th ed. Macmillan Publishing Company, New York. Brady, NC. and Weil, RR. 2002. The nature and properties of soils. 13th ed. Prentice Hall, New Jersey. Van Huyssteen, CW. 2009. Soil Ecology. Unpublished class notes for GKD214. University of the Free State, Bloemfontein.

For further information, please contact: Martiens du Plessis: martiens@nwk.co.za Cornie van Huyssteen: vanhuysteencw@ufs.ac.za ProAgri BNZ acknowledges Grain SA for the use of this series which originally appeared in Afrikaans in SA Graan/Grain.