Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

The unsaturated conductivity is surely one of the most important soil properties, since it determines the rate at which water flows through the soil towards the roots. Near saturation, K is high, but falls drastically as the soil becomes drier, irrespective of soil texture (Fig. 3.18). The reason for the decline is that water flows appreciably slower in the finer pores. Since sandy soils contain more coarse pores than heavier soils, K is higher in sandy soils near saturation. However, the coarse pores lose water more rapidly than the finer pores. Consequently, the decline of K in sandy soils is much steeper, compared to heavier soils. At a certain matric potential, K in the sand will become lower, compared to the heavier soil. From this point onwards, K will be lower in the sandy soil, compared to the heavier soil, although the matric potential is the same in both soils (Fig. 3.18). The practical implication of this is that grapevines in a sandy soil will experience more water constraints, compared to ones in heavier soil at the same matric potential. Therefore, it is evident that guidelines for irrigation scheduling by means of tensiometers will differ between sandy and heavier soils. Based on the foregoing, the ideal situation would be if grapevine water status could be related to K for different soils. Unfortunately, the methods used to determine K are complicated and time consuming. Therefore, K cannot be determined for soils on a routine basis by commercial laboratories. Due to this, there is almost no K data available for soils in the grape growing regions.

10 -3

Sand

10 -4

Clay

10 -5

10 -6 K (cm/sec)

10 -7

100

200

Ȍ m

(-cm)

FIGURE 3.18. Examples of the relationship between K and Ψ m as sand and clay dry out, respectively (redrawn from Hillel, 2004). The vertical dashed line illustrates the similarity in K when Ψ m reaches a suction of -50 cm. Since K is closely related to soil texture, very little can be done to improve the unsaturated flow of water in soils, particularly in the fine pores. Where naturally compact soil is loosened during soil preparation, the fine pores will probably

IRRIGATION OF WINE GRAPES 69