Handbook for Irrigation of Wine Grapes in South Africa

Chapter 3

Water related soil properties

3.1 INTRODUCTION Plant root systems absorb water containing dissolved nutrients from the soil via the fine hair roots. Consequently, the soil’s ability to absorb, conduct, store and lose water are important considerations when it comes to practical irrigation management for sustainable wine production. Naturally, soils may differ substantially within a region, or even within a vineyard block. The soil variation will have an impact on the way irrigation needs to be managed with respect to irrigation intervals, volumes of water per irrigation, etc . Therefore, it is necessary to know, and understand, the different soil properties that are related to water. Since the other water related soil properties are all related to soil texture, it is one of the most important soil properties, as will be discussed below. TEXTURE Texture refers to the size distribution of the particles smaller than 2 mm in diameter that constitute the solid component of soils. The latter can be separated into clay, silt and sand. The sand fraction is further separated into fine, medium and coarse sand. Soil texture can be classified into various classes according to the percentages of the particles present in a soil sample. The percentages are referred to as the particle size distribution. Furthermore, soil texture triangles can be used to determine the nature of the sand (Fig. 3.1A), as well as the soil textural class (Fig. 3.1B). Determining particle size distribution for accurate texture classification is a tedious process, and needs to be carried out by soil laboratories. However, soil texture can also be classified according to the physical appearance and properties when the soil is manipulated by hand in a certain way (Fig. 3.2). Such “hand” texture classifications are commonly used, particularly when soils are being mapped. With a bit of experience, hand classification can provide fairly accurate textural classes rapidly at a low cost. 3.2

54 CHAPTER 3 – WATER RELATED SOIL PROPERTIES