Handbook for Irrigation of Wine Grapes in South Africa

Chapter 5

If soils are irrigated with diluted winery wastewater over time, the soil pH (KCl) can increase by approximately 1 to 2 units in the absence of rainfall, irrespective of clay content (Mulidzi et al. , 2015b). The high amount of basic cations, particularly K and Na, applied via the wastewater seems to be the reason for the pH (KCl) increase (Fig. 5.20). In soils where leaching occurred upon the application of simulated rainfall, the pH (KCl) tended to decline slightly, whereas in the Lutzville sand and Robertson clay where no leaching occurred, the pH (KCl) showed a slight incline (data not shown). Since winery wastewater used for irrigation contained a high organic load, compared to the river water (Mulidzi, 2016), these cations were probably present in the form of organic salts. Such salts can produce OH - anions via decarboxylation that will increase the soil pH as illustrated in Figure 5.22 (Rukshana et al. , 2011). Organic acids present in the wastewater may also be a source of organic anions via the dissociation of H + which can increase the soil pH via decarboxylation (Fig. 5.22). If this happens, the soil might initially contain higher levels of H + , but the pH will increase over time as more OH - is formed (Rukshana et al. , 2011). The organic load in the wastewater could be a further source of organic N. These compounds will also produce OH - anions which can increase the soil pH if ammonification occurs in soil (Fig. 5.22).

Plant residues

Soil matrix O HO O O HO O O HO O O HO O O HO O O HO O O HO O O HO O

Effect of initial soil pH & pHBC and pKa of compound

Soluble compounds O rganic acids R-CO O H ( Acetic, malic, citric & benzoic acids) O rganic anions R-CO O - ( Potassium citrate)

H +

OH -

Dissociation

O rganic anions R-CO O -

R-H + CO 2

Decarbox ylation

Association

H +

OH -

2H +

O rganic N ( Glucosamine)

NH 4

NO 3 -

+

Ammonification

Nitrification

CO 2

H + /OH - ?

O ther compounds

Initial effect ( Physiochemical process)

Temporal effect ( Biological process)

FIGURE 5.22. Diagram illustrating possible mechanisms of soil pH changes upon addition of model compounds (redrawn from Rukshana et al ., 2011).

Figure . Diagram illustrating possible mechanisms of soil pH changes upon addition of model compounds ( after Rukshana et al. , 2014) .

IRRIGATION OF WINE GRAPES 135