FERTILISER GUIDELINES FOR THE WINE INDUSTRY

TABLE 1. Minimum soil phosphorus concentrations required for grapevines grown in different soil pH regimes, as applicable for different extractants # .

Extractant Olsen Citric acid Mehlich III Bray I Bray II mg/kg

Soil pH KCl

Soil texture class

Sandy Loamy Clayey Sandy Loamy Clayey Sandy Loamy Clayey Sandy Loamy Clayey

– – – – – –

25 30 35 25 30 35 25 30 35 25 30 35

25 30 35 20 25 30 20 25 30 18 21 25

20 25 30 15 20 25 10 12 15 10 12 15

20 25 30 20 25 30 20 25 30 15 18 21

<5.0

5.0-6.0

3

10 12 15 10 12 15

6.0-7.0

>7.0

Sandy (0-6% clay) / Loamy (6-15% clay) / Clayey (>15% clay) # Data on the relative extractability of P with different extractants were supplied by C.P. Beyers, Nitrophoska. Bray I: 0.025M HCl + 0.3M NH4F Bray II: 0.1M HCl + 0.3M NH4F Mehlich III: 0.2M acetic acid (CH 3 COOH) + 0.25M NH 4 NO 3 + 0.015M NH 4 F + 0.13M HNO 3 + 0.001M EDTA Olsen: 0.5M NaHCO 3 Citric acid: 0.05M citric acid (C 6 H 8 O 7 ) Mostly, a simplified approach is followed by laboratories serving the wine industry, namely when the soil pH KCl < 7.0, either a citric acid, Bray I, Bray II or Mehlich III extraction is conducted and similar norms are used (Table 1). For soil with a pH KCl ≥ 7.0, an Olsen extraction is often done, and the norms in Table 1 are used. The logic is that an Olsen extraction is less aggressive and has a higher pH, theoretically reflecting the lower rate of P release in the root zone at higher soil pH conditions. Olsen, however, has been shown to extract only 5-7% of citric acid extractable P. In reality, this means that the total P in the soil can become excessively high (e.g. if extracted with Bray I or Bray II), while the Olsen P remains below the norm.

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