Soil Management: Soil Acidity Management

This aspect involves two major issues : one, soils with low pH and two, soils acidified by our management practices such as manuring and terracing. It also involves the crop species since cocoa is sensitive to low pH and high Al saturation while oil palm and rubber are not. In fact, high soil pH which generally goes with high exchangeable Ca and Mg may be detrimental to rubber yield since the named nutrients can caused unstable latex and pre-coagulations.

Excluding acid sulphate soils which will be discussed in part two of this lecture, soils of pH less than 4.5 or Al saturation more than 30% should be limed if cocoa is grown on them. This is because good responses to liming were obtained by Lam and Lim (1991) and Lim and Ho (1994) for such soils cultivated with cocoa. Oil palm and rubber can tolerate low acidity, and pH 3.5 does not seem to influence crop productivity.

Soil acidification is generally a natural process of soil formation such as leaching of nutrients, nutrient uptake by plants and pollutants. However, soil acidification which is generally regarded as soil degradation can occur through fertilisation with acidifying fertilisers such as ammonium sulphate as shown.

(NH4) 2SO4 + 8 0 —> 2 NO3 + H 2 SO4 + 2H20

Kee et al . (1993) showed that this process occurs in oil palm agroecosystem where soil pH decreased from 4.2 to 3.8 after 7 years of NK applications (Table 8). Further reduction occurred a month after fresh application of NK fertilisers. However, K uptake did not seem to be influenced by such low pH (Kee et al ., 1993). Although further trial is necessary to ascertain this, some plantation sector has taken the precaution to avoid soil acidification by applying NK fertilisers in the avenues of fully mature oil palms. EFB mulching and liming are known to increase soil pH and might be used if the degree of acidification is found to be detrimental.

Table 8 : Effect of NK fertiliser on the soil acidity of Musang series soils.
Treatment Site

Depth (cm)

0 – 15

15 – 30

Without NK fertiliser Palm circle

4.15

4.07

Interrow

4.49

4.37

Frond heap

4.48

4.36

With NK fertiliser Palm circle

3.35

3.43

Interrow

4.27

4.14

Frond heap

4.38

4.33

SE for treatment

0.07

0.05

SE for site

0.04

0.04

SE for interaction

0.06

0.04

Source : After Goh et al. (1995)

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