Page 87 - Ilmu Tanah

74 The Chemistry and Fertility of Soils under Tropical Weeds

Root excretion may not only intensify the release of nutrients from mineral
(non-organic) sources, but also from organic sources due to the presence of root-
excreted soil enzymes and increased microbial and microfaunal activities (Luo et
al., 2014; Salam, 2014). Soil enzymes like phosphatases, arylsulfatase, proteases,
etc may avail nutrient elements from organic sources. In general, soil enzymes
catalyze the transformation of structural organic matter elements that are not
available to root absorption to inorganic free nutrient elements that are available
to plant root absorption. The greater the activities of soil enzymes in the soil
environment the greater the plant nutrient elements needed by plants are sufficed
from organic sources of nutrients. Therefore, the capacity of vegetation roots in
obtaining plant nutrient elements from soils is supported not only by the root
+
excretion of H and organic acids as well as the evolution of CO 2 , but also by the
root production of soil enzymes.

1)
Table 5.1. Organic acids related to soil organisms .

No. Condition/Location Organic Acids

1 Rhizosphere CO 2 , HNO 3 , Protein, Amino Acids (Aspartic,
Glutamic); Vitamin, Polysacharides
(Polygalacturonic Acid); Sugar and Sugar Acids
Phenolic Acids, Aliphatic Acids (Oxalic, Malic,
Citric, Tartaric); Growth Inhibitors, Pheromones,
and Repelling Substances
2 Microorganisms Oxalic, Citric, Formic, 2-Keto Glukonic, Laktic,
Malic, Tartaric Acids, and Lichen Acids
3 Various Conditions Oxalic and Citric Acids
4 Aerobic Phenolic Acids
5 Anaerobic Acetic, Butiric, Formic, Fumaric, Succinic, and
Laktic Acids
6 Below plant residues Alliphatic Acids (Oxalic, Citric, Malic); and
and canopy Phenolic Acids (Galic, Vanilic, Hydroxybenzoic)
1)
After Robert and Berthelin (1986)

As mentioned previously, weeds are different from other vegetation in that
this vegetation is more competitive and destructive. Weeds may have higher root-
+
to-shoot ratios (Salam et al., 2019) and therefore may produce more H , organic

Abdul Kadir Salam and Nanik Sriyani – 2019