Page 27 - Ilmu Tanah

14 The Chemistry and Fertility of Soils under Tropical Weeds

grow more extensively and competitively. They produce more biomass and
+
probably more H ions, organic acids, and CO 2 that acidify the soil environment.
Their acidifying effect may be greater in soils planted with weeds with high root-to-
shoot ratios. High root-to-shoot ratios and production of acidifying substances may
cause high capability of weeds to dissolve soil minerals containing nutrient
elements such as K, Ca, Mg, and micronutrients.
Arachis pintoi showed higher root-to-shoot ratio than did Asystacia gangetica,
Widelia sp., Paspalum conjugatum, and Pennisetum purpureum (Ontia, 2018; Salam
et al., 2019). A. pintoi showed the highest effectiveness in stimulating the release
of K from infertile soil of Ultisol Tanjung Bintang from South Lampung and from
fertile soil of Mollisol Jabung from East Lampung. Their effectiveness follows the
sequence of A. pintoi > A. gangetica > Widelia sp. > P. conjugatum > P. purpureum.
The released K was reported to be well-correlated with the dry-weight of root and
shoot.
The conceptual mechanisms of weed roots in affecting the soil properties are
described in Fig. 1.10. As pointed out previously, the weed roots may release the
+ + 2+ 2+ 2+ 2+
H ions in exchange of the absorption of cations including K , Ca , Mg , Cu , Zn ,
etc. The root respiration may also release CO 2 that upon reaction with water
+
molecules produces H ion as shown by Eq. 1.5.

+ -
CO 2 + H 2 O  H + HCO 3 ……. Eq. 1.5

+
These processes may increase the concentration of H ions, which may then
attack the nutrient element position in the structure of soil minerals. As reported,
+
the released K and other nutrient elements are intensified by the presence of
+
more H ions (Johnston and Olsen, 1972; Manley and Evals, 1986; Salam, 1989;
Najafi-Ghiri and Jabari, 2013). The excreted organic acids by weed roots may also
lower the ambient soil pH and intensify the effectiveness of weed roots in
stimulating the release of nutrient elements from soil minerals matters.
The production of various enzymes by weed roots and microorganisms
stimulated by the presence of weed roots, such as phosphatases, arylsulfatase,
proteases, etc., may accelerate the decomposition of soil organic matters releasing
various nutrient elements from the organic matter structures. This process of
decomposition along with the weathering of soil mineral matters may accumulate
+
the soluble K and other nutrient elements in the soil solution.

Abdul Kadir Salam and Nanik Sriyani – 2019