Page 45 - Ilmu Tanah

32 The Chemistry and Fertility of Soils under Tropical Weeds

temperature, and C/N ratios give an indirect effect of the enhancement of organic
matter decomposition. As stated in Salam (2014), soil water, soil N, and soil
temperature influence the growth of soil enzyme producing microorganisms. In
general the soil microbial population and activity, and thereby the activities of soil
enzymes, increase with the increase in water content, N content, and soil
temperature. In particular for phosphatases, their activities below the optimum pH
increase with the increase in soil pH. Optimum pH is the value of pH at which the
activity of phosphatases is maximum.

2.3 The Effects of Weeds on Nutrient Element Release

from Soil Minerals and Organic Matters

Chemicals released by weed roots may directly and indirectly affect the soil
properties that might intensify the releases on nutrient elements from soil solids.
The depletion in soil pH in the root zones of weeds may directly increase the
process of soil mineral weathering releasing nutrient elements into the soil water.
Salam et al. (2019) report that several weeds stimulate the release of K from soil
minerals. The effectiveness of weed plants in stimulating the release of soil K were:
A. Pintoi > A. gangetica > Widelia sp. > P. conjugatum > P. purpureum. A. pintoi is
suggested to be the most effective in stimulating the release of K from soil minerals
as proven by the positive values of Exch. K [ Exch. K = Final Exch. K (After
Planting) – Initial Exch. K (Before Planting). This phenomenon is related to its
higher Root-To-Shoot Ratio compared to others.
The increase in soil water cations is not only attributed to the weathering of
soil minerals but also to the decomposition of soil organic matter directly speeded
by various soil enzymes excreted by weed roots and their associated
microorganisms. Therefore, the increase in soil exchangeable K in the root-zones of
A. pintoi reported by Salam et al. (2019) might have also been related to this
phenomenon.
Combined with decrease in soil CEC in response to the decrease in soil pH, the
increase in nutrient element concentration in soil water may indirectly cause the
increase in soil exchangeable cations. However, the presence of weed-roots
absorbing cations may cause the real exchangeable cations in soils to decease. This
phenomenon is reported by Salam et al. (2019), the soil exchangeable K was lower
in the presence of weeds like A. gangetica, Widelia sp., P. conjugatum, and P.
purpureum. A. pintoi is different, demonstrating its effectiveness in stimulating the

Abdul Kadir Salam and Nanik Sriyani – 2019