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The Chemistry and Fertility of Soils under Tropical Weeds 99

and Available P in soils of the primary forest are reported to be much higher than
those in the secondary forest and agriculture lands (Table 6.5, Table 6.7) (Salam et
al., 1998b). Al these elements in the primary forest had been preserved by
minimum disturbance and high litter inputs and organic matters of residues from
various vegetation, while those in agricultural lands had been degraded by soil
tillage and low organic matter inputs. Therefore, forest conversion may shift the
soil fertility related to the contents of organic matters, organic C, total N, and
available P.

4.5
4
3.5
Available P (mg kg -1 ) 2.5
3

2
1.5
1
0.5
0
Primary Secondary Coffee Cultivated
Forest Forest Plantation Land

Topsoil Subsoil

Fig. 6.4. The decrease in soil P by deforestation.

Soils of coffee plantation also show different organic C contents if planted with
different weeds or managed by different tillage system. Those planted with
grasses (for example Paspalum conjugatum) are reported to be higher in organic
matter content than the lands with natural vegetation or lands of control without
weeds (Salam et al., 2001). In general, soil tillage systems also affect soil organic
matter content (Table 6.6). In general, organic matter and C are well-preserved in
minimum soil tillage but drastically decreases in conventional tillage soils (Klein and

Abdul Kadir Salam and Nanik Sriyani – 2019