Page 130 - Ilmu Tanah

The Chemistry and Fertility of Soils under Tropical Weeds 117

There are several forms of K present in the soil environment but not all forms
of K in soils are available to plants and are classified into Slowly Available, Slightly
Available, and Readily Available. The soil K in the mineral structure is Slowly
Available (Table 7.3). Those classified as Slowly Available (primary minerals) is
about 100 to 3000 times greater than those on the soil exchange sites
(Exchangeable K). Therefore not all K is available and can be absorbed by plant
roots. To use this source, this slowly available K must be released from the
structure of the minerals through weathering processes.

Table 7.3. The forms and availability of K in the soil environment*.

Sources Classification Concentration

1 Primary Minerals: Slowly Available 0.1 – 3%
-1
- K Feldspars (20.000 – 60.000 kg ha )
- Muscovit

- Glauconit
- Biotite
-1
2 Fixed-K Slightly Available 0  100 kg ha
-1
3 Exchangeable K Readily Available 200 kg ha
(Fertile Soils)
4 Dissolved K Readily Available 1–50 ppm
(Soil Solution)
-1
(0.4-20 kg ha )
 Total varies depending on parent materials and weathering stage
 Granitic Soils > Calcareous Soils
 Old Soils (Latosol) < Young Soils
*Adapted from Corey (1964)

The relationships between all forms of soil K is depicted in Fig. 7.2. As
explained previously, the primary minerals and organic matters are the main

Abdul Kadir Salam and Nanik Sriyani – 2019