Page 127 - Ilmu Tanah

114 The Chemistry and Fertility of Soils under Tropical Weeds

excretion and plant residues are therefore very important to enhance the soil K
availability. Infertile soils may have sufficient K if treated with enough organic
matters.

Table 7.1. Plant nutrient reserves in soil and their releasing mechanisms*.

Nutrient Releasing Dissolved
Reserve Forms
Elements Mechanisms Forms
N Organic Matters Decomposition NH 4 +
+ +
NH 4 in Mica and Dissolution NH 4
Vermiculite Interlayers
+
Exchangeable NH 4 Cation Exchange NH 4
2- 2-
S Organic Matters Decomposition S , SO 4
Sulfides (Ex. FeS) Oxidation SO 4 2-
2-
Gypsum (CaSO 4 ) Dissolution SO 4
-
P Organic Matters Decomposition H 2 PO 4 , HPO 4 2-
-
Al-P, Fe-P, Ca-P Ligand Exchange H 2 PO 4 , HPO 4 2-
-
Dissolution H 2 PO 4 , HPO 4 2-
2+
+
2+
K, Ca, Mg Silicate Minerals (Ex. Weathering, K , Ca , Mg
Feldspars) Dissolution
Carbonates, Sulfates Dissolution Ca 2+
+
Organic Matters Decomposition K
Fe, Mn, Zn, Hydroxide Precipitates Dissolution
Cu Adsorbed by Oxides of Desorption Cations and
Fe, Al, and Mn Dissolved
Chelates Dissociation Chelates
Exchangeable Ion Exchange
B, Mo Adsorbed by Oxides of Desorption H 3 BO 3
2-
Fe and Al and Clay MoO 4
Minerals
*Taken from Singer and Munns (1987)

The plant nutrient elements are released as free ions into the soil solution
through several mechanisms (Table 7.1) controlled by several factors as listed in

Abdul Kadir Salam and Nanik Sriyani – 2019