Aluminium, Hydrogen, and Transition Metals

Aluminium in soils is closely connected to soil acidity and is also discussed in the chapters on acid soils and ion-water reactions. The acidity of acid soils is due to the reactions of water with exchangeable Al3+ on the surface of soil particles. The strong Al-water reaction repels H+ from the water molecules iuto the soil solution. This can create soil acidities as low as pH 4.5. Stronger acidity means other H+-yielding reactions—organic acids from soil organic matter decay, sulfur and sulfide oxidation, phosphate fertilizers, ammonia oxidation, acid rain, and Fe- and Mn-water reactions—are active. 

Transition metals (groups IB through VIIB and group VIII of the periodic table, Fig. 2.1) are distinct from the elements at either end of the periodic table in that electrons are added to and removed from inner electron orbitals. The chemistry of the transition metals therefore changes more subdy from element to element than elements having election changes only in the outer, a and p, orbitals. In addition, many transition can have more than one oxidation state in soil. 

Under aerobic conditions, almost all of the transition metals plus aluminium and beryllium, essentially all of the metals in the periodic table except the alkali and al- 

In anaerobic soils, the individual chemistry of the ions is more distinctive. The transition metal ions in the middle of each period of the periodic table—chromium, manganese, iron, nickel, cobalt, and copper—can reduce to lower oxidation states, while the end members—scandium, titanium, and zinc—have only one oxidation state. The lower oxidation states are more water soluble but still tend to precipitate as carbonates and sulfides, or associate with organic matter, thus reducing their movement but increasing then plant availability. 

Small amounts of transition metal and A1 ions are associated with clay surfaces and soil organic matter. The amounts of exchangeable and water soluble ions increase with soil acidity. The approximate order of trivalent cation retention is 

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