fertilizer is generally lower than those of nitrogen and potassium fertilizers. Thus,
increasing the efficiency of P fertilizers, typically lower than 20%, is a concern. P
accumulates in the plow layer soils of farmlands, and with an increase in P accumulation, a portion of the P is released gradually to rivers, lakes, and bays causing
eutrophication. The natural resource of P fertilizers is phosphate rock. Economically
mineable phosphate rock appears to be limited (Amundson et al. 2015), although
estimates of the world P reserves require further research (Edixhoven et al. 2014). As
70–80% of phosphate rock is consumed as fertilizers, a sustainable management
method of P or recycling of P is desirable for farmlands. This section introduces the
forms of P related to soil–plant systems, except for those discussed in Chap. 5.
6.4.1 Apatite and Related Reactions
Apatite is a major mineral of phosphate rock. The general chemical formula of
apatite is Ca 5 (PO 4 ) 3 X (X ¼ F
À , Cl
À , OH
À , and others). Apatite is largely divided
into apatite of sedimentary and igneous origins. Apatite of sedimentary origin is
microcrystalline, whereas apatite of igneous origin is highly crystalline.
Figure 6.28 shows an example of a sedimentary-origin apatite from Florida, USA.
As shown in the optical micrograph (Fig. 6.28a), the major particles are subrounded
and have various sizes, although some are broken. The particles have mostly grayish
color, but some are light-brown or some other color. These particles are apatite,
except for a small amount of colorless transparent quartz. As shown in the magnified
SEM image (Fig. 6.28b), the sedimentary apatite appears to be an aggregate of
microcrystals. According to the EDX spectrum (Fig. 6.28c), the Florida apatite has a
significant amount of F
À
. The powder XRD pattern (Fig. 6.28d) is identical to the
pattern of the reference fluoroapatite (Fig. 6.28e), except that a small amount quartz
is included.
Apatite is converted to fine powder or to more soluble P fertilizers, such as Ca
(H 2 PO 4 ) 2 ÁH 2 O, (NH 4 ) 2 HPO 4 , and others, by adding acids.
Fresh apatite particles are found in young volcanic ash soils (Fig. 6.29) (Nanzyo
et al. 1997; Nanzyo and Yamasaki 1998; Nanzyo et al. 2003) or granitic soils
(Fig. 2.18). These have an igneous origin and are more crystalline than those of
sedimentary origin. As apatite is a minor constituent in volcanic ash and has high
particle density, heavy liquid is used to concentrate apatite particles. An example of
volcanic ash, from Mt. Pinatubo (1991), is shown in Fig. 6.29 (Nanzyo et al. 1997).
An SEM image of the heavy fraction of the Pinatubo ash is shown in Fig. 6.29a, and
four P-rich particles were identified using element maps. The particle labeled “b” and
one other particle are almost entirely composed of apatite because their shapes in the
element maps of P (Fig. 6.29d) and Ca (Fig. 6.29e) are very close to those in the
SEM image (Fig. 6.29a). The particle labeled “c” is a composite particle with a Fe–
Ti oxide (Fig. 2.15), as shown in Fig. 6.30. The EDX spectra (Fig. 6.29b, c) of the
dashed squares labeled “b” and “c” in Fig. 6.29a support the conclusion that these
grains are apatite.
6.4 Phosphates Related to Soil-Plant Systems
163
increasing the efficiency of P fertilizers, typically lower than 20%, is a concern. P
accumulates in the plow layer soils of farmlands, and with an increase in P accumulation, a portion of the P is released gradually to rivers, lakes, and bays causing
eutrophication. The natural resource of P fertilizers is phosphate rock. Economically
mineable phosphate rock appears to be limited (Amundson et al. 2015), although
estimates of the world P reserves require further research (Edixhoven et al. 2014). As
70–80% of phosphate rock is consumed as fertilizers, a sustainable management
method of P or recycling of P is desirable for farmlands. This section introduces the
forms of P related to soil–plant systems, except for those discussed in Chap. 5.
6.4.1 Apatite and Related Reactions
Apatite is a major mineral of phosphate rock. The general chemical formula of
apatite is Ca 5 (PO 4 ) 3 X (X ¼ F
À , Cl
À , OH
À , and others). Apatite is largely divided
into apatite of sedimentary and igneous origins. Apatite of sedimentary origin is
microcrystalline, whereas apatite of igneous origin is highly crystalline.
Figure 6.28 shows an example of a sedimentary-origin apatite from Florida, USA.
As shown in the optical micrograph (Fig. 6.28a), the major particles are subrounded
and have various sizes, although some are broken. The particles have mostly grayish
color, but some are light-brown or some other color. These particles are apatite,
except for a small amount of colorless transparent quartz. As shown in the magnified
SEM image (Fig. 6.28b), the sedimentary apatite appears to be an aggregate of
microcrystals. According to the EDX spectrum (Fig. 6.28c), the Florida apatite has a
significant amount of F
À
. The powder XRD pattern (Fig. 6.28d) is identical to the
pattern of the reference fluoroapatite (Fig. 6.28e), except that a small amount quartz
is included.
Apatite is converted to fine powder or to more soluble P fertilizers, such as Ca
(H 2 PO 4 ) 2 ÁH 2 O, (NH 4 ) 2 HPO 4 , and others, by adding acids.
Fresh apatite particles are found in young volcanic ash soils (Fig. 6.29) (Nanzyo
et al. 1997; Nanzyo and Yamasaki 1998; Nanzyo et al. 2003) or granitic soils
(Fig. 2.18). These have an igneous origin and are more crystalline than those of
sedimentary origin. As apatite is a minor constituent in volcanic ash and has high
particle density, heavy liquid is used to concentrate apatite particles. An example of
volcanic ash, from Mt. Pinatubo (1991), is shown in Fig. 6.29 (Nanzyo et al. 1997).
An SEM image of the heavy fraction of the Pinatubo ash is shown in Fig. 6.29a, and
four P-rich particles were identified using element maps. The particle labeled “b” and
one other particle are almost entirely composed of apatite because their shapes in the
element maps of P (Fig. 6.29d) and Ca (Fig. 6.29e) are very close to those in the
SEM image (Fig. 6.29a). The particle labeled “c” is a composite particle with a Fe–
Ti oxide (Fig. 2.15), as shown in Fig. 6.30. The EDX spectra (Fig. 6.29b, c) of the
dashed squares labeled “b” and “c” in Fig. 6.29a support the conclusion that these
grains are apatite.
6.4 Phosphates Related to Soil-Plant Systems
163