2 Macronutrient Elements (P, K, Ca, Mg) in Composted Solid Wastes and Compost-amended Soil

Applications of Composted Solid Wastes for Farmland Amendment



131



Table 4.4

Procedure of sequential extraction experiment (33, 37, 40–43, 45–49, 84, 85)

Fraction



Procedure



Water-soluble

Exchangeable

Carbonate-bound

Fe–Mn oxides-bound

Organic matter-bound



5 g + 50 mL H2 O, 125 rpm, 25–30◦ C, 24 h

50 mL 0.05 M KNO3 25–30◦ C, 125 rpm, 24 h

Room temperature (25–30◦ C), 40 mL 1 M NaOAc (pH 5.0), 24 h

100 mL 0.04 M NH2 OH · HCl in 25% HOAc (∼ 96◦ C), 2 h

15 mL HNO3 + 30% H2 O2 (pH 2), 85 ± 2◦ C, 2 h; a second 15 mL

aliquot of 30% H2 O2 (pH 2), 85 ± 2◦ C, 3 h; 25 mL 3.2 M NH4 OAc in

20% HNO3 , diluted to 100 mL, 30 min



potassium phosphate is also water-soluble. High rate of water-soluble potassium in composts

is also due to high solubility of element K in water.

Calcium is well found in HEC (2.03–2.22 × 105 mg Ca/kg), much higher than in other

composts (several 104 mg Ca/kg SSC, SMC, SMMC, and GC, and even several 103 mg

Ca/kg SPC and CEC). Calcium is known as one major component of chicken feed additives,

since a certain level of dietary calcium is good for increasing survivability and reducing leg

abnormalities (70). Water-soluble calcium has been observed evidently in various composts,

and especially mostly in SSC (5.9 g Ca/kg). The second highest level (several hundreds mg/kg)

of water-soluble calcium has been detected in HEC. The contents of water-soluble Ca in other

composts are found mostly less than 100 mg/kg, only with two exceptions (GC-2 and CEC-2).

Total Mg is at a similar level, ranging from 1.12 × 103 to 1.43 × 104 mg/kg, and no

significant difference can be found. Water-soluble Mg is found mostly in SPC (2,900 and

1,900 mg/kg) and SSC (3,300 mg/kg), but at the level of several 100 mg/kg in others, with two

exceptions (GC-2 and CEC-2).

One simple comparison is made in Table 4.5, which is helpful for imagining the distributions of P, K, Ca, and Mg in composts. Generally speaking, both K and Mg are elements that

easily form water-soluble inorganic salts, resulting in high level of water-soluble K and Mg

in composts. On the other side, both P and Ca easily form the salts of less solubility, often

leading to lower contents of water-soluble P and Ca in composts.

2.2.2. P, K, Ca, and Mg in Composts-Amended Soil



As in Fig. 4.3, there is no significant change in the contents of total and water-soluble

phosphorus between the soil amended with or without SPC and GC, which can be attributed to

relatively low loads of SPC and GC to the farmland, although the phosphorus content is much

lower in background soil (2.19 and 2.80 g P2 O5 /kg for NSPS, and 1.73 g P2 O5 /kg for GSB)

than in SPC (47.2–57.7 g P2 O5 /kg) and GC (6.1 g P2 O5 /kg). Comparatively, the applications

of SMC and SSC have caused element phosphorus increases in amended soils. For SMC, the

application caused the significant total phosphorus increases in the soil, while the results of

SSC application indicate that the phosphorus accumulation seems have been well affected by

rainfall. SSS-2 (1 year) contained more phosphorus than SSS-1 (6–7 years), indicating that the



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Table 4.5

Comparison of P, K Ca and Mg distributions in composts

SPC



GC



SSC



SMC



SMMC



CEC



Total P

Water soluble P

Rate (sol./total)

Total K

Water soluble K

Rate (sol./total)

Total Ca

Water soluble Ca

Rate (sol./total)

Total Mg

Water soluble Mg

Rate (sol./total)

HIGH ⇒ LOW



Fig. 4.3. Water-soluble and total P in amended soil (81).



HEC



Applications of Composted Solid Wastes for Farmland Amendment



133



Fig. 4.4. Water-soluble and total K in amended soil (81).



content of phosphorus in soil is affected by rainfall much more strongly than by application

period.

Figure 4.4 indicates that the applications of SPC, GC, SMC, and SSC have hardly resulted

in total and water-soluble K accumulations in amended soils. Although composts were popularly rich in total and water-soluble K (Fig. 4.2), the outstanding solubility of K-containing

compounds made it impossible for K to stay in the soil in large quantity.

Figure 4.5 exhibits slight Ca increases in the soil amended with SPC, GC, and SMC, except

SSC. This is because the element Ca was a little more abundant in SPC, GC, and SMC than

in the blank soil. SSC also contained more Ca, but no increase of Ca in soil has been found.

Figure 4.6 shows that the applications of SPC and GC have hardly caused the Mg increase

in soil, as mainly may be attributed to the high solubility of Mg-containing salts. Element Mg

in SMC-applied soils has revealed an irregular appearance. Moreover, significant Mg increases

have been observed in SSC-amended farmlands. Main cause is the much higher level of Mg

in SSC (1.12 × 104 mg Mg/kg soil) than that in blank soil (10 mg Mg/kg soil).



2.3. Micronutrient Elements (Fe, Mn, Cu, Zn) in Composted Solid Wastes

and Composts-amended Soil

2.3.1. Fe, Mn, Cu, and Zn in Composts



Figure 4.7 shows total Fe, Mn, Cu, and Zn contents in various composts.



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Fig. 4.5. Water-soluble and total Ca in amended soil (81).



Fig. 4.6. Water-soluble and total Mg in amended soil (81).