Competitive and noncompetitive adsorption of silicate and phosphate by two acid Si-deficient soils and their effects on P and Si extractability

Figures & data

Table 1. Some chemical properties of the two soils used.

Measurement	Soil 1	Soil 2
pH(water)	5.0	5.0
EC (dS m^–^1)	0.12	0.18
Organic C (g kg^–^1)	11	17
Dithionate-extractable Fe (mg kg^–^1)	8.6	11.0
Dithionate-extractable Al (mg kg^–^1)	1.6	2.1
Exchangeable Ca (mmolc kg^–^1)	13.4	16.1
Exchangeable Mg (mmolc kg^–^1)	2.8	2.3
Exchangeable K (mmolc kg^–^1)	1.1	1.5
Exchangeable Na (mmolc kg^–^1)	0.31	0.42
BSES-P (mg kg^–^1)	42	106
Resin-extractable P (mg kg^–^1)	14	22
CaCl2-extractable Si (mg kg^–^1)	9.4	11.1

Figure 1. Silicate and phosphate adsorption isotherms at pH 5.0, 5.5, 6.0 and 6.5 for the two study soils. Standard errors of the mean shown.

Figure 2. Effect of pH on the concentration of Si and P in equilibrium solutions after addition of Si and/or P at an initial concentration of 1.6 mM. Equilibrium was with either monoelement (Si or P) or multielement (Si + P) solutions. Standard errors of the mean shown.

Table 2. Langmuir and Freundlich isotherm constants and coefficients of determination (R²) for adsorption of Si and P onto the soils at pH 5.0, 5.5, 6.0 and 6.5.

Element	Soil	pH	Langmuir isotherm			Freundlich isotherm
			qmax (mmol g^−1)	b (L mmol^−1)	R^2	Kf	1/n	R^2
Si	Soil 1	pH 5.0	4.45	0.27	0.52	0.92	0.76	0.96
		pH 5.5	5.02	0.32	0.78	1.15	0.74	0.98
		pH 6.0	11.3	0.17	0.59	1.58	0.86	0.99
		pH 6.5	12.7	0.27	0.83	2.61	0.82	0.99
	Soil 2	pH 5.0	2.85	0.51	0.92	0.98	0.55	0.95
		pH 5.5	18.4	0.10	0.51	1.71	0.91	0.99
		pH 6.0	22.7	0.11	0.66	2.20	0.91	0.99
		pH 6.5	47.6	0.09	0.95	3.72	0.95	0.99
P	Soil 1	pH 5.0	11.9	1.98	0.99	7.04	0.34	0.99
		pH 5.5	11.1	2.07	0.99	6.86	0.33	0.99
		pH 6.0	10.7	2.12	0.99	6.72	0.32	0.99
		pH 6.5	10.2	2.11	0.99	6.54	0.32	0.99
	Soil2	pH 5.0	15.9	5.47	0.99	12.2	0.26	0.98
		pH 5.5	15.3	6.30	0.99	11.8	0.27	0.96
		pH 6.0	14.6	6.21	0.99	11.5	0.27	0.97
		pH 6.5	14.2	6.32	0.99	11.1	0.26	0.97

Figure 3. Concentrations of resin-extractable P and P adsorption isotherms for soil 1 treated with three rates of Si (R1, R2 and R3 = 300, 600 and 1200 mg Si kg⁻¹ respectively) as CaSiO₃ which gave soil pH values of 5.4, 5.6 and 6.5 or an appropriate amount of Ca(OH)₂ to give the same pH values. Standard errors of the mean for values on adsorption isotherm shown. Values of extractable P followed by the same letter are not significantly different P ≤ .05.

Figure 4. Concentrations of CaCl₂-extractable Si and Si adsorption isotherms for soil 1 treated with three rates of P (R1, R2 and R3 = 30, 60 and 100 mg P kg⁻¹ respectively) as Ca(H₂PO₄)₂ which gave soil pH values of 5.3, 5.4 and 5.5 or an appropriate amount of Ca(OH)₂ to give the same pH values. Standard errors of the mean for values on adsorption isotherm shown. Values of extractable Si followed by the same letter are not significantly different P ≤ .05.