Rationale

Phosphorus uptake by plants is affected by both soil and plant factors. Soil properties that affect P uptake are labile P, soil water content, soil pH, CaCO₃, clay, and amorphous and complexed Fe, Al and Mn. Phosphorus reactions and processes affecting solubility and movement in soils are not only important for plant nutrition but are important factors that affect nutrient management plans developed to protect water quality and environmental parameters. Field observations suggested different plant P availabilities at similar soil test P concentrations (NaHCO₃-P). This poster summarizes selected information that partially explains the observed responses.

Approach

The data were obtained from field experiments conducted between 1983 through 1990, from a series of greenhouse experiments, and from surface soil samples obtained recently from arid, calcareous soils in the Pacific Northwest. All soil samples were air dried and crushed before extraction for NaHCO₃-P (STPC) and 0.01 M CaCl₂-P (Solution-P). After extraction, P concentrations were determined by the Murphy-Riley procedure. Phosphorus sorption isotherms were determined across a range of P solution concentrations in 0.01 M CaCl₂ after 24 h continuous shaking at 22°C. The P buffer capacity was estimated as the EBC, defined as the slope of the P-adsorption isotherm at the indigenous equilibrium P concentration, i.e., y = 0. Lime was determined as acid equivalent. Additional experimental details are given in the appropriate publication.

Figure 1. Phosphorus uptake by Stephens winter wheat (Triticum aestivum L.) at different growth stages grown under field conditions. At early plant development, P uptake on the high lime plots was less than the low lime plots at similar STPC concentrations (A). Differences disappeared at soft dough but maturity differences persisted. Lime differences disappeared when uptake was plotted against soil solution-P concentrations (B).

Figure 2. Effect of increasing lime concentration on sudangrass (Sorghum bicolor L, Moench) grown at 32 ppm STPC in a Portneuf silt loam. Phosphorus uptake was 52, 32, 21 and 8 mg P/pot for 0, 3, 9 and 15 % lime, respectively. Corresponding solution-P concentrations were 0.28, 0.26, 0.19 and 0.08 ppm, while EBCs were 35, 51, 74 and 190 L/kg, respectively.

Figure 3. The relationship between STPC and solution-P for selected soils. Lime equivalent shown in parenthesis. The specific relationship depends upon soil type, lime equivalent, and the physical and chemical characteristics of the lime and other soil components. The Millville soil’s lime is predominantly dolomite rather than calcite. Note the wide range of STPC’s for a solution-P concentration of 0.1 ppm.

Figure 4. The relationship between lime equivalent and STPC needed to have a soil solution-P concentration of 0.1 ppm. This relationship indicates that the STPC should increase 1.4 ppm for each percent the lime equivalent increases. The one soil requiring a STPC of 50 ppm for 19.5% lime was a recently exposed subsoil, while the other anomaly was for the Millville soil containing dolomite, which behaved like a soil containing 5-7 % lime.

Discussion

The solution P concentration at the soil-root interface before uptake should be similar to that in the bulk soil (estimated by solution-P). At a given STPC, this concentration decreased as the acid equivalent lime concentration increased. An increase in lime concentration increased the EBC. The effective diffusion coefficient for P is inversely related to the P buffering (estimated by EBC). Therefore, an increase in EBC decreases both the solution-P concentration and the distance that P will move to the plant root hair to reduce plant P uptake. A recent study across a wide range of western calcareous soils also suggests that the organically complexed Fe and Mn is closely related to the P sorption maximum (Leytem and Westermann, 2003). In order to provide the same level of available P the degree to which the sorption capacity is P saturated must be increased as lime concentration increases. Normally this is done by increasing the STPC or the P fertilization rate. Amrani et al., (1999) also showed that adding the buffering capacity to a P requirement model increased the recommendation accuracy. The NaHCO₃ test is a non-equilibrium extraction of primarily sorbed or labile P (Olsen et al., 1954) and appears insensitive to changes in the soil’s buffer capacity caused by lime. Most Ca-P minerals found in soils are not very soluble in this extractant even though the Ca-ion activity is near zero.

In Conclusion:

• Increasing acid equivalent free lime decreased the soil solution P concentration and increased the P buffering capacity (EBC).
  
• Increasing the STPC approximately 1.4 ppm P for every percentage of free lime may compensate for the lime effect on P availability.
  
• Additional studies are needed to better identify lime, and organically complexed or hydrous oxides of Fe, Mn and Al effects on P sorption and availabilities in calcareous soils.

Selected References

Amrani, M., D.G. Westfall and L. Moughli. 1999. Commun. Soil Sci. Plant Anal. 30:129-144.

Leytem, A.B. and D.T. Westermann. 2003. Phosphate sorption by Pacific Northwest calcareous soils. Soil Sci. (Accepted for publication).

Murphy, J. and J.P. Riley. 1962. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27:31-36.

Olsen, S.R., C.V. Cole, F.S. Watanabe and L.A. Dean. 1954. Extraction of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circ. 939. U.S. Gov. Print. Office. Washington, DC. 19 p.

Westermann, D.T. 1987. Lime effects on P availability. Proc. 38th Annual N.W. Fertilizer Conference, Pasco, WA, July 14-16, 1987. p. 79-85.

Westermann, D.T. 1992. Lime effects on phosphorus availability in a calcareous soil. Soil Sci. Soc. Amer. J. 56:489-494.