the K,CO, method was unsuitable, since it extracted relatively much more phosphorus than was available to plants. Rhoades (23) leached soils with a solution of 0.01 N Na2CO3 buffered with sufficient HBO, added to give it the same pH as the soil. For most calcareous soils this solution was largely a mixture of sodium bicarbonate and sodium borate with pH values between 7 and 8.5. This method was not proposed for acid soils. A new method for extracting soil phosphorus with sodium bicarbonate solutions is described in this circular. The relationship of the extractable phosphorus to yield response in greenhouse and field experiments and to a measure of plant available phosphorus, or the 'A' value (12), is shown. The NaHCO3-soluble phosphorus is compared with the methods of Bray (4), Truog, CO2-soluble (26), and H2O-soluble (2) by means of correlation analysis with 'A' values. DEVELOPMENT OF METHOD THEORETICAL BASIS Several studies have been conducted to show the relationship of pH to phosphate solubility in calcareous soils (6, 13, 22, 26) and in systems containing excess CaCO3 (1, 7). The results show that the solubility of the calcium phosphate is a function of pH and Ca++ activity, with a minimum solubility between pH 7 to 7.5 and an increase in solubility on both the alkaline and acid side of this pH range. On the alkaline side the solubility of phosphorus in the presence of solid phase CaCO3 is a function of the Ca++ activity, whereas on the acid side of the minimum solubility range in pH the solubility of phosphorus is a function of both the H+ activity and the Ca++ activity (7). An increase in H+ activity increases the solubility of phosphorus and an increase in Ca++ activity decreases the solubility of phosphorus. One of the objectives in research on soil tests has been to eliminate or diminish the secondary precipitation reactions that may occur during the extraction process with dilute acids (8). McGeorge and others (18) stated that the solvent action of CO2 on phosphorus in calcareous soils was mainly a function of reaction and that the H+ activity must be sufficient to overcome the common ion effect of Ca++ in precipitating a calcium phosphate. Thus, two reactions with opposite effects on phosphate solubility occur when CO2 is bubbled through a calcareous soil:water suspension, i. e., an increase in solubility caused by a decrease in pH and a decrease in solubility when precipitation of a calcium phosphate occurs as a result of the increase in Ca++ activity. Stanberry (26) showed that both reactions affected phosphate solubility on a calcareous Sagemoor fine sandy loam, since the concentration of phosphorus was higher at both pH 8.5 and 5.8 than at intermediate pH values. Time-of-extraction studies on this same soil with CO2 showed that there was a maximum solubility of phosphorus in 5 and 30 minutes, respectively, for low and high phosphate fertility levels. A decrease in solubility of phosphate following the maximum level led to the conclusion that a calcium phosphate precipitated from supersaturated solutions of the CO2 extract. 6.939 AGRICULTURE ESTIMATION OF AVAILABLE PHOSPHORUS GRILLBRE LIBRAN Secondary precipitation reactions may occur with extractants that increase the H+ activity of the calcareous soil:water system, because 30 a concomitant increase in the Ca++ activity occurs. Since the solubility of phosphate may be increased in this system with a decrease N3Cn Ca++ activity, the secondary precipitation reactions should be largely eliminated with an extractant that decreases the Ca++ activity. Gardner and Kelley (13) indicated that in the presence of CaCO3 and excess carbonate ions the increase in phosphorus solubility in 1percent K2CO3 solutions on calcareous soils was a result of the repression of the Ca++ activity. The K,CO, method is not applicable to acid soils, however, probably because forms of iron and aluminum phosphates unavailable to plants are dissolved as a result of the high pH of the extracting solution. The Ca++ activity in a soil:water system is repressed with NaHCO3 solutions at pH 8.5, and the undesirable effects of the high pH of 1percent K2CO, solutions are avoided. The ratio of HCO3- to CO2- is 63 to 1 in a solution of NaHCO3 at pH 8.5. The influence of NaHCO3 concentration on the molar concentrations of HCO3-, CO; ̄, and Ca++ in a CaCO3-H2O system at pH 8.5 is shown in table 1. The calculations are based on the mass-action law, using 4.82X10-9 as the solubility product of calcite (11) with no corrections for salt effect. In a 0.5 molar solution of NaHCO3, the equilibrium calcium concentration is 857 times smaller than in a pure CaCO3-H2O system in equilibrium with atmospheric CO2. The main effect of the NaHCO3 in calcareous soils, therefore, is to decrease the Ca++ activity, which in turn increases the solubility of phosphorus. In acid and neutral soils the phosphorus present in the form of calcium phosphates would probably become more soluble in the presence of NaHCO3, owing to a repression of the Ca++ activity, assuming that solid phase CaCO, would be formed at pH 8.5. Since the calcium phosphates are known to be the most available forms of phosphorus in these soils, the NaHCO3 method may be applicable to acid and neutral as well as to calcareous soils. The main effect of the NaHCO3 on acid and neutral soils probably would be through ionic competition of HCO3-, CO3-, and OH- ions for phosphate adsorbed on the surface of soil particles. According to *Kurtz and others (16) the replacing ability of HCO3- for phosphate was greater than the replacing ability of acetate or sulfate ions on acid soils. Laatsch (17) stated that HCO3- ions displaced sorbed phosphate from colloidal surfaces in soils. TABLE 1.-Influence of NaHCO3 concentration on the molar concentrations of HCO3-, CO3, and Ca++ in a CaCO3-H20 system at pH 8.5 1 Solubility of calcite in H2O at atmospheric pressure of CO2 (11). The amount of phosphorus on the surface of soil particles, which readily exchanges with P32 in the soil solution, was shown to be highly correlated with plant available phosphorus, or 'A' values (21). The relationship between the surface or exchangeable phosphorus measured by P32 and the phosphorus soluble in NaHCO, is shown in NaHCO3-Soluble Phosphorus (pounds PO, per acre) 150 100 50 100 200 300 400 500 600 700 FIGURE 1.-Relationship between surface phosphorus and phosphorus soluble in 0.5 molar NaHCO, solutions on 25 soils. figure 1 for 25 soils. The pH of these soils ranged from 5.0 to 7.6. The NaHCO3 extracts about 50 percent of the surface phosphorus. The data indicate that the NaHCO3-soluble phosphorus is closely related to a form of phosphorus, which in turn is highly correlated with the amount of phosphorus available to plants. EXTRACTION DETAILS Time of Extraction Five soil types, representing acid to highly calcareous soils, were selected for this study (table 2). Samples from plots receiving applications of phosphate in the field are included to illustrate the effect of phosphate-fertility level. The extracting solution was 0.5 molar NaHCO3 solution at pH 8.5 and the soil: solution ratio was 1:20. |