Sodium Bicarbonate Phosphorous

The sodium bicarbonate (NaHCO3) method is based on the carbon dioxide reaction theory in which CO2 (carbon dioxide) given off by roots, soil organisms, etc., combines with H2O (water) to form H2CO3 (carbonic acid). This is weakly effective in dissolving alkaline phosphates, and there are some other chemical changes affecting calcium phosphates and other compounds which contribute to the results. The weak extraction of dilute NaHCO3 correlates rather well with crop response to acid phosphate fertilizers and the need for additional phosphorus on slightly acid (down to pH 6.5) soils and on slightly basic (7.0 + pH) to highly basic soils. In basic soils the phosphorus exists in large part as alkaline earth phosphates. Acidification of the soil or lowering of the pH tends to dissolve these and bring the phosphorus into solution. In acid soils the phosphorus generally has a significant fraction combined with iron and aluminum as phosphates. These exist when the pH is acidic. When the pH is raised toward the neutral point by the addition of an alkali element (sodium, potassium) or an alkaline earth (calcium, magnesium) these iron and aluminum phosphate compounds tend to dissociate, and the phosphorus becomes more available. This is the effect of a sodium bicarbonate test (extraction at pH 8.5) on acidic soils, and accounts for the large test values reported from acid soils.

In general phosphorus may be considered to exist as relatively insoluble compounds on both sides of a soil reaction point of approximately 6.6 pH. The solubility of the phosphorus in these compounds increases as the pH approaches this point, and this is the region of maximum phosphorus solubility and long term availability.

Strongly acidic soils have the major portion of the phosphorus as weak-acid-insoluble compounds which are soluble in basic solutions. As the natural pH of the soil rises proportionately less of these compounds are present and proportionately more of the alkaline phosphate compounds. These latter compounds are less soluble in basic solutions. As a result the critical values for crop needs as shown by the bicarbonate test will change as the soil pH changes. At a soil pH of approximately 6.2 or less a bicarbonate reading of 60-70 ppm P can be considered adequate for most crops and usually no further crop response can be detected from added phosphorus. As the soil pH rises, the critical value of the extractable phosphorus level will decrease. At the neutral point 1:20 bicarb extraction phosphorus test reading at 25-30 may be considered as adequate for most field crops with a high yield goal.

As a result of this change in critical values the bicarbonate test is difficult to interpret on acid soils and is usually replaced by the sodium acetate extraction. We do not recommend the use of the sodium bicarbonate test for naturally acidic soils with pH below 6.2.

For high value, high yield crops such as potatoes, maintenance applications of 80-100 lbs. P2O5 are recommended even for high soil test readings. However, if the soil test results indicate extremely high readings additional phosphorus applications may be detrimental because it may induce a deficiency of other elements, such as zinc.

The ranges given below indicate adequacy readings for low phosphorus demand crops to adequacy readings for most high demand crops. Some agronomists prefer to add 5-10 points to the ranges of values given for soils above pH 7.1 as an offset to expected reversion of added phosphorus during the growing season. This practice is not proven, but may have merit.

The accompanying table (below) indicates relative ratings for different soil test reading.

The next page contains phosphorous recommendation charts.s

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