Chemistry and availability of nonexchangeable ammonium and potassium in some midwestern soils

Albert Edmond Cox, Purdue University

Abstract

Many soils in the Midwestern USA can retain ammonium (NH$\sb4\sp+$) and potassium (K$\sp+$) in nonexchangeable (fixed) forms in the interlayer of micaceaous clay minerals. Nonexchangeable NH$\sb4\sp+$ and K$\sp+$ (especially K$\sp+$) can contribute significantly to crop nutrition during the growing season. Sodium tetraphenylboron (NaBPh$\sb4$) mimics plant nutrient uptake by precipitating solution NH$\sb4\sp+$ and K$\sp+$ and facilitating the release of nonexchangeable pools. We modified a NaBPh$\sb4$ procedure by substituting copper (Cu$\sp{2+}$ for mercury (Hg$\sp{2+}$) in the recovery of precipitated NH$\sb4\sp+$ or K$\sp+$, and refined the method further for routine use. The modified method released 71 percent of the nonexchangeable NH$\sb4\sp+$ and up to 10 times more nonexchangeable K$\sp+$ in some Midwestern soils compared to the standard methods for determining nonexchangeable NH$\sb4\sp+$ and K$\sp+$. Additional studies were conducted to evaluate the utility of the modified method to (i) determine release rates in soils, (ii) estimate K$\sp+$ distribution and illite content in soil particle size fractions, and (iii) measure the dynamics of plant available K$\sp+$ in soils. Potassium release in soils during incubation in NaBPh$\sb4$ was almost complete in 7 d and was well described by four mathematical models. The release of soil K$\sp+$ was mainly by intra-particle diffusion and was correlated to loss of soil illite. Sodium tetraphenylboron extracted K$\sp+$ mainly from the clay-sized fraction, and illite content estimated by the procedure correlated well to tedious conventional approaches (sodium pyrosulfate fusion and x-ray diffraction methods) currently in use. Because NaBPh$\sb4$ extracts both exchangeable and nonexchangeable K$\sp+$, the modified method (5-min. extraction period) proved superior to the conventional 1 M NH$\sb4$OAc soil test method for predicting K$\sp+$ availability to winter wheat (Triticum aestivum L.) in a greenhouse study. In addition, the NaBPh$\sb4$ method was able to measure unit changes in soil K$\sp+$ status that can result from crop removal or K$\sp+$ fertilization. These studies show that the modified NaBPh$\sb4$ method can be used to evaluate nonexchangeable NH$\sb4\sp+$ and K$\sp+$, and has potential both as a K$\sp+$ soil test and for determining illite content in soils for K$\sp+$ management purposes.

Degree

Ph.D.

Advisors

Joern, Purdue University.

Subject Area

Soil sciences|Agronomy

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