Date of Award

Fall 2013

Degree Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

Brad Joern

Committee Chair

Brad Joern

Committee Member 1

Jim Camberato

Committee Member 2

Darrell Schulze

Committee Member 3

Hao Zhang


Nitrogen (N) is the most limiting essential nutrient for crop growth. Numerous studies have been performed to improve N fertilizer recommendations. Accurate prediction of soil N supply has been found to be one of the most important factors that determine optimum fertilizer N rates. Seven soils collected from various locations across Indiana at four different depths were tested for N mineralization potential. Results showed that laboratory mineralizable N in the top layer (0-15 cm) of the seven soils ranged from 50 to 68 mg N kg-1 soil. In addition, more than 50% of the total mineralizable N was contributed from the 15 to 60 cm depths. Different methodologies used for estimating soil N supply capacity were also compared in this study. We found that soil N mineralization estimated from long-term static laboratory incubation was correlated to crop N uptake under greenhouse conditions. Some chemical indices such as Illinois Soil Nitrogen Test, anaerobic-N, and Hot KCl-N also showed promises in predicting laboratory N mineralization potential. However, the mineralizable N estimated from laboratory incubations did not show any relationship with soil N supply in the field, which can be attributed to large weather variations under field conditions. Therefore, a process-based weather-driven N transformation and loss model was developed to improve the prediction of optimum in-season fertilizer N rates. So far through simple regression analyses from existing N response studies we found that yearly plant N uptake simulated from this model was highly correlated to yield data under field conditions (R2 > 0.95 for any site year, R2 > 0.80 for combined site years).

Potassium (K) is also one of the most important essential nutrients for crop growth. The availability of K in the soil determines K fertilizer recommendations. Potassium ions can be fixed between the layers of 2:1 clay minerals in the soil, which decreases the availability of K for plant uptake. We conducted two studies to evaluate the impacts of different factors on soil K availability. One was to assess the effect of anhydrous ammonia (AA) injection on soil K fixation, and the other was to evaluate the effect of soil moisture on soil K test levels. Results of the first study showed that the injection of AA dramatically decreased the nonexchangeable K concentration in some soils up to 4.5 cm away from the injection point, but did not significantly affect the exchangeable K concentration in the soil. In the study about effects of moisture on soil test K (STK) levels, we found that soils with initially high exchangeable K concentrations fixed K upon drying, while soils with initially low exchangeable K concentration released K upon drying. The equilibrium soil K level at which no change in STK occurs upon drying varied with soils (106 to 241 mg kg-1), and was positively related to the predicted soil K critical value. However, the mechanisms affecting K release/fixation still require more study.

Included in

Soil Science Commons