ABSTRACT
This chapter provides a discussion on nitrate-nitrogen soil tests for estimating the nitrogen rates to be applied to improve crop yields, increase use efficiency, and decrease environmental impacts while solving soil fertility problems. Phosphorus tends to be more variable than nitrogen and potassium. This is interesting because of the role that phosphorus plays in photosynthesis and energy exchanges within crops. One would expect it to show the least variability, but there is evidence that once high-yield levels are reached, crop yield per unit of uptake variability may be least for potassium. Soil tests can be used with a fair degree of accuracy to predict the need for fertilizer phosphorus applications. Also, soil phosphorus levels can be increased through applications of phosphate fertilizers and/or manure. The Rothamsted Experiments in the UK, perhaps the oldest known soil fertility plots in the world, provide testimony on the use of liming materials and fertilizers to maintain and improve soil fertility and soil productivity.
