Potato tops and buckwheat as potential nutrient contributors to subsequent crops

The nutrient value of crop residue and the previous crop potential to improve nutrient availability to the current crop can be important for successful crop production. Field studies were conducted at Pettibone and Karlsruhe, ND, in 2005 and 2006 to determine the fertilizer N value (FNV) of potato (Solarium tuberosum L. var. Russet Burbank) tops to barley (Hordeum vulgare L.) at Pettibone, and corn ( Zea mays L.) at Karlsruhe compared with urea. A second study was conducted in Fargo and Mandan to determine by laboratory fractionation the quantity of P solubilized by buckwheat (Fagopyrum esculentum Moench). Spring wheat (Triticum aestivum L.) was grown as a reference crop with buckwheat in the Fargo trial, and at Mandan, buckwheat was grown after sunflower (Helianthus annus L.). Barley and corn response to spring-applied urea-N fertilizer and dry potato tops was determined from barley leaf N, grain N and yield, corn leaf N and yield in 2005; and from barley grain protein and yield, and corn yield in 2006. In 2005, potato tops (2.2 T) produced the same amount of barley leaf N at the 5-leaf stage (2.53%) and grain N (2.02%) as urea fertilizer (28 and 64 kg N ha-1), respectively. In 2006, estimated FNV of 2.8 T potato tops to produce 15.4% barley grain protein was 47 kg N ha-1 as urea. The estimated FVR to produce 1.34% corn grain N (2.8 T of residue) was 51 kg N ha-1. We determined that 2.2 T ha-1 potato tops returned to the soil would produce about as much grain protein as urea fertilizer rates between 14 and 47 kg N ha-1. In the second study, calcium-bound P pool contained the highest amount of P and contributed the most P to the available pool. Buckwheat solubilized sparingly soluble apatite P into available forms and took up significantly (p< 0.05) more P than wheat from the inorganic fractions but not in the organic P pool. Thus, the ability for buckwheat to solubilize P from less soluble inorganic P pools to soluble forms useable by a following rotation crop was supported in this study.