Sean M. Wallace, Texas Agrilife Extension Service, Lubbock, TX and Calvin Trostle, Soil & Crop Sciences, Texas AgriLife Extension Service, Lubbock, TX
In general technical nitrogen requirements have not been established for castor in the United States. Brazil and India have each shown nutrient requirements for castor although the climate, soil types, cultivars, and cultivation techniques are vastly different than what would be used regionally in the United States. Our objective is to measure yield response to nitrogen fertilization on irrigated castor in West Texas. Field tests were conducted 2010-2011 at Lubbock, TX (Texas Tech University Quaker Research Farm, and Texas AgriLife Extension and Research Farm), and 2010 at Pecos, TX (Texas AgriLife Research Farm), using an RCDB layout with five replication of five treatments (0-40-80-120-160 lbs/A of N added in the form of UAN). Spring soil samples were collected from each plot (0-6”, 6-12”, 12-24”, 24-36”, and if possible 36-48”, 48-60” depths) and analyzed for nitrate and ammonium nitrogen. The 2011 drought reduced the yields enough to likely overshadow possible significant differences. The 2010 TTU Quaker Research Farm Site showed significant increases in yield from the 80 and 120 lbs/A N vs. the 0 lbs/A N treatment (P = 0.05), with soil test to the 36” depth showing an accumulation that averaged around 45 lbs/A of nitrate nitrogen. The 2010 AgriLife Lubbock Research Farm site soil test results show large accumulations of soil nitrate nitrogen to the depth of 36” averaging near 150 lbs/A, with no significance differences between N treatments. The 2010 AgriLife Pecos Research Farm site soil test results also showed large accumulations of soil nitrate nitrogen to the depth of 36” averaging near 250 lbs/A. Yields for this site were low and differences were not meaningful, significant differences between treatments were found between the 0 and 40 lbs/A treatment vs. the 160 lbs/A treatment (P = 0.05). The sites with low levels of rainfall and irrigation showed N treatments ineffective at increasing yield. Test sites with higher levels of rainfall and irrigation show the treatments of 80-120 lbs/A of nitrogen to significantly increase the yield. High subsoil nitrate nitrogen levels masked any yield response to nitrogen fertilizers.