TURNIP GROWTH, LEAF YIELD, AND LEAF NUTRIENT COMPOSITION RESPONSES TONITROGEN FORMS

`Shogoin' turnip plants (Brassica rapa L.) were grown in sand culture under five nitrate:ammonium (NO3:NH4) ratios (N:N of 1:0, 3:1, 1:1, 1: 3, 0:1). The leaves expressed symptoms of NH4 toxicity (reduced growth and curly leaves with dark-green areas surrounding yellow spots) when NH4 was the dominant nitrogen (N) form. Increasing NO3 in the nutrien t solution significantly (p<0.01) increased leaf and root fresh weight and dry weight. Leaf nutrient concentration and composition of all el ements analyzed, except N and calcium (Ca), responded quadratically (p <0.01) to N03:NH4 ratios, and the highest values were observed with th e 1:0 [for molybdenum (Moll, 3:1 ([or magnesium (Mg)l, 1:1 [for boron (B), coper (Cu), iron (Fe), manganese (Mn), and zinc (Zn)l or 1:3 [for phosphorus (Pi and potassium (K)1 treatments. Nitrogen and Ca leaf co ncentration responses were linear and highest at 0:1 and at 1:0, respe ctively. Cultural practices and fertilizer applications should maintai n N03 as the dominant N form in the root zone, and the continuous use of NH4- based or NH4-releasing fertilizers is not recommended for the production of high yields of turnip greens.