Foliar Nutrition and Leaf Chlorophyll Index of Container-Grown Shade Trees in Response to Controlled-Releaser Fertilizer

Nutrition management is one of the most important components of managing ornamental tree nursery crops.  Controlled-release-fertilizers (CRF) provide growers with a means to ensure adequate nutrition is available for tree growth through the growing season while reducing the potential for off-site nutrient movement.  The goal of the present study is to determine the response of trees from two ash alternative species; ‘State street^’ maple (Acer miyabei ‘Morton’) and ‘Harvest gold^’ linden (Tilia cordata × mongolica ‘Harvest gold’) to controlled-release fertilizer (15-9-12, Osmocote-Plus, Scotts, Inc. Marysville, OH).  One and one-half inch (4 cm) caliper nursery liners were planted in 25 gallon (95 L) containers in 80:20 (v:v) pine bark and peat moss and grown on for two years.  Each spring containers were top-dressed with 200, 275, 350, 425, 500, or 575 grams of CRF.  Response variables measured included caliper and height growth, foliar nutrition and SPAD chlorophyll index.  Fertilization rate did not affect (P>0.05) total stem caliper growth or height growth of trees from either species.  SPAD index values showed a pronounced seasonal pattern, increasing through the spring and then reaching a plateau through the summer.  Mid-summer SPAD values varied between species (P<0.001) with mean values averaging 5-6 units higher in linden trees than in maple trees.  Fertilization increased (P<0.05) SPAD values of trees from both species during the midsummer period.  SPAD index values were highly correlated (P<0.001) with foliar nitrogen concentration of trees from both species.  For maple trees, SPAD was correlated (P<0.01) with foliar phosphorus, potassium, and sulfur.  For linden trees, SPAD was correlated (P<0.05) with foliar phosphorus and iron.  Foliar magnesium and calcium were negatively correlated (P<0.05) with foliar potassium suggesting an antagonism between uptake of potassium and the other cations.  The relative lack of growth response of trees from both species suggests that the lowest rate (200 gram per container) of CRF may be adequate to produce acceptable caliper growth in these trees.  The increase in foliar nutrients, including nitrogen, phosphorus and potassium, without a concomitant increase in growth suggest luxury consumption of these elements had occurred.   Rates of potassium addition should be monitored closely given the apparent antagonism between uptake of potassium and uptake of magnesium and calcium, particularly in linden trees. The broad plateau for SPAD levels observed indicates that comparative samples may be collected at any point during the summer.