Growth of Juniperus and Potentilla using Liquid Exponential and Controlled-release Fer

Juniperus scopularum Sarg. (Rocky Mountain juniper) and Potentilla fruticosa L. ‘Gold Drop (gold drop po ten til la) plants grown in containers had sim i lar or bet ter morphology, higher nitrogen con cen tra tions and contents, and high er N-use ef ficien cy when grown with liquid fertilizer ap plied at an exponentially in creas ing rate as compared to the same amount of N applied via controlled-release fertilizers. More im por tant ly, plants grown with a half-exponential rate were similar to those grown with controlled-release fer til iz er but with a higher N-use efficiency, indicating that this type of fertilization may be a method for reducing the amounts of applied nutrients in nurseries and sub se quent nutrient discharge. from 30% to 60% of the total nitrogen applied (Dumroese et al., 1995). Oth er researchers have also found that ex po nen tial ly fertilized plants had a higher affinity for absorbing nutrients, more stress resistance, and were better able to compete with weeds (Timmer, 1997; Timmer and Aidelbaum, 1996; Timmer and Armstrong, 1987). Therefore, less applied fertilizer to plants with a higher affinity for ab sorb ing nutrients should result in reduced nutrient runoff from nurseries. My objective was to compare growth of two species growing in 2.8-L containers and fertilized with controlled-release fertilizers or fertigated with two rates of exponential fer til i za tion. Materials and Methods My experiment used two species (Juniperus scopularum Sarg. and Potentilla fruticosa L. ‘Gold Drop ) grown under three fertilizer re gimes [controlled-release fertilizer (CRF; con trol); full-rate exponential; half-rate ex po nen tial]. Both species are commonly grown in the western United States and represent de cid u ous and coniferous plants. The controlled-release fer til iz er rate was the oper a tion al rate for these two species at Jayker Whole sale Nursery (Meridian, Idaho). The total amount of N supplied with the full-rate ex po nen tial treat ment equaled that supplied by the controlled-release fertilizer. A halfexponential rate was included to determine if less applied N cou pled with the reported enhanced N-use efficiency of exponentially fertilized plants could result in ac cept able growth of these species. To calculate exponential fertilizer re gimes, biomass and N con cen tra tion of typical plants at the start of a growing season were needed. In spring, I harvested four rep li cates of five plants of Juniperus and Potentilla (20 plants total of each species). Juniperus liners were 1-year-old seedlings grown in 340-mL containers as part of a 2-year crop at the Univ. of Idaho Forest Research Nursery, Moscow, Idaho. Hard wood Potentilla ramets were fallcollected and root ed during winter in an unheated polyhouse at Jayker Wholesale Nursery and were 6 months old when the experiment began. Roots of both species were carefully washed, sep a rat ed from shoots, and both roots and shoots were dried 72 h at 60 °C to achieve constant weight. Nitrogen con cen tra tion was determined by Scotts Testing Lab o ra to ry (Allentown, Pa.). The growth substrate for both spe cies was custom-mixed at Jayker Wholesale Nursery and consisted of the following: composted fine-grind fir bark ( 1 cm; 65%); sawmill waste (sand–sawdust mixture; 20%); and lo cal source cinders ( 0.6; 15%). Using a blending machine, I amended 0.76 m of sub strate with 4.3 kg 14N–6.1P–11.6K (14–14–14 Apex Blue Polyon Coated Fertilizer, 4-month release rate at 21 °C, J.R. Simplot Co., Boise, Idaho), 0.9 kg Apex Mi cro n u tri ent For mu la tion (J.R. Simplot Co.), 0.9 kg dolomitic lime, 1.8 kg MgSO4, and 0.9 kg elemental S (99.5%). The growth substrate for the exponential treat ments was amended with all of the fertilizers except the Apex Blue Polyon Coated Fer til iz er. Once the growth substrate was pre pared and appropriate fertilizers included, nine plants of each species were randomly assigned to each of three fertilizer treatments and plant ed into 2.8-L (#1) containers. On 1 May, I ran dom ly placed potted plants within an open-sided growing area with a polycarbonate roof. All containers were watered to saturation, allowed to drain for 0.5 h, and weighed to obtain a saturated weight. At least three ran dom ly chosen plants from each species × treat ment combination were weighed daily, and plants were irrigated (control plants with con trolled-release fertilizer) or fertigated (ex po nen tial fertilizer) when the average container weight within a species × treatment com bi na tion was 80% of saturated weight. The volume of water added brought the substrate back to saturation with minimal leaching. Leachate was collected for 0.5 h in plastic trays beneath each container and then reapplied to the plant. Every 6 weeks, I slowly watered the growth substrate in each container to saturation with plain water and determined a new saturated weight as described above. The basic formula for exponential fer til iza tion is: