Fruit tree orchards are often fertilized following calendar-based fertilization programs instead of rational fertilization programs based on actual tree needs. Factors such as the ripening season or the amount of nutrients removed from the tree annually with management practices such as thinning, pruning, or harvesting are not often considered and consequently many peach orchards in the Southern U.S. are overfertilized. This project proposes to determine the amounts of nutrients lost by peach cultivars of different ripening seasons in all the nutrient removal events (pruning, thinning, harvest, and leaf fall) for estimating tree nutrional demands. Understanding crop demands would lead to recommendations that optimize orchard fertilization by maintaining a nutrient balance between inputs and outputs so that potential fruit yields are achieved while fertilizer application costs, labor costs (because of pruning of excessive vegetative growth), and environmental problems caused by overfertilization are reduced.
The goal of this project is to provide fertilization guidelines for peach growers in Southern U.S. that optimize efficiency of fertilization, reduce orchard costs, and environmental impact.
With this purpose, the following specific objectives are (1) to assess peach tree nutritional needs in six cultivars of three ripening seasons; and (2) to prepare guidelines for fertilization programs and disseminate results among growers, extension agents and researchers.
The experiment was carried out at Clemson University Musser Fruit Research Farm. Eighteen mature fruit-bearing peach trees of six cultivars of three ripening seasons were used [Early-season trees (harvested at the end of May): ‘Spring Snow’ and ‘Desiree’; mid-season trees (harvested in July): ‘Sweet N Up’ and ‘Coralstar’; late-season trees (harvested in August): ‘Snow King’ and ‘Snow Gem’]. All trees were maintained in a perpendicular V system with 1.5 m between trees and 6 m between rows. The orchard was irrigated with microjets, and disease, pest and weed control were performed according to currently recommended rates and practices for commercial peach orchards in the Southeastern U.S. Granular fertilizer was pplied at the following rates: 195 lb/ac (or 214 kg/ha) 19-19-19 N-P-K in March and 125 lb/ac (or 137 kg/ha) 19-19-19 in July.
Samples of the following vegetative and reproductive organs were collected: wood at winter pruning, fruitlets at thinning, fruit at harvesting, and leaves during leaf senescence. Also, a leaf analysis was performed in July to monitor the nutrient status of the tree. At these removal events, the fresh weight of all the pruned wood, fruitlets, harvested fruit and fallen leaves was recorded, and a sample between 2 and 5 kg (depending on the organ) was collected. Samples were washed, (sliced and divided into different tissues for the case of fruit) and oven-dried at 70 C for 7-60 days (depending on the organ), ground to fine powder, and analyzed for N, P, K, Ca and Mg following standard determination methods. This project supported the collection of data for one year but the graduate student had already collected information for two more years with the support of the SC Peach Council.
Early-season peach trees lost more nutrients compared to mid- and late-season peach trees. When calculated on a per-tree basis, total nutrient removal were (average of three years, in g/tree, for early-/mid-/late-season cultivars, respectively):
N: 86.9 / 80.7 / 80.0
P: 15.3 / 11.9 / 11.8
K: 149.6 / 131.9 / 139.8
Ca: 110.5 / 88/9 / 81.0
Mg: 37.0 / 29.6 / 28.4
Considering these values and the tree spacing of the orchard (1.5 x 6 m), the equivalent annual removal of nutrients per acre would be (for early-, mid- and late-season cultivars, respectively):
N: 86.0 / 79.9 / 79.2 lb N/ac
P: 15.2 / 11.7 / 11.7 lb P/ac
K: 148.1 / 130.6 / 138.4 lb K/ac
Ca: 109.4 / 88.0 / 80.2 lb Ca/ac
Mg: 36.6 / 29.3 / 28.2 lb Mg/ac
Recycling nutrients from tissues removed from the tree back to the soil-tree system is very important for improving the long-term sustainability of the farm. For instance, shredding prunning wood, and leaving thinned fruitlets and decomposing fallen leaves on the ground is not only of importance for improving physical and biological characteristics of the soil but also to return to the soil a remarkable portion of the nutrients removed from the tree, thus, improveing soil nutrient pools. For instance, if all nutrients present in pruned wood, thinned fruitlets and fallen leaves are returned to the soil, the only nutrients actually removed from the tree soil system would be those taken out with the harvest (for instance, for N would be 17.0 / 18.6 / 26.4 lb/ac for early-, mid-, and late season cultivars). These amounts 1) are considerably higher than those currently recommended and applied by most growers, which for mature trees are around 60-70 lb/ac; and 2) consider the effect of the cultivar and its ripening season (e.g. for the field in our study, the early-season cultivars would have needed 35% less N than the late-season cultivars). Considering the average commercial peach farm in the southeastern U.S. is between 500 and 5,000 acres, the amount of fertilizer that could be saved could really impact the long-term profitability and sustainability of the farms. This is important if we consider that symptoms of over-fertilization (especially N over-fertilization) such as excessive vegetative growth that shade the lower branches of the tree (and require more pruning and increase labor expenses) are common.
Furthermore, we learned that trees that ripen at different times also have different nutrient resorption (recycling) patterns. An early-season peach tree that is harvested in May have more time to accumulate nutrients in their leaves throughout the summer before mobilizing them back to the reserves (before leaf senescence) than a late-season tree that is harvested at the end of August, for instance. Thus, we have observed that, for the same amount of biomass (i.e. nutrient concentration of), early-season peach trees had more nutrients removed with their pruned wood and during leaf abscission before dormancy than late-season peach trees. Thus, chipping and incorporating these materials into the soil is even more important for early-season cultivars. This is one more reason why farmers should consider fertilizing their orchards differently based not only on the crop they got but estimating amount of wood removed through pruning and ripening season.
Educational & Outreach Activities
The results of these project have been partly disseminated to several growers and extension agents in conversations during field visits but they have not been fully disseminated yet, as we recently finalized analyses. Nevertheless, the supervisor of this project has a 30% extension appointment and work with growers almost on a daily basis and will continue disseminating these results in future grower meetings and field visits.
A journal article titled “Ripening season affects tissue mineral concentration and nutrient partitioning in peach trees” by the graduate student working on this project and the supervisor has been accepted by the Journal of Plant Nutrition and Soil Science, is currently in press, and will be published shortly. Also, a brief article summarizing these results is about to be published in the grower’s magazing “The Peach News”.
The graduate student gave three presentations this past year, one at a regional conference (Southern Region – American Society for Horticultural Science, in Jacksonville, FL, in February 2018) and two at a national conference (American Society for Horticultural Science Annual Conference, in Washington, DC, in July-August 2018). Two of her presentations were awarded as second best presentation at both the regional conference and the national conference. The supervisor of this project has discussed this data with graduate students at Clemson University in an invited lecture this year. Furthermore, the same week this project report is being submitted, he will be presenting these data at an invited lecture on fruit tree growing to a class of undergraduate and graduate students from Washington State University (Pomology, by Dr. Lisa DeVetter, WSU).
This project has started to fuel up conversations between researchers, extension agents, and some farmers on the impact of optimization of fertilization on farm sustainability and profitability. Both graduate student and advisor will use these data for the revision of current guidelines and preparation of fertilization recommendations as sustainable practices to improve soil quality, and tree performance. Furthermore, as discussed in the results and discussion section, optimization of fertilization must be integrated with other orchard management practices such as pruning (and pruned wood management), which is one of the most labor intensive and costly practices.
Farmers have not adopted practices coming from this research yet since we recently finalized analyzing all the data and getting conclusions. However, both graduate student and advisor are committed to translate these results to farmers and extension agents in the short- and mid- term. This project has been synergistic to other side projects that we recently started related nutrient use efficiency, water use and its impact on nutrient leaching, and soil health. After all, we consider sustainable agriculture as a multidisciplinary area that needs to be studied thru a systems approach.