By Dario J. Chavez
The southeastern United States possesses a unique environment in terms of variable soil and humid climate. Although humid, there has been incidences of several droughts throughout the years. A lack of proper irrigation scheduling under these circumstances can result in huge losses in production and in the state’s economy.
Since 2000, the Southeast has had five exceptional droughts (2000, 2002, 2008, 2012 and 2016). In 2019 and 2020, growers suffered small periods of drought. A period of drought can be detrimental for peach trees during the first years after planting in the field, especially in areas where the current practice is to start irrigation during the third or fourth year after planting, as is commonly done in the Southeast.
Current fertilizer recommendations for peaches are variable and their origins are difficult to pinpoint. Adequate fertilization will reduce leaching and runoff of nutrients. Optimal fertilization can result in a balanced mineral nutrition program that is critical to maintain yields, fruit quality and efficient plant growth.
Among the nutrients in fertilizer, nitrogen (N) is the main compound. It has been shown previously that excessive N levels can result in excessive growth of plants that can create further shading to fruit and fruiting wood. This results in reduction of yields and excessive use of resources for pruning and other canopy management tools.
A team has conducted peach research for the last six years at the University of Georgia Griffin Campus to develop irrigation guidelines and improve current fertilizer recommendations. A 2-acre plot of Julyprince cultivar grafted on Guardian™ rootstock (145 plants per acre) was planted in July and August of 2015 in Griffin. Julyprince is a high-yielding cultivar ripening in early July with excellent quality and size. It is probably the standard for that ripening season in the region.
The experiment evaluated several treatments: Irrigated versus non-irrigated plants; drip irrigated versus micro-sprinkler irrigated plants; and four different granular fertilizer rates (levels of 25%, 50%, 100% and 200% of the current recommended rate). The amount of nitrogen in each level was as follows: 14, 29, 58 and 115 pounds per acre for 1-year-old plants; 20, 43, 85 and 171 pounds per acre for 2-year-old plants; and 22, 44, 87 and 174 pounds per acre for 3-year and older plants, respectively.
The irrigation was controlled using sensor-based software, which read the soil moisture close to the roots of the plants and activated/deactivated the irrigation system as needed to keep the soil moisture above the threshold established in the software. Both the drip irrigation and micro-sprinkler irrigation were set to have the same amount of water delivered to the plants, 8 gallons per hour.
RESEARCH RESULTS
The first year of data collection was 2016, which was characterized by drought during the spring and summer seasons. This year was a good example of uncharacteristic conditions in which 20% less rain than historically normal was recorded. For the research, this was a key factor to determine the effect of a drought in early orchard establishment. From a growers’ perspective, drought is never desired; but from a research perspective, it was excellent.
The 2017 (after April), 2018 and 2019 seasons were rainy, with precipitation events close to the historical normal. Even though the 2017, 2018 and 2019 seasons were not excellent for our research because of the frequent rain, the effects of the 2016 drought in plants continued to be observed in 2017 and 2018. In 2019, the fourth year after planting, irrigation began for the non-irrigated treatment following current practices in the southeastern United States.
Trunk cross-sectional area and canopy volume (Table 1) are good representations of overall plant growth in peaches. When evaluating both parameters, clear differences were observed, especially in 2016 (the drought year). Drought negatively affected non-irrigated plants. Plants were visually smaller and less dense when comparing irrigated vs. non-irrigated (Figure 1) trees. The drought in 2016, when plants were very young, affected their growth. This continued even two years after the drought.
Differences among drip and micro-sprinkler systems were not consistent across years for trunk-cross sectional area and canopy volume. However, drip irrigation used less water than micro-sprinkler irrigation (approximately 35% water savings), because it loses less water through evaporation. No consistent differences were observed for fruit yield among irrigation systems, but in 2018 a freeze negatively affected the drip-irrigated plants. Similarly, fruit quality and fruit size characteristics were not affected across seasons for all the treatment comparisons.
In comparing different fertilization levels, no differences were observed for plant growth, fruit yield, fruit quality and fruit size over three seasons. Furthermore, the cumulative N partitioning per tree (evaluating total N removed per tree during winter and summer pruning, fruit thinning, harvest and defoliation) for three years was not affected by fertilizer levels. Similarly, researchers did not observe an increase in leaf and fruit nutritional status associated with the different fertilization levels studied over three years.
In 2017 and 2019, irrigated plants had higher commercial yields than non-irrigated plants. This difference is believed to be attributed to the drought in 2016 and its effect in plant growth across all seasons. Moreover, a difference of approximately 40 pounds per tree of commercial yield was observed when comparing cumulative yields across three seasons between irrigated and non-irrigated plants. This yield gain after three seasons clearly represents an economic gain for peach producers.
NEW RECOMMENDATIONS
Based on the results and experiences observed in this long-term study, the following new recommendations for Georgia and the southeastern United States have been established:
- Irrigation availability from the time of planting is optimal. This will ameliorate any possible effects caused by a drought if it were to occur during the first years of establishment.
- Continue with the standard fertilizer application schedule but reduce the amount to 50% of the recommended rate as described in the Southeastern Peach, Nectarine, and Plum Pest Management and Culture Guide.
- Fertilizer decisions should be made every year based on leaf analysis and plant growth characteristics. If leaf nutrient analysis in July is found within the sufficient range or below, then fertilizer application should be made. If the N level is above the optimum range, then July low-input fertilization should not be applied.
Dario J. Chavez is an associate professor at the University of Georgia in Griffin.