Glenn B. Fain, Ken M. Tilt, Charles H. Gilliam, Harry G. Ponder, and Jeff L. Sibley

Irrigation systems, schedules, and growth substrate are major parameters affecting plant growth. A more efficient alternative to the standard practice of overhead irrigation is cyclic irrigation through a spray stake in an individual container. Spray stake irrigation can cause excessive leaching, if not properly monitored, due to the high application rates of emitters. Spray stake application efficiency can be increased by using cyclic irrigation. With cyclic irrigation, a plant’s daily water allotment is subdivided into more than one application with prescribed intervals between applications. With conventional irrigation practices, the daily water allotment is applied in a single application.

Cyclic irrigation may improve irrigation application efficiency by allowing time for water to move through the micropore system of a container substrate. Growers using cyclic irrigation can expect greater plant utilization of applied nitrogen (N) and reduced water and nutrient loss from containers. Earlier work at Auburn has shown a 47% reduction in N leached from nursery pots irrigated with cyclic treatments compared to a single overhead application.

Pot-in-pot production, introduced around 1990, is a nursery production method that combines some of the benefits of both field and container production. A “socket” pot is permanently placed in the ground and a containerized plant is then placed inside the “socket” pot. Limited research has been done to determine potential benefits of cyclic irrigation in pot-in-pot production.

This study was conducted to determine the effects of cyclic microirrigation and pinebark substrates amended with coconut coir or peat on container effluent and growth of Red SunsetTM Maple (Acer rubrum ‘Franksred’).

METHODS
Seventy-two bare root liners, 5 to 6 feet in height, of Acer rubrum ‘Franksred’ were planted in 15-gallon containers in April 1997 in full sun. Three different substrate combinations were used: 100 percent pinebark, pinebark:peat (4:1 by volume), and pinebark:coconut coir (4:1 by volume). Each substrate was amended with 7.7 lbs per cubic yard dolomitic limestone and, after planting, trees were topdressed with 11.8 ounces of 15-9-11 plus slow release fertilizer (O. M. Scotts Co., Inc., Marysville, Ohio). Initial height and trunk caliper (diameter) were taken after trees were planted, and final growth measurements were taken at the termination of the study on September 23, 1997.

For each substrate, three irrigation treatments were compared: application of a given volume in a single application at 10:00 a.m.; the same volume divided into three applications at 10:30 a.m., 1:00 p.m., and 3:30 p.m.; or the same volume divided into six applications beginning at 8:00 a.m. with 90 minutes between cycles. Initial irrigation volume, April to mid-June (period one) was 2.5 quarts per plant; from mid-June to mid-July (period two) the volume was increased to 4.5 quarts per plant; and from mid-July to harvest (period 3) the volume was increase to 5.8 quarts per plant. Irrigation was applied through maxi-jet spray stakes (Maxijet Inc., Dundee, Florida) with a Bowsmith model HPC6 pressure compensating emitter (Bowsmith Inc., Exeter, California) at a rate of 13.5 ounces per minute. Total leachate volume was collected from four replications of all treatments on a biweekly basis throughout the study. Soluble salts and pH readings were taken from all containers monthly. Leachate subsamples (3.4 ounces) were collected biweekly and frozen for N analysis at the end of the study. Total inorganic-N concentrations were used to calculate total inorganic-N lost per container (volume x concentration).

RESULTS
For the three substrates tested, total airspace was higher for pinebark at 21.8% than it was for pinebark:coir at 16.9%. Water-holding capacity was greatest for pinebark:peat and pinebark:coir. There were no differences in total porosity between substrates. Pinebark:peat had the lowest bulk density (Table 1).

Table 1. Airspace, Water-holding Capacity (WHC), Total Porosity, (TP) and Bulk Density (BD) of Container Substrates
	Substrate physical properties1
Treatment	Airspace2	WHC3	TP4	BD5
Pinebark	21.8	42.8	64.6	0.292
Pinebark:peat (4:1)	19.2	47.4	66.6	0.254
Pinebark:coir (4:1)	16.9	47.3	64.2	0.298
1 Substrate physical properties determined using the North Carolina State University Porometer. 2 Airspace: Percent volume filled with air after substrate is saturated and allowed to drain for 60 minutes. 3 Water-holding capacity: Percent volume filled with water after substrate is saturated and allowed to drain for 60 minutes. 4 Total porosity: Percent volume of the substrate comprised of pore space. 5 Bulk density: Ratio (g/cm3) of mass of dry solids to bulk volume of substrate.

Irrigation application efficiency was highest for pinebark:peat among substrate treatments for period two while pinebark and pinebark:coir were similar. During periods one and two, irrigation application efficiency was greatest for the six-cycle treatment followed by the three-cycle and single, respectively (Table 2). During period three, both cyclic treatments had the greatest irrigation application efficiency among all substrates. These results are consistent with prior research showing increased irrigation application efficiency with cyclic irrigation. Within the single irrigation treatment, pinebark:peat had the highest irrigation application efficiency followed by pinebark:coir and pinebark, respectively. Irrigation application efficiency appeared to increase as the season progressed, possibly due to increasing plant needs and environmental conditions.

Tree growth was affected by substrates and irrigation treatment (Table 3). Shoot dry weight was about 8% greater with plants grown in pinebark:peat compared to plants grown in pinebark. Plants grown in pinebark:peat had a 17 and 12% greater height increase than those grown in pinebark:coir and pinebark, respectively.

Plants grown with cyclic irrigation had the greatest shoot dry weight among irrigation treatments with plants in the three-cycle and six-cycle having 23 and 17% greater shoot dry weight respectively than plants grown with a single irrigation application (Table 3). With trunk diameter, plants receiving three-cycle and six-cycle irrigation treatments had a 23 and 26% greater diameter increase respectively than plants grown with a single application irrigation. Tree height was also affected by irrigation treatment. Plants grown with three-cycle irrigation had a 16% greater height increase than plants grown with a single irrigation application. Our results support a previous study showing an increase in growth of ‘Okame’ Cherry (Prunus × incamp) with cyclic compared to a single irrigation application. Irrigation treatment had no effect on substrate pH. Irrigation treatment had an effect on electrical conductivity, with the six-cycle treatment having the highest electrical conductivity for the July and August samples (Table 4). More irrigation cycles and greater efficiency, or reduced leaching fraction allowed salts to accumulate in the substrate. However all electrical conductivity readings were below thresholds where root damage might be expected to occur.

Cyclic irrigation reduced total N leached by a minimum of 89% in June and August when compared to a single irrigation application (Table 4). While N concentration was generally higher in cyclic treatments, reduced leachate volume (i.e., greater irrigation application efficiency) resulted in less N leached. For example with the six cycle irrigation in August the N concentration was 34.6 mg/liter; however, total N leached per pot was 0.2 mg/pot. This is a 99% reduction compared to the single irrigation application. These data suggest greater retention of N with cyclic versus a single irrigation application. This agrees with previous work which showed a decrease in N leached when using cyclic irrigation compared to a single irrigation application. Leachate N concentration was greatest for 100% pinebark in June at 9.3 mg/liter compared to 5.5 and 6.0 mg/liter for pinebark:peat and pinebark:coir, respectively. There were no other differences between substrates on leachate N (data not shown).

With increasing emphasis on water quality as well as quantity used, growers should consider changing management practices to improve irrigation application efficiency of container-grown trees. Cyclic irrigation is one method of improving water quality by reducing runoff and nutrient loss from containers. Reducing N leached during production is an important environmental goal. Reduced leachate volume and increased N retention in the substrate may allow for more effective use of controlled release fertilizer and thereby reduce potential negative impacts on the environment.

Both cyclic irrigation and pinebark:peat (4:1 by volume) substrate increase irrigation application efficiency by reducing leachate volume in a pot-in-pot production system. Our results indicate that cyclic irrigation may lead to increased growth in production of specimen trees. Both six- and three-cycle irrigation produced increased growth of Acer rubrum ‘Franksred’ compared to a single irrigation application. The pinebark:peat substrate (4:1 by volume) produced increased shoot dry weight over 100% pinebark and pinebark:coir (4:1 by volume). Leachate N concentration increased with cyclic irrigation; however, due to the reduced leachate volume with cyclic irrigation, less N was leached. Furthermore, many growers of large container plants can apply cyclic microirrigation methods without major changes in existing equipment.