Evaluating Three Invasive Algal Species as Local Organic Sources of Potassium for Pak Choi (Brassica rapa, Chinensis Group) Growth

The application of locally available invasive algae biomass as a fertilizer for crop production in Hawaii is being investigated as a substitute for imported chemical fertilizers. Three closely related greenhouse trials were conducted to determine if the algae served as a source of potassium (K) on growth, yield, and K mineral nutrition in pak choi (Brassica rapa, Chinensis group). In the first trial, three algal species (Gracilaria salicornia, Kappaphycus alvarezii, and Eucheuma denticulatum) were applied at five rates of K, each to evaluate their effects on growth and K nutrition of pak choi plants. The pak choi was direct seeded into 0.0027-m pots containing peatmoss-based growth media. In trial 2, pak choi was grown in peat media at six rates of K provided by algae (E. denticulatum) or by potassium nitrate (KNO3). In trial 3, the six rates of K were provided through algae (K. alvarezii), KNO3, and potassium chloride (KCl) and were compared for growth and K nutrition. Results from the first greenhouse trial showed no significant differences among the three algal species in yield or tissue K content of pak choi. However, plant yield and tissue K concentration were increased with application rates. The maximum yield and tissue K were observed when K was provided within the range of 250–300 kg·ha. Similarly, in Expts. 2 and 3, there were no significant differences between commercial K fertilizers and algal K species for yield. Only K rates were significant for yields and tissue K concentrations. It was concluded that K in the invasive algae was similarly available as K in commercial synthetic fertilizers for pak choi growth in terms of yield and tissue K content under our experimental conditions. To meet increasing demand for locally available inputs, the use of seaweed as a source of crop nutrients is a viable option (Pramanick et al., 2014). Many growers in the Pacific region are requesting such an organic fertilizer (Radovich et al., 2012). Marine algae are used in agricultural and horticultural crops, resulting in many beneficial effects on yield and quality (Dhargalkar and Pereira, 2005). In many countries, seaweed and seaweed-based fertilizers are still used in both agriculture and horticulture (Verkleij, 1992; Zodape, 2001). Not only providing nutrients, these seaweeds improve soil structure and humus content when they are composted and applied to the soil (Haslam and Hopkins, 1996). Seaweed liquid fertilizer (SLF) of Dictyota dichotoma was found to increase the yield, growth of roots and shoots, number of roots, and maturity time of Abelmoschus esculentus L. (okra) at low concentrations (Sasikumar et al., 2011). Seaweed concentrate prepared from Ecklonia maxima (Osbeck) Papenfuss has improved the growth of tomato seedlings when applied as a soil drench (Crouchand van Staden, 1992). It was observed that the application of SLF of Ascophyllum nodosum increased the chlorophyll of cucumber cotyledons and tomato plants (Whaphamem et al., 1993). In Hawaii, the most commonly found macrosized invasive species are G. salicornia (Gorilla Ogo), K. alvarezii (Kappaphycus), and E. denticulatum (Eucheuma); all are dominant alien algal species in Hawaiian reefs (Smith et al., 2002). They show potential for use as a locally available source of K (Ahmad et al., 2016; Radovich et al., 2012). Several farmers in Hawaii have been using the algal species for some crops, but there is a lack of information about the availability and optimal rates of K for crop growth from these algal species. The overall objective of this research was to evaluate three invasive algal species on yield and K mineral nutrition of pak choi, and then to compare one algae species with KNO3 and/or KCl. Materials and Methods Three greenhouse experiments were conducted at the University of Hawaii’s Magoon Research Facility (lat. 21 18#22$N, long. 157 48#37$W). The three algal species were obtained from the Department of Land and Natural Resources on Oahu Island and were dried in the oven at 70 C for 72 h. The dried algae were ground into a fine powder using a coffee grinder. The required amounts of K provided from each algal species and for each application rate were calculated based on the K analysis report received from the Agricultural Diagnostic Service Center (ADSC) at University of Hawaii, Manoa (Table 1). The amount of K required from algae per plant based on rates was calculated based on pak choi plant density of 71,659 per hectare with a spacing of 0.30 m between plants and 0.45 m between rows. The required amount of K per plant was divided by the % of K present in each species. For trials 1 and 2, we used pak choi (B. rapa, Chinensis group) cv. Bonsai and for the trial 3, cv. Mei Qing choi Table 1. Mineral nutrient concentration in Kappaphycus alvarezii, Eucheuma denticulatum, and Gracilaria salicornia used in these trials. Nutrient K. alvarezii E. denticulatum G. salicornia N (%) 0.26 0.57 0.8 C (%) 18.12 18.54 22.05 P (%) 0.04 0.07 0.16 K (%) 20.34 18.02 14.1 Ca (%) 0.21 1.55 4.62 Mg (%) 0.41 0.76 0.78 Na (%) 3.35 3.82 3.88 Fe (mg·L) 173 72 375 Mn (mg·L) 10.5 14 210 Mean values with n = 2. The tissue samples were analyzed by the ADSC laboratory at the University of Hawaii, Manoa. Received for publication 23 June 2016. Accepted for publication 14 Dec. 2016. This research has been funded by USDA-WSARE project SW11-055 and the Hawaii Department of Agriculture. We thank Robert E. Paull and Susan Miyasaka for internal review of the manuscript. Corresponding author. E-mail: theodore@hawaii. edu. 436 HORTSCIENCE VOL. 52(3) MARCH 2017 was used. All the seeds were obtained from Harris Seeds of Harris Seeds Company (Rochester, NY). Seeds were planted in 0.0027-m size pots. Trial 1. Three algal species (G. salicornia, K. alvarezii, and E. denticulatum) were applied to supply five rates of K at 0, 1.17, 2.35, 3.51, and 4.70 g/plant (which is equivalent to 0, 84, 168, 252, and 336 kg·ha), with ‘‘0’’ being the control treatment without K fertilizer. The other major nutrients, nitrogen (N) and phosphorus (P), were provided through tankage (bone meal) (10– 3.7–0.8) and triple superphosphate (0–45–0) at constant rates for all the plants at the normal required rates at 168 and 112 kg·ha, respectively (Hemphill, 2010). The actual amounts provided were 22.72 g/plant of bone meal and 3.74 g/plant of triple superphosphate. The experiment was set up as a complete randomized design (CRD) with five replicates. Three to four seeds were directly sown to each pot filled with peat-based media (Sunshine Mix No. 4, SunGro, Agawam, MA) on 15 Apr. 2013. A week after seedling emergence in all pots, plants were thinned to one per pot. Overhead irrigation was provided twice a day for 10 min. Plants were harvested on 27 May 2013, 6 weeks after emergence. Trial 2. This trial was also set up as a CRD with three replications and two fertilizers, E. denticulatum and KNO3. On the basis of the results of trial 1, in which no significant differences were found between the three invasive algal species, E. denticulatum was randomly selected and used as a source of K fertilizer and compared with KNO3 synthetic fertilizer. The two K fertilizers were applied to supply six rates of K at 0, 1.56, 2.35, 3.12, 3.91, and 4.70 g/ plant (which is equivalent to 0, 112, 168, 224, 280, and 336 kg·ha), with ‘‘0’’ being the control treatment without K fertilizer. The other major nutrients, N and P, were provided through urea (46–0–0) and triple superphosphate (0–45–0) at constant rates for all the plants at the normal required rates at 168 and 112 kg·ha, respectively. Urea was provided at 5.10 g/plant to provide N at 2.35 g/plant for all pots, except the pots treated with KNO3 which received 3.93, 3.37, 2.79, 2.22, and 1.63 g/plant of urea. The triple superphosphate was provided to all the pots at 7.77 g/plant to provide 1.56 g of P per plant. The same procedures were followed as in trial 1 with the following exception. Plants were watered to excess of pot capacity (150 mL) through a drip system once a day. Seeds were sown on 28 Jan. 2014 and were harvested on 11 Mar. 2014, 6 weeks after emergence. Trial 3. This was also a CRD with four replications of three fertilizers, K. alvarezii, KNO3, and KCl, to supply six rates of K at 0, 1.56, 2.35, 3.12, 3.91, and 4.70 g/ plant (which is equivalent to 0, 112, 168, 224, 280, and 336 kg·ha). Three to four pak choi seeds were sown into each pot filled with peatmoss on 6 Mar. 2015. The same amount of N and P were provided from urea and triple superphosphate as used in trial 2 with adjustments made in KNO3treated plants. The same procedures were followed as in trials 1 and 2 with the following exceptions. A deionized (DI) water system (US Water system, Indianapolis, IN) filter was fitted in the greenhouse connecting to the regular greenhouse irrigation system producing DI water at 0.0075 m per minute. The collected DI water was used to irrigate the pots using a watering can, 4 times per week. Plants were harvested on 17 Apr. 2015, 6 weeks after emergence. Plant harvest and measurement. For trials 1 and 2, aboveground fresh weights (tops) were immediately measured after harvest and data were recorded. The plants were then placed in an oven (Precision Mechanical Convection Oven; Thermo Scientific, Waltham, MA) at 70 C and were dried for 72 h to constant weight, and the dry weights were recorded. The dried tissue samples were analyzed for K and other nutrients at the ADSC laboratory, University of Hawaii at Manoa, using an inductively coupled plasma spectrometer (Optima 7000DV, Waltham, MA). The fresh petioles harvested from trials 1 and 2 were also analyzed for leaf sap K concentrations using the portable ion-specific electrode (B731LAQUAtwin Cardy meter; Horiba Scientific, Irvine, CA). The fourth leaf plus midrib were collected from each plant immediately after the harvest and pressed in a garlic presser to extract plant sap. Each 1 mL of sap was diluted with DI