Field experiments at two sites determined that for Romaine lettuce on the soils tested, no further P fertilizer application is need if extractable P is above 650 ppm

Deenik, J.

Hamasaki, R.

Shimabuku, R.

Uchida, R.

ScholarSpace at University of Hawai'i at Manoa

Soil and Crop ManagementSept. 2007SCM-19Published by the College of Tropical Agriculture and Human Resources (CTAHR) and issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperationwith the U.S. Department of Agriculture. Andrew G. Hashimoto, Director/Dean, Cooperative Extension Service/CTAHR, University of Hawai‘i at Mänoa, Honolulu, Hawai‘i 96822.An equal opportunity/affirmative action institution providing programs and services to the people of Hawai‘i without regard to race, sex, age, religion, color, national origin, ancestry, disability,marital status, arrest and court record, sexual orientation, or status as a covered veteran. CTAHR publications can be found on the Web site <http://www.ctahr.hawaii.edu/freepubs>.Romaine lettuce is an important vegetable crop forfarmers in the Waimea area of the island of Hawai‘iand in Kula on Maui. These areas are suited for this cropbecause they combine a favorable climate, character-ized by warm sunny days and cool nights, with fertilevolcanic ash soils. Given the dramatic rise in the cost offertilizers since 2004, careful management of fertilizerinputs is an important part of growing a successful andprofitable crop.A soil test for phosphorus (P) is a useful tool farmerscan use to determine whether their fields require P fer-tilizer or whether the soil contains a sufficient reserveof P for good crop growth. The soil P concentration as-sociated with optimum growth is a critical piece of in-formation that allows forthe diagnosis of sufficiencyor deficiency; this concen-tration is called the soil Pcritical level. The CTAHRAgricultural DiagnosticService Center (ADSC) hasused a general value of 85ppm as the P critical levelfor all vegetable crops inlight soils such as volcanicash soils. Given resultsfrom field trials recentlyconducted in Waimea show-ing that the soil P criticallevel for head cabbage was450 ppm (Deenik et al.2006), we undertook fieldPhosphorus Fertilizer Management for Romaine LettuceGrown in Fertile Volcanic Ash Soils of Hawai‘iJ. Deenik1, R. Hamasaki2, R. Shimabuku3, and R. Uchida41Department of Tropical Plant and Soil Sciences; 2,3Department of Plant and Environmental Protection Sciences,2Kamuela Extension Office, 3Kahului Extension Office; 4Agricultural Diagnostic Service Centertrials in Waimea and Kula to determine the soil P criti-cal concentration for romaine lettuce production in vol-canic ash soils.Two field experimentsWe conducted two field experiments to evaluate romainelettuce yield response to increasing concentrations ofsoil P at the Lalamilo Research Station in Waimea,Hawai‘i, and the Kula Research Station on Maui. Cli-matic conditions and soil type are similar at both sta-tions (Table 1). At the Lalamilo site we imposed four Ptreatments equivalent to 0, 30, 60, 120, and 240 lb P peracre arranged in a randomized complete block with fourreplicates. At the Kula site, five P treatments (0, 598,2384, 3336, and 4765 lb Pper acre) were imposed toensure a wider range in tar-get soil P concentrations atthe high end (high quanti-ties of P fertilizers are re-quired in these volcanic ashsoils to overcome their highP adsorption capacity).Phosphorus as triple super-phosphate (TSP) was broad-cast evenly across each plotand tilled in to a depth of 6inches. Nitrogen, as urea,was broadcast with the TSPat 50 lb N per acre and side-dressed twice during thecropping cycle at 75 lb NRomaine lettuce production in Lälämilo, Hawai‘iUH–CTAHR Phosphorus Fertilizer Management for Romaine Lettuce . . . SCM-19 — Sept. 20072per acre each time. Four-week-old lettuce seedlings(Lactuca sativa cv. Paris Island cos) were transplantedon September 26, 2006 at Lalamilo and on January 26,2007 at Kula. Harvest was on November 5, 2006 atLalamilo and March 27, 2007 at Kula. At harvest, 25plants from each plot were cut at the soil surface andweighed. Two plants were randomly selected from eachplot, carefully washed to remove soil, dried at 70°C for72 hours, and analyzed for nutrient concentration at theADSC. Composite soil samples (0–6 inches depth) werecollected from each plot immediately before transplant-ing and at harvest and analyzed for pH, extractable P,and nutrient cations (K+, Ca2+, Mg2+, Na+) at the ADSC.What we foundRomaine lettuce growth showed a strong response toincreasing soil P concentrations at both experimentalsites. There was a steep linear response in lettuce growthas soil P concentration increased to approximately 500ppma followed by a plateau in growth as soil P concen-tration increased to more than 1500 ppm (Fig. 1). Theerror bars associated with the points in Figure 1 showthat there was considerable variability in soil P concen-tration in the plots that received higher P inputs. We useda non-linear regression technique to fit a linear-plateauequationb to estimate the soil P critical level shown inFigure 1.This analysis indicated that the soil P critical concen-tration for romaine lettuce in the volcanic ash soils ofWaimea and Kula is in the range of 446–644 ppm (545± 99 ppm). This means that we would expect lettuce toshow a positive growth response to P fertilizer whensoil P concentration is less than 550 ppm, but we wouldnot expect a growth response when soil P concentrationis greater than approximately 650 ppm. In other words,on fields where soil P concentration is greater than 650aIn the volcanic ash soils of Waimea and Kula, which are character-ized by low bulk density (0.5 g/cm3), 1 ppm is equal to approxi-mately 1 lb per acre.bThe linear-plateau equation (Shuai et al. 2002) allows for simulta-neous fitting of two linear equations, one characterizing the linearincrease in growth as soil P increases and another associated withthe yield plateau, to reveal the node, the point where the two equa-tions meet. The X-coordinate associated with node corresponds tothe soil P critical level. The fitting process also provides an estimateof the variability associated with predicted node expressed as a stan-dard error.ppm, the farmer can expect no added benefit to yieldfrom P fertilizer applications.RecommendationAgricultural soils that are high in phosphorus, eitherbecause of their natural fertility or the accumulation ofyears of fertilizer P applications in excess of crop re-quirements, often do not need additional P to supportgood crop yields. In earlier on-farm trials with head cab-bage in Waimea and Kula, we showed that soils withsoil P concentrations above 450 ppm did not need moreP fertilizer to obtain good yields (Deenik et al. 2006).Similarly, the results with Romaine lettuce provide abasis to propose a revised soil P critical concentrationof 650 ppm for its production in the volcanic ash soilsof Waimea and Kula. If soil analysis for P is 650 ppm ormore, the ADSC will advise that additional P fertilizerapplication is not recommended for a Romaine lettucecrop.Thus with the use of a soil test, Romaine lettuce farm-ers can determine when P fertilizers are necessary andapply them to reach the target of 650 ppm. When soilsshow higher P levels, farmers can save money by notapplying P fertilizer and still be confident that their yieldswill remain high.ReferencesDeenik, J., R. Hamasaki, R. Shimabuko, S. Nakamoto,and R. Uchida. 2006. Phosphorus fertilizer manage-ment for head cabbage. University of Hawai‘i atMänoa, College of Tropical Agriculture and HumanResources, publication SCM-16, 6 p. www.ctahr.hawaii.edu/oc/freepubs/pdf/SCM-16.pdfShuai, X.F., Z.J. Zhou, and R.S. Yost. 2003. Using seg-mented regression models to fit soil nutrient and soy-bean grain yield changes due to liming. Journal ofAgricultural Biological and Environmental Statistics8(2):240–252.AcknowledgmentsOur research was made possible through funds obtainedfrom the Natural Resource Conservation Service (CESUCooperative Agreement no. 68-3A75-4-72 Mod. 2) andthe Office of Economic Development of Maui County(Grant Agreement no. G2279). We also thank Drs. G.Uehara, N.V. Hue, and R.S. Yost for their constructivecomments.3UH–CTAHR . . . Grown in Fertile Volcanic Ash Soils of Hawai‘i SCM-19 — Sept. 2007Figure 1. Romaine lettuce growth response to increasing soil P concentration in volcanic ash soils in Waimea and Kula.Note: The data were fit with a linear-plateau equation (y = a + b*(if x<=node,x,node) using non-linear regression in SigmaPlot (version 8). Eachpoint is a mean of four replicates, and the error bars represent the standard error of the mean.Table 1. Selected climatic and soil data for the CTAHR Lalamilo and Kula Research Stations.Annual Average Initial soilSoil Soil Elevation rainfall temperature conditionsStation taxonomy texture (feet) (inches)  (°F)Min. Max. pH P (ppm)Lalamilo(Waimea series) Silt Loam 2500 30 59 73 5.3 354(±0.02) (±12)Kula(Kula series) Loam 3000 30 52 78 6.0 133(±0.1) (±3)Numbers in parentheses are standard errors of the mean.Medial,amorphic,isothermicHumicHaplustands

Phosphorus Fertilizer Management for Romaine Lettuce Grown in Fertile Volcanic Ash Soils of Hawai'i

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Phosphorus Fertilizer Management for Romaine Lettuce Grown in Fertile Volcanic Ash Soils of Hawai'i

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