Philippine Journal of Crop Science 2003, 28(1): 39-48 Copyright 2005, Crop Science Society of the Philippines Released January 2005 SOIL FERTILITY LIMITATIONS OF IFUGAO RICE TERRACES TAHERE A SIGARP, NENITAV DESANIER01, JIMMY C CABIGAT2, EMILIO H ABAYAO2 & ALFONSO B CAYONG2 'Philippine Rice Research Institute, Maligaya, Science City of Munoz, Nueva Ecija. 2 Local Government Unit, Ifugao In 2002 and 2003, a total of 68 soil samples were collected from Ifugao rice terraces in 3 municipalities (Banaue, Hungduan and Kiangan) and evaluated for fertility status through 2 methods: (a) minus-one-element test (MOET) and (b) soil analysis (SA). For SA, the samples were dried and sieved while for MOET the samples were kept moist. MOET setups were prepared in 2 replicates planted with PSB Rc44 and Palawan (local variety) following procedures described by Descalsota et al (2001) and Sigari et al (2002). Soils with relative shoot dry weight (RSDW) of 80% or less were considered nutrient-deficient. Likewise, SA was done to measure soil texture, pH, %OM, %0C, total N (%), C:N ratio, available P, exchangeable K, available Zn, S, Cu and soluble Fe. Soils with total N < 0.2%, Olsen P < 6 ppm, exchangeable K < 0.2 cmol/ kg soil, available Zn < 1 ppm, available S < 6 ppm, and available Cu < 0.1 ppm were identified as nutrient-deficient. By MOET, N deficiency was found common to all soils. In Banaue, 31.9% of the soils were recorded as deficient in P, 78.6% in K, 31.9% in Zn, 61.2% in S, and 17.8% in Cu. In Hungduan, 54.7% of the soils were deficient in P, 66.6% in K, 41.9% in Zn, 69.4% in S, and 33.3% in Cu. In Kiangan, 33.6% of the soils were deficient in K, 16.4% in Zn, 38.4% in S, and 28.3% in Cu. The results of SA revealed that all the soils were acidic (pH range 4.8-6.8), mostly with medium texture, and had medium to high levels of OM. In Banaue, soil C:N ratio averaged 16.1, in Hungduan 14.8 and in Kiangan 11.4. In Banaue, 54.1% of the soils were N- deficient, 67.6% P-deficient, 37.8% K-deficient, and 5.4% S-deficient. In Hungduan, 23.8% of the soils were N-deficient, 57.1% P-deficient, 44.4% K-deficient and 4.8% S-deficient. In Kiangan, 100% of the soils were N-deficient and 20% were K-deficient. With data combined, N-, P-, K-, S, and Zn deficiencies appeared to be the major soil fertility limitations of Ifugao rice terraces. Zinc deficiency appeared to be associated with a high amount of OM in the soil, and other limiting factors, making Zn unavailable to the rice plants. Use of organic nutrients with adequate amounts of P and K and improvement of cultural practices to alleviate N-, Zn- and S-deficiencies are recommended. Keywords Ifugao rice terraces, MOET, nutrient deficiency, rice terraces, soil analysis, soil fertility INTRODUCTION partly because of adverse agro-climatic conditions and partly because of local cultural Geographically, the rice terraces of Ifugao management practices, use of varieties with low- Province, northernLuzon, make up a yielding potential, and poor soil fertility. While magnificentlandscapesculpturedintotheuse of improved rice cultivars with high-yielding mountainsides and have been declared a Worldpotential and tolerancetoadverse climatic HeritageSite(1995).Agriculturally,theirconditions can address the problems of low production is very much below their potential.temperature and low solar radiation, actual Rice yields in such relatively cool, elevated areasyield may not be increased if proper cultural are very low, 2.3 t/ha (CECAP & Philrice 2000), management practices are not adapted and soil fertilityisnot improved appropriately. Past The overall objective of this study was to studies revealed that with appropriate nutrientgenerate data on which to base soil fertility and water management, a rice yield of 5-6 t/harecommendations for Ifugao rice terraces. The could be obtained. In the same studies, it was specific objectives were as follows: shown that rice yield increased with additional a)Evaluateand comparethefertility organic nutrients, but the increases were not status of soils in the rice terraces of proportional to the amount of organic nutrients selected Ifugao towns, those of Banaue, added and in some cases, were not significant at Hungduan and Kiangan. all. Thus, Sigari et al (2002) assumed that either b)Compare and combine the results of two the soil was infertile or the fertility limitation methods of determining soilfertility: was not confined to nitrogen but included other minus-one-element test (MOET) and soil nutrients as well. In both cases,rice yield analysis (SA). increases would be less than that expected as c) Based on theresults, recommend a the green manures were added to the ricefields. fertility management scheme for the rice Thus it appeared necessary to have a detailed terraces to improve yield and increase data on the soil fertility status of rice terraces. income of Ifugao rice farmers. Such data would provide clues in formulating an integrated nutrient management package to MATERIALS & METHODS ensure increased rice yield and higher farmer income. During the years 2002 and 2003, a total of In the province, the farmers recycle almost 68 soil samples were collected from Ifugao rice all the rice straw and weeds into the soil. Toterraces in 3 municipalities,ie, Banaue (39 some extent this helps return nutrients taken up samples), Hungduan (19 samples), and Kiangan by rice plants and weeds. Use of organic matter (10samples).From eachlocation,10soil in the long run, however, may result in the samples were randomly collected down to a 20- accumulation of organic substances especially cm depth and the soil thoroughly mixed. A small under cool climate and continuous flooding. portionof each sample was air-dried and Accumulation of organic substances may in turnpulverized for soil analysis and the rest were aggravate soil acidity, and make some elements kept moist for the MOET. such as Zn and Cu less available to plants A nethouse was installed at the Phil Rice (Dobermann & Fairhurst 2000). CentralExperimentStationinMaligaya, Soil fertility limitations are evaluated byScience City of Mu floz, Nueva Ecija, for the fieldexperiments,planttissueanalysis, MOET setup. Each setup contained 8 plastic observations on the incidences of deficiency orpots (20 cm in diameter and 18 cm in height) per toxicitysymptomsandsoilanalysis.Soil replication, each filled with 4 kg wet soil and analysis has been widely used in diagnosis ofassigned for the following nutrient formulas: soil fertility limitations. complete (soil supplied with the N, P, K, Zn, S, In recent years, a biological technique calledand Cu elements), control, minus N, minus P, theminus-one-elementtest(MOET) was minus K, minus Zn, minus S and minus Cu developed by Phil Rice soil scientists (Descalsota (Descalsota et al 2001). With adequate soil, each et al 1999), with 6 elements under consideration setup was prepared in 2 replications. All the (N, P, K, S, Zn and Cu), whereby experimentalnutrients except N were mixed 3-5 cm into the plants are grown from seed to seed with a soil surface before seeding. The N was added 10 different fertilizer element deliberately missing daysafterseeding (DAS).Initially,5pre- in each treatment; observations on plant growth germinated seeds of rice cultivars PSB Rc44 and arecomparedwithknownsymptomsof Palawan (local variety) were sown into each pot; elemental deficiency. The technique is based onlater, the growing seedlings were thinned to 3 the principle that plant growth is determined orper pot at 10 DAS. The soil was kept submerged limited by the most limiting nutrient in the soil. and water depth increased to 2-3 cm as the The responses are partly manifested by reduced seedlings grew taller. Weekly observations were plantheight,reducedtillercount,delayed made regarding the occurrence of deficiency maturity, smaller panicles and by the presence symptoms and plant growthfactors(plant ofdistinctdiscolorationsuchaschlorosis, height and tiller count). necrosis and/or streaks (Descalsota et al 2000). At 60 DAS, the plants were cut at the base and oven-dried.Soils where total shoot dry 8.3%. C:N ratio averaged 16.1, ranging from 10.3 weight was less than 80% of shoot dry weight ofto 24.1; total N averaged 0.19%, from 0.09% plants grown with complete fertilizer (given all (very low) to 0.38% (medium), Olsen P averaged the 6 elements) were classified as nutrient- 12.7 ppm, from 2.6 ppm (low) to 41.7 ppm (high); deficient (Descalsota et al 2000). exchangeable K averaged 0.25 cmol/ kg soil, Likewise soil samples were analyzed for from 0.1 cmol/ kg soil (low) to 0.97 cmol/ kg soil texture (particle size density), pH (1:1 water: (high); available Zn averaged 5.7 ppm, from 1.1 soil) organic carbon and organic matter content ppm (marginal)to47.7 ppm (veryhigh); (usingthe Walkey-Black method),total Navailable S averaged 20.2 ppm, from 5.2 ppm (Kjeldahl method), available P (Olsen method), (low) to 81.6 ppm (very high), available Cu exchangeable K (ammonium acetate), availableaveraged 13.2 ppm, from 1.9 ppm (medium) to Zn (DTPA), available S (turbidimetric method),21.2 ppm (high); soluble Fe averaged 248 ppm, available Cu (DTPA) and soluble Fe (ammonium from 34.1 ppm (high) to 441.5 ppm (very high). acetate extraction method). Soils with total N <As to texture, 82.4% of the soils were loam and 0.2%, available P < 6 ppm, exchangeable K < 0.2 17.6% were silt loam (Table 2). cmol/ kg soil, available Zn < 1 ppm, available S < In Hungduan, soil pH averaged 5.3, ranging 6 ppm and available Cu < 0.1 ppm Cu, werefrom 4.7 (acidic) to 6.0 (slightly acidic); OM considereddeficientfor thoseparticularcontent averaged 6.0%, from 3.2% to 12.0% elements.Soilscontaining medium or high (unfavorably high); C:N ratio averaged 14.8 from amounts of nutrients were also identified on the 11.0 to 18.3; total N averaged 0.28%, from 0.14% basis of accepted criteria (Table 1).