YIELD RESPONSE OF SELECTED IRRIGATED RICE VARIETIES TO VARYING N, P AND K FERTILIZER REGIMES

Rice production has been inconsistent in Mwea irrigation scheme due to poor crop management practices and reduced soil fertility. Proper rice nutrition conserves the environment, increases sustained crop production, farmer’s crop yield and profits. A field experiment was conducted at MIAD Center, in Kirinyaga County, during 2016-17 and 201718 to determine the response of rice varieties to different N, P and K fertilizer treatments. The trial was conducted in randomized complete block design, with three replications of 13 N, P and K fertilizer regimes as the main plots treatments and varieties Basmati 370 and BW 196 as the sub-plot treatment. The N kg ha-1: P2O5 kg ha-1: K2O kg ha-1 fertilizer treatment ratios used were: 00:00:00, 60:40:40, 80:60:60, 100:80:80, 60:40:00, 80:60:00, 100:80:00, 60:00:40, 80:00:60, 100:00:80, 00:40:40, 00:60:60 and 00:80:80. Plant height, number of tillers hill-1, panicle length, and grain yield responded positively to fertilizer application, but 1000-grain weight did not. Variety BW 196 recording shorter plants and panicles, more tillers hill-1, higher grain weight and yield than variety Basmati 370. 00:40:40, 00:60:60, 00:80:80, 60:40:00 and the no-fertilizer control treatments recorded the least number of tillers hill-1. 60:40:40, 80:60:60, 100:80:80, fertilizer treatments had longer panicles than the no fertilizer control and 00:40:40. Except for 100:80:00 and 00:40:40, in the first season and 60:40:00, 80:60:00, 100:80:00 in the second season, all the fertilizer regimes increased grain yield relative to the control. The highest grain yield was realized in fertilizer regime 80:60:60, 100:80:80, 80:00:60 and 100:00:80. 80:00:60 is the recommended fertilizer regime.


Introduction
Traditionally, rice crop was prominent only in Asia, but over the years, it has also gained prominence in Africa's farming systems and diet. In Africa, rice is ranked fourth after maize, sorghum and millet, in terms of acreage grown. Its production is ranked second after maize (FAOSTAT, 2012). Several studies have shown that Kenya has the capacity of producing over 9 t ha -1 of rice (Saito et al., 2013). In spite of that, the mean rice yields in Kenya between the year 2005 and 2009, was 2.5 t ha -1 (Onyango, 2014). This translates to a yield gap of over 6.5 t ha -1 .
According to reports from MOA (2009), the rice production sector in Kenya faces several setbacks, such as: poor infrastructure, high costs of farm inputs and machinery, uncoordinated marketing, inadequate rice crop management skills, high disease incidences, land degradation and nutrient losses. In Mwea irrigation scheme, the average crop production has been fluctuating due to reduced soil fertility, soil degradation and use of inappropriate crop management practices by farmers (Nyamai et al., 2012). Formerly, farmers in Mwea used to burn one part of their farm and spread ashes across the paddy fields. Because of the increasing demand for animal feed, straw is baled and sold; hence, organic matter is transported to a different location. Husks are also disposed elsewhere after dehulling (Muhunyu, 2012). The mass transportation of organic matter from the rice fields, combined with permanent water logging and rice monoculture reduces the inherent capacities of the soil to supply nutrients, and hence results in long-term detrimental effects on crop production.
Balanced application of inorganic fertilizers is important for increased yields and sustained production of quality rice. It aids in reducing injury from infestation of pests and diseases, and minimizes expenditures that are to be used in controlling them. Nutrient requirements vary with varying varieties of rice (Dobermann and Fairhurst, 2000). In Mwea, at least all farmers use inorganic fertilizers, with 86% using DAP (Diammonium Phosphate) as basal fertilizer, 93% using SA (Sulphate of Ammonia) as a top dresser. Only 2% use MOP (Muriate of Potash) as a basal fertilizer (Muhunyu, 2012). Irrespective of the soil nutrient status and rice variety planted, most farmers apply 50 kg acre -1 of either DAP or SA, to their fields. Such application of blanket fertilizer could have significant negative effect on the performance and yield of rice. Good nutrition could lower the current rice yield gaps (6.5 t ha -1 ) in Kenya. Considering these perspectives, the current study was set up to monitor the response of selected rice varieties to varying nitrogen, phosphorus and potassium fertilizer regimes.

Materials and Methods
The present experiment was conducted for two seasons; between the years 2016 and 2018. It was setup in MIAD center, in Kirinyaga County. MIAD is located to the northeastern region of Nairobi, Kenya's capital, and to the southeastern part of Mount Kenya (Ngige, 2004). Before setting up the trial plots, soil at the experimental site was collected from different homogenous units, at two depths (0-15 and 15-30 cm), and bulked into two samples. The soils were analyzed for macro and micronutrients, pH, electrical conductivity and organic carbon. Very low levels of potassium (68.00, 37.10 ppm) and phosphorus (3.06, 0.93 ppm), and considerably low levels of Boron (0.21, 0.33 ppm) and zinc (1.02, 1.12 ppm) were reported at both depths.
Meteorological data was collected from MIAD meteorological station, during the research period. The highest rainfall (481 mm) was recorded in November 2017 and the lowest (0.00 mm) in January 2017. The mean temperature ranged from 21.75 to 28.79 o C. The highest relative humidity was 86.47% (November 2018) and the lowest was 69.74% (January 2017).
Each 9 m 2 nutrient plot was bunded and lined with a 500 gauge polythene bag (45 cm deep), to minimize mixing of nutrients and to maintain a uniform water depth. Plants in each plot had a 20 x 20 cm spacing.
To monitor the response of the crop to varying inorganic fertilizer regimes, data was collected from a sample of 10 plants (IRRI, 2014). Observations on plant height and the number of tillers hill -1 were collected at 30 and 75 DAT. Plant height measurements were taken from the soil surface to the apex of the tallest panicle, excluding awns. The tiller count hill -1 was counted on the ten marked hills plot -1 . At maturity, paddy rice was harvested, and dried to 14% moisture content. Grain yield was weighed and the weight of 1000 well developed seeds were also recorded. Using the 15 th edition of Genstat software , the collected data was subjected to analysis of variance. The Least Significant Difference (LSD) test (at P≤0.05) was used to compare means between treatments.

Results and Discussion
A positive response of varieties Basmati 370 and BW 196 to nitrogen, phosphorus and potassium fertilizer, in plant height, number of tillers hill -1 , panicle length and grain yield was recorded in this study. This could be due to the low levels of these plant nutrients in the soils in the study area. In the present study applying 80:60:60, 100:80:80 and 100:00:80 fertilizer treatments augmented plant height relative to the nofertilizer control. N omission treatments (00:40:40, 00:60:60, 00:80:80) did not have a significant effect on plant height relative to the no fertilizer control. The increased plant height in fertilizer regime 80:60:60 and 100:80:80 could be explained by the sufficient supply of rice nutrients. The inadequate supply of N in 00:40:40, 00:60:60, 00:80:80 resulted in plant heights resembling those of the no-fertilizer control (Table 1); because nitrogen deficiency must have disrupted metabolic processes of the plants (Dobermann and Fairhurst, 2000).
From the results of this study, fertilizer application had an insignificant effect on 1000grain weight of rice (Table 3). A thousand grain weight is not prone to effects by environmental factors; it is controlled genetically (Yoshida, 1981). This explains why BW 196 had higher 1000-grain weight than Bs 370. In the current study, higher (4.86 t ha -1 ) mean grain yield was registered in BW 196 than Basmati 370 (4.04 t ha -1 ). The yield registered for variety Bs 370 in the current study is in the range (4.1 to 6 t ha -1 ) reported by Ndiiri et al. (2013), from farmers' fields in Mwea. All the fertilizer treatments augmented grain yield relative to the control with an exception of 100:80:00 and 00:40:40, in the first season and 60:40:00, 80:60:00, 100:80:80 in the second season (