Fertilizer prescription for achieving targeted rice yield in inceptisol soils

Introduction

Rice serves as a staple food for nearly half the global population, including 60% of India's people, who are particularly vulnerable to rising rice prices. It also underpins the livelihoods of most Indian farmers. Over the past five decades, rice production in India has nearly tripled, playing a vital role in the country's nutritional security. However, the annual compounded growth rate (ACGR) of rice production has declined, from 3.55% during 1981-1990 to 1.74% during 1991-2000, despite achieving a record production of 99.50 million tons with a productivity of 2.20 tons per hectare in 2008-2009. Rice is cultivated in nearly all Indian states, with West Bengal, Uttar Pradesh, Madhya Pradesh, Bihar, Odisha, Andhra Pradesh, Assam, Tamil Nadu, Punjab, Maharashtra, and Karnataka contributing 92% of the total area and production. Yet, India remains among the countries with the lowest rice yields. Seventy percent of the 414 rice-growing districts report yields below the national average, indicating that even after the advent of high-yield technologies, a significant portion of rice-growing areas remains low-producing. States like Bihar, Odisha, Assam, West Bengal, and Uttar Pradesh account for 60% of these low-productivity areas. Alarmingly, 32% of irrigated rice lands also produce low yields (Tiwari, 2002) [12] . Rice-based cropping systems significantly contribute to food production, but current systems often involve imbalanced and inadequate fertilizer use, with blanket recommendations that overlook soil fertility and productivity variations. Achieving future productivity gains and improving input efficiency will require tailored soil and crop management technologies designed for individual farms or fields. Research has highlighted significant field variability in soil nutrient supply, nutrient use efficiency, and crop responses. Managing this variability remains a critical challenge for enhancing the productivity of intensive rice-cropping systems (Rao, 2011) [7] .

Materials and Methods

On-farm testing trials were conducted in the village of Parsiya, Naugarh block, Chandauli district, Uttar Pradesh, India, during the Rabi season of 2022-23 on alluvial soil (Inceptisol). Soil samples from the 0-15 cm depth were collected, dried, sieved through a 2 mm mesh, and analyzed for physico-chemical properties as per Jackson (1973) [3] . Available nitrogen was determined using the alkaline permanganate method (Subbiah and Asija, 1956) [11] , available phosphorus by the Olsen method (Olsen et al., 1954) [5] , and available potassium by the ammonium acetate method (Hanway and Heidal, 1952) [2] , all described in Jackson (1973) [3] . Five fertilizer treatments were tested: Control, Farmers' practice, General recommended fertilizer dose, Soil Test Crop Response (STCR) for 38 q ha⁻¹, and STCR for 43 q ha⁻¹, using the rice variety HUR-917 (Malviya Sugandha Dhan 917). The targeted yields for the crop were set at 38 q ha⁻¹ and 43 q ha⁻¹ based on the variety's yield potential. Pre-sowing soil samples were analyzed following standard procedures. The soil resource inventory of the study area is provided in Table 1. The required quantities of nitrogen, phosphorus, and potassium were calculated using fertilizer adjustment equations (Singh et al., 2014a) [8] .

Where: The fertilizer doses were determined based on soil test values for nitrogen (N), phosphorus (P 2 O 5 ), and potassium (K 2 O) available in the soil, and applied according to the soil test crop response (STCR) method for various yield targets. The treatments included: (i) a control group, (ii) farmer's practices, (iii) the general recommended dose, (iv) STCR-based fertilizer doses for a yield target of 38 q ha -1 , and (v) STCR-based fertilizer doses for a yield target of 43 q ha -1 . The fertilizer application was calculated based on the grain yield target (T), which was adjusted using the available N, P, and K from the soil, and the quantities of N, P 2 O 5 , and K 2 O to be added via fertilizers (FN, FP 2 O 5 , and FK 2 O, respectively). For the STCR treatments, one-third of the nitrogen (N) was applied as basal fertilizer along with the full dose of P 2 O5 and K 2 O, while the remaining half of the nitrogen was applied 27 days after sowing. The final nitrogen dose was applied at the panicle initiation stage. Urea was used for nitrogen, single super phosphate for phosphorus, and muriate of potash for potassium. The rice variety used for both the STCR and other treatments was HUR-917, also known as Malviya Sugandha Dhan 917.

Results and Discussion

Soil characteristics

The soil in the study was alluvial (Inceptisol) with a pH ranging from 7.50 to 7.65. The organic carbon content varied between 0.33% and 0.37%. The soil was classified as medium in available nitrogen, with values ranging from 209.00 to 210.80 kg ha -1 , low to medium in available phosphorus, ranging from 16.70 to 17.10 kg ha -1 , and medium to high in available potassium, ranging from 183.90 to 155.90 kg ha -1 , as shown in Table 1. While these soils are generally considered fertile, they are deficient in nitrogen and organic matter (humus), but they have moderate levels of phosphorus and potassium. Note: Rice @ Rs. 20.00/kg, N @ Rs. 17.39/kg P 2 O 5 @ Rs. 56.25/kg, K 2 O @ Rs. 26.66/kg.

A minor modification was made to the ready reckoner. FP refers to the farmer's practice, which represents the fertilizer doses typically applied by farmers in the area. GRD stands for the general recommendations provided by the agricultural department of the district, based on soil test values. B:C ratio refers to the benefit-cost ratio, which measures the profitability of the inputs used in farming.

Yield targeting of Rice based on soil test

Experimental data from follow-up trials conducted as frontline demonstrations in farmers' fields during the 2022-23 period are provided in Table 2. The data from these field experiments helped determine the nutrient requirements for producing one quintal of rice grain yield, as well as the percentage contribution of nutrients from soil (%CS) and fertilizer (%CF). These parameters were used to develop fertilizer prescription equations for NPK alone. The nutrient requirements for producing one quintal of rice were 6.26 kg of N, 1.12 kg of P 2 O 5 , and 3.78 kg of K 2 O. The contributions of nutrients from soil and fertilizers were found to be as follows: for nitrogen (N), 25.41% from soil and 117.03% from fertilizers; for phosphorus (P 2 O 5 ), 40.99% from soil and 35.42% from fertilizers; and for potassium (K 2 O), 19.67% from soil and 45.47% from fertilizers.

It was observed that the contribution of potassium from fertilizer was higher than from the soil. This increased potassium uptake could be due to the interaction effects of higher nitrogen (N) and phosphorus (P) doses, combined with the priming effect of starter potassium (K) doses applied in the treated plots. This likely facilitated the release of soil potassium, leading to higher uptake from native soil sources (Ray et al., 2000) [6] . Similar findings of higher potassic fertilizer efficiency were also reported for rice in alluvial soils (Ahmed et al., 2002) [1] . The yield targets of 38 and 43 q ha -1 were achieved with lower applications of N and P 2 O 5 fertilizers, but higher applications of K 2 O, compared to the doses applied in farmer's practices and soil-based recommendations. For example, in the alluvial soil of West Bengal during the winter season, the highest rice yield reached 6.0 t ha -1 regardless of nitrogen levels, but this could be increased to 7.4 t ha -1 with higher applications of potassium fertilizers (Tiwari, 2002) [12] . This improvement is likely due to higher nitrogen use efficiency and better nitrogen recovery by the crop with increased potassium application (Marschner, 1995) [4] . For rice variety HUR-917 (Malviya Sugandha Dhan 917), the target yields of 38 and 43 q ha -1 were successfully achieved, as shown in Table 2, with yields surpassing the expected targets in all cases. In all locations, grain yields of rice based on the general fertilizer recommendations (GRD) were lower than those achieved with the 38 and 43 q ha -1 targets. These results align with findings from Singh et al. (2014) [8] and Singh et al.

(2015) [9] . Among the two yield targets, the 43 q ha -1 target showed a higher response ratio compared to the 38 q ha -1 target, though both targets resulted in higher yields. This could be attributed to better efficiency of applied NPK fertilizers at lower yield target levels (Singh et al., 2014; Singh et al., 2015) [8,9] .

Post harvest soil fertility status

Post-harvest soil analysis showed a significant build-up and maintenance of available nitrogen (SN), phosphorus (SP), and potassium (SK) under the STCR treatment, compared to farmer practices and general fertilizer recommendations. Despite the higher nutrient removal in the STCR treatment due to the increased yield, post-harvest soil fertility was better preserved in the STCR plots. The highest post-harvest soil nitrogen was observed in Location-1 (Smt. Phuleshari W/O Sri. Bhagwandas, Village-Parsiya) with 240.00 and 233.00 kg ha -1 for the 38 and 43 q ha -1 targets, respectively. Soil potassium was highest in Location-2 (Smt. Phulmati Devi W/O Sri. Jayshankar, Village-Parsiya) with 215.00 and 210.00 kg ha -1 , and soil phosphorus was highest in Location-1 (Smt. Phuleshari W/O Sri.

Bhagwandas, Village-Parsiya) with 19.50 and 18.1 kg ha -1 for the respective yield targets, as shown in Table 3. The increased nutrient build-up under the STCR treatment was attributed to the balanced application of chemical fertilizers in combination with organic manure. The combined use of inorganic fertilizers improved both the chemical and physical properties of the soil, potentially contributing to enhanced and sustainable production. Greater profits, along with the maintenance of soil fertility, were achieved when fertilizers were applied according to the appropriate yield targets year after year using the STCR approach (Singh et al., 2015) [9] . For the efficient use of applied fertilizers, it is important to consider additional factors such as soil pH, organic carbon content, and other parameters that influence soil nutrient retention. These factors play a crucial role in developing an effective fertilizer schedule and ensuring optimal nutrient absorption and assimilation by the plants. This study aims to provide guidelines for the appropriate amount of fertilizer to use in rice cultivation. A yield equation based on soil health will not only promote sustainable crop production but also help farmers optimize the use of costly fertilizers, taking into account their financial situation and the prevailing market prices of the crop.

Acknowledgement

AcknowledgementsThe authors would like to express their gratitude to the Indian Institute of Soil Sciences, Bhopal, for providing financial support through the AICRP on STCR project during the course of this study.

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