How fertiliser types affect rice yield and soil under SRI

What fertilizer type changes under SRI

SRI is a crop-management system rather than a fertilizer recipe. Its core practices usually change seedling age, plant spacing, soil aeration, irrigation and weed management, so the root zone in an SRI field can differ substantially from a continuously flooded, densely planted rice field. Nutrient supply therefore interacts with a management environment in which roots may explore a larger volume of soil and alternate wetting can change oxygen availability, nutrient transformations and microbial activity. Fertilizer performance can consequently vary according to field conditions, nutrient sources and the wider management system.

The 2005 study compared fertilizer sources while maintaining equivalent nominal quantities of nitrogen, phosphorus as P2O5 and potassium as K2O. This design addressed whether the form in which nutrients are supplied can alter crop and soil responses. Coordinated chemical and organic fertilization promoted plant development and grain yield, improved physical and chemical soil status, and increased fertilizer-use efficiency. The results support the role of fertilizer source as an important component of nutrient management under SRI.

Mineral fertilizers and organic materials deliver nutrients through different pathways. Soluble mineral sources can provide nitrogen, phosphorus and potassium quickly and predictably, which is valuable when crop demand rises rapidly. Organic inputs such as manure, compost, crop residues or vermicompost contain carbon and nutrients that become available as they decompose. They can also influence aggregation, water retention, cation exchange, pH and microbial activity. The practical advantage of an integrated program is that different nutrient-release patterns and soil effects can complement one another under suitable field conditions.

What field trials say about grain yield

Integrated programs can match or exceed mineral-only treatments

A two-season SRI field experiment at Varanasi, India, tested 50%, 75% and 100% of the recommended fertilizer dose together with nitrogen supplied through farmyard manure or vermicompost. Srivastava et al. (2014) reported that 100% of the recommended fertilizer dose increased grain yield by 19.0% over the 50% dose. Increasing organic-source nitrogen to 45 kg N/ha also improved yield components and grain yield relative to lower organic-N rates. Importantly, 75% of the recommended fertilizer dose plus 45 kg N/ha through vermicompost produced a markedly high grain yield that was statistically at par with several treatments combining the full recommended fertilizer dose with farmyard manure or vermicompost.

Purely organic nutrition is not automatically sufficient

Reddy et al. (2013) reported a mean grain yield of 76.56 q/ha under SRI with 100% NPK compared with 64.76 q/ha under the traditional method, while purely organic nutrient management under SRI did not produce the highest grain yield. In Japan, Chapagain and Yamaji (2011) found that SRI changed several plant traits, but overall grain yield did not differ from conventional management; conventional-inorganic management produced comparatively high grain yield, while some SRI-organic combinations improved other plant characteristics and economic returns. These studies show that SRI can continue to benefit from readily available nutrients depending on soil and crop conditions.

How fertilizer source changes soil properties

Soil responses are one of the strongest reasons to compare fertilizer sources over more than a single harvest. In a four-year paddy experiment, Mi et al. (2018) found that adding cattle manure to NPK increased 2014 rice grain yield by 11.4% relative to NPK alone, while adding rice straw increased yield by 9.3%. The cattle-manure treatment also raised soil pH by 0.16 to 0.29 units and cation exchange capacity by 17.4% to 21.9%, and it produced the highest available phosphorus concentration in the surface layer. Other organic amendments in the same study produced different pH responses, demonstrating that organic amendments can affect soil properties in distinct ways.

Chesti et al. (2015) reported that 100% NPK plus 10 t/ha of farmyard manure increased grain yield to 5.36 t/ha, compared with 4.96 t/ha under 100% NPK alone, while also increasing soil organic carbon and available nitrogen, phosphorus and potassium. Long-term rainfed-lowland work by Banik et al. (2006) adds an important qualification: the highest mean yield was 3.53 t/ha with inorganic fertilizer and 3.47 t/ha with cow dung, a difference that was not statistically significant, yet several soil-quality indicators were highest under cow dung. Yield and soil quality can therefore move together, but they do not have to rank treatments in the same order.

Selected results from rice fertilizer studies

Why results vary across sites and water regimes

The response to fertilizer is filtered through the soil. A review by Turmel et al. (2011) found that SRI yield responses differed with soil fertility and soil type: yield advantages were more apparent on highly weathered, infertile Acrisols and Ferralsols, while differences were not evident on more fertile Gleysols, Luvisols and Fluvisols in the studies analyzed. This pattern shows that the baseline soil environment can determine how strongly a management change affects nutrient capture and crop performance.

Water management is another part of the interaction. Li et al. (2024) compared different organic-to-inorganic fertilizer ratios under conventional and controlled irrigation. With equal nitrogen application, the treatment with the highest organic fertilizer ratio yielded 2.7% more rice than chemical fertilizer alone and 22% more than the unfertilized control. Controlled irrigation increased yield by 6% compared with conventional irrigation and also changed soil temperature, electrical conductivity, nitrogen and pH. The experiment demonstrates that fertilizer-source effects can change when the water regime changes.

Broader evidence also reveals environmental trade-offs. A 2024 global meta-analysis of 211 papers and 852 observations found that replacing part of inorganic fertilizer with organic substrate increased crop productivity by 3.04% on average and improved several biological soil-fertility indicators. It also reduced several nitrogen-loss pathways, but paddy-field methane emissions increased by 41.2%. These findings show that higher productivity and improvements in soil characteristics can occur alongside changes in greenhouse-gas emissions. Fertilizer strategy can therefore be evaluated across crop performance, soil quality and environmental outcomes.

Equal NPK does not mean equal nutrient availability

One of the easiest mistakes in fertilizer comparisons is to assume that equal total N, P and K means equal nutrition for the crop. Total nutrient content describes what is present in the material, not when it becomes available to roots. Mineral nitrogen can be immediately soluble, while organic nitrogen must be mineralized. Phosphorus availability depends strongly on soil chemistry, and potassium may be released at different rates from residues, manures and mineral salts. Two treatments can therefore contain the same nominal nutrient amount yet produce different nutrient concentrations in soil solution during critical growth stages.

Organic materials also bring non-nutrient properties into the comparison. Carbon inputs can support microbial biomass and contribute to aggregation; some manures can buffer acidity and increase cation exchange; residues with high carbon-to-nitrogen ratios can temporarily immobilize nitrogen. Compost maturity, moisture, storage losses and nutrient analysis all matter. Different organic materials can therefore produce different soil and yield responses even when they are applied alongside the same mineral fertilizer program. The four-year paddy study illustrates the point: cattle manure, rice straw, green manure and spent mushroom compost produced different soil and yield responses despite all being organic additions to NPK.

The same logic applies to fertilizer-use efficiency. A higher efficiency value may reflect better synchronization between nutrient release and crop uptake, a stronger root system, less nutrient loss from the soil, or a lower nutrient input used to produce a similar yield. The 2005 study reported that coordinated organic and chemical nutrient supply increased fertilizer-use efficiency, supporting the role of integrated management in improving nutrient use under the tested conditions.

The evidence does not show that organic or mineral fertilizer wins everywhere; it shows that nutrient source, dose, soil and water management interact.

How to read an SRI fertilizer study

A fertilizer trial becomes much more informative when the treatment labels are translated into actual field conditions. This is especially important in SRI, because changes in planting density, irrigation, weeding and root-zone aeration may occur at the same time as changes in fertilizer. When comparing studies or deciding whether a result is transferable, the following variables are particularly important:

1. Starting soil fertility, pH, texture and organic matter, because these determine how strongly a field may respond to added nutrients or carbon.
2. Actual N, P and K rates and whether equal nutrients refers to total nutrient content or estimated plant-available amounts.
3. Organic input type, maturity, nutrient analysis and application timing, including whether manure, compost, residues or vermicompost were used.
4. Water regime, seedling age, spacing and other SRI practices that can alter root growth, aeration and nutrient transformations.
5. Trial duration and whether soil measurements represent a one-season response or changes accumulated over several crop cycles.

These variables explain why coordinated organic and chemical fertilization can perform well under SRI conditions while producing different responses across environments. Research across multiple rice-production systems supports integrated nutrient management while also demonstrating that outcomes vary according to site, amendment type, fertilizer rate and water regime.

What the evidence supports for nutrient planning

Across the evidence reviewed here, integrated nutrient management is a credible option for rice systems because it can combine the rapid nutrient availability of mineral fertilizers with the longer-term soil effects of organic materials. Several field studies found higher grain yield or improved soil nutrient status when manure or vermicompost was combined with mineral fertilizer. At the same time, other experiments found comparable yields between organic and mineral programs or comparatively high yields under mineral-only management. The supported conclusion is therefore conditional: integrated programs can work well, but the best balance is site-specific rather than fixed by the SRI label.

The 2005 study fits that broader picture: fertilizer source affected both crop and soil outcomes under SRI, and the combined organic-plus-chemical treatment was reported as the strongest overall option. More recent research supports the same mechanism and shows multiple cases where integration improved yield or soil properties, while also demonstrating variation among sites and management systems. For practical nutrient management, the main takeaway is to adjust the organic-mineral balance according to local soil, water and crop responses rather than treating either fertilizer category as universally superior.

Frequently asked questions

Callum Hebden — author

Callum Hebden is an editorial byline of the CAB Direct Editorial Team for agriculture explainers. At CAB Direct, Callum covers agricultural markets and prices, farm subsidies and support schemes, land use, and the regulation and farm-level economics of GM and...