Agroecological Nutrient Management Strategy

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This entry is adapted from the peer-reviewed paper 10.3390/su16020845

Rice self-sufficiency is central to Indonesia’s agricultural development, but the country is increasingly challenged by population growth, climate change, and arable land scarcity. Agroecological nutrient management offers solutions though optimized fertilization, enhanced organic matter and biofertilizer utilizations, and improved farming systems and water management.

agroecology biochar biofertilizers climate change greenhouse gases

1. Introduction

Indonesia heavily relies on rice as a staple food, serving as the main component of almost every meal and constituting a fundamental part of the daily diet. Rice cultivation plays a pivotal role in the country’s economy, providing employment to millions in rural areas and supporting livelihoods and local economies. Food security, defined as equal access to sufficient, safe, and nutritious food necessary for all people ^[1], has been an economic, social, and research interest in the past few decades. However, rice self-sufficiency has remained the primary indicator of food security in Indonesia ^[2].

Rice production faces challenges due to the high and increasing population ^[3], land scarcity, and climate change. Its low management level results in a wide yield gap, representing the difference between the actual yield and the attainable yield in rice production ^[4]. Researchers believe that agroecological nutrient management, combined with the control of paddy field conversion, will be the solution for attaining sustainable rice self-sufficiency in Indonesia.

Agroecological nutrient management refers to a sustainable and holistic approach to managing nutrients in agricultural systems. It involves the integration of ecological principles with agricultural practices to optimize nutrient use efficiency, minimize environmental impacts, and enhance long-term agricultural sustainability. It takes into account the complex interaction between crops, soil, microorganisms, and the environment to promote nutrient cycling, reduce nutrient losses, and improve soil fertility. It is a management approach based on agroecological principles that aim to increase farm productivity through the wise use of external inputs, sustaining ecosystem services, and valorizing ecological processes and ecosystem services ^[5]^[6]. This approach reduces reliance on chemical fertilizers and enhances the use of organic matter, biofertilizers, and the efficient use of water ^[7]. A recent study showed that agroecological nutrient management with a higher input of organic matter increased soil pH, the availability of potassium, calcium, and magnesium, potassium concentration in leaves, and mycorrhizal colonization ^[8]. Another study that increased reliance on legumes, integrated crop–livestock production, and use of soil amendments resulted in enhanced soil organic matter (and soil carbon) accrual. The effects in the long term were more consistent in terms of increase yields, yield stability, profitability and food security ^[9]. An agroecological approach is important for restoring degraded lands ^[5]^[10] and overcoming the yield gap ^[4]. Increasing concerns about the loss of natural habitats and biodiversity emphasize the importance of producing more rice on existing cropland by improving the efficiencies of nutrients, energy, water, and other inputs in an agroecological process ^[5]^[11]^[12].

As a populous country, dependence on rice import may make Indonesia vulnerable to international market price fluctuations. Although Dawe (2008) ^[13] doubted that price volatility and world market price distortion would be significant, the government policy is that national production must be increased to meet demand, and import must be eliminated or kept to a minimum.

There are two approaches to increasing rice production: extensification and (sustainable) intensification. Extensification is becoming more and more difficult due to arable land scarcity ^[14]. Furthermore, the conversion of existing rice-producing areas is occurring at an alarming rate ^[15], resulting in shrinking paddy field areas. Hence, intensification has a greater opportunity to contribute to increased rice production. The actual average rice yield of about 5.2 t ha⁻¹ is far below the potential yield of irrigated rice, ranging from 8.3 t ha⁻¹ to 11.7 t ha⁻¹, or the rain-fed rice yield, ranging from 7.9 t ha⁻¹ to 12.1 t ha⁻¹ ^[16]. These data clearly show a great opportunity to close the yield gap to increase national rice production.

Yuan et al. (2022) ^[17] suggested that yield is attainable up to 80% of potential yield under irrigated or 70% of potential yield under rain-fed rice systems. Increasing yield to above 80% of the potential yield may not be feasible economically, and it may pose a threat to the environment due to the excessive input it requires ^[11]^[12].

2. Agroecological Nutrient Management

2.1. Nitrogen, Phosphorous and Potassium Fertilization

N, P, and K fertilizers are essential for increasing paddy rice yield and national rice production. However, fertilizer applications that are unbalanced and insufficient have contributed to low fertilizer efficiency and are ineffective for increasing crop yield.

Until at least early 2010, the uniform recommendation of 600 kg urea ha⁻¹ (276, N), 300 kg SP-36 ha⁻¹ (47, P), and 150 kg KCl ha⁻¹ (78, K) was still common ^[18], and this may have led to the excessive or deficient status of some or all nutrients. In early 2020, Indonesia developed a soil nutrient status map at a scale of 1:50,000. In Figure 1, researchers demonstrate only the map of high-status phosphorous (P) and potassium (K) for Indonesia.

Plant tissue analysis, also known as plant tissue testing, is recommended to complement soil tests or soil nutrient status maps. By measuring the nutrient level in the plant tissue, one can evaluate nutrient deficiency or nutrient surplus and address these imbalance problems accordingly. When farmers encounter unexplained issues with a plant, such as stunted growth or discolorization, tissue analysis can provide valuable information to diagnose these problems ^[19].

Plant tissue tests can complement soil tests. Plant tissue test results are compared against specific nutrient critical levels to guide fertilization practices aimed at adjusting plant tissue nutrient concentrations to meet these critical levels. For instance, Dobermann and Fairhurst (2000) ^[20] proposed critical levels of N, P and K for rice at 2.2%, 0.20%, and 1.4%, respectively.

In a study on nutrient sufficiency, P. Grassini ^[21] analyzed the concentrations of N, P, and K in rice plant tissues from samples collected in major paddy field production areas. The findings showed higher concentrations of N and P, but lower concentrations of K than the critical levels. This discovery emphasizes the necessity of regularly conducting soil tests in conjunction with plant tissue tests.

2.2. Micronutrient Fertilization

One of the most limiting factors in rice tillering and spikelet sterility is Zn deficiency. The critical level of Zn in soil is 0.8 milligrams (mg) kg⁻¹ using diethylenetriaminepentaacetic acid (DTPA) extraction ^[20]. In soil, Zn deficiency causes stunted plant growth and reduces crop yield ^[22]^[23]. The recommended dose for Zn fertilization is 5–10 kg Zn ha⁻¹ and can be applied in the form of zinc oxide (ZnO), zinc chloride (ZnCl), or zinc sulfate (ZnSO₄) ^[24], or by dipping rice roots into a solution of 0.05% ZnSO₄ for five minutes ^[23]. Zn deficiency can also be alleviated by recycling rice straw because 60% of Zn in plant tissues is stored in straw ^[20].

Copper plays an important role in N, protein, and hormone metabolisms; photosynthesis and respiration; pollen formation; and activating the ligninolitic enzymes ^[20]. Cu deficiency affects grain formation, causes chlorosis and a loss of turgor in young leaves, and may favor the incidence of disease outbreak ^[22]^[23]. As much as 5–10 kg Cu ha⁻¹ can be applied for a five-year period, or rice seedling roots or rice seeds can be soaked in 1% copper sulfate (CuSO₄) solution for one hour ^[20].

Plants require B for carbohydrate metabolism, sugar transport, lignification, nucleotide synthesis, respiration, germination, and seed production extraction ^[25]. It is essential for the germination of pollen grains, the growth of pollen tubes, and seed and cell wall formation, and it promotes plant maturity ^[26]. Boron deficiency reduces plant height and may lead to the failure of panicle formation ^[23] and light chlorosis, the death of growing points, and deformed leaves with areas of discoloration ^[22].

In plants, Fe promotes the formation of chlorophyll. The reactions associated with Fe include the redox reactions of chloroplasts, mitochondria, and peroxisome ^[27]. The critical limit of Fe is less than 5 mg kg⁻¹ soil (using DTPA + calcium chloride [CaCl₂], pH 7.3 extraction). This condition is generally found in alkaline soils or in soils with a very high Fe-to-P ratio ^[20]. A deficiency of Fe can be seen as chlorosis or yellowing between the veins of young leaves ^[22].

Manganese functions as a part of certain enzyme systems. It helps in chlorophyll synthesis and also increases the availability of P and Ca. It has similar properties to Mg and can substitute for Mg in some enzyme systems ^[27]. The optimum Mn concentration in plants also decreases the incidence of diseases. A deficiency of Mn is characterized by chlorosis or yellowing between the veins of new leaves ^[22].

Molybdenum is required in the smallest amount of all the essential micronutrients. It is required to form the “nitrate reductase” enzyme, which reduces NO₃⁻ to NH₄⁺ in plants and is the first step of incorporating inorganic NO into organic N compounds. Other functions of Mo are present in nitrogenase (N fixation) and nitrate reductase enzymes; it plays an important role in plant nodulation and is needed to convert inorganic phosphates to organic forms in plants ^[26]. A deficiency of Mo results in a disrupted N metabolism ^[27].

Chlorine is required for turgor regulation, electrical charge balance, resisting diseases, and photosynthesis reactions ^[26]. A deficiency of Cl may cause chlorosis and wilting in young leaves ^[22].

Silicon is beneficial to the mechanical and physiological properties of plants and helps them overcome biotic and abiotic stresses. It enhances root water uptake, which helps regulate aquaporin activity and gene expression ^[28]^[29]. In addition, Si provides resistance against pathogens and pests as well as tolerance of droughts and heavy metals, and enhances quality and yield ^[22].

Most micronutrients can be supplied by organic matter ^[30]; hence, it is good practice to utilize organic matter where available. Although irrigation water can also contain micronutrients, care must be taken with water quality, especially for nonconventional irrigation water sources, because the water may contain heavy metals and other harmful elements ^[31].

2.3. Enhanced Use of Organic Fertilizers

Organic fertilizers can be used in the form of organic matter, biofertilizers, or bioorganic fertilizers.

2.3.1. Organic Matter

Organic matter plays an important role in enhancing soil properties and crop yield. The level of organic matter in soil can be increased by adding high-quality organic matter, such as composted plant residues or barnyard manure, into rice soils. Organic matter has many beneficial functions for soil, such as increasing soil water-holding capacity, improving soil structure, releasing macro- and micronutrients, improving soil biological activities, and increasing soil carbon stock ^[32]^[33]^[34]. For example, the application of 2 t ha⁻¹ dry weight of manure would provide about 16 kg N, 14 kg P, 31 kg K, and 16 kg Ca ^[35], in addition to other macro- and micronutrients. Organic matter application can increase organic C content from 0.78% to 0.83%, as well as increase soil CEC. Meanwhile, the application of 5 t ha⁻¹ rice straw containing 9 kg N and 26 kg K increased the rice grain yield from 2.39 t ha⁻¹ to 4.14 t ha⁻¹ compared to plots without rice straw recycling ^[36].

Rice straw typically contains 0.5% to 0.8% N, 0.07% to 0.12% P, and 1.16% to 1.65% K ^[37]. If all the rice straw yield (at a national average of 5 t ha⁻¹) is recycled, this equates to recycling 25 kg ha⁻¹ to 40 kg ha⁻¹ of N, 3.5 kg ha⁻¹ to 5.9 kg ha⁻¹ of P, and 58 kg ha⁻¹ to 83 kg ha⁻¹ of K per crop season. Almost 80% of K absorbed by rice plants is stored in rice straw. Therefore, it is highly recommended to return straw to the paddy field to prevent potassium depletion and provide a substantial amount of N, as well as a decent amount of P ^[32].

However, under saturated conditions, the addition of fresh organic matter may induce methane (CH₄) emissions. Furthermore, the application of easily decomposable organic matter, such as that containing high carbohydrates, can enhance N microbial immobilization. Conversely, the addition of organic matter containing high cellobiose and cellulose (intermediately decomposable compounds) will lead to a lower rate of N immobilization. On the contrary, when the added organic matter is dominated by recalcitrant compounds, such as lignins and tannins, it does not affect N immobilization. Notably, the C-to-N ratio per se is not a determinant of N immobilization ^[38].

Before applying organic fertilizers to soil, partially decomposed straw (compost) is recommended. Straw composting can be accelerated by adding N to reduce the C-to-N ratio. The decomposition process can also be accelerated by adding molasses to the windrows of straw ^[39]. This allows decomposition to take place in situ, hence reducing the labor needed for processing. By doing so, transportation costs and associated greenhouse gas (GHG) emissions can be reduced. Combining the use of organic fertilizers and inorganic fertilizers can increase rice yield significantly compared to only organic manure application ^[40]^[41].

Ando et al. (2022) ^[41] reported that applying inorganic fertilizer (NPK) with a proper amount of slaked lime (Ca(OH)₂) and rice straw compost is the most efficient fertilizer management system in paddy soils. Further, the application of lime and the recycling of straw increased rice yield, likely due to their effect on soil fertility and plant N uptake ^[42]. Another study also reported that incorporating rice straw or crop residues enhances both soil organic carbon (SOC) content and soil health, ameliorates climate change, and also increases beneficial soil microbe activities ^[43]^[44]. Rice straw application increased soil microbial respiration in the rhizosphere because it functions as a substrate and energy source for microbes ^[44]. Applying rice straw in combination with inorganic N fertilizer (80% N through rice straw and 20% through mineral fertilizer) resulted in maximum enzymatic activity compared to only crop residue or only mineral fertilizer application ^[43].

2.3.2. Biofertilizers

Biofertilizers are microorganisms capable of increasing nutrient availability and improving soil health. The use of biofertilizers can increase soil nutrient availability and can also reduce soil pathogens ^[44]. The following sections discuss several types of biofertilizers that have been developed in Indonesia.

2.3.3. N-Fixing Biofertilizer

N-fixing biofertilizer comprises the beneficial miroorganisms that convert N₂ into NH₃ ^[45]. The process of converting N₂ into NH₃ via diazotrophic microbes allows the total N content to be replenished and the fixed N regulates crop growth and yield ^[46]. N-fixing biofertilizer has been demonstrated in free-living microorganisms with anaerobic fixation (e.g., Clostridium pasteurianum) and aerobic fixation (e.g., Azotobacter chroococcum). Other prokaryotic organisms include cyanobacteria and archaebacteria. Symbiosis in the root systems of nonleguminous plants (e.g., Frankia spp.) and leguminous plants (e.g., Rhizobium and Bradyrhizobium) and an associative fixation between nonsymbiotic microorganisms (e.g., Azospirillum spp.) growing on the root systems of nonleguminous plants, but without forming nodules, also increases N availability to plants ^[45]. N-fixing groups also include green S bacteria, firmibacteria, actinomycetes, and proteobacteria. Nitrogenase is the key enzyme that carries out the conversion of dinitrogen into NH₃ during the process of N fixation ^[45]^[46].

Biological nitrogen fixation plays an important role in restoring soil fertility and ecosystem sustainability. N-fixing bacteria can provide 50–70 kg N-urea ha⁻¹ to crops ^[47] and is important for restoring agroecological functions and reducing the yield gap in staple food production by increasing soil fertility and generating income for farmers ^[48].

A study by Razie and Anas (2008) ^[49] showed that Azotobacter and Azospirillum inoculation increased rice growth with their ability to fix N₂ from the atmosphere. The increase in the total N content of soil was followed by an increase in the total N content of the plant tissue. Azotobacter spp. and Azospirillum spp. also increase root and shoot growth in rice plants. When Azotobacter is used as a biofertilizer and is combined with 50% NPK fertilization, it significantly increases the growth and production of grain and matches the growth and production of grain with 100% NPK fertilization ^[50].

N-fixing blue-green algae (BGA), such as Nostoc sp., Anabaena sp., Tolypothrix sp., and Aulosira sp., have the potential to fix N₂ and are used in paddy fields ^[51]. BGA are photosynthetic prokaryotic microorganisms which are capable of N fixation because they contain nitrogenase. They also benefit rice plants by producing growth-promoting substances ^[52]. BGA may contribute 30–40 kg N ha⁻¹ to the ecosystem. Grain yield increased from 2000 kg ha⁻¹ to 2300 kg ha⁻¹ with algalization, and from 3000 kg ha⁻¹ to 3200 kg ha⁻¹ under the treatment of 50 kg Urea-N ha⁻¹ with algalization. In general, algal inoculation (where effective) increases yield by about 14% ^[53]. In the plants treated with BGA, N uptake was higher than those of the untreated control ^[52]. A study by Setiawati et al., 2020 ^[54], demonstrated that the application of green manure increased soil N content from 0.10% to 0.20% and organic C content from 0.8% to 2.0%.

The application of a 50% dose of NPK fertilizer, along with 7 t ha⁻¹ of Azolla and 25 t ha⁻¹ of a compound biofertilizer containing Azotobacter, Azospirillum, N-fixing bacteria, and P-solubilizing bacteria, matched the yield of full-dose NPK fertilizers. This implies that Azolla and the compound biofertilizers compensated for about 50% of N, P and K needs, relative to a full NPK dose (138 kg ha⁻¹ of N, 7.8 kg ha⁻¹ of P, and 41 kg ha⁻¹ of K) ^[55].

Study by Setiawati et al., 2020 ^[54] reported that using 10 t ha⁻¹ of goat manure and 10 t ha⁻¹ of goat manure in combination with either 10 t ha⁻¹ of Azolla or 2 t ha⁻¹ Sesbania green manure, or 5 t ha⁻¹ Azolla plus 1 t ha⁻¹ Sesbania green manure, did not show a significantly different response of rice grain yield, ranging from 4.4 to 5.8 t ha⁻¹. This implies that nutrients supplied by 10 t ha⁻¹ goat manure sufficed for the crop’s needs, and that additional green manure was not necessary with such a high-level goat manure application.

Similarly, another study ^[56] using either 10 t ha⁻¹ or 20 t ha⁻¹ fresh Azolla pinnata or powdered compost of either 2.5 or 5 of Azolla pinnata did not affect rice grain yield relative to the control treatment without Azolla application. This pot study applied blanket fertilizers equivalent to 46 kg ha⁻¹ of N, 8 kg ha⁻¹ of P, and 41 kg ha⁻¹ of K.

In general, however, when paddy fields are inoculated with BGA, rice grain yield increases by 7% to 22% ^[57]^[58]^[59]^[60].

2.3.4. Phosphate-Solubilizing Microbes

Phosphate-solubilizing microorganisms (PSMs) play a critical role in the soil’s P cycle. They achieve this by mineralizing organic P, solubilizing inorganic P minerals, and storing a significant amount of P in biomass ^[61]^[62].

PSMs, whether phosphate-solubilizing bacteria (PSB) or phosphate-solubilizing fungi (PSF), produce organic acids such as citric acid, glutamate, succinate, lactate, oxalate, glyoxylate, malate, fumarate, tartrate, and α-ketobutyrate. These acids can bind Ca, Al, and Fe, facilitating phosphate availability to plants in the form of dihydrogen phosphate (H₂PO₄⁻) ^[63].

PSMs isolated from bulk soils and rhizospheres have been shown to hydrolyze P by releasing phosphatases, playing a major role in P mineralization in most soils ^[61]. Puspitawati and Anas (2013) ^[64] succeeded in isolating PSM capable of dissolving phosphate sources such as calcium phosphate [Ca₃(PO₄)₂], aluminum phosphate (AlPO₄), and ferric phosphate (FePO₄), increasing the solubility of P from 65% to 135%. The application of PSM to paddy soils significantly increased rice plant growth and P uptake, and reduced chemical P fertilizer usage by up to 50%.

2.3.5. K-Solubilizing Microbes

One way to increase the solubility of K from K-containing minerals or rocks involves the use of K-solubilizing microbes (KSMs), including K-solubilizing bacteria or K-solubilizing fungi. Various groups of rhizobacteria and fungi are involved with the solubilization of K minerals in the soil system ^[65]. A consortium of rhizobacteria, including Bacillus edaphicus, Bacillus mucilaginosus, Bacillus circulans, Acidothiobacillus ferrooxidans, Paenibacillus sp., Pseudomonas, and Burkholderia demonstrates effective K-solubilizing abilities ^[65]. Aspergillus terreus also exhibits a high capacity to dissolve minerals and rocks containing K. These microbes are ubiquitous, and their presence depends on structure, texture, organic matter, and related soil properties ^[65]^[66].

Overall, biofertilizers that have been well developed in paddy fields include free-living or symbiotic microbes which help N fixation and PSMs. Among the various biofertilizers, N-fixing microbes (i.e., BGA) are some of the most commonly used and can increase grain yield by 200–450 kg ha⁻¹ ^[53]. Scaling up their application is an important strategy for increasing rice yield.

2.3.6. Bioorganic Fertilizers

Bioorganic fertilizers are organic fertilizers that are improved by adding beneficial microbes such as N₂ fixers, PSMs, KSMs, and antagonistic microbes. Bioorganic fertilizers are used to reduce chemical fertilizer use, improve soil properties, and reduce environmental pollution.

The application of rice straw enriched with Azotobacter was able to reduce the use of NPK fertilizers by 25%. The use of organic straw fertilizer enriched with Azotobacter without artificial fertilizers increased soil NO₃⁻ and NH₄⁺. The use of rice straw enriched with Azotobacter and a 100% dose of NPK fertilizers was able to increase the rice grain harvest compared to the application of N, P, and K without organic fertilizer ^[51]. Bioorganic fertilizers reduced inorganic fertilizer use by about 25–30% and increased rice yield by about 25% ^[67]. The application of 50% NPK and bioorganic fertilizer increased 2.35 t ha⁻¹ grain yield and 3.39 t ha⁻¹ straw yield compared to the application of 50% NPK without bioorganic fertilizer. This yield increase was attributed to the supply of P and K from organic fertilizers and the increased availability of these nutrients due to the solubilizing microbes.

2.4. Other Locally Exploitable Nutrient Sources

Besides chemical and organic fertilizers, there are many other sources of nutrients, including soil ameliorants such as agricultural lime, biochar, and crushed natural minerals, also called stonemeal.

Lime, especially dolomitic lime, has been widely used in Indonesia. Dolomitic lime can improve the microbial activity of acidic soils and increase crop yields ^[68]^[69]. Besides providing Ca and Mg, dolomitic lime also increases soil nutrient availability, such as N, and P, in acid soils ^[69]^[70]^[71].

Liming and applying straw increased soil N availability and the activities of soil enzymes involved in both C and N cycling and, hence, rice yield ^[42]. The application of dolomitic limestone increased K, Ca, Mg, and Mn content in the leaves ^[72]. Additionally, calcite or dolomite lime can be used as a soil amendment on rain-fed paddy fields with a low pH or low Ca and Mg contents ^[73]. Lime application also can reduce cadmium (Cd) and lead (Pb) accumulation in rice ^[74]^[75]^[76]^[77]^[78].

Biochar is a char produced from organic matter via thermal processing under depleted O₂ concentrations (pyrolysis) ^[79]^[80]. Applying biochar in combination with cattle manure increased soil organic C; CEC; available P; exchangeable K, Ca, and Mg; and nutrient uptake by crops ^[81]. Biochar also can be used as a soil amendment in acidic soils to improve soil P and reduce exchangeable Al and soluble Fe, as well as to significantly increase rice biomass ^[82] and yield ^[83].

Biochar improves soil’s physical, chemical, and biological properties. Biochar improves soil’s chemical properties by increasing nutrient retention and availability. Biochar decreases soil bulk density and increases saturated hydraulic conductivity. Incorporating biochar into soil increases the microbial population in the microsphere because the biochar surface changes soil conditions and makes it favorable for microbial activity in the soil ^[84]^[85].

Rice husk biochar (RHB) contains 1.12 mg kg⁻¹ of N, 0.98 mg kg⁻¹ of P, 184 mg kg⁻¹ of Mg, 168 mg kg⁻¹ of Si, 225 mg kg⁻¹ of Ca, and 176 mg kg⁻¹ of K. Therefore, its application to the soil improves the soil’s nutrient status. The application of 10 t ha⁻¹ RHB significantly increases rice grain yield to 4.6 t ha⁻¹ compared to 2.6 t ha⁻¹ under the control treatment without biochar. Both the RHB and control plots received 25 kg ha⁻¹ of K, 26 kg ha⁻¹ of P, and 150 kg ha⁻¹ of N ^[86]. Another study using rice straw biochar resulted in 20–22% higher rice grain yield ^[87].

Stonemeal (i.e., ground stone containing high amounts of K and other macro- and micronutrients) has not been widely exploited in Indonesia. Considering the high price of conventional K fertilizer (KCl), the use of stonemeal seems promising. Stonemeal can rejuvenate nutrient-poor soils because some types of stonemeal are rich in K, P, Ca, Mg, and various micronutrients ^[88]^[89].

Indonesia possesses K-bearing minerals, such as K-feldspar and leucite (an aluminosilicate mineral), that contain most essential nutrients, including 4–20% of K ^[71]. Various techniques such as mechanochemistry, leaching, alkali fusion, and bioleaching can process these minerals into K fertilizers ^[90]. In Pati Regency, Central Java, Indonesia, K-bearing rocks are prevalent in alkaline and subalkaline formations characterized by low silica content and high alkalinity. These formations, including leucite, plagioclase, pyroxene, and opaque minerals, exhibit a potassium oxide (K₂O) content between 1.94% and 8.61% ^[91]. This aligns with a prior study in Pati Regency noting K₂O content between 1.92% and 8.79% in leucite, augite, pyroxene, quartz, and sanidine rocks ^[92].

It will be necessary to conduct a survey of basic cation-rich (especially K) minerals, followed by field testing and an economic feasibility assessment of stonemeal. If this study proves successful, its potential application extends not only to paddy rice but also to other agricultural sectors, such as oil palm and other intensively managed crops.

2.5. Improved Water Management Systems

Sustainable intensification to increase food security will require proper water management in addition to the use of high-yielding varieties and agronomic practices ^[93]^[94]. To increase water availability and mitigate the effects of drought in agricultural areas, irrigation management will need to be improved, as will the management of proper planting time ^[95]^[96]^[97]^[98]. Water is responsible for increased agricultural production, so it is crucially important to improve irrigation and water use efficiency ^[99].

The System of Rice Intensification (SRI) is one water use efficiency strategy ^[100]^[101]^[102]. The SRI emerged from on-farm experimentation in Madagascar, where existing norms of paddy rice were radically amended by reducing planting density, improving soil with organic matter, reducing the application of water, and the early transplantation of rice seedlings. Organic fertilizers are used in addition to chemicals. The SRI can increase N and P availability and soil microbial C activity ^[103]^[104]. The SRI method has a positive influence on rice root development, optimizing nutrient absorption in the soil ^[105].

The SRI improves plant physiological processes and characteristics, including longer panicles, more grains per panicle, a higher proportion of grain filling, deeper and better distributed root systems, and a higher number and larger leaves ^[106]^[107], as well as improved water-use efficiency ^[107]^[108].

However, despite the voluminous research data showing the dramatic advantages of the SRI, its adoption at farmer level is sluggish because the method requires significant additional labor input and specific management techniques that are challenging for smallholders ^[109]. Some studies have also reported no significant increase in paddy yields under the SRI compared to conventional rice management systems ^[110]. Incentives for SRI implementation are lacking; SRI rice does not command a premium price and it requires greater water regulation efforts than conventional systems ^[96]^[111]. Controversies persist in the literature regarding the associated benefits of grain yield and the SRI’s reduced water use ^[112]^[113]^[114].

2.6. Farming Systems and Crop Rotation

In an agroecological nutrient management system for paddy rice, diverse crop rotations can benefit soil health, nutrient cycling, and overall sustainability. In irrigated paddy rice systems in Indonesia, there are typically two to three crops per year. The third crop usually consists of secondary crops such as maize, soybean, peanut, or various vegetable crops ^[115]. This third crop improves nutrient utilization, cuts off pest and disease pressure, and increases overall yield and diversity. For instance, rotating lowland (flooded) rice with upland (oxic) maize improves root colonization by Archaea and bacteria ^[116].

Azolla, a water fern, can be used as green manure in paddy fields. Its symbiotic association with the cyanobacterium Anabaena azollae enables it to fix nitrogen from the atmosphere. Azolla is either incorporated into the soil before rice transplantation or grown as a dual crop along with rice ^[117].

The ‘Minapadi’, or rice–fish farming system, can be an effective rice farming system for increasing profitability if up to 4 t ha⁻¹ of compost is added ^[118]. Rice–duck culture is another example of an integrated farming system in paddy rice ^[119]. Ducks feed on weeds, dead leaves and pests (such as plant hoppers and leafhoppers) in the field, during which process they stir up the water and soil and fertilize the field so that soil nutrients are increased and the need for the application of fertilizers and pesticides is lowered ^[120]. Furthermore, rice–fish–duck is a promising ingenious system worth testing ^[121].

The key to success in these farming systems is to tailor crop rotations based on local agroecological conditions, crop compatibility, and the specific needs of the rice paddy system. Diversified rotations help maintain soil fertility, reduce pest and disease pressure, and improve overall resilience in paddy rice agriculture.

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