Rice is one of the world’s most consumed cereal crop. It is a staple food for many Asian, African, Latin American and Caribbean countries; with approximately 90% of the world’s production and consumption of rice coming from Asia (1). Rice is usually classified according to its growing environment (Lowland and upland rice grown in lowlands and uplands) or according to its water source (rain fed and irrigated rice). Organic rice farmers use organic manure and cropping systems such as rice-legume intercropping or crop rotation to improve or maintain soil fertility. This article will discuss some of these methods used in rice production.
Some types of organic manure used in rice cropping are green manure, farm yard manure and organic compost.
Green manure/cover cropping: Here leguminous crops are grown as cover crops and then slashed and left on the soil surface to decompose and increase the soil organic matter. An additional benefit to this system is that the legume crops fix nitrogen in the soil as they grow. A common green manure used in rice production is Azolla or duckweed fern. Azolla is a freshwater fern which forms a symbiotic (mutually beneficial) relationship with blue – green algae called Anabaena azollae. Anabaena azollae lives inside Azolla and has the remarkable ability to capture atmospheric nitrogen and convert it to ammonia which is taken up by Azolla. Nitrogen fixed in Azolla becomes available to rice when Azolla is decomposed. Azolla is used extensively in China and many other Asian countries in paddy rice production. Sesbania is also a green manure used in rice production. It enriches the soils nitrogen content through its nitrogen fixing ability. Azolla and Sesbania when incorporated into the soil release nitrogen gradually; 70% of their nitrogen is made available within 20 days (5).
Farm yard manure: Farm yard manure can be a mixture of animal dung, urine, straw and litter used as bedding stock. The richness of this manure depends on some factors such as (a) the nutritional value of food given to the animal; feeds rich in protein and minerals produce rich dung (b) the quantity of straw used; a high straw content ensures slow decomposition of manure (6). Farm yard manure increases the organic carbon levels of the soil (8).
Compost: Compost is made up of decomposed organic materials. Organic materials can be anything from crop residues to animal droppings or dung. Rice residue compost is used in organic rice production. This compost consists of rice straw and husk (by products from milling), legume crop residues and animal waste. These components are piled up in layers to make a heap. The heap is kept moist and either left to stand or stirred once in two weeks to facilitate decomposition.
Important points to note about organic manure
• Compared to inorganic/artificial fertilizers organic manure often have a lower concentration of major plant nutrients (nitrogen and phosphorus). However, they have the advantage of enriching the soils with micro nutrients, enzymes and micro-organisms often not present in artificial fertilizers (7).
• Organic manure if applied inappropriately could cause environmental pollution. Application of manure at a time when crop growth is slow and uptake of nutrient is low can result in leaching (washing downwards the soil profile) of nitrate and phosphorus into ground water. Furthermore, application of manure on anaerobic soils can lead to nitrogen loss to the atmosphere through denitrification (reduction of nitrate to nitrous-oxide and nitrogen gas).
• Nitrogen present in organic manure is in its organic form and not readily available to the crop. It becomes available only after undergoing the process of mineralization, where decomposition by micro-organisms transforms organic nitrogen to its inorganic form (ammonium nitrate).
Rice-legume cropping: Rice-legume intercropping is a common practice in upland rice production and especially among growers in developing countries. The motivation for this practice is usually that growers get the benefit of yields, from more than one crop at the end of the growing season. Additionally, as this involves intercropping with legumes the grower also enjoys the benefits of increased soil fertility and quality.
1. The roots and other crop residues of legumes when decomposed by micro organisms help improve the soil organic matter content therefore improving the soil carbon content and soil quality for the next growing season.
2. Leguminous crops (as previously mentioned in this article) have nitrogen fixing ability. Legumes intercropped with rice have the ability to fix between 100-200 kg of nitrogen per hectare from the atmosphere within a year (2, 9). Common leguminous crops intercropped with rice include groundnut, mung bean, soybean black gram, pigeonpea, and cowpea.
3. Finally, scientific research has shown some evidence of nitrogen transfer from legume crops to non-legume crops in the root zone (4). Here Legumes release soluble nitrogen compounds which can be taken up by non-leguminous crops.
Bibliography and further reading
1. Facts and Figures on Food and Biodiversity-https://www.idrc.ca/en/article/facts-figures-food-and-biodiversity.
2. Matusso et al (2012) Potential role of cereal-legume intercropping systems in integrated soil fertility management in smallholder farming systems of sub-Saharan Africa. https://www.ku.ac.ke/schools/environmental.
3. Gowda et al (2001) Legumes in rice-based cropping systems in tropical Asia constraints and opportunities. https://pdf.usaid.gov/pdf_docs/PNACM398.pdf.
4. Thilakarathna et al (2016) Belowground nitrogen transfer from legumes to non-legumes under managed herbaceous cropping systems. Agronomy for Sustainable Development 36:58.
5. Ventura et al (1987) Azolla and sesbania as biofertilizers for lowland rice. Philipines Journal of Crop Science 12:61-69.
6. Lockhart JAR and Wiseman AJL (1988) Introduction to crop husbandry including grassland 6th edition. Pergamon Press, Oxford. Pp 63.
7. Cover crops-https://www.knowledgebank.irri.org.
8. Chettri et al (2003) Effects of farmyard manure, fertilizers and green manuring in rice-wheat systems in Bhutan: Results from a long-term experiment. Experimental Agriculture. 39: 129-144.
9. Toomsan et al (1995) Nitrogen fixation by groundnut and soyabean and residual nitrogen benefits to rice in farmers’ fields in Northeast Thailand. Plant and soil 175: 45-46.