Fertilizer International 495 Mar-Apr 2020
31 March 2020
Rice fertilization
CROP NUTRITION
Rice fertilization
Rice is one of the world’s most popular food staples, providing around one-fifth of the total global calorific value of human diets. Cultivation and consumption is particularly prevalent in Asian countries. We look at the nutrient needs of this widely-grown cereal.
Rice (Oryza sativa) is the primary food staple for more than half of the world’s population. More than 3.5 billion people depend on rice for more than 20 percent of their calorific intake – with Asia, South America and sub-Saharan Africa being the largest rice-consuming regions globally.
Rice can be grown in a wide range of latitudes and under many different soil, climate, and hydrological conditions. But it is mainly grown in the humid and sub-humid climates of tropical and sub-tropical regions.
World production
World production of milled rice more than tripled between 1960/61 and 2017/18, increasing from 151 million tonnes to 495 million tonnes. (The equivalent rough rice tonnages rose similarly, from 221 million tonnes to 740 million tonnes.) These increases have been delivered largely through yield improvements – the global average rice yield having more than doubled from just 1.8 t/a in 1960/61 to 4.6 t/ha by 2017/18 – while world harvested area increased by just over one-third over this period, from 120 million hectares to 163 million hectares. Global production and yields have continued to grow modestly over the last decade (Figure 1).
RICE: Key facts
- Rice is the world’s second largest staple crop behind corn (maize).
- It is grown and produced on a vast scale, with a total of 740 million tonnes (495 million tonnes milled) harvested in 2017/18 from an area of 163 million hectares.
- Four major types of rice are grown worldwide.
- Indica, the most common type – being responsible for 75 percent of global production – is grown in the tropics and sub-tropics of India, Central and Southern China, and the Philippines.
- Aromatic rice (Jasmine and Basmati) accounts for 15 percent of global production and is mainly grown in Northwest India and Pakistan.
- Japonica (8% global production) originates from Northern and Eastern China and is grown extensively in cooler subtropical and temperate zones.
- Glutinous rice (<2% global production) is grown mainly in Southeast and East Asia.
- Rice production underwent significant growth during the middle 20th century as part of the ‘Green Revolution’ in Asian agriculture.
- The decades between the 1930s and the late 1960s saw major advances in agricultural inputs and practices – such as the introduction of improved, high-yielding, disease-resistant seeds, more extensive irrigation, wider fertilizer and pesticide use, and the shift to mechanisation.
RICE MARKET MONITOR
Global rice production at 512 million tonnes was largely unchanged in 2019, according to the latest assessment from the UN FAO. This production volume is down just half of one percent from 2018’s all-time record high. On the consumption side, world rice utilisation for 2019/20 looks likely to rise by one percent year-on-year to a record high of 514 million tonnes. This is despite a 1.3 million tonne downward revision in March.2
World rice stocks at the close of 2019/20 were recently revised upwards to 182 million tonnes, marginally below their record opening levels. Lower closing stocks in Thailand have been outweighed by higher than anticipated reserves in India. Record-breaking public sector carry-ins and local procurement suggests India will account for much of the foreseen rise in export inventories in 2019/202 .
World rice trade this year (January-December 2020) is forecast to recover by 3.6 percent year-on-year to reach 46 million tonnes. Higher exports from India and mainland China underpin much of the predicted recovery2 .
Five countries – China, India, Indonesia, Bangladesh and Vietnam – account for more than 70 percent of global production and consumption (Figure 2). Of these, China and India alone are responsible for more than half of global rice production and consumption. Rice provides up to half of dietary calories for hundreds of millions of people in Asia – and is therefore vital for food security. Rice has also emerged as the fastest growing food staple in sub-Saharan Africa1
Cultivation practices
Rice is unique among cereal crops because its root system is adapted to flooded and largely anaerobic soil conditions. Rice crops are grown in four main environments:
- Irrigated systems
- Rainfed lowland areas
- Rainfed upland areas
- Flood-prone deepwater environments.
Irrigated rice is grown in bunded fields known as paddies. These are surrounded by a small embankment that keeps the water in. This productive growing practice delivers 75 percent of world rice production while covering some 54 percent of the world’s harvested area for rice1 .
Cultivation in rainfed lowland areas provides a further 20 percent of the global rice harvest, with rice grown in rainfed upland areas contributing an additional four percent to the world total. Deepwater rice is grown in flooded conditions in which the water depth is more than half a metre for at least one month of the year. Although it contributes just one percent to the global rice harvest, more than 100 million people in South and Southeast Asia rely on deepwater rice for their sustenance1 .
Fertilizers account for 20-25 percent of total production costs in lowland rice systems. Therefore, increasing rice yields by adopting suitable nutrient management practices has become an essential part of modern rice cultivation.
Plant growth and yield
The traditional method for cultivating rice is in paddy fields. These are flooding to a depth of 5-25 cm during or after the placement of young seedlings. This simple method, although requiring the channelling and damming of water, reduces the growth of weeds and deters rodents and pests.
Rice plants grow a main stem and four or five side stems known as tillers. Primary tillers grow from the lowermost nodes of the transplanted seedlings with further secondary and tertiary tillers emerging from higher nodes as the stem grows. Each tiller grows a flowering head known as a panicle. It is the panicle that ultimately produces harvested rice grains3 .
Rice seedlings grow rapidly, taking 4-5 months to reach maturity with plants eventually growing to a height of 90 cm. By late summer, rice grains begin to appear in long panicles on the top of the plant. By the end of summer, these grain heads are mature and ready to be harvested3 .
Fertilizer application rates and timings are critical to successful rice cultivation. High-yielding modern rice cultivars are highly dependent on the supply of essential nutrients in adequate amounts4 .
Rice typically requires 14.7 kg N, 2.6 kg P and 14.5 kg potassium per tonne of grain yield. Nitrogen and potassium supply is particularly critical when panicles start to emerge – typically about 60 days before harvest for tropical rice. Insufficient nitrogen at this growth stage reduces the number of spikelets per panicle, resulting in a loss of yield. Inadequate K supply at this stage also negatively influences yield by affecting both grain filling and the number of panicle spikelets3 .
The global average yield of irrigated rice is 5 t/ha, although yield averages vary widely – nationally, regionally, and seasonally. Skilled rice farmers in the tropics can achieve rice yields of 7-8 t/ha in the dry season and 5-6 t/ha in the wet season. The productivity of rainfed upland and deepwater rice is much lower at around 1 t/ha4 .
This article mainly focusses on rice fertilization in irrigated and rainfed lowland systems – as these account for about 92 percent of total rice production and 80 percent of the global harvested area.
The cultivation of rice under flooded soil conditions affects nutrient availability, uptake, use efficiency and fertilization practices. Average N, P, and K nutrient use efficiency for rice cultivation in five key growing countries was estimated at just 33 percent, 24 percent, and 38 percent, respectively. These use efficiencies were based on field trials with 179 farmers in China, India, Vietnam, Indonesia and the Philippines. Nevertheless, the understanding of nutrient management in lowland rice has progressed rapidly in recent decades, although further research is required4 .
Rice deficiency symptoms
Deficiency symptoms vary according to the mobility of the nutrient within the plant. For mobile nutrients (nitrogen, phosphorus, potassium and magnesium), deficiency symptoms appear in oldest (lower) leaves first. This is because mobile nutrients tend to migrate to the youngest leaves which act as sinks. In contrast, deficiency symptoms for immobile nutrients (calcium, iron, manganese, zinc and sulphur) appear in youngest (upper) leaves first because these nutrients are locked within the oldest leaves and parts of the plant3 .
Nitrogen for yield capacity
Nitrogen is generally the most yield limiting nutrient in rice production4 as it is associated with:
- Plant height
- Panicle numbers
- Leaf size
- Spikelet numbers
- The number of filled spikelets – this largely determines the yield capacity of the rice plant.
Modern high yielding rice varieties, which typically produce around 5 t/ha of grain, can remove about 110 kg of nitrogen from the soil4 . Prolonged nitrogen deficiency causes severe plant stunting, reduces tillering and depresses yield3 .
Nitrogen is applied to rice in split applications. The number of applications and overall application rate can be adjusted to meet nitrogen demand. This is generally estimated using leaf colour charts3 .
In water-seeded rice, where the soil is flooded for periods during the growing season, most of the nitrogen fertilizer is generally applied pre-planting and prior to flooding. At this stage, around 65-100 percent of the total nitrogen application is typically applied, as an ammonium (NH4+ ) fertilizer source, usually when rice plants are at the 4-5-leaf growth stage.
Nitrogen fertilizers placed on dry soil need to be flooded-in immediately, or shallow incorporated and then flooded within 3-5 day. Incorporation will protect against ammonia volatilisation and nitrification/denitrification losses as long as the flood is maintained. Soils need to remain flooded for at least three weeks to maximise the uptake of early applied nitrogen3 .
Sufficient nitrogen should be applied at the pre-planting stage to ensure no additional nitrogen is needed by rice plants until the panicles start to emerge. Additional nitrogen should be topdressed at this stage, or when nitrogen deficiency symptoms appear3 .
The application of nitrogen to irrigated lowland rice can be very inefficient, however. Nitrogen use efficiency generally varies between 20-80 percent with an average of about 30-40 percent4 .
To improve fertilizer efficiency, the International Rice Research Institute (IRRI) has developed a site-specific nutrient management (SSNM) approach for rice fertilization. IRRI suggest that rice crops requires about 50 kg/ha of nitrogen fertilizer for each tonne of additional grain yield. Using this relationship, optimum nitrogen application rates can be calculated using target rice yields of 5.5 t/ha in the dry season and 6.5 t/ha in the wet season. SSNM recommends applying nitrogen as three split applications, with an early application of about 20-30 percent of the total requirement. The remaining 70-80 percent is split between two subsequent applications, based on demands of the rice crop, as determined from leaf colour using leaf colour charts3 .
Phosphorus for roots, flowering and ripening
Phosphorus supply is critical for maximising rice grain yields. It is particularly important during early vegetative growth stages because of its role in tillering, root development, early flowering, and ripening. Early application is essential for root elongation3 .
Phosphorus-deficient plants are stunted, have abnormal bluish green foliage, erect leaves, relatively few tillers and poor root mass. Development of the canopy is also slowed and plant maturity delayed. Plant tissue testing is the best tool for diagnosing deficiency3 .
Above-ground phosphorus uptake by high-yielding rice varieties may approach 60 kg/ha, but more commonly lies in the range 25-50 kg/ha. Some 60-75 percent of total plant phosphorus resides in the panicles at maturity4 .
Phosphorus fertilizers are typically soil-applied when the land is being prepared pre-planting or pre-flooding. Recommendations are usually made on the basis of soil tests levels and yield expectations. For flooded rice in Asia, phosphorus is normally applied at a rate of 26 kg/ha to maximise yields, while in the United States between 10-40 kg/ha is applied. At the extreme end, soils with high P-fixing capacities may require applications as high as 97-175 kg/ha4 .
Highly water-soluble single and triple-super phosphates (SSP/TSP), diammonium phosphate (DAP), and sometimes monoammonium phosphate (MAP), are the phosphate fertilizers most commonly applied to rice4 .
Potassium for grain number and weight
Potassium improves root growth and plant vigour, helps prevent lodging, and enhances crop resistance to pests and diseases.
In high yielding rice systems, potassium is often the most limiting nutrient after nitrogen4 as it promotes:
- Tillering l Panicle development
- Spikelet fertility
- Uptake of nitrogen and phosphorus
- Leaf area and leaf longevity
- Disease and pest resistance
- Root elongation and thickness
- Stem thickness and strength
- Resistance to lodging.
Potassium plays a particularly valuable role in improving grain number and grain weight due to its influence on photosynthesis and other plant functions3 .
Modern high-yielding rice varieties take-up potassium in larger amounts than any other major nutrient. In Asia, for rice crops yielding 5 t/ha, total potassium uptake is around 100 kg/ha – although this is concentrated in the straw at maturity, not the harvested crop. Total plant potassium uptake may even exceed 200 kg/ha for rice yields greater than 8 t/ha3 .
Potassium recommendations are usually based on soil test results. An application of 50 kg/ha is normally applied to maximise flooded rice yields. Straw needs to be factored in as an important additional nutrient source when calculating K requirements, given that 80 percent of potassium remains in the straw after harvest4 .
Applying potassium after deficiency symptoms appear can be relatively ineffective, as these generally deliver only limited yield improvements3 .
Major deficiencies and their correction
Inadequate nutrient supply is a major yield-limiting factor in rice cultivation. Phosphorus, zinc or iron deficiencies – or the presence of excess of salts like iron or aluminium – affect rice yields over about 50 million hectares of rice land in Asia4 .
Micronutrient deficiencies are mainly associated with silty and sandy loams and other high pH soils (>7.5), but do not generally occur in acid and slightly-acid clay soils (pH = 5-6.53 ).
Zinc deficiency is associated with low organic matter soils with high levels of available phosphorus. Waterlogged soils are also particularly susceptible. Deficiency in rice after transplanting is a widespread phenomenon limiting productivity in lowland growing conditions. Rice yield losses due to Zn deficiency can range from 10-60 percent. Inadequate soil zinc levels limit tillering in rice and, consequently, the number of panicles4 .
Broadcasting zinc sulphate (10-25 kg/ha of ZnSO4·H 2 O or 20-40 kg/ha of ZnSO4·7H 2 O) over the soil surface is recommended when deficiency symptoms are observed. Foliar sprays of zinc sulphate solution (200L of 0.5% solution per hectare) are an effective emergency treatment for Zn deficiency in growing plants4
“Fertilizer applications and timings are critical for successful rice cultivation.”
Sulphur deficiency is also widespread in many rice-growing regions, including India, Brazil and Southeast Asia, affecting both the number of panicles and panicle length. Sulphur applied at a rate of least 10 g/ha is reportedly necessary, with ammonium sulphate and single superphosphate being good sources. Up to 20-40 kg/ha of sulphur can be applied on severely deficient soils4 .
Gypsum is also widely-applied as both a sulphur (sulphate) and calcium source. However, calcium deficiency in rice usually signals unfavourable growing conditions, rather than inadequate supply to the roots. Deficiencies can develop due to waterlogging, soil salinity and root disease, for example, or because of excess potassium or ammonium supply. Deficiencies are relatively rare in irrigated rice systems but are common in leached acid soils in both upland and lowland areas3 .
Calcium plays an important role in cell wall strength and the functioning of cell membranes. Deficiencies occur in the youngest leaves and growing points.
Boron deficiency typically causes floret sterility, resulting in reduced grain yields in rice. The panicles in boron-deficient rice plants can also fail to emerge from the boot. Deficiency can be corrected by applying soluble boron sources – such as borax – typically supplying B at a rate of 0.5-3 kg/ha. Borax can be broadcast and incorporated before planting, top-dressed, or applied as a foliar spray4 .
Iron deficiency commonly occurs in rainfed dry nurseries, or when rice is grown under upland conditions. Rice seedlings are most susceptible before flooding. Iron deficiency is best treated by applying iron sulphate (FeSO4 supplying Fe at a rate of 30 kg/ha) next to rice rows. Iron sulphate can also be broadcast alongside crop residues, green manures, or animal manures. Foliar applications (solution of 2-3% FeSO4 or Fe chelates) can also cure deficiencies4 .
Manganese deficiency is observed in:
- Upland rice
- Alkaline and calcareous soils with low organic matter status
- Degraded paddy soils high in Fe
- Acid uplands (oxisols, ultisols)
- Leached acid sulphate soils
- Leached sandy soils
- Excessively limed acid soils.
Deficiency can be corrected by foliar applications (MnSO4 solution) – or by applying manganese (5-20 kg/ha as sulphate or oxide) in bands along rice rows with an acidifying starter fertilizer such as ammonium sulphate4 .
References
