The following points highlight the seven nutrients required for rice production and which were considered as the major plant nutrients for enhancing the crop productivity. The nutrients are: 1. Nitrogen 2. Phosphorus 3. Potassium 4. Sulphur 5. Zinc 6. Iron 7. Boron.
Nutrient # 1. Nitrogen:
Nitrogen, the most important nutrient for rice, is universally limiting the rice productivity. It seems that majority of Indica varieties are adapted to relatively low levels of nitrogen in the region of 25 kg N ha-1. Higher quantities of nitrogen usually result in lodging and consequent yield loss. On the other hand, Japonica varieties respond to heavy nitrogen application up to 120 kg N ha-1 or even higher.
Tanaka (1958) from his studies on rice mineral nutrition concluded that the nitrogen metabolism of Indica rices becomes disturbed at the reproductive stage due to excess nitrogen and they are not able to utilise nitrogen efficiently for grain production. Japonicas utilises nitrogen efficiently for grain production under high levels of nitrogen and gives greater yield.
Forms of nitrogen:
Rice plant depends mainly for its nitrogen upon the decomposition of organic matter under anaerobic conditions and in the early stages of growth takes up nitrogen in the form of ammonia (NH4+-N), the stable form of nitrogen in submerged soils.
At early growth stages and up to 200 ppm N, rice grows better with ammonia than with nitrogen. Many experiments could be quoted to prove that application of nitrogen as nitrates in the early stages of growth is without effect or is even deleterious to the plant.
This may be because nitrates are easily leached and lost to plants, while the deleterious effect may be due to conversion of nitrate to nitrite. After panicle initiation and at 100 ppm N, nitrate is better source for rice than ammonia. At 20 ppm N, however, ammonia is as good as nitrate. Therefore, at realistic levels of nitrogen concentration in soil solution, ammonia appears to be better than or as good as nitrate throughout the life cycle of rice plant.
The utilisation by rice of ammonia and nitrate nitrogen is affected by the reactions of the medium but plants supplied with ammonia make better growth at all reactions than those supplied with nitrate. Rice absorbs ammonia in preference to nitrate from a solution that contains both.
Nitrogen Losses:
When ammonia or ammonia forming nitrogen fertiliers are applied on soil surface they undergo several changes. As an example, when urea is applied on soil surface with a pH 7.0 or above, substantial part of applied nitrogen could be lost due to ammonia volatilization. About 8.5 per cent of applied urea-N loss has been reported as lost due to ammonia volatilization.
Others reported higher losses ranging from 20 to 50 per cent, especially during seedling stage of rice. Due to land submergence and consequent reduced soil condition, several species of bacteria use nitrate as a source of oxygen (O2) and reduce it to N2 or N2O through the process of denitrification. Denitrification losses are severe under alternate flooding and drying.
Of the various solid nitrogen fertilisers, urea is most susceptible to runoff losses when it is surface broadcast, since it is highly soluble chargless organic molecule, which cannot be retained by soil particles. Fertilisers containing NH4-N even when applied to moist soil surface get adsorbed on the exchange complex, thus minimising the nitrogen losses. Appreciable amount of nitrogen will be lost by leaching in upland rice, while such loss will be around one per cent under lowland conditions.
There are two stages in the growth of rice crop when nitrogen is most needed: early vegetative and panicle initiation stages. Fertilising the crop during early vegetative growth promotes tillering leading to higher yield. Application at panicle initiation or early booting stage will help the plant produce more and heavier grains per panicle. Panicle initiation occurs approximately 55 days before expected maturity date.
Generally, rice plants, regardless of variety, mature in about 30 days after flowering. Panicle initiation begins approximately 25 days before flowering. Normally, these three stages: panicle initiation-flowering-maturity are more or less constant in duration regardless of time of planting. The panicle initiation stage, thus, may be determined approximately by adding the duration of these three stages.
Efficiency of Fertiliser Nitrogen:
Fertiliser use efficiency is the output of any crop per unit of the nutrient applied under a specified set of soil and climatic conditions. According to Barker (1977), fertiliser efficiency is increase in yield per unit of fertiliser nutrient applied.
Agronomists usually express the efficiency of fertiliser nitrogen in kg of rough rice produced kg-1 of applied nitrogen. Physiologists, on the other hand, define the efficiency of nitrogen utilisation in kg of rough rice produced kg-1 of nitrogen absorbed. These two efficiencies can be related by introducing a third parameter, percentage of nitrogen recovery.
Efficiency of fertiliser nitrogen (kg rice kg-1 N applied) = Percentage of nitrogen recovery (kg absorbed N kg-1 applied N) x Efficiency of utilisation (kg rice (kg rice kg-1 N absorbed)
The percentage nitrogen recovery varies with soil properties, methods, amounts and timing of fertiliser application and other management practices. It usually ranges from 30 to 50 per cent in the tropics. The efficiency of utilisation for grain production in the tropics is about 50 kg rough rice kg-1 nitrogen absorbed and this appears to be constant regardless of the yields achieved.
In temperate regions, the efficiency is about 20 per cent higher than in tropics. Using values for the recovery percentage and utilisation efficiency obtained for tropics, the efficiency of fertiliser nitrogen can be calculated.
Efficiency of fertiliser nitrogen = (0.3 ~ 0.5) x 50
= 15 ~ 25 kg rice kg-1 applied N.
Improving Nitrogen use Efficiency:
Fertiliser use in Asia now accounts for nearly half of the world’s consumption of fertiliser and more than two-third of this is applied to cereals, mostly rice. Agronomic evidence consistently shows that nitrogen use efficiency (NUE) in rice production is low and increasing the same could be a major avenue for improving the yield and increasing system sustainability. The NUE of applied fertilisers ranges from 20-40 per cent depending on soil and other fertiliser management practices. Several implementable practices have been formulated for improving the NUE.
Split application:
Efficiency of fertilisers will be low if the entire recommended rate is applied, especially as basal dose, due to losses of nitrogen. Efficiency due to split applications is well established. Application of nitrogen should be according to the needs of crop at different stages of its growth. Two split applications are recommended for short and medium duration varieties, while three appears to be ideal for long duration varieties.
More than three splits are recommended for sandy soils because of excessive loss of nitrogen through leaching. The ideal stages of application, based on the needs of the crop, are tillering (around 20 days after planting) and at panicle initiation.
Deep placement:
Transformation of ammonia fertilisers when applied on soil surface (oxidised zone) and consequent losses indicate necessity for its deep placement in the root zone (reduced zone) for efficient use. When applied basally, it should be incorporated into the reduced zone during final land preparation. When applied as top dressing, it should be placed 3-4 cm deep in the soil.
Nitrification inhibitors:
There are chemicals, which can retard the process of nitrifican and reduce losses by leaching and denitrification. Widely tested chemicals include N-Serve (2-chloro 6 (trichloromethyl) pyridine), AM (2-amino-4 chloro 6 methyl pyrimidine), DCD (dicyadiamide) and ST (sulphathiazole).
Increase in NUF and consequent improvement in the productivity of rice has been reported. In addition, nitrification inhibiting properties of neem (Azadirachta indica) seed extract and neem cake have been developed for neem cake coated urea.
Slow release nitrogen fertilisers:
These fertilisers release nitrogen slowly thereby allowing its uptake before it is lost from the soil. There are two types: coated conventional fertilisers such as sulphur coated urea (SCU), polymer coated urea (PCU) and neem cake coated urea (NCU) and chemicals with inherent slow nitrogen release properties such as IBDU (isobutylidene diurea), urea form etc. These are effective in increasing the NUE. However, their high cost inhibits their wider user under field conditions.
Urea super granules (USG):
USG of different shapes and sizes have been tested for their efficiency under field conditions. They can increase the NUE by 20-30 per cent in lowland rice. Problems in their placement, in the absence of low cost equipment, presently limit their use under field conditions. INM and balanced fertiliser use are equally effective in improving nitrogen use efficiency.
Response of Rice to Nitrogen:
Integrated nutrient management and balanced fertiliser use are equally effective in improving the nitrogen use efficiency.
Investigations in India, as a result of long series of experiments, have come to the conclusion that rice in all circumstances responds to the application of nitrogen. Several factors influence response of rice to applied nitrogen. High yielding varieties are more responsive to applied nitrogen than traditional varieties.
Response to nitrogen is higher during dry season than monsoon season due to greater number of sunny days during the period from flowering to maturity. Grain yield per unit area increases with decrease in spacing up to certain extent, after which there is no change or a decrease depending on the variety.
Optimum spacing at a low nitrogen level is closer than that at higher levels. Response to nitrogen is more pronounced at wide spacing than at close spacing and is more prominent is dry season than in rainy season. Low responsive varieties have a wider optimum spacing than high responsive varieties.
Upland rice receives little or no fertiliser because of the risk involved in input investment. Traditional varieties are unresponsive to nitrogen, which tend to increase dry matter production without increasing grain production. However, many experiments with improved varieties have shown response to increased nitrogen supply. In general, application of 40-60 kg N ha-1 in three splits is optimum both for rainfed upland and rainfed lowland situations during monsoon season.
Irrigated rice responds to higher levels of nitrogen since drought or deep flooding are not the problems either during monsoon season or dry season. Response to nitrogen, in general, varies from 40-60 kg ha-1 in fertile soils of delta areas to 80-100 kg ha-1 in light soils of low fertility during kharif.
In dry season, optimum rate of application, in general, is 100 and 120 kg N ha-1 for short duration varieties and medium and long duration varieties, respectively. In delta areas, option rate is around 80-100 kg ha-1. There are instances of latest medium and long duration varieties responding up to 200 kg ha-1 or even higher in the recent past.
As such there is need for revising fertiliser recommendations based on the location specific responses. On equal nitrogen basis all the fertilisers are equally effective in increasing the yield, provided losses from applied fertilisers are minimised.
Nutrient # 2. Phosphorus:
Phosphorus content in moist Indian soils is within 200-800 ppm P and the range of organic P as per cent of total P is 10-30 per cent. Deficiency of phosphorus occurs widely in soils of low or high pH: acid soils, acid sulphate soils, calcareous soils and alkali soils.
Inorganic Phosphorus:
Inorganic P in soils is bound to Ca, Al and Fe. The other inorganic forms are occluded and reductant soluble P. The Ca-P contributes to 40-50 per cent or even more of total P in much neutral, alkaline and calcareous soil, while Fe-P and Al-P, generally, constitute 8-10 per cent of the total P in soils.
Reductant soluble and occluded forms can be twice as dominant in acid red and laterite soils than on neutral alkaline alluvial soils or black soils. The Ca-P is more concentrated on coarser soil particles with less surface activity and hence lesser availability to crop. On the other hand, Fe-P and Al-P are associated with colloidal fraction having high surface activity and hence more availability to plants.
Phosphorus in Soil Solution:
Depending on soil type, land submergence increases the concentration of P in soil solution from 0.05 ppm to about 0.6 ppm and subsequently decreases the same.
Increase of P solubility in submerged soils has been attributed to:
a. Reduction of ferric phosphate to ferrous phosphate.
b. Hydrolysis of Al and Fe phosphate at higher soil pH.
c. Dissolution of the appetite because of higher CO2 pressure in soil solution.
d. Disorption of P from clay and oxides of Fe and Al.
e. Release of occluded-P by reduction of hydrated ferric oxide coating.
f. Displacement of P from ferric and Al-P by organic anions.
Subsequent decrease of P in soil solution may be due to resorption of P on clay or aluminum hydroxide and an increase in pH. The ionic species of P in soil solution depend on pH. Within a pH range of 4-8, the principal species are H2PO4– and HPO42-. At pH 7.2, H2PO4– and HPO4 2- are present in equal proportions. At pH 5.0, however, H2PO4– is the dominant species and HPO42- is almost absent.
Deficiency of phosphorus can be diagnosed by means of the following symptoms:
a. Leaves of the plant turn dark green and sometimes purple green.
b. In extreme cases, the plant begins to turn yellow from lower leaves and finally dies away.
c. Plant height may be nearly the same as that of normal one but the tiller number is much smaller than normal.
d. Silica content of leaves increases and consequently the plant becomes erect.
e. Heading date is usually later than normal.
Improving Phosphorus use Efficiency:
The partial productive efficiency of phosphorus for grain is higher at early growth stage than at later stages, since it is needed for root and tiller development. If sufficient phosphorus is absorbed at early growth stage, it can be easily redistributed to growing organs later.
Method of application:
As indicated above, phosphorous requirement is high in the early stages. As such, the general practice is to apply phosphate fertilisers during final puddling/ploughing. Dipping roots of rice seedlings in phosphate slurry has also been recommended.
Time of application:
The water-soluble P fertilisers (superphosphate, DAP) should be, generally, applied as basal dose before sowing/transplanting. If not, they can be top dressed within 30 days after planting. For acid soils, ground rock phosphate should be applied 3-4 weeks prior to planting.
Sources of phosphorus:
All the phosphatic fertilisers used in India, except nitro- phosphates contain almost all of their phosphorus in water soluble form. In lowland rice soil, there is relatively little significant difference among phosphatic sources, except acid or extremely alkaline soils as evident from field experiments in different parts of the world. Superphosphate seems to be a good source in all types of soils except those extremely acidic. On these acid soils, rock phosphate is better than superphosphate.
As compared with single superphosphate, the efficiency of rock phosphate on acid soils is reported to be 65-135 per cent. Rock phosphate should be incorporated into the soil 3-4 weeks before planting for better efficiency. Particle size of rock phosphate is important for its efficiency. Smaller the particle sizes, greater the efficiency. Mixing with compost, incubating with phosphate solubilising bacteria and mixing with pyrites increases its efficiency.
Response of Rice to Phosphorus:
Many investigations around the world report that lowland rice fails to respond to phosphatic fertilisers even though upland rice grown on the same soil show positive response, since flooding usually increases the availability of soil phosphorus. Application of 30-40 kg P2O5 ha-1 is the general recommendation for rainfed rice. For irrigated rice, the recommended rate is 40-60 kg P2O5 ha-1 depending on the rate of nitrogen application.
Nutrient # 3. Potassium:
The requirement of rice crop for potassium is much greater than for either nitrogen or phosphorus. Over 80 per cent of the absorbed potassium by the plant is found in straw. Need for potassium is most likely to occur on sandy soils.
Sources and Time of Application:
Land submergence reduces ferric and manganus forms of iron and manganese leading to increase in their concentration in soil solution and these ions exchange with potassium on the exchangeable complex and release it in soil solution.
Hence, the availability of native potassium increases in rice soils on flooding. Potassium chloride is the principal fertilisers source to rice. Potassium sulphate may be used in areas of sulphur deficiency. Sodium can substitute for potassium to a limited extent.
The partial productive efficiency of potassium for grain is high at early growth stages, decline and becomes high again at later stages. Since rice require large quantities of potassium, its availability to crop is necessary up to heading. Potassium should be applied during final land preparation. There are sporadic instances of response of rice to split applications especially on soils of low CEC with excessive drainage.
Deficiency of potassium can be visualized from the following symptoms:
1. Reddish brown spots appear along the veins of leaves and this phenomenon proceeds from lower leaves to upper leaves.
2. Deficiency is frequently accompanied by helmenthosporium leaf spots from tips of the leaves.
3. Borders of the leaves turn yellow and begin to die.
4. Plant is shorter and leaf colour is greener than normal and tillers number may be a little smaller than that for normal plant.
Potassium deficiency is often associated with iron toxicity, which is common on acid and acid sulphate soils. Its deficiency also occurs on poorly drained soils.
Response of Rice to Potassium:
Generally, response of rice to added potassium is not marked as for nitrogen and phosphorus. Most rice soils in Asia do not need potassium as much as N or P and only small and variable increase in yield is obtained with added potassium fertiliser. Rice soils in high rainfall areas that are deficient in potassium may respond to addition of potassium fertilisers.
In India, highest response to added potassium (about 1.5 t ha-1) has been obtained with 60 kg K2O ha-1 on sandy soils. As of now, application of potassium fertilisers is on insurance or balancing principle in several instances.
Location specific fertiliser schedules have been formulated for different states in the country. Recommended fertiliser schedules for different agroclimatic zones in Andhra Pradesh are given in Table 1.16. Higher recommended rates for Rayalaseema are under well irrigation.
Nutrient # 4. Sulphur:
Sulphate is reduced to sulphide in flooded soils leading to decline in soil sulphate concentration. Thus, the availability of soil sulphur decreases as soil reduction proceeds.
Symptoms of sulphur deficiency in plants are similar to that of nitrogen but can be differentiated:
1. In most crops, the younger leaves turn yellow due to sulphur deficiency as against older leaves turning yellow due to nitrogen deficiency.
2. Its deficiency cannot be corrected even with heavy nitrogen application.
3. The sulphur level in plant tissue is lower while nitrogen levels are higher.
The critical sulphur content in straw for maximum dry weight varies from 0.16 per cent at tillering to 0.07 per cent at flowering and to 0.06 per cent at maturity. The critical N: S ratio for maximum dry weigh also varies from 23 at tillering to 13 at maturity.
Though, sulphur deficiency seldom encountered in acute form, it has been observed in some areas and a moderate deficiency may possibly be more wide spread than is commonly thought, since it is added to soil through irrigation water, atmosphere and precipitation in appreciable quantities. A little less than 2.0 kg ha-1 is required to produce 1.0 t of rice. Soils deficient in sulphur respond to applications ranging from 20 to 50 kg S ha-1 depending on the levels of deficiencies.
Nutrient # 5. Zinc:
Nene (1966) in India first reported Zn deficiency in lowland rice. Since then it has been recognised as a widespread nutritional problem throughout the world. Next to nitrogen and phosphorus efficiency, zinc deficiency now ranks first among the nutritional disorders.
Deficiency of zinc in lowland rice occurs in near neutral to alkaline soils, particularly colcareous soils. Availability of both soil and applied zinc is much higher in upland soils than in submerged soils. Soil submergence causes substantial decrease in zinc concentration in the soil solution. Rice crop removes 30-40 g Zn t-1 of grain.
Deficiency and critical limits of zinc:
Deficiency in lowland rice usually appears within 2-3 weeks after sowing or transplanting.
Later, the crop may recover spontaneously:
1. Midribs of the younger leaves, especially at the base become chlorotic.
2. Brown blotches and streaks appear on lower leaves followed by stunted growth. If the deficiency is severe, entire leaf becomes rusty.
3. Size of leaf blade is reduced but that of leaf sheath is little affected.
4. Inspection of the Zn deficient rice field shows brown, rusty and stunted plants in patches.
A possible solution to zinc deficiency is to drain the field, however, at the cost benefits of land submergence. The plant available Zn in Indian soils as extracted with DTP solution ranges between 0.08 and 20.5 ppm and 0.6 ppm is considered critical level. The critical level of zinc deficiency in plant is about 15 ppm.
Higher concentration (more than 80 ppm) often leads to zinc toxicity. Rice varieties differ in their susceptibility to Zn deficiency. Jaya, Basmati, Sita, Maduri and IR 62 are most susceptible, while HM 484, UPR 238 and TNAU 801790 are least susceptible.
Source and response of rice to zinc:
Zinc chloride (45%), zinc oxide (70%), zinc sulphate (21%) and Zn EDTA (12%).
Zinc is, generally, applied to soil at sowing or transplanting. When the deficiency is noted in standing crop, 0.5 per cent Zn SO4 spray is recommended. However, on zinc deficient soils, basal application is preferred to foliar application to avoid damage to the crop by the time deficiency symptoms are noticed.
Dipping rice roots in 2-4 per cent ZnO suspension has been found to be very effective. In general, it is customary to apply 25 kg Zn SO4 ha-1 at the time of land preparation for each crop or 50 kg ha-1 once in an year if two rice crops are grown.
Nutrient # 6. Iron:
Iron deficiency is wide spread in calcareous high pH soils, compact soils with restricted drainage and sandy soils with low organic carbon. It is more frequent on upland than on lowland rice. With the same colcareous soil, upland rice may suffer severe iron deficiency, while lowland rice may grow normally. Toxicity of iron is likely to occur on acid sandy and acid sulphate soils besides organic soils such as peaty soils.
Iron deficiency and critical limits:
It is relatively easy to diagnose the iron deficiency.
1. Old leaves stay green and new leaves turn yellow.
2. Chlorosis occurs first between the veins on the leaf and then the veins themselves turn yellow and consequently the hole leaf suffer from chlorosis.
3. If the deficiency is severe, emerging leaves become pale yellow and if the deficiency is not corrected they become white and the plant often die.
4. If ferric salt is sprayed on leaves, the sprayed spots recover and turn green.
The concentration of ferrous iron in soil solution increases sharply after submergence. After four weeks of submergence, strongly acid soils build up in the soil solution concentrations as high as 300 ppm Fe2+ and then show an exponential decrease. In general, the concentration of iron in soil solution is controlled by soil pH, organic matter content, iron content itself and duration of land submergence.
Source and response to iron:
Rice crop removes around 160 g iron t-1 of grain. Iron chlorosis can be better controlled by foliar spray of 0.5 per cent Fe SO4. Sometimes, as many as six sprays may be necessary. The common sources of iron are Fe SO4 (19% Fe) Fe DTPA and Pyrite (FeS2) containing 20-23 per cent iron.
Among the iron carriers tested for rice, Fe SO4 proved superior to pyrite. Addition of FYM or H2SO4 recorded highest yield and iron uptake in calcareous Inceptisols, whereas 2.5 t ha-1 iron pyrite was superior in improving rice yields in Vertisols.
Nutrient # 7. Boron:
Deficiency of born is noticed in Tarai and Teesta alluvial, red and laterite and Alfisols of West Bengal, Orissa, Madhya Pradesh and in calcareous, alluvial and red yellow catenary soils of Bihar. Available boron in Indian soils ranges from traces to 8 ppm. However, saline-alkali soils of Punjab may contain as high as 70 ppm.
Boron deficiency and critical limits:
Like iron and zinc, boron is relatively immobile in plants. The deficiency symptoms appear first on growing parts of the plant.
1. Tips of the emerging leaves become white and rolled as in the case of calcium deficiency.
2. White necrotic streaks appear on the youngest and next leaf which may coalesce to form large white irregular patch.
3. The growing point may die in a severe case but new tillers continue to be produced.
The critical level of boron deficiency in rice is 18-20 ppm, while the toxicity level is 400 ppm. Rice crop removes 6.6 g boron t-1 of dry matte. Boron toxicity symptoms included chlorosis at the tips of older leaves, especially along margins, followed by appearance of large dark brown spots, which ultimately turn brown and dry up. Toxicity usually appears six weeks after planting.
Sources and response to boron:
Important sources of boron are borax (11%) boric acid (17%), boron frites (2-6%), sodium tetraborate (14%) and sodium tetraborate anhydrous (20%). Foliar application is usually recommended. Borax 0.2 per cent solution, with lime, twice the weight of borax is recommended to avoid phototoxicity. In general, borax and sodium tetraborate are used for soil application, while boric acid is used for foliar spray.