Nutritional summary of rice

Nitrogen

Nitrogen affects all parameters contributing to yield (e.g. spikelet number per panicle, percentage of filled spikelets, grain protein content) and, thus, a sufficient N supply is necessary for achieving high yields. At the same time, N deficiency is the most commonly detected nutrient deficiency symptom in rice. 

Nitrogen is an essential constituent of amino acids, nucleic acids and chlorophyll. Deficient plants are stunted and older leaves or the whole plant is yellowish green.

In general, N shows the highest effect on yield increase compared to other nutrients. 

Selection of appropriate N rate is important for attaining the max. economic yield and minimizing the impact on the environment.

Rice N requirements depend on expected yield levels, which in turn are related to soil and climate (esp. solar radiation) and the supply of other nutrients.

Nitrogen affects all parameters contributing to yield (e.g. spikelet number per panicle, percentage of filled spikelets, grain protein content:

In flooded rice soil NH4+ is the main available N-form. However, both N forms are taken up efficiently by the rice plant. Researchers show that adding NO3- to nutrient solution increased N growth and N acquisition considerably. NO3-N should not be applied as base dressing as it would be subjected to high denitrification losses. In contrast, fully established rice takes up NO3-N very efficiently before large parts of the applied NO3-N would be subjected to denitrification.

General N recommendation 

• Apply about 15-20 kg N/t grain yield target (less in rainy season than in dry season). 
• Divide N fertilizer recommendations into 2-3 (wet-season) or 3-4 (dry-season) split applications. 
• Fertile soils (>3.4 t/ha grain yield without N fertilizer) often do not require basal N at sowing of transplanting. 
• Hybrid rice always requires basal N. 
• The greatest N requirement is between midtillering and panicle initiation. 
• In contrast to P and K fertilizers, the residual effect of N fertilizer is small.

Phosphorus

Its major functions are in energy storage and transfer and maintenance of membrane integrity. P is mobile within the plant and promotes tillering, root development and early flowering. It is particularly important in early growth stages (e.g. for tillering). P fertilizer is required when the rice plant‘s root system is not yet fully developed and the native soil P supply is small.

Deficiency symptoms: 

• Stunted dark green plants with erect leaves and reduced tillering. 
• Stems are thin and spindly and plant development is retarded. 
• Young leaves appear to be healthy but older leaves turn brown and die. 
• Moderate P deficiency is difficult to recognize in the field.

Causes of P deficiency: 

• Low indigineous soil P-supplying power. (e.g. acid sulfate soils in which large amounts of active Al and Fe result in the formation of insoluble P compounds at low pH.) 
• Insufficient application of mineral P fertilizer. 
• P deficiency is more likely in direct-seeded rice where plant density is high and root systems are shallow.

General P recommendation 

To maintain yields of 5-7 t/ha and replenish P removed with grain and straw, fertilizer P rates should be in the range of 15-30 kg P/ha (35-70 kg P2O5/ha). 

Foliar P applied at heading is an efficient measure to boost yields on low P soils.

Potassium

K has among others functions in osmoregulation, enzyme activation, regulation of transpiration by stomata and the transport of assimilates. K provides strength to plant cell walls as it is involved in the lignification of sclerenchyma tissues. K has no pronounced effect on tillering but increases the number of spikelets per panicle, percentage of filled grains and 1000-grain-weight.

Deficiency symptoms 

• Dark green plants with yellowish brown leaf margins or dark brown necrotic spots first appear on the tips of older leaves (K is very mobile within the plant and is re-translocated). 
• Other symptoms may be: greater incidence of lodging, increased incidence of diseases, early leaf senescence, leaf wilting, and leaf rolling, particularly under conditions of high temperature and low humidity. Large percentage of sterile or unfilled spikelets. 

General K recommendation: 

To maintain yields of 5-7 t/ha and replenish K removed with grain and straw, fertilizer K rates range between 20-100 kg K/ha (24-120 K2O/ha). 

Hybrid rice always requires larger applications of K (60-120 kg K2O/ha on most soils) than inbred modern varieties.

Calcium

Ca is among others important for the build-up and functioning of cell wall membranes and the strength of cell walls.

Deficiency symptoms:

• Ca is not re-translocated to new growth; thus, symptoms usually appear first on young leaves.
• Chlorotic-necrotic split or rolled tips of younger leaves.
• Ca increases resistance to diseases such as bacterial leaf blight.

Ca deficiency is common in acid, strongly leached, low CEC soils. Ca is probably an important factor associated with poor plant growth on acid sulfate soils (Rorison, 1973). It was observed that Ca and Mg in plants grown on acid sulfate soils were highly correlated to dry matter production (Attanandana, 1982). 

General Ca recommendation 

• Returning straw to the field helps to maintain plant available Ca. 
• Application of YaraLiva Tropicote (CaNO3) at a later growth stage increases Ca plant availability.

Sulfur

S is a constituent of the amino acids cysteine, cysteine and methionine. Sulfur is contained in the plant hormones thiamine and biotin (which are involved in carbohydrate metabolism).

Deficiency symptoms: 

• Pale green plants, light green colored young leaves. 
• The tips of young leaves may become chlorotic. 
• In contrast to N deficiency, leaves are a paler yellow in S-deficient plants. 
• S deficiency symptoms are similar to those of N deficiency. Plant tissue analysis can be important to identify S deficiency! 
• S deficiency symptoms need to be detected early in order to prevent a decrease in yield. 
• Critical levels for S deficiency: Between tillering and flowering, either a concentration <0.1% S in the shoot or a N:S ratio of >15 indicate S deficiency (Blair et al. 1980, In: Dobermann et al. 1998)

Management 

• Incorporate straw instead of removing or burning it. 
• Sulfate-S is immediately available, whereas elemental S must undergo oxidation before becoming available. 
• Because S is required for tiller production, it must be applied early, preferably at sowing, like P. Delaying application until maximum tillering can reduce yield by about one-third. 
• Apply 10 kg S/ha with moderate S deficiency and 20-40 kg S/ha on soils with severe S deficiency.

Zinc

Zinc is essential for several biochemical processes such as auxin metabolism, chlorophyll production, enzyme activation, cytochrome and nucleotide synthesis, maintenance of membrane integrity. 

Zn deficiency is the most widespread micronutrient disorder in rice. Its occurrence has increased with the introduction of modern varieties, crop intensification, multiple cropping, use of high analysis (zinc-free) fertilizers, increased Zn removal. Zinc deficiency has been reported in lowland rice in Brazil, in India and in the Philippines (Fageria et al. 2011).

Deficiency symptoms 

• Dusty brown spots on upper leaves of stunted plants appearing 2-4 weeks after transplanting, with uneven plant growth. 
• Under severe Zn deficiency, tillering decreases or may even stop. 
• Typical Zn rice tissue levels are between 25 and 100 ppm with deficiency symptoms appearing when levels fall below 20 ppm (In: Naik and Das, 2007).

General Zn recommendation: 

• Soil surface application is more effective than incorporation into soil because Zn stays longer plant available. 
• Zn sulfate is the most commonly used source. 
• Recommendation vary between 2 - 6 kg Zn/ha (Dobermann and Fairhurst, 2000) and 5 - 10 kg Zn/ha (Fageria, 2011). 
• Foliar spray has the advantage that less Zn becomes plant unavailable in the soil. 
• Apply 0.5 - 1L YaraVita Zintrac 700/ha as foliar spray at active tillering and panicle initiation. 
• Zinc seed treatment has the advantage that small amounts of Zn to rice seeds are more economical and convenient than soil or foliar applications.

The grain yield was significantly improved by the use of Zn seed treatment compared with the control and was equal to the yield from the standard recommendation in the US of 11 kg Zn ha-1 PPI. (Slaton et al. 2001).

Boron

Boron has a primary role in cell wall biosynthesis, and structure and plasma integrity. It is required for carbohydrate metabolism, sugar transport, lignification, nucleotide synthesis and respiration.

Deficiency symptoms: 

• Reduced plant height 
• Tips of emerging leaves are white and rolled (similar to Ca deficiency symptoms) 
• If plants are B deficient during reproductive stage, panicle and spikelet formation, grain set and grain filling are depressed. (Lordkaew et al. 2013) 

Boron (B) is not re-translocated, i.e. all boron found in the grain has to be available during the reproductive stage.

For a rapid treatment of B deficiency, apply 0.5–3 kg B ha-1broadcast before planting, topdressed, or as foliar spray. 

High leaching/percolation should be avoided since B is very mobile in flooded soils and can move below the root zone.