Nutrient management






General nutrient management for N, P and K







Understand how much nutrients (NPK) the crop needs to produce a ton of grain per hectare and translate that amount to what you have to apply on the field.




What are the most common nutrient deficiencies and why?





N, P and K are the most needed nutrients in rice


Nitrogen (N) promotes rapid plant growth and improves grain yield and grain quality. N is generally the most limiting nutrient as it is needed in large amounts and is readily lost as a gas (volatilized) and by leaching (washing out of the root zone).


Nitrogen deficiency is the most common nutrient problem. The plants give a lower yield due to limited photosynthesis and assimilation.



Phosphorus (P) is essential for root development, tillering, early flowering, and ripening. It is especially deficient in sandy soils with low organic matter contents, in very acid soils and in alkaline soils.





P deficient plants are stunted, tillering is reduced, the leaves often have a blackish green color, and maturity is delayed.



Potassium (K) is essential for root growth and plant vigor. It also helps against lodging and improves the crop resistance to diseases. Potassium can quickly become yield limiting in high yielding rice systems where most of the straw is removed because rice straw is rich in K!


K deficient plants are stunted dark green plants with yellowish brown leaf margins older leaves can have necrotic tips and margins, and lodging is more frequent.



Zinc and sometimes sulfur deficiencies are much less frequent but can be important locally.


Nitrogen, phosphorous and potassium make the rice plant strong and healthy with many tillers which result in a high yield.




The different benefits of N, P and K to the rice plant




How much nutrients does the crop need?


In the previous lessons we explained that the crop needs nutrients to grow and mature normally.

The rice crop needs the following uptake of N, P and K to produce 1 ton of grain per hectare:


  • Nitrogen (N):15-20 kg


  • Phosphorous (P): 2-3 kg


  • Potassium (K): 15-20 kg (If all straw remains in the field and is evenly distributed, K requirements can be reduced to 3-5 kg/ha per ton of grain yield.




So, how much fertilizer do you need to apply?


In every field, you will get a certain grain yield even if you donít apply any fertilizer. The nutrients for this yield are supplied by the soil. If you want to increase the yield, it is necessary to apply fertilizers.


However, it is very important to understand that of all the fertilizer you apply on the field, the crop can only take up a fraction.


For nitrogen, for example, even with good management about half the applied fertilizer (50%) is lost. We said above that the plants need to take up 15 to 20 kg of nitrogen per hectare to produce 1 ton of grain. This means we have to apply twice as much fertilizer, or 30 to 40 kg of N per hectare, to produce an extra ton of grain!


So, imagine:











  • This example uses 15kg as needed N uptake. The needed N crop uptake ranges from 15 to 20 kg N per ton of grain/hectare.

  • These calculations assume nitrogen is the major factor limiting yields.






What do the different numbers of NPK on bags mean?


Vietnamese example of

a bag containing NPK


When you buy a bag of fertilizer there is always an indication of the percentage of N, P and K in that bag. These numbers tell you how many kilograms of N, P2O5 and K2O a 100 kg bag contains. If you calculate which nutrients you need for your field you can then buy the right bag of fertilizer.



How much kg N, P and K do the following bags contain per bag of 100 kg?


Check your answer by clicking here

15 kg of N


15 kg of P2O5

15 kg of  K2O

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46 kg of N


0 kg of P2O5

0 kg of  K2O



Note: Remember these numbers are always on the basis of 100 kg (in %). If you buy bags with a different weight than 100kg, you need to make some extra calculations. (Example: For bags of 50kg, divide the NPK numbers by 2 to get the actual amount in kg. For bags of 25 kg, divide the NPK numbers by 4.)






When and how should you apply N, P and K?


On most soils, yields can be increased by applying fertilizer because the soil does not supply enough nutrients. In most cases, Nitrogen (N), Phosphorus (P) and Potassium (K) need to be applied to the crop to get higher yields.


N management: The crop needs N during the whole vegetative phase. But because N gets easily lost (volatilization and leaching) not all the N should be applied in one application. Therefore, N is applied in 2 or 3 portions (splits), before or short after crop establishment, at early tillering, and panicle initiation. Before establishment, N is incorporated into the soil (basal), later it is applied on top of the soil (top dressed).


P management: The crop needs P especially early in the crop development. Apply all P either basal or top dress all P short after establishment (usually not later than 2-3 weeks after seedling emergence or transplanting).


K management: The crop needs K mostly early in the crop development. Apply all K basal or if the rate is very high (more than 100 kg K/ha) apply half as basal and half together with the second N application.





More information on how, when, and how much N, P and K should be applied follows.






Next lesson


In the next two lessons we will discuss nutrient management in more detail for irrigated rice and rainfed rice.