One of the central themes in plant nutrition is to find the particular growth stage when supply of a particular nutrient leads to highest yield per unit amount of the absorbed.
Partial productive efficiency of nitrogen for grain production is defined as:
Partial productive efficiency of nitrogen absorbed at period (n-1) to (n) = [Y(n) – Y(n-1)]/[N(n) – N(n-1)]
where,
Y(n) = grain weight of the treatment corresponding to period (n)
Y(n-1) = grain weight of the treatment corresponding to period (n-1)
N(n) = total nitrogen uptake of the treatment corresponding to period (n)
N(n-1) = total nitrogen uptake of the treatment corresponding to period (n-1).
In other words, partial productive efficiency is the amount of grain production per unit of nitrogen absorbed at a particular growth stage. The greater the value of partial productive efficiency, the higher the efficiency of nitrogen absorbed to produce grain. Solution culture technique provides a convenient means to study partial productive efficiency since, the nutrient supply can be controlled easily.
From the classical work by Kimura and Chiba (1943), the following important conclusions were drawn:
1. Nitrogen absorbed at early stages is used to produce more straw than grain.
2. Nitrogen absorbed at later stages is used to produce more gain than straw.
3. Partial productive efficiency for both grain and straw is higher when the nitrogen supply is lower.
4. There are two peaks for partial productive efficiency for grain: the first around 20-25 days after planting when the nitrogen concentration in the culture solution is high and the second around twenty to ten days before heading when the nitrogen concentration is moderate.
5. When the nitrogen concentration is high, there is no second peak. Thus, the most efficient time to supply nitrogen for grain production varies with the level of nitrogen supply.
If very limited nitrogen should be applied at about 20 days before heading, when the supply is moderate, nitrogen may be given twice-around 20 days after planting and again at about 20 days before heading. When nitrogen is abundant, application at early growth stage is relatively more efficient for grain production.
The capacity of soil to hold applied nitrogen is an important consideration in determining the efficiency of basal versus split application of nitrogen fertilisers. Soils with montmorillonite clays have higher nitrogen retaining capacity than those with kaolinite or allophone.
For soils with low nitrogen holding capacity, split application of fertiliser should result in high nitrogen recovery and hence higher yield than a basal application. On the other hand, split applications may not be better than a basal application in soils when the applied ammonia is held by clay.
Observation of changes in leaf colour and shape is believed to be the important means for diagnosing the nutritional conditions of rice plant.
Symptoms of the deficiency of nitrogen are indicated below:
1. Younger leaves become small in size and turn pole green starting from the tip.
2. Older leaves first turn yellow, then brown and finally dry up. Yellowing order always proceeds from below to above without exception.
3. Plant growth as a whole becomes very poor, height is short and tiller number is small.
4. Culms and leaves look stiff and leaves are thin, narrow and erect.