Nitrogen Excess

Effect on plants

• causes excessive growth

• plants become more attractive to insects and diseases

• reduces stem strength

Signs

• plants look overly green

• may be healthy, but also may be lodged

• may have thin stems

• may have increased disease or insects

• plants in patchy pattern across the field

Importance/Occurrence

• negative implications on the environment

• decrease farm profits

• it is used where fertilizers are relatively cheap and where farmers don’t understand the amount of nitrogen required relative to their yield goals

Symptoms

• Plants look overly green

• May be healthy, but also may be lodged at maturity (especially in direct-seeded rice)

• May have thin stems

• May have increased disease (e.g., bacterial leaf blight, sheath blight, blast) or insects (leaffolder)

• Patchy pattern resulting from uneven application across the field

Confirmation

Check the field and/or ask farmer about the rate of N applied.

Problems with similar symptoms

P deficiency will produce dark green leaves that may be confused with excessive N application; however P deficient plants produce less tillers and have stunted growth.

Why and where it occurs

Excess nitrogen is typically used where fertilizers are relatively cheap and where farmers don’t understand the amount of nitrogen required relative to their yield goals and the right time of N application.

Mechanism of damage

Excessive nitrogen causes “luxuriant” growth, resulting in the plant being attractive to insects and/or diseases/pathogens. The excessive growth can also reduce stem strength resulting in lodging during flowering and grain filling.

When  damage is important

Damage can be important if it results in lodging during heading or grain fill or if the attack from diseases or insects is increased at vegetative phase. The excessive use of N also has negative implications for the environment and lowers farm profits.

Economic importance

Excess N does not tend to be a widespread problem. Where it does occur, it can increase pest and disease problems, requiring higher pesticide use to control them. Pesticide-related health risks are also high. If crops lodge, harvest cost increases and grain quality is poor. If excess N moves to the environment then the indirect costs can be quite high.

Management principles

Farmers should apply sufficient N to meet the plants needs. On average this equates to around 20 kg N for each t of grain produced. The farmer needs to know how much N is coming from the soil and other sources (e.g., water or bacteria in the soil or water) and then apply the additional N to meet the yield goal. The Leaf color chart is a simple tool ensuring that sufficient but not excessive N is applied. It helps farmers to determine the right time of N application based on crop need and soil N supply.

Selected references

1. Balasubramanian V, Morales AC, Cruz RT, Abdul Rachman S. 1999. On-farm adaptation of knowledge-intensive nitrogen management technologies for rice systems. Nut. Cycl. Agroecosyst. 53:59-69.
    

2. FFTC (Food and fertilizer technology center). 1994. Fertilizer use and sustainable food production. FFTC for the Asian and Pacific region, 14 (June 1994):4-5.
    

3. IRRI-CREMNET. 2000. Progress report for 1998 and 1999. IRRI, Los Baños, Philippines.  

Contributors

V Balasubramanian and M Bell

Related Links

Leaf Color Chart