Nitrogen Fertilizers

Misuse of Nitrogen Fertilizers - Case Study

There are many types of nitrogen fertilizers, each is characterized by different nitrogen forms and as a result, have a different effect on plants.

In this case study we demonstrate how the wrong choice of nitrogen fertilizers can have detrimental effect on crop.

Although this particular case deals with strawberries, the same conclusions apply to most crops. 

Field Characteristics

Main crop is strawberries, grown in clay-loam soils. The water retention capacity of the soil is high and soil pH is 8.5.

Temperatures at the field are high: 35-42º C (95-107.6 ºF). 

Fertilization and irrigation regimens

Nitrogen fertilizers application - A base application DAP (Di-ammonium phosphate) was applied, followed by applications of urea.

Irrigation - Drip irrigation. Water applied in relatively small amounts (20-30 m3/hectare) given at short intervals (every 2 days). 

Observations in the Field

• Over 30% plant losses. 

• Throughout the field, plants are stunted, too small and not vigorous.

• Root system is not developed and shows symptoms of black root rot, i.e. roots are black, and have no root hairs.

• Plants show symptoms of severe iron deficiency.

Discussion

Urea Applications

Urea is a NH2 based nitrogen fertilizer. Urea undergoes hydrolysis in soil:

(NH₂)₂CO + 2H₂O  ------>   (NH₄)2CO₃  

(NH₄)2CO₃ + 2H⁺  ------->   2NH₄⁺  + CO₂ +  H₂O

Under conditions of high soil pH, high temperatures and high soil moisture, a large portion of the ammonium nitrogen (NH₄⁺), that is formed, converts into ammonia (NH₃):

NH₄ + OH ---> NH₃ + H₂O

Ammonia is a gas which tends to volatilize. High concentration of ammonia is toxic to roots, especially of young plants. As a result of the field conditions described above, a high percentage of the nitrogen that was applied in its urea form was lost to the atmosphere, while ammonia concentration at the root zone reached toxic levels.

In addition, in the hydrolysis process of Urea, bicarbonate (HCO₃^-) is formed, increasing soil pH locally. In young plants, which have a shallow root system, this might increase the chance of iron deficiencies.

Applications of Ammonium Nitrogen

Both DAP and urea are sources of nitrogen in the ammonium form - NH₄⁺  (urea converts into ammonium). The metabolism of NH4⁺ in plant requires oxygen , but as temperature increases, oxygen becomes less soluble in soil solution and its concentration is decreased.

Research has shown that in root temperature of 32º C, application of 100% NH₄⁺, causes strawberry plants to gradually deteriorate until their complete death. The damage is related to oxygen shortage in root cells

 

Irrigation Regimen 

The high water retention capacity of the soil, combined with the short irrigation interval, also reduced the oxygen concentration in the soil solution, resulting in serious damage to the strawberry plants.

DAP as a High Phosphorus Fertilizer 

DAP contains 48% P₂O₅. Applying a high concentration of phosphorus to a shallow layer of soil, can result in an immediate precipitation of phosphorous with iron.  This makes iron unavailable to plants and can explain the sudden iron deficiency symptoms. 

Conclusions  

Under the local conditions, the choice of nitrogen fertilizers, as described above, had proved to be very problematic and might be the reason for plant death. 

Basing the plant nutrition on NH₄⁺ nitrogen and urea explains the damage to the root system and the undeveloped plants.

In addition, the irrigation regimen applied to that type of soil, kept root system under constant low-oxygen stress, enhancing the damage caused by the inappropriate choice of fertilization regimen.

The same fertilization under different field conditions may have better results.