Which type of damage do you expect urea should do to soil? Urea will be transformed to ammonium in the soil and is then taken up by plants.

Hi Ulises Lopez; Following link from EPA may be helpful to you.

http://www.epa.gov/iris/toxreviews/1022tr.pdf

Good luck

Dear Ulises,

Soil acidity is one important chemical characteristic that influence soil productivity and fertility. Among the processes in natural N cycle, nitrification (conversion of NH4+→NO2-→NO3-) is a source of soil acidity.

Many tropical soils are highly weathered, low in soil organic matter and low in cation exchange capacity (CEC). In these soils, continuous use of Ammonium based inorganic fertilizer may lead to further acidification and decline in soil fertility.

Urea, when applied to soil, forms Ammonium (NH4+) which then undergoes Nitrification. Therefore, continuous use of Urea fertilizer, accompanied by lack of adequate return of crop residues (source of organic matter) to some tropical soils can lead enhance soil acidity.

Following attachment is one example that describes a study in this area:

Susantha.

Methodology of life cycle assessment (LCA) may serve your purpose.

In this article you find the toxic influence of urea on seed germination:

http://www.pnas.org/content/86/21/8185.full.pdf

Evidence that the adverse effect of urea fertilizer on seed

germination in soil is due to ammonia formed through

hydrolysis of urea by soil urease

Effects of different amounts of purified urea, urea

fertilizers, and urea fertilizer impurities on germination

of seeds in Dickinson soil

I agree with the answers. But the question of quantitative determination remains unsolved. I am waiting for good relies.

Before we attempt to quantify the 'damage' we need to know whether such 'damage' exists. If such 'damage' does exist we need to know the type of damages. If urea is applied at the correct rates (e.g. 200 kg/ha/year to dairy grazed pasture at four split doses) there is unlikely to be any damage, either to soil or plant.

It is well known that livestock urine does contain urea and dairy cows can deposit significant amount of urea in urine patches (estimated as 2000 kg urea/ha). Nitrogen transformation in cow urine patch has been well documented.

As suggested by other workers in their answers, ammonium and nitrate will be generated following urea application to soil. Both of these forms can be taken up by plants. However, in excessive amounts nitrate can leach beyond plant root zone causing leaching and potential ground water contamination.

In terms of 'damage' to soil, excessive urea hydrolysis will cause temporary high alkalinity in soil as a result of high ammonia (NH3) release. This can cause organic-C dissolution and C leaching. Once NH4 released from urea has been nitrified into nitrate (NO3), 2H+ are released as a by-product of nitrification process which could increase soil acidity in the long-term. Also there is evidence of cation (Ca++) leaching from soils as a result of anion (NO3-) leaching.

As for the plants, high NH3 released from urea could kill foliage (you may have observed this from dog urine patches on lawns). As for plant physiology, nitrate as a source of N to produce amino acids could demand plant energy compared to the uptake of ammonium. This is because once absorbed by plants nitrate has to be reduced by enzymes to ammonium form to produce amino acids.

Dear Lopez

Kindly browse the following .

http://archive.lib.msu.edu/tic/tgtre/article/1995apr3.pdf 

This may be the damage caused to the soil. Normally best management practices if followed in toto, may alleviate these damages.

Dear Lopez

From my little work on effects of urea on selected soil chemical properties both at incubation and field trials, it is possible to measure the damages done on soils by urea. This measurements can be determined by monitoring changes in concentrations or values of parameters such as soil pH, organic carbon, exchangeable bases especially Ca and Mg, exchangeable acidity, soluble aluminum, aluminum saturation and base saturation percentages. The final values of these parameters after application of known quantities of urea preferably at different rates can be compared with their initial values at the outset of the trial, simple regression analysis can be of help. My findings have shown urea reducing soil pH from 5.7 to 4.7 in an alfisol and from 4.6 to as low as 3.2 in an ultisol. Its damaging effects is more on soil organic matter/carbon depletion with values dropping to as low as 4.11 g/kg from initial 8.4 g/kg. All these will sure culminate into yield reduction in the long run.

Hello Yetunde und Ulilis, I think Yetunde gave a verv impressive and logic answer. I was surpised that in tropical soil the effect is so high. The effects here in Austria seems not be so high and can seen on germination of sugar beet or maize with high fertilzer dosis during germination time.

Greetings from Vienna in Austria.

Johann 

Judging by the most recent answers I get the impression that urea is considered as a harmful fertiliser by some of our colleagues. Using our well developed and documented knowledge in urea, it can be used as efficiently and effectively as other nitrogenous fertilisers.

If using urea as a fertiliser in germination or raising seedlings, urea should be applied to soil 2-3 days prior to sowing seeds or planting seedlings. This 2-3 day window will allow the temporary alkalination caused by the release of ammonia from urea to neutralise. Within this time ammonia (NH3) would have stabilised as ammonium (NH4).

High dissolution of organic-C in soils is caused by high application of urea. I would like to see colleague Yetunde's rate of application. As I mentioned in my previous answer, the amount of urea deposition in cow urine patches is 10 times greater than the conventional fertiliser application rate. Despite this, in New Zealand, well established dairy pasture soils which are grazed 9 months/year have very high organic-C content. In comparison, vegetable and cropping soils which are cultivated regularly have 4-5 times lower level of organic-C than the dairy pasture counterparts. What this observation illustrates is that organic-C can be easily depleted in soils by constant cultivation because of oxidising and mineralising organic-C than highly concentrated urea in cow urine leaching organic-C.

In terms of effect of acidity caused by urea, the focus should be accorded to all ammonium based fertilisers. All ammonium based fertilisers (e.g. ammonium sulphate, di-ammonium phosphate, ammonium nitrate) will cause soil acidification. The acidity causing effect is NOT unique to urea. We all know that acidity is caused by NH4 being nitrified to NO3, during which reaction two protons (2H+) are produced. There are two ways of avoiding acidification; (a) use nitrification inhibitors (which can inhibit NH4 being oxidised to NO3) when using urea or any ammonium based fertilisers or (b) avoid urea and ammonium-based fertiliser altogether and use nitrate based fertilisers (e.g. KNO3). As you all know the use nitrate based fertilisers is not preferred because of high loss of applied-N by leaching. Therefore, the ongoing use of urea or ammonium based fertilisers to sustain food production or farming is inevitable.

Many countries are aware of the issue of acidification and such an issue is managed by regular liming. Soil acidification is also an issue in outdoor livestock farming (e.g. clover based pasture) even if nitrogenous fertilisers are not used. But this can be managed by regular liming as well.