Hi Gaurav

Biuret can be toxic to plants. Here is an article on it from IPNI

https://www.ipni.net/ppiweb/bcrops.nsf/$webindex/AFE7446D8AF92F988525732D0038FDCE/$file/07-3p06.pdf

John

Dear Gaurav Singh,

Urea is suitable for all crops and soils. Being non-polart, this is an ideal Fertilizer, but urea having more than 1.2% Biuret it is very harmful for certain crops like pineapple, Citrus, Tobacco and coffee which are sensitive to Biuret, urea with a lower Biuret content of 0.5 to 0.8% is preferable for application as a foliar spray. Chlorosis occurs on the leaf of the plants, when Biuret more than 1.5%.Fertilizer grade urea   contains the impurity biuret which is toxic to the plant especially when applied as a leaf dressing. Biuret more than 1.5 % can damage the root of the plant and also useful bacteria and chock the capillary of the root cell, Nitrogen carrying capacity will be reduced.

The biuret formed at following conditions in Urea process.

1.       High Temperature.

2.       High Concentrations

3.       High residence time

Formation of biuret takes place when urea is heated to its melting point it starts decomposition with evolution of ammonia preumably,urea first isomerizes which dissociates into isocynic acid and ammonia.

•          CO(NH2)2    =  NH4CNO       +              NH3.

•          (UREA)           (AMM.CYNATE) 

•          The isocynic acid reacts with urea to form biuret.

•          NHCO  +  CO(NH2)2    =    NH2CONHCONH2

•          In the presence of excess ammonia biuret is formed at substantially lower rate by direct reaction between urea molecules.

•          2CO(NH2)2 =  NH2CONHCONH2+NH3.

•             (UREA)                    BIURET                      AMMONIA

•          BIURET + UREA    =   TRIURET  + AMMONIA

Regards,

Prem Baboo

to be honest: the biuret was/is a long ago hoax from fertilizer market competition: we made foliar spraying with biuret solutions without any harmful effects: my suggestion: just make your own simple trials with biuret purchased from a chemical store!

 Dear colleagues , very interesting discussion on old but important topic. I have quickly gone through the literature provided by  Drs. John and Baboo .Literature provided by Baboo is old but the note of Dr.Mikkelesen of IPNI is the latest (2007).Dr.Schnug comments will make us think and do some new research.The problem of Biuret in urea is old as it was realized in early fifties.In initial years urea contained around 5% biuret and the limit fixed for safe use was 2%.From literature, now the urea plants can manufacture urea containing 1% biuret which is safe for many crops even for foliar application.Now the biuret problem is for crops which are sensitive to biuret concentrations of as low as 0.3-0.5%.The sensitive crops mentioned in the literature are   citrus,pineapple, avocado,potato and tobacco.So  some new studies are needed to prove the sensitivity of crops to biuret concentration in urea and the upper limit for toxicity if any.For soil application,it seems there is no problem for any crop.

 In some of the crops , tip burn as result of urea spray is very common . In some perennial fruit crops , low biuret urea is used a foliar spray to induce flowering for elevating De-novo arginine biosynthesis for better floral primordia formation . Citrus is one of such crops.

excerpts from publication of  Mikkelesen (2007, Better Crops). In the past, urea manufacturing processes sometimes resulted in fertilizers with elevated biuret concentrations. In high concentrations, biuret interferes with internal N metabolism and hinders protein formation in plants. Biuret is degraded by many soil microorganisms, but the rate is relatively slow. Modern urea manufacturing typically results in biuret concentrations less than 1.0 to 1.3%, which does not pose problems for most uses. There are some plant species that appear to be especially sensitive to biuret, so “low-biuret” urea should be used for foliar application in these situations.(PDF enclosed for further reading )

Exerpts from work of Carles ( J.Ann. Physiol. Veg. 2: 5-18). The effects of urea sprays containing various concentrations of biuret were studied on several plants, including cucumbers, tomatoes and orange trees. Biuret was shown to be toxic only above a certain concentration, namely about 4.4 g. of biuret per 100 g. of urea, and below this level it could have a beneficial effect on growth and yields. Toxic levels of biuret interfered with the absorption of mineral elements but not with their translocation. This induced deficiency of mineral elements and disturbed amino acid synthesis and the functioning of the citric acid cycle.

But surely , your own trial would be the best course of action ,considering such a varied crop response towards biuret toxicity , especially while using as foliar spray.  

Dear Scientists, thank you very much for your detail nicely explanation regarding the  biuret problem, yes it is toxic in some crops when apply through foliar, and through soil application also have some problem....  

Biuret in urea can cause agronomic problems if placed near the seed or even if added preplant in bands where seeds will later be planted.

Most manufacturers of urea keep biuret content low by keeping high temperatures to a minimum. Biuret content is typically around 0.3%, although urea of foreign origin appears to be higher.

High heat is normal during the manufacture of urea. If heat exceeds 200 degrees F there is a slight conversion of urea to biuret, but this takes place only during the manufacturing process. No such conversion happens in storage or in the soil.

Biuret converts to ammonia, but conversion is much slower than for urea. Since biuret remains in the soil for several weeks, the potential for seed damage continues beyond the brief period of conversion of urea to ammonia. The major damage of biuret is to germinating seeds. There is little damage through plant absorption, although some citrus crops have been affected.

Also see the attachment........old paper

Dr.Singh, as per current knowledge(your attached paper) one should not bother for biuret  in urea  either for soil or foliar application(a little doubt for citrus).Is it correct?

Urea was the economic "shark" in the competeing fertilizer market and biuret the stigma its competitors put on it!

Dr. Schnug I understand your answer.Some body can do a review of the work done on citrus or any other sensitive crop for seed germination or foliar application of urea containing low concentrations of biuret (0.2-0.6%) to conclusively  say scientifically that biuret is not harmful to crops or do a few experiments if needed.Even the article of James,G.R and Oomen,C.J.(attached by Dr.Singh) contains over 40 research papers.It needs to be published with appropriate discussion  in some reputed scientific Journal.

Very informative PDf Dr Singh . Astonishingly , the biuret toxicity was identified in crops like avocado , citrus , pineapple etc in 1950s , but there is hardly any precise mechanistic insights available to explain the biuret toxicity when used as foliar spray . Biuret does interfere with  N-metabolism . Lets listen more to few of our colleagues , what they have to say ..Dr Schnug , how can we break  the  inseparable chemistry between shark and stigma...

There is also problem in case of fertigation especially the mixture WSF grades where urea is also used as a raw material. The Biuret % is very sensitive because it directly hits the feeding roots. If biuret % will be more then it may burn the feeding root system. Parallelly, it will impact the microbial population.