I believe, the important thing is that urea should not be applied at the surface of the soil.  Even if urea is placed at 1-2 cm depth, it is not prone to ammonia volatilization losses.  Thus, depending upon the availability of machinery for urea application, it can be applied from say 2 cm to 5-6 cm deep in the soil. Urea placed deep in the soil even by its application just before applying irrigation is also good; the irrigation water transports urea to a depth.  But it may not be feasible in maize. 

Dear Chaoqiang Jiang

In order to improve nitrogen use efficiency, you can divide the urea into three parts and apply it at three stages.

- place seeds of maize and a part of solid urea in 4-5 cm depth of soil during sowing.

- apply another part of urea preferably as a solution by foliar spray or with irrigation water at mid stem elongation stage.

- apply the last part of the urea at tasseling stage.

Regards

Kazem Ghassemi-Golezani

I believe, the important thing is that urea should not be applied at the surface of the soil.  Even if urea is placed at 1-2 cm depth, it is not prone to ammonia volatilization losses.  Thus, depending upon the availability of machinery for urea application, it can be applied from say 2 cm to 5-6 cm deep in the soil. Urea placed deep in the soil even by its application just before applying irrigation is also good; the irrigation water transports urea to a depth.  But it may not be feasible in maize. 

In the UK the advice generally given is to place 10 - 15 kg N/ha below the seed at drilling to encourage early growth.  The remainder of the nitrogen should be applied to the soil surface as soon as the maize has emerged.

Urea can be applied to soil as solid or solution or to certain crops as foliage spray. To blend urea into the soil with to a depth at which ammonia losses will not occur so that nitrogen use efficiency may increase. In general, for summer maize most recommended depth is  8-16 cm. Please find below the reference for your answer.

Yang et al (2016). Journal of Plant Nutrition and Fertilizers 22, 830-837. Doi: 10.11674/zwyf.14409.

Urea supplies mainly nitrogen which isa mobile nutrient and it should be applied between 2 to 3 cm in depth covered with soil to avoid volatilization .however, the quantity of urea should be applied in phases to avoid luxury consumption by crops and leaching  into underground water .Excessive application of urea will encourage luxury consumption and excessive vegetative growth of crops as well as delayed crop maturity ,hence,the application may cause soil acidity and destruction of soil properties and there is need for soil testing to determine the quantity of urea to use .The planting of leguminous crops will fix nitrogen into soils and reduce the cost purchasing urea and prevent  destruction of soil properties

As N is very mobile in the soil considering both method and time of application is very important. Urea as a source of nitrogen is also easily soluble in the soil. So in contrast to phosphorus it should not be applied deeper in the soil since it can be easily leached away before the full development of roots to take up the available N in the soil. As a split application, urea is usually applied as top dressing when maize plants reached at knee high stage, normally after cultivation and weeding in the presence of sufficient moisture. Overall, time of application is very important in terms of improving efficiency of N though depth of application may have an effect on N-use efficiency. Whether granule or pelleted is also another factor that affects the efficiency of N.

Your can apply at a depth of 4-5 cm but this varies with the type of the soil as N can easily leach in sandy loam soil so a reasonable depth is of great importance. 

Hello,

The optimum depth of application of urea is variable. It depends on the moment of application, the rain and the type of urea. In Argentina we applied urea to 5 cm of depth at the beginning of spring for early sowing maize. For later sowing maize is applied on the surface and the rain is incorporated into the soil.

Greetings.

Improving the N use efficiency is very important to sustain economic and environmental sustainability. As you know, the global nitrogen use efficiency is very low which is about 33%. That means the other 67% is unaccounted for, It is either lost as gasses to the atmosphere or via leaching. In terms of money, the N loss is estimated to be over USD15 billion (Raun  et al. 1999 and 2002). So, application of  N fertilizer with organic amendments may optimize the efficiency of N instead of applying it alone. Besides, use of organic amendments helps enhance soil water retention and use efficiency, which is also very vital for the availability and uptake of nutrients. Thus, a comprehensive approach seems feasible and sustainable to enhance fertilizer use efficiency and nutrient balances in the soil for soil quality and sustainable agriculture.