Which method is NOT used to incorporate urea fertilizers?

Incorporating urea fertilizers is crucial for maximizing their effectiveness in supplying nitrogen to crops while minimizing environmental impacts such as volatilization. The method of spreading urea on the surface without incorporation does not facilitate this integration into the soil, which can lead to nitrogen loss due to ammonia volatilization.

Now, when looking at the other methods: injection involves applying the fertilizer directly into the soil, effectively reducing losses and ensuring that the nutrients are available to plant roots. Tillage incorporates the fertilizer into the soil through physical mixing, which can enhance nutrient uptake by crops. Rainfall can also play a role in incorporating urea as precipitation can help dissolve the urea, moving it into the soil.

Since spreading on the surface without incorporation allows for the high potential for loss of nitrogen and does not effectively utilize the benefits of urea, this method distinctly stands out as one that is NOT used to incorporate urea fertilizers effectively.