Was wondering what is the lowest soil temp 46-0-0 or 21-0-0 will be converted and available for root growth? I realize root growth probably stops as the ground freezes, but at what temp will N no longer convert to a form the plant can use?
The plant will continue to convert the N to sugars (carbs.) as long as the roots are continuing to grow. The roots will grow as long as the ground is not frozen... you know that already.
As I understand it, the problem you have is TIME. The colder it is the slower the roots grow. The less N will be taken into the plant and less N already in the plant will have time to be converted. The plant will continue to convert N already absorbed by the roots when the ground thaws (at a very slow rate).
So the answer is... 33 degrees. Just remember, although the roots are still growing, they are growing very slow. Therefore, the amount of N uptake is significantly reduced. So, a lot of the N is being lost to leaching or runoff before it has time to be taken into the plant when app. is applied during extended cold periods.
Top growth comes to a halt at a specific soil and air temperature depending on the type of turf.
Cool season grasses in my area are mostly stagnant when the temp goes below 40. Therefore any N you apply during that time is most likely going to leach through the soil profile before the plant can utilize it. If your climate provides for some moderate temps from time to time, the N might be of some value. But you are correct that water soluable is the method of choice in colder conditions.
The N will be available regardless of temperature, but the issue is how much of it actually is wasted.