Agronomy News & Views by Owen Beever
October 5, 2011
High Soil Test Nitrogen
Soil tests seem to be generally showing relatively high levels of nitrogen in fields that weren’t seeded this past season. In some cases the levels are very high – 200-300 lbs N/ acre.
Given the summer we experienced it is not too surprising to see the high N levels. Mineralization - the soil process that changes organic nitrogen to inorganic nitrogen (the form which plants can use eg/ NO3) is driven by a number of factors but the major ones are soil temperature and moisture. Obviously these fields had a good supply of moisture. We’ve experienced one of the warmest summers in some time so conditions have been quite favorable for a build-up in the nitrogen supply. We saw something similar happen in the hot summers of the 1980’s when soil nitrogen levels often reached 200-300 lbs / acre even though the fields had been cropped.
There are likely going to be differences depending if fields were cultivated and the timing. Those cultivated earlier are more likely to show higher nitrogen levels. Cultivation allows oxygen into the soil which enhances the mineralization process – kind of like stirring up the coals in a fire causes it to burn more quickly. For those fields that have been in a reduced till or zero till system for a few years there will be an organic matter build-up at the soil surface which will enhance the nitrogen supply.
Soil testing is a good tool and a good guideline. Keep in mind though it is not a perfect science. This is not a reference to the soil analysis in the lab. The analysis is done under controlled conditions and provides consistent results. What is not perfect is what happens in the field. Most fields have a fair amount of variability. The more rolling and uneven the field the more variability exists but even fields that appear flat and uniform can vary a good deal. This variability makes the sampling process more difficult. No matter how well the sampling is done or which system is used there are going to be spots in the field that have higher levels of N than the soil test indicates and spots that have less.
Because of the variability I’ve always had some discomfort with going to a zero level of nitrogen fertilizer when soil test results show high soil N. My approach would be to still apply some minimum amount - perhaps 30-50 lbs / acre. This will help account for any areas that happen to be lower in soil N such as upper slopes.
Just one other thought – the mineralization process shuts down or is very slow at temperatures of 5 C. That normally happens by about mid Sept. The warm weather lately will likely have kept the process going later than normal adding to the soil nitrogen levels.