Garth Butcher

Over the past five years I have noticed an increase in nitrogen levels on my long term zero tillage fields. This observation is based on soil tests taken every year in early October after harvest. This has been a pleasant surprise but where application rates were no adjusted downward it has created problems such as higher than desired protein levels in malt barley, lodging, delayed maturity, and environmental concerns. Not realizing the cost savings of a reduced nitrogen fertilizer application is also a concern.

I will discuss why I think this change has taken place on my farm, but please keep in mind that these are only speculations.

An example of the changes in soil nitrogen levels can be seen on SW 19-16-25 which has a clay loam soil with an organic matter of 7.5 percent and a pH of 7.9.

1994. average nitrate level – 22 lb/acre

2002. average nitrate level – 41 lb/acre

This change over two, 5-year periods is typical of other long term zero tillage fields on my farm. The fact that the example above has been zero tilled for over 20 years, I feel is a significant. Land that I have more recently begun to farm doesn’t show the same level of increase in nitrogen available. This is probably due to the fact that long term zero tillage fields have higher organic matter levels. Couple this with more active mineralization* of the organic matter due to changing soil characteristics, and you have more nitrogen available to plants. This more active mineralization is probably due to higher concentrations of soil organisms and a moister soil surface. Remember, most of the mineralization takes place within a few inches of the surface and requires moisture as well as oxygen and warmth.

Adopting zero tillage is probably not the only factor that has contributed to higher soil nitrogen levels. Other factors may have come into play.

Pulse crops in my rotation are undoubtedly contributing. Soil tests after field peas on my farm show higher levels of soil nitrogen. This effect seems to have been amplified since I began using granular inoculants about seven years ago. While only improving yields marginally, the amount of plant growth and the volume of nodulation on the roots has noticeably increased. The contribution of the pea crop can now be observed for two years after this crop. Previously the effect of the peas was noticeable for only one year.

My rotations have changed somewhat over the years, with an increase in oilseeds and pulses and a decrease in cereal crops. The residues of these non cereals have a higher nitrogen content and therefore may be contributing to the increase in nitrogen in the soil.

There may be one other factor contributing to the increase in mineralization of organic matter on my farm. This is a factor that could affect agriculture in all of the Northern Great Plains – Climate Change.

We have experienced somewhat warmer temperatures over the past years. Although it hasn’t been true throughout the Northern Great Plains, in our area, we have had higher than average moisture. Both factors speed up mineralization.

In the past, in our more northern area, mineralization of organic matter to nitrogen has been at a slow pace. If global warming is eminent, mineralization should speed up if there is adequate moisture in the soil. This could mean that lower rates of nitrogen fertilizer may be required. This could benefit our bottom line ultimately and could draw down organic matter levels as has been experienced in warmer areas of the Great Plains. Zero Tillage and continuous cropping with relatively lower soil temperatures could buffer this effect somewhat.

The above is more or less conjecture. But I feel that one thing is true, to assume that soils are reacting exactly as they did in the past is a mistake. Soil testing and monitoring and analyzing the results is ever important.