Anthony Bly
Corn growers and agronomists have been noticing earlier than normal corn nitrogen (N) deficiency appearing on lower leaves this year. Some N deficiency symptoms later in the season are ok; however, corn plants exhibiting N deficiency during mid-season might be an indication of N loss through several of the following pathways. N could have been lost through volatilization, leaching, or denitrification. The soil nitrate-N could also be positionally unavailable because it could be in the soil near the surface that is dry, while actively growing corn roots might be lower in the soil extracting water. However, if leaching rain occurred in June, then nitrogen should be available from deeper in the soil profile. Or, since corn N rate recommendations were developed across variable environments for N mineralization from organic matter, the cooler than normal year may have reduced soil microbial activity therefore reducing the amount of N provided by the soil. Nitrogen provided by the soil through microbial mineralization of organic matter is extremely unpredictable and greatly influenced by environmental conditions such as soil temperature and moisture.
Soil respiration of carbon dioxide (CO2) has been shown to be a very good way to monitor soil microbial activity. Many researchers have correlated CO2 soil respiration and temperature. Optimum soil temperatures are between 80 and 85 degrees Fahrenheit for soil microbial respiration. Four inch soil temperatures from several locations are shown in the graph below. Only one site (Parkston) entered into the optimum soil temperature zone for soil microbial activity, and only for short periods of time. All other sites stayed well below the 80-85 degree temperature range. Therefore, lower nitrogen supply from soil mineralization could be expected this year. This may also be confounded by cropping systems with more surface cover, as soil temperatures are even cooler when compared with bare, unprotected soil surfaces. However, the trade-off between exposed, unprotected soil and covered, protected soil that is cooler should be an easy choice, because conserving our soil resource through soil health improvement is vitally important to the future successes of crop production.
Figure 1: Four inch soil temperatures.
Source : SDSU