Cover Crops (IMAGE) Penn State Caption This is how the cover crop mix looked at one study site. From the same seed mixture, cover crop mixture expression varied greatly across farms and researchers hypothesized that this variation was correlated with soil inorganic nitrogen concentrations and growing days. Credit Ebony Murrell, Penn State Usage Restrictions non commercial use only License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.