Delaware Agronomy Blog

University of Delaware Cooperative Extension

Exchangeable Cation Uptake by Irrigated and Rainfed Soybeans (2021-2022)

Although Ca, Mg, and K are all exchangeable nutrients that are considered plant available, soil chemistry and plant root interactions result in different uptake and bioavailability. Within the soil, Ca and Mg can move with soil water or by diffusion, while the lower K concentrations do not readily move with soil water. This results in differences in uptake for soils with adequate moisture versus those under drought stress. Understanding how concentrations of each nutrient, the soil CEC, and soil moisture content interact is important for giving future nutrient recommendations.

Irrigated and rainfed dry corners in a Delaware field.

Average Delaware Growing Degree Days (2019-2021)

The standard method used to follow and predict corn growth stages is using growing degree days (GDD). This is a calculation that uses average daily temperatures measure accumulated heat over the growing season. Using GDD works better than days from planting because cool spring temperatures slow early planted corn, while corn planted later in May can have a more linear growth pattern.

For the past three seasons in Georgetown we have followed our research plots and have these values as the average GDD for our area (Table 1). They will be similar to those found in other states, but represent averages and ranges for our region. You may find GDD values on our regional mesonet (DEOS) or through the Climate Smart Ag page at Cornell (edit the site location).

Growing Degree Days (GDD) Average Accumulation to Reach Corn Vegetative and Reproductive Stages.

Cover Crop Biomass and Termination Considerations

Jarrod Miller, Amy Shober, & Mark VanGessel

The benefits of cover crops to the following corn crop can include additional nitrogen (N) or weed suppression, but maximizing these benefits requires later termination to build greater biomass. These N and weed control characteristics are especially appealing this season as input costs are relatively high while supplies are relatively low. However, growers should take the time to estimate the additional costs of allowing a more robust cover crop to accumulate this spring, as surface residues reduce proper seed placement as well as limit seed to soil contact. This article will discuss the management of cover crops for both maximizing N benefits as well as weed suppression in the following corn crop.

Aerial picture of cover crop plots

Cover crop plots at the Carvel Research and Education Center.

Maximizing Cover Crop Biomass

Estimating the amount of N that could be available requires knowledge about 1) total cover crop biomass and 2) cover crop C:N ratio, both of which are affected by termination timing. The longer a cover crop is allowed to grow, the greater the amount of cover crop biomass will accumulate. Continue reading

Estimation of Cover Crop Nitrogen with Drones

Jarrod O. Miller, Assistant Professor and Extension Specialist, Agronomy, jarrod@udel.eduAmy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality,,  Jamie Taraila, Graduate Research Assistant; 

As part of a Northeastern SARE graduate student grant, we used a drone to predict the nitrogen (N) that may be present in cover crops prior to burndown. We flew fields in Laurel, Georgetown, and Harbeson with a readily available consumer drone (Phantom4) equipped with a standard (RGB) camera. Each of the fields were flown prior to cover crop burndown (late April to late May) resulting in 25-100 images per field that had to be stitched together into one image (Figure 1a). The camera captured different wavelengths of light (i.e., red, green, blue) that were reflected by plants which were transformed into the Visible Atmospherically Resistant Index (VARI). This allowed us to estimate plant biomass by comparing VARI values to cover crop biomass that was collected in the field. We collected 10 samples per field, which were dried, weighted, and then analyzed for N content by the UD soil testing lab.

Continue reading

Evaluating Cover Crop Growth with Drones

 Jamie Taraila, Graduate Research Assistant; Jarrod O. Miller, Assistant Professor and Extension Specialist, Agronomy,; Amy L. Shober, Professor and Extension Specialist, Nutrient Management and Environmental Quality,


As part of a Northeast SARE sponsored project, we evaluated cover crop growth response (biomass) to various seeding rates. We also followed cover crop growth through the winter using drone imagery. We planted NRCS recommended rates of rye (100 lbs/acre), clover (15 lbs/acre), and a rye/clover mix (40 lbs/acre rye, 10 lbs/acre clover) at the UD Carvel Research and Education Center in Georgetown following corn harvest in late September 2020. We also planted the same cover crops at reduced rates of 25, 50, and 75% the NRCS recommendation. By the time we sampled cover crop biomass in April, there was very little difference in biomass from the 25 to 100% (full) rates. In fact, the biomass response curves show that there was little increase in biomass above the 25% seeding rate (0.25) for each cover crop type, as the curve is relatively flat above this rate (Figure 1). Continue reading

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