Tracy M. Blackmer

Transmittance and Reflectance Measurements of Corn Leaves from Plants with Different Nitrogen and Water Supply

Summary Nitrogen is essential for crop production, but also contributes to eutrophication of surface waterand degradation of drinking water quality. Modern corn production requires relatively large quantities of N, which are generally supplied by fertilizers. Over-application of N fertilizers and animal wastes frequently results in nitrate leaching. Synchronizing N availability with crop N need offers the potential to protect the environment without sacrificing production. Tools are needed to rapidly and easily monitor crop N status to make timely decisions regarding fertilizer application. Analytical and optical techniques were evaluated with greenhouse grown corn at silking to evaluate several methods to monitor crop N status. A portable chlorophyll meter was used to measure chlorophyll content of leaves by means of transmittance measurements. Leaf N concentration and chlorophyll meter readings were positively correlated, but were also affected by water stress and hybrid differences. Water stress decreased chlorophyll meter readings but increased leaf N content and diffusive resistance. Nitrogen stress decreased leaf N concentration, chlorophyll meter readings, and diffusive resistance. Both water and N stresses affected crop reflectance measurements. Reflectance values in the green and near IR portions of the spectrum were inversely related to crop N status. Water stress increased reflectance in red, green, and near IR wavelengths. Water stress by N status interactions were significant for chlorophyll meter readings as well as reflectance measurements. Both leaf reflectance and chlorophyll meter measurements provided a good indication of N status for adequately watered plants, but the relationships were poor for plants grown under prolonged water stress.

Techniques for monitoring crop nitrogen status in corn

Abstract Nitrogen (N) fertilizers used in corn production contribute to the occurrence of nitrate to ground water. Tissue testing is a tool that can aid in fertilizer N management. Several tissue testing techniques were evaluated for their potential to aid in N management of irrigated corn in Nebraska. Leaf N content provided a traditional means of identifying N deficiencies in corn. Leaf N concentrations were difficult to interpret because of luxury consumption and the lack of consistent critical levels across hybrids and growth stages. Chlorophyll meters showed little sensitivity to luxury consumption and provided instantaneous results, but required an in‐field reference fertilized with extra N. Chlorophyll meter readings were highly correlated with grain yield and provided early detection of a N deficiency. The combination of chlorophyll meter data and irrigation demonstrated the merits of a “fertilization‐as‐needed”; approach to N management. The post‐harvest stalk nitrate test accurately identified s...

Aerial Photography to Detect Nitrogen Stress in Corn

Summary Economic and environmental benefits can result from improved nitrogen (N) management in corn (Zea mays L.) production. This research project was conducted to determine the utility of aerial photographs to detect N deficiency in a crop canopy caused by natural soil variability and variable fertilizer N application rates. Chlorophyll meter readings and digitized aerial photograph data were compared to corn grain yield and stalk nitrate concentrations from a 60-ha field in Central Nebraska. In addition to natural field variability, 30.4-m long treatments were imposed through the center of the field by applying fertilizer at rates of 0, 56, 112, 168, and 224 kg N ha -1 . Grain yield, chlorophyll meter readings, photographic brightness and stalk nitrate samples were collected. Despite a significant effect from fertilizer treatments, chlorophyll meter readings did not correlate well with grain yields. The brightness of the red component in a digitized color photograph showed a significant inverse relationship (r 2 = 0.42) with grain yield. Stalk nitrate concentrations at harvest, when compared to red brightness from the digitized photograph, provided a better confirmation of N status than grain yield. Aerial photographs appear to be capable of detecting management induced variability as well as reduced yield portions of a field that result from natural variability.

Chlorophyll meter readings in corn as affected by plant spacing

Abstract Heightened environmental consciousness has increased the perceived need to improve nitrogen (N) use efficiency by crops. Synchronizing fertilizer N availability with maximum crop N uptake has been proposed as a way to improve N‐use efficiency and protect ground water quality. Chlorophyll meters (Minolta SPAD 502) have the potential to conveniently evaluate the N status of corn (Zea mays L.) and help improve N management. A potential problem with the use of chlorophyll meters is the effect of within‐row plant spacing on meter reading variability. Chlorophyll meter readings and leaf N concentration of irrigated corn at anthesis and grain yield at harvest were measured on plants grouped into eight within‐row plant spacing categories. Leaf N concentration was not affected by plant spacings, but chlorophyll meter readings and grain yield per plant increased as plant competition decreased and N fertilizer rate increased. These data indicate that avoiding plants having extreme spacings can greatly incre...

Reliability of Chlorophyll Meter Measurements Prior to Corn Silking as Affected by the Leaf Change Problem

The normal practice of using chlorophyll meters involves measuring the youngest fully developed leaf of a corn plant (Zea mays L.) before silk emergence and then switching to the ear leaf. A discontinuity exists of chlorophyll meter measurements associated with leaf change prior to the silking stage. The lag of silk emergence within a cornfield prior to silk emergence makes it practically impossible to take chlorophyll measurements on leaves at the same growth stage. Therefore, the change in leaves being measured introduces errors when chlorophyll meters are used prior to silk emergence to diagnose nitrogen (N) deficiencies. We report results from three field studies to assess the difference in chlorophyll measurements between the uppermost fully developed leaf and the ear leaf near the time of silk emergence and identify possible problems associated with the difference when chlorophyll meters are used to estimate the need for in‐season fertilization in cornfields.

Fertilizer bands affect early growth of corn.

Nitrogen (N) applied in bands across cornfields often induces differences in plant height, leaf color, and growth stage of corn (Zea mays L.). Especially during wet springs, plants growing immediately over the bands are often noticeably taller and greener for a short period. Plants growing between the bands experience N deficiency until their roots reach the bands. The impacts of such short periods of N deficiency on plant early growth have received little attention. We studied the effects of tracks left by fertilizer applicator on corn growth stage, plant height, and leaf chlorophyll meter readings (CMRs) in a field where conditions seemed favorable for a fertilizer-induced advancement in growth stage. Measurements showed that the reduced plant height or leaf color attributable to a temporary N deficiency was mainly associated with the delay of growth stages and might have little influence on final grain yield.