G. V. Johnson

Optical Sensor‐Based Algorithm for Crop Nitrogen Fertilization

Abstract Nitrogen (N) fertilization for cereal crop production does not follow any kind of generalized methodology that guarantees maximum nitrogen use efficiency (NUE). The objective of this work was to amalgamate some of the current concepts for N management in cereal production into an applied algorithm. This work at Oklahoma State University from 1992 to present has focused primarily on the use of optical sensors in red and near infrared bands for predicting yield, and using that information in an algorithm to estimate fertilizer requirements. The current algorithm, “WheatN.1.0,” may be separated into several discreet components: 1) mid‐season prediction of grain yield, determined by dividing the normalized difference vegetative index (NDVI) by the number of days from planting to sensing (estimate of biomass produced per day on the specific date when sensor readings are collected); 2) estimating temporally dependent responsiveness to applied N by placing non‐N‐limiting strips in production fields each year, and comparing these to the farmer practice (response index); and 3) determining the spatial variability within each 0.4 m2 area using the coefficient of variation (CV) from NDVI readings. These components are then integrated into a functional algorithm to estimate application rate whereby N removal is estimated based on the predicted yield potential for each 0.4 m2 area and adjusted for the seasonally dependent responsiveness to applied N. This work shows that yield potential prediction equations for winter wheat can be reliably established with only 2 years of field data. Furthermore, basing mid‐season N fertilizer rates on predicted yield potential and a response index can increase NUE by over 15% in winter wheat when compared to conventional methods. Using our optical sensor‐based algorithm that employs yield prediction and N responsiveness by location (0.4 m2 resolution) can increase yields and decrease environmental contamination due to excessive N fertilization. *Contribution from the Oklahoma Agricultural Experiment Station.

Switchgrass Response to Harvest Frequency and Time and Rate of Applied Nitrogen

Abstract Switchgrass (Panicum virgatum L.) is currently being evaluated as a raw material for producing fuel, chemicals, and electricity. Switchgrass biomass is bound by the growing environment that includes fertility. More information is needed on sustainable switchgrass production as influenced by nitrogen fertility and harvest management. Field experiments were initiated at Chickasha and Perkins, OK in 1996 and 1998, respectively to evaluate switchgrass response to applied nitrogen (N) at rates of 0, 112, 224, 448, and 896 kg ha−1. In addition, harvest frequency and time of N application were evaluated. Yield maximums and the greatest N, potassium (K), phosphorus (P), and sulfur (S) uptake values were achieved with 448 kg N ha−1 applied all in April and harvested three times. In fact, harvest frequency was the most important factor affecting yields over the course of these studies with average dry matter yields of 16.3, 14.7, and 12.9 Mg ha−1 yr−1 for three, two, and one harvest yr−1, respectively. No significant change in soil organic carbon was detected over time. Although dry matter yields were found to decline with time, total N uptake did not. Forage N concentration was found to be greater in later years, thus increasing production costs. While yields were highest (18.0 Mg ha−1) with 448 kg N ha−1 applied all in April and three harvests, applying 0 N and harvesting three times produced almost as much total biomass (16.9 Mg ha−1). This limited response to N is possibly explained by the evolution of switchgrass under low N conditions. Increasing forage concentrations of K, magnesium (Mg), P, and S were noted with increasing yields, indicating a potential for response to these nutrients. #Contribution from the Oklahoma Agricultural Experiment Station.

NITROGEN FERTILIZATION OPTIMIZATION ALGORITHM BASED ON IN-SEASON ESTIMATES OF YIELD AND PLANT NITROGEN UPTAKE

Current methods of determining nitrogen (N) fertilization rates in winter wheat (Triticum aestivum L.) are based on farmer projected yield goals and fixed N removal rates per unit of grain produced. This work reports on an alternative method of determining fertilizer N rates using estimates of early-season plant N uptake and potential yield determined from in-season spectral measurements collected between January and April. Reflectance measurements under daytime lighting in the red and near infrared regions of the spectra were used to compute the normalized difference vegetation index (NDVI). Using a modified daytime lighting reflectance sensor, early-season plant N uptake between Feekes physiological growth stages 4 (leaf sheaths lengthen) through 6 (first node of stem visible) was found to be highly correlated with NDVI. Further analyses showed that dividing the NDVI sensor measurements between Feekes growth stages 4 and 6, by the days from planting to sensing date was highly correlated with final grain yield. This in-season estimate of yield (INSEY) was subsequently used to compute the potential N that could be removed in the grain. In-season N fertilization needs were then considered to be equal to the amount of predicted grain N uptake (potential yield times grain N) minus predicted early-season plant N uptake (at the time of sensing), divided by an efficiency factor of 0.70. This method of determining in-season fertilizer need has been shown to decrease large area N rates while also increasing wheat grain yields when each 1m2 area was sensed and treated independently.

Nitrogen Response Index as a Guide to Fertilizer Management

Abstract The efficiency with which fertilizer‐nitrogen (N) is transferred to grain‐N in cereals is usually less than 50% and averages 33% worldwide. Two long‐term N fertility experiments were evaluated to examine temporal changes in nitrogen use efficiency (NUE) and their causes. Averaged over 30‐yr, non‐irrigated winter wheat NUE was 49% at a 22.4 kg N ha−1 yr−1 rate and decreased to 34% at a rate of 112 kg N ha−1 yr−1. The average NUE for a 15‐yr irrigated corn experiment was 30.6% at the lowest N input (90 kg N ha−1) and decreased to 18.3% at a rate of 270 kg N ha−1 yr−1. Low NUE values are a result of excess N present in the soil–plant system. The extent to which N is present in excess is determined by the potential yield and how much of that yield will be supported by non‐fertilizer N, presumably mineralized from soil organic matter. For both experiments there was greater temporal variability in non‐fertilized (check‐plot) yields (CV of 31.4 for wheat and 34.3 for corn) than in plots where maximum yields were obtained from N‐fertilizer (CV of 20.1 for wheat and 15.7 for corn). The yield of unfertilized plots was not related to the maximum yield of fertilizer plots over time. A response index (RI) was calculated by dividing the maximum yield of fertilized plots by the yield of unfertilized plots to determine the extent of fertilizer need and response for a given year. For both wheat and corn the RI was unpredictably variable (CVs≈35) over time, and ranged from lows of about 1.1 to a high of 4.1 for wheat and 3.5 for corn. In general, low RI values resulted when unfertilized yields were high, which even occurred after 10 to 30 yr without N fertilization. Low RI values may be more common in farmers fields where N is applied annually. Except at the lowest N rates, NUE increases for each rate as RI increases because N inputs are less likely to be excessive. Nitrogen management strategies that increase NUE may only be possible to evaluate for site‐years when RI is substantially greater than 1 (e.g. >1.5). Since response to N fertilizer is strongly dependant on supply of non‐fertilizer N in a given year, any N management strategy that includes a reliable in‐season predictor of RI should dramatically improve NUE in cereal production. #Contribution from the Oklahoma Agricultural Experiment Station

Evaluation of Green, Red, and Near Infrared Bands for Predicting Winter Wheat Biomass, Nitrogen Uptake, and Final Grain Yield

Abstract Presently normalized difference vegetative indexes (NDVI) based on red (RNDVI) or green (GNDVI) reflectance are commonly used to evaluate plant health, biomass, and nutrient content. This study was conducted to determine which of these two indexes is more correlated with biomass, forage nitrogen (N) uptake, and final grain yield of winter wheat. Three experimental sites were established in Oklahoma in the fall of 2001 at Stillwater. Spectral reflectance measurements were taken at Feekes growth stage 4, 6, and 10.5 followed by winter wheat forage harvest. When evaluated at specific stages of growth, RNDVI was consistently more highly correlated with biomass than GNDVI. Green NDVI and RNDVI were more highly correlated with forage N uptake than with dry biomass at each stage of growth, but neither index appeared to have a comparative advantage over the other. Both indexes were highly correlated with final grain yield and grain N uptake across all locations. Neither index appeared to have a sizeable advantage over the other, suggesting that either will perform equally well when predicting forage N uptake, grain yield, and grain N uptake in winter wheat. Red NDVI does appear to be a better predictor of forage biomass, specifically at earlier stages of growth. #Contribution of Okla. Agric. Exp. Stn.

Effect of nitrogen rate on plant nitrogen loss in winter wheat varieties 1

Abstract Gaseous nitrogen (N) loss from winter wheat (Triticum aestivum L.) plants has been identified, but has not been simultaneously evaluated for several genotypes grown under different N fertility. Two field experiments were initiated in 1993 and 1994 at the Agronomy Research Station in Stillwater and Perkins to estimate plant N loss from several cultivars as a function of N applied and to characterize nitrogen use efficiency (NUE). A total of five cultivars were evaluated at preplant N rates ranging from 30 to 180 kg·ha‐1. Nitrogen loss was estimated as the difference between total forage N accumulated at anthesis and the total (grain + straw) N at harvest. Forage, grain, straw yield, N uptake, and N loss increased with increasing N applied at both Stillwater and Perkins. Significant differences were observed among varieties for yield, N uptake, N loss, and components of NUE in forage, grain, straw, and grain + straw. Estimates of N loss over this two‐year period ranged from 4.0 to 27.9 kg·ha‐1 (7.7...

Bray/Kurtz, Mehlich III, AB/D and ammonium acetate extractions of P, K and MG in four oklahoma soils

Abstract Multi‐element soil extraction solutions offer increased convenience in soil testing laboratory operations. The recently developed Mehlich III and ammonium bicarbonate‐DTPA multi‐element extraction solutions were each compared with the more conventional Bray/Kurtz extractant for P determination and with 1N ammonium acetate for K and Mg determinations. The latter two solutions are single and tri‐element extractants in current use by the Oklahoma State University Soil Testing Laboratory. The Bray/Kurtz extractant was also compared with the 1N ammonium acetate for K and Mg determination. A total of 310 soil samples from four soil types, which included five long‐term soil fertility experiments, were used for these comparisons. All sites have been in continous wheat production and received various N‐P‐K fertilizer treatments for at least: five years prior to soil sampling. AIT soil samples were extracted in triplicate for each extractant. Results showed that extraction of elements, by all solutions wer...

Growth stage, development, and spatial variability in corn evaluated using optical sensor readings

ABSTRACT Knowing the exact stage of growth where expressed plant variability is at a maximum might lead to the identification of times when in-season fertilization could have the greatest impact. One field experiment was initiated to measure daily plant growth and spatial variability in corn (Zea mays L.) over the entire growth cycle using optical sensor readings (normalized difference vegetative index, or NDVI) collected every 0.05 m in length, 0.6 m wide from 4 corn rows, 27 m in length. Averaged over all 4 rows, plants were spaced 21 ± 7 cm apart. For each row and sensing date, the mean, standard deviation, and coefficient of variation (CV) were computed from the NDVI readings. Eighteen days after planting, NDVI values were near 0.20, and later peaked near 0.81, 54 days after planting at the 10-leaf growth stage (V10). Coefficients of variation were found to peak much earlier, 33 to 35 days after planting at the 6-leaf growth stage (V6), (31% to 34% during this period). Expressed spatial variability decreased from >30% at V6 to just under 10% at the 11-leaf growth stage (V11). Immediately following V11, a distinct increase in CVs was found just following the initiation of tasseling (VT), but lasted for only 2 days. Expressed spatial variability was greatest at the V6 growth stage, and this peak in the within-row-by-plant variability may be the precise growth stage at which treating that variability will have the greatest impact.

Late-season Prediction Of Wheat Grain Yield And Grain Protein

Pre-harvest prediction of winter wheat (Triticum aestivum L.) grain yield and/or protein could assist farmers in generating yield maps and reliable product marketing. This study was conducted to determine the relationship between spectral measurements (taken from Feekes growth stage 8 to physiological maturity) and grain yield and grain protein. Spectral measurements were taken using photodiode detectors and interference filters for near-infrared (NIR) and red spectral bands. The study was conducted over 2 years at seven locations where existing field experiments were already in place across Oklahoma. Spectral readings were taken at Feekes growth stages 8, 9, 10.5, 11.2, and 11.4. The normalized difference vegetative index (NDVI) was calculated. In both cropping cycles, NDVI was well correlated with grain yield, grain N uptake, straw N uptake, and total N uptake at Feekes growth stages 9 and 10.5 (R2>0.5). However, by Feekes 11.2 no relationship between NDVI and grain yield or N uptake was observed. In 1999–2000 at Feekes 11.4 (harvest), NDVI and grain yield were poorly correlated. Across locations and years, no consistent relationship existed between NDVI and grain N or straw N at any stage of growth. Grain N and straw N could not be reliably predicted using NDVI at any stage of growth. #Contribution of the Oklahoma Agricultural Experiment Station.

Nitrogen Balance in the Magruder Plots Following 109 Years in Continuous Winter Wheat

Abstract The Magruder plots are the oldest continuous soil fertility wheat research plots in the Great Plains region, and are one of the oldest continuous soil fertility wheat plots in the world. They were initiated in 1892 by Alexander C. Magruder who was interested in the productivity of native prairie soils when sown continuously to winter wheat. This study reports on a simple estimate of nitrogen (N) balance in the Magruder plots, accounting for N applied, N removed in the grain, plant N loss, denitrification, non‐symbiotic N fixation, nitrate (NO3 −) leaching, N applied in the rainfall, estimated total soil N (0–30 cm) at the beginning of the experiment and that measured in 2001. In the Manure plots, total soil N decreased from 6890 kg N ha−1 in the surface 0–30 cm in 1892, to 3198 kg N ha−1 in 2002. In the Check plots (no nutrients applied for 109 years) only 2411 kg N ha−1 or 35% of the original total soil organic N remains. Nitrogen removed in the grain averaged 38.4 kg N ha−1 yr−1 and N additions (manure, N in rainfall, N via symbiotic N fixation) averaged 44.5 kg N ha−1 yr−1 in the Manure plots. Following 109 years, unaccounted N ranged from 229 to 1395 kg N ha−1. On a by year basis, this would translate into 2–13 kg N ha−1 yr−1 that were unaccounted for, increasing with increased N application. For the Manure plots, the estimate of nitrogen use efficiency (NUE) (N removed in the grain, minus N removed in the grain of the Check plots, divided by the rate of N applied) was 32.8%, similar to the 33% NUE for world cereal production reported in 1999. #Contribution from the Oklahoma Agricultural Experiment Station.