Submitted to: Precision Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/26/2011
Publication Date: 4/26/2011
Citation: Bausch, W.C., Brodahl, M.K. 2011. Strategies to evaluate goodness of reference strips for in-season, field scale, irrigated corn nitrogen sufficiency. Precision Agriculture. (DOI)10.1007/s11119-011-9230-9. Interpretive Summary: Nitrogen (N) fertilizer has received attention for a long time as a potential source of ground and surface water pollution because of the large amounts of N fertilizer used on agricultural crops. Excessive N fertilizer applications typically occur when uniform rates of N are applied over an entire field when areas within the field actually need less because high residual amounts exit in the soil profile and when N fertilizer is applied in the fall or early spring when N fertilizer is not required by spring planted crops. In-season N management studies have been conducted for many years to alleviate low N use efficiency by various crops such as corn. Some of the best methods that exist require use of a non-N-limiting reference area to make N recommendations. However, placement of reference areas in fields with spatially variable soils and the impact this variability has within the reference area are not fully understood. This study evaluated three strategies to determine goodness of reference areas in two commercial, center pivot-irrigated corn fields with spatially variable sandy soils. Use of the whole reference area without regard to soil differences in the reference area typically designated corn in the sandiest areas as N deficient when in fact it wasn’t. Using NRCS soil survey maps to delineate major soil types within the fields improved corn N status designations but tended to designate corn in lighter soil texture areas within a mapping unit as N deficient. Apparent soil electrical conductivity (ECa) has been shown as a good predictor of soil texture for non-saline soils. Using this as a surrogate for soil texture and creating soil variability maps based on ECa classes, corn N status throughout the reference areas was typically classified as N sufficient regardless of soil differences within the reference area. This technique was best of the three strategies evaluated for determining normalizing values to be applied to like areas in a field with spatially sandy soils.
Technical Abstract: The nitrogen (N) sufficiency approach to assess plant N status for in-season N management requires a non-N-limiting reference to make N recommendations. Use of reference strips in fields with spatially variable soils and the impact this variability has within N enriched reference strips are not well understood. Consequently three strategies were evaluated to determine goodness of reference strips in two commercial center pivot-irrigated corn fields with spatially variable sandy soils. Strategies were to use (i) the median value from the whole reference strip regardless of soil type, (ii) the medium value within NRCS mapping units in the reference strip, and (iii) the median value within apparent soil electrical conductivity (ECa) classes as a surrogate for soil texture differences in the reference strip as normalizing values. A sufficiency index (SI) calculated from radiometer measured canopy reflectance data (SIsensor) and from SPAD chlorophyll meter data (SImeter) at two growth stages during corn vegetative growth were used to assess N sufficiency within the N enriched reference strips. Use of the whole reference strip as a normalizing value indicated corn in the sandier soils within the reference strip was N deficient. NRCS soil mapping units used as normalizing values improved N sufficiency designations of corn in the reference strip for the different soil types contained within the reference strip but tended to designate corn in lighter texture areas within a mapping unit as N deficient. The ECa as a surrogate for soil texture typically performed best for classifying corn N sufficiency throughout the reference strip and is recommended as a method to obtain reference strip normalizing values in fields with spatially variable sandy soils.