Land application of organic materials for soil management in Nebraska is important.

• Organic N applied annually to Nebraska cropland is equal to 150 lb/ac N applied to about 1.3 to 1.6 million acres.
• Beef feedlot manure is important but other livestock manure, but organic municipal and industrial wastes in total are also important. We refer to all as manure in the following.
• The availability of applied organic N and the fertilizer N substitution values of applied organic materials is not well predicted (Table 1).
• The uncertainty of applied organic N availability leads to over-application of fertilizer N resulting in low efficiency of applied N use.
• Canopy sensor guided in-season N application practices have been validated for corn produced on unmanured fields and may be a way to greatly improve N use efficiency for applied organic N.
• Application of 30 to 60 lb/acre of fertilizer N pre-plant followed by in-season fertilizer N application according to crop need may greatly improve applied N recovery and use efficiency.
• The canopy sensor practices may need calibration for manured fields, possibly with variations due to manure type and years since manure was applied.

The research objectives

1. Validate or adapt canopy sensor guided in-season N application practices for fields with manure or other organic material applied.
2. Improve the prediction of the fertilizer N substitution values for organic materials

The Methods

• Two sets of trials
  • Set 1 trials were at Brule (2014-2016) and Eastern Nebraska Research and Extension Center (ENREC) (2015-2017) with 0, intermediate and high levels of composted or stockpiled feedlot manure applied. Each manure level had a set of N ramp treatments with 27 lb/acre N rate increments to 107 lb/ac, and a high N reference strip of 178 lb/ac, applied before planting.
  • Set 2 trials were at 6 locations in eastern Nebraska with 8 different organic materials during 2016 and 2017 (Table 2). No pre-plant fertilizer N was applied to the organic material treatments.
• The crop was continuous corn with no tillage
• Crop canopy reflectance was sensed for NDRE (Normalized Difference Red Edge Index) at V12 to v14 and plots were split for with and without sensor guided in-season N application.
• The algorithm for interpretation of sensor readings was:
  
  
  
  
  
  
  
• Grain yield and other variables were measured.

Results I. Crop canopy sensor guided in-season N application for manured fields.

• The in-season N rate at ENREC was 43% and 17% less with manure applied compared with no manure for the year of application and for the mean of the 2^nd and 3^rd year after application, respectively. The in-season N rate was 23% less with manure applied at Brule.
• The sufficiency index algorithm over-estimated N need when the pre-plant N rate was 0 or 27 lb/acre (30 kg/ha) (Fig. 1).
• Yield potential was lost if the crop was too stressed by low pre-plant N application.
• Pre-plant N of 55 lb/acre prevented excessive low N stress while allowing for NDRE differences expressive of crop N need and determination of in-season N application rates.
• The best time for canopy reflectance sensing was determined to between V12 and V14.
• The above SI algorithm worked well (Fig. 1). Data analysis has not yet finalized improvements to the algorithm either for manured or unmanured fields.
• In a preliminary comparison of the above algorithm with another used in Nebraska, the latter tended to under-apply in-season N. This needs further data analysis.

Results II. Improve the prediction of the fertilizer N substitution values for organic materials.

• Similar for rainfed and irrigated
• 77% greater for a loamy sand compared with silt loam and silty clay loam soils
• Not affected by C:N ratio for these materials but a big effect of cellulose and lignin contents so that N substitution was 2.3 times more for livestock manure compared with municipal biosolid
• Not reduced by composting compared to uncomposted.

Compared with previous estimates of FNS (Table 1, 2)

• FNS was 28% higher for cattle manure
• FNS was 67% higher for compost.