Improving fertilizer P and N use efficiency in sweet corn (2013)

Research report to the Oregon Processed Vegetable Commission

Dan Sullivan
OSU Dept. of Crop and Soil Science

Ed Peachey and Aaron Heinrich
OSU Dept. of Horticulture

A. Enhanced efficiency nitrogen fertilizers for sweet corn.

This project evaluated the efficacy of nitrogen (N) supplied by Enhanced Efficiency Fertilizer (EEF) products (urea +  additive) as compared to untreated urea.  We measured product efficacy in retaining plant-available N in the root zone, and increasing ear yield and plant N uptake.  Soil ammonium + nitrate-N in soil was measured for 8-wk following pre-plant broadcast fertilizer application.  Field trials were conducted on-farm (Monroe) and at OSU Veg Farm. All EEFs were dry fertilizer products. 1) ESN. Polymer coated urea. Polymer coat reduces the rate at which urea-N is converted to leachable nitrate-N. 2) Duration 45.  Polymer coated urea.  Polymer coat is thinner (designed to release N more rapidly than ESN). 3) Super-U.  Urea fertilizer treated with chemical urease and nitrification inhibitors. The urease inhibitor slows the conversion of urea to ammonium-N by the soil enzyme, urease.  The nitrification inhibitor slows the microbially-mediated conversion of ammonium-N to leachable nitrate-N in soil. 4) Nitrapyrin (one trial).  Chemical nitrification inhibitor. Inhibits microbial conversion of ammonium-N to nitrate-N in soil. Ear yield and crop N uptake were not increased by any of the EEF formulations (5% probability level).   Nitrogen fertilizer rates evaluated in 2013 (80 to180 lb/ac) were apparently in excess of that required for maximum ear yield.  All EEF products slowed urea-N conversion to nitrate-N during the first 2 wk following application, providing some protection from early season leaching. After 2-wk, product efficacy varied.  All products released most of the fertilizer N as nitrate prior to growth stage for side-dress N fertilizer application. Duration-45 was the least effective; the other products should be evaluated in 2014 at lower urea-N application rates.  

B. Improving fertilizer P use efficiency in sweet corn

This is the second year of field trials evaluating corn yield response to starter P fertilizer application rate.   In 2013, field trials were conducted at the OSU Vegetable Research Farm (Corvallis) and in a grower field.   Starter P fertilizer rates were 0, 15, 30, 60 and 120 lb P2O5 per acre. All treatments received the same N and K applications. Soil test P (Bray P1 method) was 42 ppm at OSU Veg Farm (Corvallis I), and 135 ppm at the grower field (Corvallis II).   Ear yield and quality did not increase with P fertilization across the six field sites evaluated in 2012 and 2013 suggesting that starter P fertilizer rates can be reduced without compromising ear yield. Five of the 6 sites where this research was conducted (2012-13) exceeded 50 ppm soil test P (Bray P-1 method), the soil test level at which little or no starter P is recommended in OSU Guide EM 9010.  In future, alternative starter fertilizer formulations may play a role in delivering the desired rate of starter N with a reduced P fertilizer rate.  Mixtures of urea-ammonium nitrate (Solution 32; 32-0-0) with ammonium polyphosphate (APP; 10-34-0) are one way to maintain N in starter fertilizer while reducing its P. This fertilizer would supply half the N as urea.  Urea fertilizers should not be placed in contact with seed.  To avoid salt damage to seeds, the OSU Guide (EM 9010) recommends starter fertilizer application at a rate less than 90 lb N+K2O in a band 2 inches below and 2 inches beside the seed row.   

C. Improving fertilizer N use efficiency in sweet corn.

Soil N mineralization is the process whereby soil organic matter decomposes, releasing plant-available nitrate-N in the process. The more N supplied by soil organic matter, the less is needed from N fertilizer.  Improved N mineralization estimates will make  more accurate N fertilizer rates possible.  This year we measured N uptake by unfertilized corn plants at an additional 6 farm locations (vs. adjacent farmer-fertilized corn), and also quantified N mineralization in the laboratory.  Specific objectives (2013) were to 1) Evaluate at-planting soil NO3-N as a predictor of N fertilizer needs, and 2) Quantify the range of typical N release rates from soil organic matter. N mineralization rates across sweet corn fields were reasonably consistent, except in unusual situations (plowdown of legume cover crop or application of dairy manure for many years).  We recommend that growers continue to use the PSNT soil test at V-6 as their primary tool for forecasting N fertilizer need.  At-planting soil testing was shown to have value for predicting N fertilizer needs (although it is probably less accurate than PSNT).  A site specific test for soil N mineralization rate was not accurate enough to recommend for commercial adoption.  

Evaluate at-planting soil NO3-N as a predictor of N fertilizer needs (2012-13): Soil NO3-N was always higher in the PSNT soil samples (corn at V-6) compared to at-planting samples. Soil nitrate increased by an average of 28 and 37 lb N/acre between planting date and PSNT sampling date in 2012 and 2013, respectively. Growers who are applying all their N fertilizer pre-plant and using the at-plant soil nitrate value with the PSNT interpretive guidance (Table 10 EM9010) should apply ~30 lbs less N/A than the PSNT recommendations (to account for soil N mineralization between plant and the PSNT).

Quantify the range of typical N release rates from soil organic matter.  Nitrogen mineralization in the field. 2011-13 (27 fields): we found that sweet corn obtained at least 40% of its N from mineralization (average = 65%), with fertilizer supplying the rest. Unfertilized plants at silking contained an average of 88 lb/acre.   This finding is in general agreement with N mineralization rates measured in the laboratory (see below).  Nitrogen mineralization in the laboratory.  At 73oF (23 oC) in the laboratory, the average soil N mineralization rate after 42 d was approximately 0.37 ppm N/day (1.3 lb N/acre-ft/day). This measured laboratory N mineralization rate is in general agreement with the rate of N mineralization observed in the field. A rate of 0.37 ppm N per day is equivalent to about 40 lb N per acre-ft in 30 days.  In general, soil N mineralization rates were not different enough among field sites to justify a routine laboratory evaluation of soil N mineralization rate.