Sewage Sludge as a Sweet Corn Fertilizer

Sewage sludge has been used as an effective source of plant nutrients for sweet corn in several short-term studies. However, the long-term effects of sewage sludge on sweet corn yields and soil chemistry have not been widely investigated. Previous results at the North Willamette Station indicated that two to three years cumulative applications of sludge were necessary before yields on sludge-treated soils equalled those on soils treated with optimal rates of commercial N fertilizers. Also, the yield response of sweet corn to banded P fertilizer could not be duplicated with sewage sludge. The experiment described below was intended to measure sweet corn yields after seven consecutive years of application of three Willamette Valley sewage sludges. Several rates of commercial N were also applied, both with and without banded P fertilizer.

Methods

The seventh annual sewage sludge treatment was applied on April 22, 1982, and plowed under within 48 hours. Sludges from the City of Portland, City of Salem, and the Unified Sewerage Agency (Rock Creek) were applied at rates approximately equivalent to 200 and 400 kg/ha of total N. In addition, the high rates of sludge were applied to plots which had been treated with lime at 13.5 MT/ha in 1976. Commercial N as ammonium sulfate was banded at planting at a rate of 56 kg N/ha. Appropriate plots were sidedressed with ammonium nitrate six weeks after seeding. Banded P was applied to appropriate plots as concentrated superphosphate banded at planting.

'Jubilee' sweet corn was seeded on May 21, 1982. Stands four weeks after planting averaged 60,000/ha. Between row spacing was 0.75 m. Plot size was 4.5 m x 9.0 m with four replications of each treatment in randomized complete block design. Yield records were taken from the centermost two rows of each plot. Ears were harvested on September 2 and graded as mature, slightly immature, or culls. Culls were not included in yield figures. Samples of kernels were taken for nutrient and trace metal analysis (data analysis not completed). Only soil pH and yield data are presented here. In addition to the sludge and commercial N treatments, several plots of uniform N rate were split by application of different rates of banded P.

Results and Discussion

Yields of mature ears increased with commercial N application to a maximum at 168 kg N/ha in the presence of banded P (Table 1). In the absence of banded P, mature ear yields also increased with N, with a maximum at 224 kg N/ha. Mature ear yields were always higher with banded P than at the same N rate without banded P; however, the differences were not usually statistically significant. Total ear yields responded similarly to increasing rates of N, with highest yields at 168 or 224 kg N/ha. Total ear yields in the presence of banded P did not generally exceed those obtained without banded P. Thus, the major effect of banded P was to hasten maturity. Trends in mean ear weight were similar to those for yields, but were not statistically significant. Most of the yield increases at higher rates were caused by a greater number of ears harvested rather than increased ear weight.

Yields on sewage sludge amended soil were approximately equal for the two rates of sludge applied and were about equal to the yields with the optimal rates of commercial N fertilizer. Apparently, cumulative applications of the low rates of sludge provided ample N for optimal sweet corn yields at this planting density. Yields did not vary significantly with the source of sludge. As with the commercial N fertilizer, banded P fertilizer significantly increased mature ear yield on plots treated with the low rate of Portland sludge.

Yields on soils treated with lime and sludge tended to be higher than those on soils treated only with sludge and tended to be higher than with optimal rates of commercial N fertilizer. Seven years cumulative application of sludge or commercial N reduced soil pH by up to 0.6 units when compared to unfertilized soil (Table 1). The low pH (5.2 to 5.5) on plots repeatedly fertilized with sludge or higher rates of commercial N may be limiting yields on these plots. Soil pH on the limed plots was nearly 1.0 pH unit higher than on unfertilized plots in 1977. In 1982, the limed and sludge treated plots had the same average pH as unfertilized plots. Apparently, the continued application of acid-forming N fertilizer and sludge mineralization has produced a significant increase in soil acidity (lowering of pH). This may have an impact on uptake of toxic heavy metals from sludge or fertilizer-amended soils.

Several plots previously treated with either chicken manure or sewage sludge were treated with several rates of banded P fertilizer in the presence or absence of starter N in the band (Table 2). Inclusion of N in the band generally had no effect on yield unless was also present. The effect of banded P was usually significant. In most cases higher mature ear yields were obtained on the subplot receiving the higher rate of banded P. Mean ear weight was not significantly affected by banded P treatment.

Sewage sludge is an effective N source for sweet corn and commercial N fertilizer is not necessary for optimal yields. However, P should still be applied in a band at planting. Careful management of soil pH will be necessary to maintain high yield and avoid heavy metal uptake.

Table 1. Soil pH and Yield of Sweet Corn on Sewage Sludge-amended Soil           Commercial  Banded P   Nominal sludge    Soil  Mature     Total    Mean ear wt.   N applied   applied    total N applied    pH  ear yield  ear yield  of all ears  (kg/ha)     (kg/ha)  (kg/ha)                   (MT/ha)   (MT/ha)       (kg)        0	        0	  0	          5.8	 9.2	   11.9        0.28  112	        0	  0	           -	16.2	   18.9	       0.30  168	        0	  0	           -	14.9	   19.8	       0.29  224	        0	  0	           -	17.3	   20.5	       0.30    0	       39	  0	          5.8	11.0	   12.4	       0.28  112	       39	  0	          5.6	16.6	   19.4        0.32  168	       39	  0	           -	20.0	   22.3	       0.33  224	       39	  0	          5.4	18.9	   20.0	       0.33  336	       39	  0	          5.3	18.7	   20.2	       0.31    0	       39	  Portland, 200	  5.5	19.8	   20.5	       0.32    0	       39	  Rock Creek,200  5.5	18.9	   19.8	       0.30    0	       39	  Salem, 200	  5.4	20.7	   22.9        0.31    0	       39	  Portland, 400	  5.3	19.6	   20.7	       0.32    0	       39	  Rock Creek,400  5.2	21.4	   22.7	       0.34    0	       39         Salem, 400	  5.3	19.1       22.5	       0.32    0	       39	  Portland, 400Z  5.9	21.8	   23.6	       0.32    0	       39	  Rock Creek 400Z 6.0	22.7	   24.5	       0.33    0	       39	  Salem, 400Z     5.7	20.9	   22.5	       0.31    0	       39	  Portland, 200Y   -	16.6	   18.7	       0.32    0	        0	  Portland, 200    -	15.5	   17.3	       0.31                                 LSD(0.05)  0.2    4.3        3.6         NS        ZSoil limed at 13.5 MT/ha in 1976.  YHalf the sludge applied in October of each year.      Table 2. Effect of Rates of Banded P Fertilizer on Sweet Corn Yields  Commercial NZ            Banded P    Mature    Total     Mean ear  applied at planting     applied to    ear       ear       weight  to main plots            subplots     yield    yield    (all ears)    (kg/ha)                   (kg/ha)   (MT/ha)   (MT/ha)      (kg)  0Y                           0       18.2      20.2        0.33                              39       17.7      20.5        0.31  56Y                          0       17.5      21.8        0.29                              78       20.2      22.5	   0.32  56Y                          0       14.9      19.4	   0.31                              39       20.7      21.6	   0.29  0X                           0       13.7      17.6	   0.29                              20       17.6      20.7	   0.31  56X                          0       19.4      21.4	   0.33                              20       19.4      19.8	   0.31  56X                         20       16.9      18.9	   0.30                              39       19.1      20.0        0.34      ZAll plots top dressed to give final N rate of 224 kg/ha.  YPreviously treated with chicken manure, 1976-1978.   XPreviously treated with sewage sludge in 1976 and 1977.  

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