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Fertilizer Requirement for Native Perennial Plants Harvested for Biomass

Study author(s): Craig Scheaffer, University of Minnesota, Department of Agronomy and Plant Genetics
Years of study: 2008 – 2009
Location(s): Lamberton MN, Rosemount MN, Austin MN

Important: for the complete report, including all tables and figures, please download using the link(s) to the right.


Field research evaluated the effect of N, P2O5 or K2O fertilizer on native prairie biomass yield, energy yield, and nutrient content. At the initiation of the trial, unfertilized biomass yields were 2.7, 1.8, and 3.0 ton/acre at Austin, Lamberton, and Rosemount, respectively, and were increased 45, 27, and 43% respectively by N fertilization. Ethanol yield per ton averaged 108 gallons/ton and decreased slightly with nitrogen fertilization. Ethanol yield/acre ranged from 211 to 323 ton/acre and was increased with nitrogen fertilization. Response to K2O and P2O5 application was less consistent and dramatic compared to N fertilizer. Concentration and content of N, P, K, and S in biomass harvested in the fall were low. Short-term harvest should not deplete soil nutrients. Producers will be able to use nitrogen fertilizer as a management tool to optimize biomass and ethanol yield.


Native perennial, warm season prairie plants have been designated by the Department of Energy as a source of biomass for energy production. They have potential for conversion to ethanol, gasification, or direct combustion. Perennial prairie species can generate significant biomass and provide many ecological services like nutrient recycling, soil erosion control, and wildlife habitat. Switchgrass, a native perennial grass, is a primary candidate for production of cellulosic fuels. In addition to pure stands of switchgrass, mixtures of native grasses (e.g., switchgrass, big bluestem) with forbs (e.g., sunflowers) and legumes (e.g., Canada milkvetch) have been recommended to provide greater long-term stress tolerance and yield stability compared to grass monocultures. Economically and environmentally sound fertilizer nutrient recommendations are lacking for native grasses and native plant mixtures proposed for biomass crop removal systems. Our objectives were to determine the N, P, and K fertilizer requirements for native perennial prairie plants used for biofuel production.


Research was conducted in established stands of native perennial plants at Austin, Lamberton, and Rosemount in 2008 and 2009. Initial soil characteristics are described in Table 1. Research that was initiated in 2008 at New Ulm was terminated due to extreme soil irregularities and due to damage to the plots from ATV traffic.

The experimental design was a randomized complete block with 4 replications per location. Plots were 10 by 10 ft. All plots received variable rates of nitrogen (N) fertilizer: 0, 50, 100, 150, and 200 lb/acre that were combined in a factorial arrangement with variable levels of P2O5 or K2O fertilizer depending on the soil type. For the low P soils, we applied P2O5 rates of 0, 30, 60, 90 and 120 lb/acre and for low K soils, K2O was applied at 0, 40, 80, 120, and 160 lb/acre. Based on soil test results (Table 1) variable P2O5 rates were applied at Austin and Lamberton. At Rosemount, variable K2O rates were applied. Fertilizers were broadcast in mid-May of each year.

Biomass yield was determined by harvesting a 3 by 3 ft area to a 3 inch height within each plot in early November each year following freezing and drying of the biomass. A 2 lb subsample was collected to a 3 inch height and oven dried. The subsample was dried and yield expressed on a dry matter basis. The subsample was then ground and was analyzed for cell wall sugars using a combination of wet chemistry(Theander et al., 1995) and Near Infrared Reflectance Spectroscopy. Equations for NIRS were developed using the software program Calibrate (NIRS 3 version 4.0, Infrasoft International, Port Matilda, PA) with modified partial least squares regression option (Shenk, 1991). Random subsets of 10 samples were chosen and subjected to conventional chemical analysis and for mineral content using wet chemistry at a commercial laboratory(Agvise Laboratories, Benson, MN). Ethanol production was determined using the Department of Energy ethanol yield calculator that is based on 5- and 6-carbon sugar content.

Soil was sampled from a 0-6 inch depth in spring 2008 from each replicate and combined for analysis for pH, P, and K. Soil was also sampled in fall of 2009 from each plot and analyzed for pH, P, and K. Soils were also sampled for soil N from 0-6 inches and 6-12 inches. All soil analysis was conducted using standard analysis  techniques (Agvise Laboratories, Benson, MN).

Differences among treatments were determined using ANOVA. Because the interaction between treatments and location was significant (P < 0.05), we analyzed each location separately. Least significance differences (LSD) were used to separate means when F tests were statistically significant (P < 0.05). For some variables, if statistically significant response to N, P, or K was indicated by ANOVA, regression analysis was conducted (SAS Institute, 2003). Data were analyzed at α = 0.05 using the MIXED procedure of SAS (SAS Institute, 2003).


The contrasting soil types, vegetation, and climate for each location resulted in a statistically significant (P<0.05) location by treatment by year interaction. Therefore, results are presented separately for each location and year.

Biomass and ethanol yield

Biomass and ethanol yield results are shown in Tables 2-10. Unfertilized yields in 2008 ranged from 1.5 ton/acre at Lamberton to 3.1 ton/acre at Rosemount. Possibility due to combined effect of consecutive years of harvest and moisture deficits yields of unfertilized treatments declined in 2009 at all locations and ranged from 1.0 ton/acre at Rosemount to 1.6 tons per acre at Austin.

Biomass and ethanol yields were increased by N applications at all locations and yield responses to N fertilization were similar at all rates of P2O5 and K2O fertilization (i.e., no significant (P<0.05) N rate by P2O5 and K2O rate interactions. Biomass yield only responded to P2O5 at Austin in 2008. Biomass yield did not respond to K2O fertilization at Rosemount. At Austin in 2009 and Rosemount in both years, a quadratic response to nitrogen fertilizer occurred indicating that we had reached a  nitrogen fertilizer rate that maximized yield. At Lamberton in both years and Austin in 2008, the response was linear indicating that the optimum rate had not been reached, but the yield increased only 11.6 lb of biomass dry matter for each pound of N fertilizer applied at Austin in 2008 and an average of 5.4 lb of biomass dry matter for each pound of N fertilizer applied at Lamberton. The biomass and ethanol responses to N are illustrated in Figures 1-6.

Ethanol yield (in gallon/ton) was calculated based on biomass sugar content and was similar over the three locations (average of 106 gal/ton). Within locations, the response to increased N rates was small with the greatest yield in gallon/ton often for the unfertilized treatment. Ethanol yield (gal/ton) declined linearly at all location except at Lamberton in 2008 when no response to N fertilization occurred. The effects of P2O5 and K2O fertility treatments on yields were limited except in 2008 at Austin where increasing P2O5 rates increased ethanol yield (Figure 1).

Ethanol yield (gallon/acre) averaged 323, 211 and 317 gal/acre at Austin, Lamberton, and Rosemount, respectively, but considerable year-year variability occurred at Austin and Rosemount. Ethanol yield per acre response to N fertilization was similar to that observed for biomass yield. The response to increasing N rate was  quadratic for both years at Rosemount and linear at Lamberton. The response was linear and quadratic in 2008 and 2009, respectively, at Austin.

Mineral concentration of the biomass

Nitrogen, P, K, and S concentration of the biomass was low and averaged 0.63, 0.08, 0.31, and 0.05%, respectively, over the three locations and years (Tables 11-23). These levels in the mature biomass harvested in fall were low relative to herbage harvested during the growing season because of nutrient translocation to belowground portions of the plant. It is likely that nutrient leaching also occurred. The response of biomass nutrient concentration to N fertilization varied with location and nutrient. Biomass N concentration increased linearly with increased N rates at all locations and years. Biomass P, K, and S concentrations were increased by N fertilization at Austin and Rosemount each year. This response was mostly linear (data not shown). At Austin and Lamberton, where P2O5 fertilizer rates were applied, P2O5 application increased P concentration of the biomass. At both locations, this response was linear in 2008 and quadratic in 2009. The linear increase in biomass K concentration due to K2O fertilization at Rosemount was significant but small. Biomass S concentration response to N fertilization was linear at Austin and Rosemount and quadratic at Lamberton.

Uptake and therefore potential removal of N, K, and P by biomass harvest is shown in Tables 24-36. Nitrogen fertilization resulted in removal of N, K, and P at all  locations. These responses were primarily linear for N concentration of the biomass, and a combination of linear and quadratic for P and K concentration of the biomass. As expected, P2O5 and K2O application that increased P concentration of the biomass also increased P and K removal. Overall, nutrient removal was low and most soils should be able to supply adequate levels of P, K, and S for continued biomass production.

Biomass sugar content

Cell wall sugar concentration information is shown in Tables 37-40. Nitrogen fertilization decreased C5 (xylan and arabinan) sugar and C6 (glucan, galactan, and mannan) sugar concentration at Austin in both years, and at Lamberton and Rosemount in 2009. Nitrogen fertilization also decreased C5 sugar content at Lamberton in 2009 and C6 sugar content in 2009. Effects of P and K fertilization were inconsistent over the locations and years. Changes in cell wall sugar content due to fertilization are reflected in the slight decrease in ethanol yield/ton of biomass with increased N fertilization rates (Table 10).

Soil properties

The effect of treatments on soil characteristics is shown in Tables 41-47. Fertilization with N, P, or K had a slight but inconsistent effect on soil pH. Average ending pH at Austin, Lamberton, and Rosemount was 6.2, 6.5, and 6.7, respectively.

At all locations, soil N levels were low reflecting plant uptake of soil N. However, nitrogen fertilization increased 0-6 inch and 6-12 inch soil nitrogen at Lamberton and Rosemount but not at Austin. The greatest effects were observed at the 150-200 lb N rates. Nitrogen fertilization had no effect on soil P or K levels. However, as expected, P and K fertilization did significantly increase soil test P and soil test K values when those treatments were applied (Table 47).

At Austin the soil test P was increased from the initial soil test of 12 ppm to 33 ppm at the end of the study for the zero phosphate rate treatment. The addition of phosphate fertilizer increased the soil test P from 32.9 ppm for the check to 48.6 ppm when 120 lbs phosphate per acre was applied for two years. At Lamberton, the initial soil test P was 8 ppm. The check after two years of this study was similar at 5 ppm. The addition of 120 lbs phosphate per acre each year of the study increased the soil test P to 21 ppm. This indicates that the phosphorous removal by the harvest of the vegetation is minimum and should not be a production problem.

At Rosemount, the initial soil test potassium was 160 ppm. After two years of production the soil test K for the check plots receiving no potash fertilizer was 128 ppm. This reduction is surprising as the potassium removal in the vegetation was small, 10 to 30 lbs per acre. The addition of potash fertilizer increased the soil test K. When 160 lbs of potash per acre was applied each year the soil test increased from 128 ppm for the check to 197 ppm.

educational activities

The results of this research were presented at the Third Crop Producer Meetings. February 8, 2010 at Fairmont, MN. The event was promoted by Rural Advantage and University of Minnesota Extension. The presentation was titled: Biomass Fertility and Yield Trials at the University of Minnesota. We plan a presentation at the Southwest Research and Outreach Center this summer and to develop educational publications on this research.

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