Hunter, L.A., Extension Educator, University of Idaho Extension
Falen, A., Research Associate, University of Idaho Soil Pedology Lab
Falen, C.L., Extension Educator, University of Idaho Extension
Kinder, C.A., Extension Educator, University of Idaho Extension
Moore, A., Soil Specialist, University of Idaho Extension

ABSTRACT

Researchers examined in-season plant available phosphorus (P) and potassium (K) from applied dairy compost in a high-elevation, dryland, organic alfalfa system. A buried bag technique was used to monitor in-season plant available P and K over two growing seasons. In 2010, the 5 and 10 tons/acre compost application rates increased the soil P levels for adequate alfalfa growth above the control. Conversely, in 2011 the levels of plant available P during the growing season were adequate or slightly below the optimal level for alfalfa growth. There was no significant difference between the 5 and 10 tons/acre plant available P throughout the two growing seasons, indicating no advantages in available soil P with the 10 tons/acre verses the 5 tons/acre. The 10 tons/acre compost rate was necessary to increase K levels to meet or slightly exceed deficiency thresholds. Research results indicated that 5 tons/acre of compost was optimal for adequate P availability, while the 10 tons/acre was optimal to increase K availability ideal for this high-elevation alfalfa production system. Composting dairy manure in southern Idaho is relatively new, and the methods used are evolving each year. Growers are encouraged to obtain laboratory analyses of the applied dairy compost to better estimate the potential nutrient availability for their soil fertility regime. 

Introduction

Both conventional and organic growers in southern Idaho are actively pursuing affordable soil additives to replace or reduce commercial fertilizer use. In organic farming systems, compost can be the primary source of soil nutrients. With Idaho being the third largest dairy producing state, dairy compost is a viable choice for farmers due to the large quantity of local dairy manure produced each year. A limited yield environment with higher elevations and a shorter growing season in south-central Idaho (USDA zone 4b-5b) increases the incentive for farmers to grow organically in order to receive higher price premiums for organic hay. Since Idaho is the second highest value US producer of alfalfa hay (NASS, 2011), organic producers will need more information on how to best utilize a local organic nutrient source for soil fertility and crop yield management.

More research is needed to understand the process of mineralization of dairy compost to help organic growers match nutrient release to crop nutrient demand (Seyedbagheri, 2010). Although there has been research done on nitrogen (N) mineralization from compost in southern Idaho, few studies have looked at plant available P and K during the growing season, or in high-elevation systems. In general, P contribution from composted manure and plant uptake is less understood (Gagnon & Simard, 1999).

The nutrient benefits of dairy compost are not always understood due to variability in compost nutrient composition and the site-specific mineralization rates that help determine when nutrients are made plant available. Composting of dairy manure in southern Idaho is relatively new and the methods used by the dairies are evolving each year. Different methods used in the composting process create variability in source-compost composition (Gagnon & Simard, 1999; Chen, Moore, & de Haro-Marti, 2012), which may limit grower adoption due to uncertainty in compost nutrient quality and optimal application rates. In this study, researchers examined in-season plant available P and K from different rates of applied dairy compost in a high-elevation, dryland, organic alfalfa system over two growing seasons. This research will help to identify optimal dairy compost application rates for producers looking to adopt this sustainable soil management practice. 

Methods

This project compared in-season available P and K in 0 (control), 5 and 10 tons/acre “as applied” rates of dairy compost. Dairy compost was fall applied on an organic, dryland alfalfa field on October 22, 2009 and October 26, 2010. The research site was a producer’s field in Picabo, ID that is considered a high-elevation farming system, with an average elevation of 5,000 ft. A commercial calibrated spreader truck applied dairy compost at rates equal to five and 10 tons/acre on 50’ x 350’ feet randomized plots side-by-side, replicated four times. Replicated control plots had no applied compost. These compost rates were as applied, not on a 100% dry matter basis. Moisture content of the compost averaged 36.4%. Therefore, the 5 tons/acre rate was 3.2 tons/acre on a 100% dry matter basis and the 10 tons acre was 6.4 tons/acre. The age of the dairy compost when applied was estimated to be four to six months old. Compost was applied each fall to the soil surface of the existing alfalfa stand and natural precipitation incorporated it into the soil. The alfalfa stand was five years old in 2009 with the first year of compost application.

A late fall compost application was considered a sustainable practice in this high-desert farming system. Colder temperature decreases microbial activity and reduces the potential for nutrient loss during the winter. In addition, the average two to three foot snow over the winter locked in the compost and the nutrients rather than promoting runoff. A spring application was not ideal in this system due to spring snow melt and wet soil conditions. Heavy equipment traffic on wet soils promotes soil compaction which is counterproductive to adding compost to the soil. Fall application of compost facilitates nutrients being available in the spring for alfalfa growth.

The compost source was dairy manure and straw bedding from local organic dairies. The composting company followed the USDA organic regulations under the National Organic Program to prepare the compost using a turned windrow system. The USDA organic regulations state that an initial C:N ratio between 25:1 and 40:1 must be established. The compost must maintain a temperature between 131°F and 170°F for 15 days using a windrow composting system, at which the pile must be turned a minimum of five times. The source compost company turned their pile 12-15 times during this process.

Researchers used a buried bag technique to measure in-season plant available P and K during the 2010 and 2011 growing seasons, at a 1-foot soil depth. Following the methodology outlined by Westermann and Crothers (1980), researchers collected an initial composite soil sample from each replicated plot on 6/28/2010 and 6/14/2011. This same day, researchers buried three mineralization bags in 2010 and four in 2011, to be pulled at a later date from each replicated plot. With additional funding in 2011, researchers were able to increase seasonal monitoring of P and K by adding an extra buried bag.  Polyethylene bags were used because researchers were monitoring nitrogen (N) mineralization in addition to available P and K. The use of mineralization bags for all in-season data collection (N, P, and K) would minimize sampling variability by monitoring nutrients at the same location within the field for each plot. In addition, the polyethylene bags would minimize nutrient volatility and leaching loss.

Researchers pulled mineralization bags on average every 46 days during the growing season. The initial composite soil sample and all mineralization bag samples were analyzed for available P and K using the Olsen’s extracting solution, 0.5M sodium bicarbonate buffered at pH 8.5 (Olsen & Sommers, 1982). The P and K solutions were analyzed using an inductive coupled plasma (ICP) spectrometer (iCAP 6000) instrument. A least squares differences (LSD) multiple comparison analysis was used for soil test P and K data.

Results and Discussion

Alfalfa removes large quantities of P from the soil (8-16 lbs P₂O₅ removed per ton of hay at 88% dry matter; Koenig et al., 2009). In addition, the range of soil P for optimal alfalfa growth is 20-25 ppm in this growing region, depending on the free lime content and the yield goal (Stark, Brown, & Shewmaker, 2002). The control plot soil analysis for the organic alfalfa showed deficient P levels (Figure 1-2). Therefore, application of dairy compost was beneficial to increase soil P levels for alfalfa growth.

In-season plant available P concentrations in 2010 were significantly different in the 5 and 10 tons/acre application rates compared to the controls at  the 0, 79, and 140 day soil sampling dates, following the fall application (Figure 1; p-values = 0.0167, 0.0157, and 0.0425, respectively). In 2011, plant available P concentrations were significantly different in the 5 and 10 tons/acre application rates compared to the controls at all sampling dates (Figure 2; p-value = < 0.0001; F value = 77.14).

In 2010, the 5 and 10 tons/acre application rates increased the soil P levels for adequate alfalfa growth above the control (Figure 1). Conversely, in 2011 the levels of plant available P in the control treatment during the growing season dropped below the ppm level for optimal alfalfa growth. There was no significant difference between the 5 and 10 tons/acre plant available P throughout the two growing seasons, indicating no advantages.

A range of 160 to 200 ppm of soil K is ideal for optimal alfalfa growth in this area. Potassium deficiencies are uncommon in Idaho soils, but can develop with fields planted with alfalfa for many years (Stark, Brown, & Shewmaker, 2002). The control plot soil analysis for the organic alfalfa showed large deficiencies of K (Figure 3-4).

In 2010, the soil K levels fluctuated little throughout the growing season (Figure 3). There was a significant difference between the 10 tons/acre application rates and the controls in 2010 for the first, third, and last soil sampling dates (Figure 3; first date, p-value = 0.0348; F value = 6.19; last date, p-value = 0.0381; F value = 6.74). In 2011, soil K levels slowly declined throughout the growing season (Figure 4). There was a significant difference between the 10 tons/acre and the control throughout the growing season (Figure 4; p-value = 0.0001; F value = 50.86).

The in-season available K levels in the control plots were variable from the 2010 and 2011 growing seasons. In 2010, all replicated plots showed an increase in the in-season available K and P at 39 days of field incubation. Being in alfalfa, the soil had not been mixed for five years. Researchers predict the combination of mixing the soil for the mineralization bags as well as the soil temperature rising increased microbial activity and stimulated an increase in available K and P in 2010 for all plots. Following 39 days, the in-season available P and K levels declined as the alfalfa crop took up nutrients. In the control plots for the 2011 growing season, the in-season available K levels started high and then declined as alfalfa took up nutrients. The control plots for both growing seasons never went above 100 ppm.

The 10 tons/acre compost rate was necessary to increase K levels to meet or slightly exceed deficiency thresholds. Therefore, the higher application of dairy compost is beneficial to maintain adequate soil K levels and minimize yield loss due to K deficiency. The differences seen in soil K between 2010 and 2011 were related to variations in compost nutrient content.

                       Figure 1. In-season plant available P over the 2010 growing season in the replicated 0, 5, and 10           tons/acre compost plots.

                        Figure 2. In-season plant available P over the 2011 growing season in the replicated 0, 5, and 10                        tons/acre compost plots.

                        Figure 3. In-season plant available K over the 2010 growing season in the replicated 0, 5, and 10                       tons/acre compost plots.

                      Figure 4. In-season plant available K over the 2011 growing season in the replicated 0, 5, and 10                       tons/acre compost plots.

Conclusions

The research results indicate the 5 tons/acre of compost is optimal for adequate P availability; while the 10 tons/acre is optimal to increase K availability ideal for this high-elevation, dryland alfalfa system. Overall, the highest (10 tons/acre) application rate showed the greatest amount of in-season plant available P and K during the two growing seasons. With the data results for in-season plant available P, a grower could decide to apply 5 tons/acre every year or apply 10 tons/acre every other year.

Differences in available soil P and K between growing years indicate variation in compost quality and environmental conditions from one growing season to the next. The research highlights the importance of obtaining a laboratory analysis of the applied dairy compost to better estimate the potential nutrient availability for the soil fertility regime. This indicates a need to improve the production of dairy compost in southern Idaho for a more uniform compost product for large-scale farm use.

Dairy compost and its ability to add soil nutrients and increase organic matter can offer both conventional and organic producers in southern Idaho a sustainable farming practice. By analyzing applied compost, growers can estimate a nutrient content dollar value from dairy compost in comparison to the cost of obtaining the same nutrients from a synthetic or other fertilizer product. Since southern Idaho soils are low in organic matter, growers should consider the unknown dollar value on increasing organic matter from compost application and the long-term benefits for soil fertility and crop production.

Researchers will continue to look at the data to indicate optimal application rates between the 5 and 10 tons/acre for increasing soil P and ultimately helping to build soil residual P and K levels from year to year. This information will help develop future economic budgets for organic and conventional producers by developing and refining alfalfa fertility guides that utilize sustainable agronomic practices. In particular, the research results will contribute to optimal compost application rates and how that relates to nutrient release of in-season P and K in a dryland, high-elevation farming system.

With Idaho being the second highest value US producer of alfalfa hay and the third largest dairy producing state (NASS, 2011), this research will help producers utilize a local organic nutrient source for soil fertility and crop yield management. In addition, all research and outreach efforts related to dairy compost utilization will help solve nutrient cycling issues for the dairy industry in southern Idaho.

ACKNOWLEDGEMENTS

Thanks to Jodi Johnson-Maynard for soil analysis funding and research support as part of a USDA Tri-State Organic Grant.

Literature Cited

Chen, L., Moore, A., & de Haro-Marti, M.E. (2012). On-Farm Composting Management. UniversityIdaho, College of Agricultural and Life Sciences, Dairy Compost Production and Use in Idaho, CIS 1190.

Gagnon, B., & Simmard, R. (1999). Nitrogen and Phosphorus Release from On-Farm and Industrial Composts. Canadian Journal of Soil Science, April, 481-489.

Koenig, R.T., Horneck, D., Platt, T., Petersen, P., Stevens, R., Fransen, S., & Brown, B. (2009). Nutrient Management Guide for Dryland and Irrigated Alfalfa in the Inland Northwest. Pacific Northwest Extension Publication. PNW0611.

NASS. (2011). Crop Values 2011 Summary. USDA. [Online] Available at http://www.nass.usda.gov/Publications/Todays_Reports/reports/cpvl0212.pdf. (Retrieved February 2012).

Olsen, S.R. and L.E. Sommers. 1982. Phosphorus. P 403-430. In: A.L. Pageetal.(eds.) Methods of Soil Analysis, part 2. Agron. Mongr. 9. 2nd ed. ASA and SSSA, Madison, WI.

Seyedbagheri, M.M. (2010). Compost: Production, Quality, and Use in Commercial Agriculture. University of Idaho, College of Agricultural and Life Sciences. CIS 1175.

Stark, J., Brown, B., & Shewmaker, G. (2002). Southern Idaho Fertilizer Guide: Irrigated Alfalfa. University of Idaho, College     of Agricultural and Life Sciences. CIS 1102.

Westermann, D.T., & Crothers, S.E. (1980). Measuring Soil Nitrogen Mineralization Under Field Conditions. Agronomy Journal, 72 (Nov-Dec),1009-1012.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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