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Production Papers

Dairy Manure Compost Improves Soil and Increases Tall Wheatgrass Yield

^a The Noble Foundation, 2510 Sam Noble Pkwy., Ardmore, OK 73401
^b Texas Agric. Exp. Stn., Texas A&M Univ. Research & Extension Center, 1229 N. Hwy. 281, Stephenville, TX 76401

^* Corresponding author (tjbutler{at}noble.org)

Received for publication December 20, 2005.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Confined animal feeding dairy operations have generated excess amounts of manure, creating a need to identify alternative uses for this plant nutrient source. The objectives in this study were to (i) study the effect of composted dairy manure on Windthorst soil (fine, mixed, active, thermic Udic Paleustalfs), (ii) evaluate two soil testing methods for measuring P when composted dairy manure is applied, and (iii) determine tall wheatgrass [Thinopyrum ponticum (Podp.) Barkworth & Dewey ‘Jose’] yield response to six rates of composted dairy manure and two rates of inorganic N fertilizer. A randomized complete block design experiment arranged in a split-plot with four replications was initiated in 2001. Main plots received a single application of composted dairy manure at rates of 0, 11.2, 22.4, 44.8, 89.6, and 179.2 Mg ha^–1, which were incorporated before planting tall wheatgrass at the rate of 17 kg ha^–1. Subplots received annual split applications of inorganic N at 224 or 336 kg ha^–1. Composted dairy manure averaged across the 2002–2003 and 2003–2004 growing seasons increased soil organic matter (OM) 54%, pH 55%, infiltration rate 550%, P 480%, and K 84% in this soil. The improved soil properties increased dry matter (DM) yields each growing season (2002–2003 and 2003–2004) up to 96 and 58%, respectively. Tall wheatgrass had similar crude protein (CP) (158–231 g kg^–1), DM yields (3858–9536 kg ha^–1), P concentrations (1.4–2.8 g P kg^–1), and P removal rates (5.4–26.7 kg ha^–1) compared to other cool-season perennial grasses.

Abbreviations: CEC, cation exchange capacity • CP, crude protein • DM, dry matter • EC, electrical conductivity • EDTA, ethylenediaminetetraacetic acid • OM, organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
APPROXIMATELY 200 000 dairy cattle in confined animal feeding operations generate an estimated 1.8 million Mg yr^–1 of manure in Erath County, TX (Van Horn et al., 1994; USDA, 2006), which can lead to an excess buildup of dairy-manure P in the immediate vicinity of the dairies. As in other intensive agricultural and livestock production systems, soils with plant-available P exceeding the levels required for optimum crop yields become problematic (Alley, 1991; Sims, 1992). To avoid environmental concerns related to P lost by surface water runoff (Sharpley and Withers, 1994), there is a need to identify alternative uses for manure and its compost, especially where soil P levels are low.

The use of compost promotes soil aggregation, which improves soil structure, increases soil pH, benefits water infiltration rate, and improves water-holding capacity (Murray, 1981; USDA NRCS, 2004). The N–P–K percentages of finished compost are relatively low, but their benefit lies in the slow release of organically-bound N and P in the soil that plants can use more effectively (Gershuny and Martin, 1992). Composting reduces the amount of weeds seeds due to the heat generated during the process and NO₃–N tends to be more stable in compost, when compared to raw manure (Helton, 2004).

Forage P uptake from soils is highly variable and is a direct function of soil P content, soil physical properties, forage biomass, and forage P concentration, the latter often species-specific (Pierzynski and Logan, 1993). The application of composted dairy manure to soils can increase plant P concentrations and forage yields resulting in greater P removal, a phenomenon observed when compost is applied to summer annual dicots (Muir et al., 2001a), annual monocots (Muir et al., 2001b), or warm-season perennial grasses (Sanderson and Jones, 1997). The efficacy of using composted dairy manure on cool-season perennial grasses, however, has not been investigated in north-central Texas.

Tall wheatgrass is a cool-season perennial grass that may have potential to provide forage of high nutritive value during the winter months, when the dominant warm-season grasses are dormant (Lauriault et al., 2002). Dotzenko (1961) and Undersander and Naylor (1987) reported Jose tall wheatgrass DM yields of 10 800 and 11 300 kg ha^–1 under irrigation, however, Malinowski et al. (2003) and Gillen and Berg (2005) reported only 900 to 3600 kg DM ha^–1. Malinowski et al. (2003) reported greater yields of Jose tall wheatgrass when clipped at stubble height of 15 cm compared to 7.5 cm the second growing season due to stand decline, and Gillen and Berg (2005) reported that Jose tall wheatgrass stands declined less rapidly when stubble height was 15 cm compared to 10 cm. A cool-season perennial grass could have an advantage over cool-season annual grasses since it does not have to be replanted each autumn, and soil erosion should be reduced because of perennial cover (Gillen and Berg, 2005). Typically soils in this region are very low in available P (Muir et al., 2001a). Composted dairy manure could serve to correct this deficiency. To assess the potential of tall wheatgrass in this environment where P availability is limiting, its P fertility requirement and uptake needs to be determined.

Two soil P testing methods, historically used to measure soil P for both agronomic and regulatory purposes, ammonium acetate-ethylenediaminetetraacetic acid (NH₄OAc-EDTA) (Hons et al., 1990) and Mehlich-3 (Mehlich, 1984), can vary considerably in their estimation of plant-available soil P, when P fertilizer (triple superphosphate) is applied (T.J. Butler, unpublished data, 2006). However, it is still unclear which soil testing method should be used to predict available soil P when composted manure is applied. The objectives in this study were to: (i) study the effect of composted dairy manure on selected Windthorst soil characteristics, (ii) evaluate two soil testing methods for measuring P in a Windthorst soil receiving composted dairy manure, and (iii) determine tall wheatgrass yield response, nutrient concentrations, and nutrient removal rates with six rates of composted dairy manure and two rates of inorganic N fertilizer.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A field study was initiated on a Windthorst sandy loam soil (Udic Paleustalfs) in north-central Texas near Stephenville (32°15' N, 98°12' W, altitude 395 m) in September of 2001 and continued until May of 2004. Initial soil test indicated pH of 5.1, 6 mg N kg^–1, 6 mg P kg^–1 (NH₄OAc-EDTA extractant), and 205 mg K kg^–1. Treatments were arranged in a split-plot randomized complete block design with four replications. There were six main treatments (compost application rate) and two subplot treatments (inorganic N fertilizer rate). Main plots were 2.2 by 15 m and received a single application of composted dairy manure before planting tall wheatgrass at a seeding rate of 17 kg ha^–1 in September 2001. Composted manure rates of 0, 11.2, 22.4, 44.8, 89.6, and 179.2 Mg ha^–1 from Producers Compost, Stephenville, TX, were manually applied and then incorporated to 15-cm depth using a roto-tiller. Compost analysis averaged 150 g kg^–1 OM, 782 g kg^–1 DM, 7.2 g kg^–1 N, 3.9 g kg^–1 P, and 15.7 g kg^–1 K. Subplots were 2.2 by 7.5 m and received inorganic N at 224 or 336 kg ha^–1 yr^–1 as a urea-ammonium sulfate blend surface-applied in equal splits during October and February each year. Tall wheatgrass was sprayed with diclofop-methyl 2-[4-(2,4-dichlorophenoxy) phenoxy]propanoate at 0.84 kg a.i. ha^–1 at the fifth-leaf stage to control annual ryegrass (Lolium multiflorum Lam.), since it would out-compete tall wheatgrass during establishment (Butler et al., 2005). Plots were not harvested during the establishment year (2001–2002), since tall wheatgrass is slow to establish.

In the second (2002–2003) and third (2003–2004) growing seasons, a portion of plot area (1 by 7 m) was harvested 15 cm above soil surface (Malinowski et al., 2003; Gillen and Berg, 2005) with an Almaco small-plot harvester (Almaco, Nevada, IA) three times (December, March, and May), when tall wheatgrass in the control (zero compost) plots were 25 to 30 cm tall. Subsamples were used to determine forage DM yield by drying approximately 400 g of plant material in a forced-air oven at 55°C until weight loss ceased. Total yearly aboveground DM production was estimated by totaling all yields from each subplot. Representative forage subsamples from each subplot and year were ground through a Wiley mill (Thomas-Wiley Co., Philadelphia, PA) equipped with 1-mm screen. Nitrogen in the forage was determined with a micro-Kjeldahl procedure (Sweeney, 1989), while the minerals (P, K, and S) were digested in nitric acid (Havlin and Soltanpour, 1989) and analyzed using a Technicon Autoanalyzer II (Technicon Industrial Systems, Tarryton NY). Nitrogen concentration was multiplied by 6.25 and reported as CP (Van Soest, 1994). Concentrations of these plant components are reported as season-long weighted averages for each subplot. Nutrient removal was estimated by multiplying the nutrient concentration and the DM yield.

Approximately 15 soil cores were taken to a 15-cm depth at the end of each growing season and composited by subplot to determine treatment differences. Soils were analyzed for pH using 1:2 ratio of soil to deionized water (Schofield and Taylor, 1995), electrical conductivity (EC) (U.S. Salinity Laboratory Staff, 1954), NO₃–N by Cd reduction (Keeney and Nelson, 1982), and P, K, S, Na, Mg, and Ca based on two soil-extractant methods, acidified NH₄OAc-EDTA (Hons et al., 1990) and Mehlich-3 (Mehlich, 1984). Elements in both extractants were measured using inductively coupled argon plasma optical emission spectrometer (ICP–OES) (Spectro Radial Modula ICP, Spectro Analytical Instruments, Marlborough, MA). Soil OM was determined by using the Loss-On-Ignition Method (LOI) (Nelson and Sommers, 1996). Infiltration rate of water into the soil was measured with a Turf-tec double ring infiltrometer (Coral Springs, FL), by averaging three readings from each subplot at the end of each growing season. The infiltration rate was determined as the amount of water per surface area and time unit which penetrated the soil (Bouwer, 1986).

Data were subjected to analyses of variance using PROC GLM (SAS Institute, 1999) with treatment differences less than P = 0.05 reported as significant. Means, where appropriate, were separated using Fisher's Protected LSD test at P = 0.05 level of significance. Polynomial contrast (PROC GLM) and regression analysis (PROC REG) (SAS, 1999) were also used on all variables to determine response curves related to compost application.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil Nutrient Status
Soil EC, S, Na, Mg, and Ca did not differ among years, compost levels, or inorganic N rates (P > 0.05), therefore that data is not reported.

Soil Organic Matter
Year, year x compost rate, year x N rate, compost x N, and year x compost x N rate interactions were not significant (P > 0.05) for soil OM, therefore data are pooled across years. Although the OM in the composted dairy manure was only 150 g kg^–1, soil OM increased (P < 0.001) curvilinear (r² = 0.37) as composted dairy manure rates increased (Table 1). Soil OM in the untreated plots averaged 13 g kg^–1 compared to 20 g kg^–1 in the plots with the greatest compost rate (54% increase). This relationship is described by:

The addition of compost OM to the soil can increase cation exchange capacity (CEC) from 20 to 70% of the original CEC (Mott, 1974; Havlin et al., 1999). However, not all composted manures increase soil OM or organic C. Helton (2004) reported that composted dairy manure low in OM (similar to this study) did not increase soil OM when surface-applied up to 71.7 Mg ha^–1 on bermudagrass [Cynodon dactylon (L.) Pers.] pasture, due to low levels of OM in parent material originating from drylot scrapings, which contain significant amounts of soil. The difference between Helton (2004) surface application of compost and the incorporation of compost could account for the increase in soil OM in this study.

Murray (1981) reported that soil pH increased with applications of compost. The soil pH increase can be partially attributed to the increase in soil OM and the high pH of the compost itself due to a high concentration of calcareous soil in the dairy compost (Helton, 2004). Soil pH decreased as inorganic N rate increased from 224 to 336 kg ha^–1, which was expected in the top 15 cm of soil (Haby et al., 1999). Dairy manure and its compost have the potential to raise pH of acidic soils or mitigate the acidification process of soils receiving N fertilizer (Sanderson and Jones, 1997; Helton, 2004).

Soil Infiltration Rate
Year, year x compost rate, year x N rate, compost x N, and year x compost x N rate interactions were not significant (P > 0.05) for soil infiltration rate, therefore data are pooled across years. Soil infiltration rate increased (P < 0.001) curvilinear (r² = 0.92) with increasing rates of composted dairy manure (Table 1). Infiltration rate increased by 100, 242, 292, 408, and 550% for 11.2, 22.4, 44.8, 89.6, and 179.2 Mg ha^–1, respectively, when compared to the control. This relationship is described by:

Increase in infiltration rate can be attributed to increased OM. In addition, improvements in soil moisture retention and infiltration rate, due to the increase in soil OM, have been reported by other researchers (Hoitink and Fahy, 1986; Boehm et al., 1993) and could result in increased crop yields. Inorganic N rate had no effect on infiltration rate.

Soil Nitrate Nitrogen
Year, year x compost rate, year x N rate, compost x N, and year x compost x N rate interactions were not significant (P > 0.05) for soil NO₃–N, therefore data are pooled across years. Composted dairy manure had little effect on soil NO₃–N levels (P > 0.05), although there was a numeric trend for greater composted manure rates having lower soil NO₃–N levels, which could be related to greater forage DM yields and N removal from those plots (Tables 1 and 2). Composting dairy manure tends to lower NH₄–N levels compared to the original manure, but NO₃–N tends to be more stable in compost. In at least one study looking at both dairy manure and its compost, however, NH₄–N concentrations were more stable in the soil than was NO₃–N (Helton, 2004). For the inorganic rate treatment, soil NO₃–N level increased (P < 0.01) by 41% at the 336 kg N ha^–1 rate (41 mg kg^–1) compared to 224 kg N ha^–1 (29 mg kg^–1).

At very low soil P levels, the Mehlich-3 extractant soil P was approximately 2.3 times greater (P < 0.0001) than the EDTA extractant P, but there were no differences (P > 0.05) between extractants when available soil P was very high with the heaviest compost rate (Table 1). The highest compost rate (179.2 Mg ha^–1), which had 124 mg P kg^–1 soil, did not exceed the maximum soil P threshold of 200 mg P kg^–1 allowed by environmental regulatory agencies in Texas (Texas Administrative Code, 1997), indicating that these low-P soils can incorporate very high rates of composted manure before this limit is reached. In contrast, Helton (2004) surface-applied 71.7 Mg compost ha^–1 to bermudagrass on a similar Windthorst soil with a control plot containing 22 mg P kg^–1 soil, and measured 231 mg P kg^–1 plant-available P in the top 5 cm of the soil, which is in excess of the 200 mg P kg^–1 soil limit (EDTA). The main difference may have been that Helton (2004) did not incorporate compost to a 15-cm depth as was done in the present study.

The Texas Cooperative Extension Soil, Water and Forage Testing Laboratory adopted the statewide use of the Mehlich-3 method in January 2004, following the determination that the NH₄OAc-EDTA method dissolved nonplant-available apatite in certain calcareous soils (T. Provin, personal communication, 2004). It appears that the Mehlich-3 extractant is suitable for this acid soil receiving large amounts of composted dairy manure. Inorganic N rate had no effect on plant-available soil P.

Soil Potassium
Year, year x compost rate, year x N rate, compost x N, and year x compost x N rate interactions were not significant (P > 0.05) for soil K, therefore data are pooled across years. Soil K levels also increased (P < 0.001) linearly (r² = 0.60) with both extractants as compost rate increased (Table 1), a phenomenon that was also observed in other compost studies (Schlegel, 1992; Helton, 2004); however, the extractants differed in their estimation of plant-available K (P < 0.0001). These relationships are described by:

The EDTA extractant measured an average of 9% greater levels of plant-available K compared to the Mehlich-3 extractant, which is the reverse trend of plant-available soil P. Mehlich-3 uses a 5-min shaking time compared to 45 min for the EDTA, resulting in a more complete release of soil-bound K (Mehlich, 1984; Hons et al., 1990) which tends to be more weakly bound to soil particles compared to P in its HPO₂ and H₂PO₄ forms (Pierzynski et al., 2005). Soil K levels were adequate even in the untreated plots; therefore it is unlikely that the increased levels of K influenced crop yield. Inorganic N fertilizer rate had no effect on available soil K.

Tall Wheatgrass
Sulfur concentrations of tall wheatgrass tissue and S levels of soil did not differ (P > 0.05) among composted dairy manure and inorganic N treatments, therefore S removal rates are not reported, since these differences were attributed to DM yield differences between years.

Forage Dry Matter Yield
Year and year x compost rate interactions were significant (P < 0.05) for DM yield, therefore data are reported by year. Forage DM yields were greater (P < 0.001) in the 2002–2003 growing season (Table 2), possibly due to greater early-season rainfall (October–December), even though total rainfall among growing seasons (September–June) did not differ (Fig. 1 ). Precipitation totaled 782 and 787 mm in the 2002–2003 and 2003–2004 seasons, respectively, however in 2003–2004, a greater portion of the rainfall occurred at the end of the season (April–June). Lauriault et al. (2002) reported that in one growing season Jose tall wheatgrass responded more efficiently to 168 kg N ha^–1 when rainfall occurred before December compared to a single irrigation in mid-January or two irrigations in mid-December and mid-February.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Monthly precipitation from August to July during 3 yr and 30-yr average trend line at Stephenville, TX.

Forage DM yield was lowest where compost was not applied (4857 kg ha^–1 in 2002–2003 and 3858 kg ha^–1 in 2003–2004) and increased by 32, 44, 64, 85, and 96% with 11.2, 22.4, 44.8, 89.6, and 179.2 Mg compost ha^–1, respectively in 2002–2003 and by 31, 33, 37, 42, and 58%, respectively in 2003–2004 (Table 2). The greatest DM yields occurred with the highest compost rate (179.2 Mg ha^–1) with yields of 9536 and 6097 kg ha^–1 in each growing season, respectively. Undersander and Naylor (1987) reported Jose tall wheatgrass yielded 11 300 kg DM ha^–1 when harvested monthly under irrigation receiving 448 kg N ha^–1. However, Lauriault et al. (2002), Malinowski et al. (2003), and Gillen and Berg (2005) reported Jose tall wheatgrass yields ranging from 900 to 3900 kg DM ha^–1 in areas receiving 400 to 600 mm annual rainfall.

In this study, forage yields did not differ (P > 0.05) between inorganic rates of N fertilizer (224 and 336 kg N ha^–1), which indicates that N was not the limiting factor for plant growth; however further study is needed to determine the optimal N rate in this environment. In a N rate study applying 0 to 720 kg N ha^–1, Dotzenko (1961) reported that Jose tall wheatgrass yield peaked at 360 kg N ha^–1, which yielded 10800 kg DM ha^–1 under irrigation. Yield varies depending on rainfall amount and distribution as well as other environmental conditions in relation to how well adapted the species is to that environment. In this study, tall wheatgrass responded positively to compost dairy manure, which improved soil fertility, especially soil P. Based on these results, tall wheatgrass has forage potential for the region, and could have an advantage over cool-season annual forage grasses since it does not have to be replanted each year.

Crude Protein and Nitrogen Removal
Year and year x compost rate interactions were apparent (P < 0.05) for CP and N removal, therefore data are reported by year. Forage CP concentration was greater (P < 0.001) in 2002–2003 (ranging from 207–231 g kg^–1) compared to 2003–2004 (158–175 g kg^–1) (Table 2); however, all CP values would be considered adequate for most livestock classes (Ball et al., 2002). Crude protein was greatest (P < 0.05) at the two highest compost rates (89.6 and 179.2 Mg ha^–1) in 2002–2003 but did not differ (P > 0.05) among compost rates in the 2003–2004 growing season (Table 2). The increase in CP in 2002–2003 with the highest compost rate is surprising since the two inorganic N fertilizer rates did not differ in CP that season. One explanation could be that the organic N from the composted dairy manure was taken up more effectively than the inorganic N, but this is speculative. These relatively high values are similar to studies that harvested immature forage and are greater than values reported for more mature cool-season perennial grasses. Read (1979) reported up to 230 g CP kg^–1 for immature cool-season perennial grasses in north Texas while Gillen and Berg (2005) reported that CP of Jose tall wheatgrass declined from an average of 211 g kg^–1 in April, 172 g kg^–1 in May to 137 g kg^–1 in June, and Hall (1998) reported that other perennial cool-season grasses showed a marked decline in CP values as harvest interval increased, with values that varied from 117 g kg^–1 for 70-d intervals to 190 g kg^–1 for 35-d intervals. In this study, CP values did not differ (P > 0.05) between the two inorganic N rates in 2002–2003. However, CP was 10% greater (P < 0.05) with the 336 kg N ha^–1 rate in 2003–2004, but this is probably not of biological importance.

The amount of N removed, which is a function of N concentration and DM yield, followed a similar trend to that of yield. In 2002–2003, N removal was greater (P < 0.001) than the 2003–2004 growing season, primarily due to the increased DM production associated with greater rainfall occurring early in the season. In both seasons, the amount of N removed also increased (P < 0.001) curvilinear (r² = 0.70 and 0.41, respectively) as compost rate increased. The amount of N removed increased up to 119%, ranging from 161 to 353 kg N ha^–1 in 2002–2003 and up to 72% increase in 2003–2004, ranging from 98 to 169 kg N ha^–1 (Table 2). Inorganic N rate did not affect (P > 0.05) N concentration or N removal in 2002–2003, however in 2003–2004, the 336 kg N ha^–1 rate removed 17% more N (P < 0.05) due to 10% greater concentration of N in the forage tissue. These data illustrate that the amount of nutrient removal is closely related to DM yield, and as yield increases, the nutrient removal rate also increases.

Phosphorus Concentration and Phosphorus Removal
Year and year x compost rate interactions were significant (P < 0.05) for P concentration and P removal, therefore data are reported by year. Forage P concentrations (Table 2) in 2002–2003 ranged from 1.8 to 2.8 g P kg^–1, which was greater (P < 0.001) than in 2003–2004 (ranging from 1.4–2.3 g P kg^–1). This greater concentration in 2002–2003 could be attributed to more soil P being available from the compost applications made at establishment, while less P was available the following growing season. In the 2002–2003 season, tall wheatgrass P concentrations increased (P < 0.001) linearly (r² = 0.37) by 6 to 56% from the lowest to highest compost rate, and in the 2003–2004 season it increased (P < 0.001) curvilinear (r² = 0.72) by 14 to 64%. These P concentrations are similar to those reported elsewhere. Undersander and Naylor (1987) reported that Jose tall wheatgrass averaged 1.9 g P kg^–1 when harvested biweekly, and ranged from 1.4 to 2.2 g P kg^–1.

The amount of P removed was greater (P < 0.001) in the 2002–2003 growing season compared to the 2003–2004 season, primarily due to greater forage yields and P concentrations resulting from compost application (Table 2). The amount of P removed increased (P < 0.001) as compost rate increased, ranging from 8.7 to 26.7 kg P ha^–1 in 2002–2003 and from 5.4 to 14.0 kg P ha^–1 in 2003–2004. In both growing seasons, removal of P increased (P < 0.001) curvilinear (r² = 0.78 and 0.83, respectively) as compost rate increased by 40 to 206% in 2002–2003 and by 50 to 159% in 2003–2004 (Table 2). However, the cumulative P recovery rates (combined P removal for both growing seasons) were 0.141, 0.106, 0.084, 0.058, 0.038 for composted manure rates of 11.2, 22.4, 44.8, 89.6, 179.2 Mg ha^–1, respectively, indicating that P efficiency is greater at lower compost rates. Inorganic N fertilizer rate had no effect (P > 0.05) on P concentration or the amount of P removed from the soil. These data indicate that tall wheatgrass could potentially be used on high P soils to remove excess P.

Potassium Concentration and Potassium Removal
Year and year x compost rate interactions were apparent (P < 0.05) for K concentration and K removal, therefore data are reported by year. Forage K concentration followed a similar trend as P, by increasing (P < 0.001) linearly in each growing season (r² = 0.27 and 0.44, respectively), ranging from 19.7 to 27.5 g kg^–1 in 2002–2003 and from 18.9 to 24.4 g kg^–1 in 2003–2004 (Table 2). Undersander and Naylor (1987) reported that Jose tall wheatgrass averaged 20.5 g K kg^–1, when harvested biweekly and 22.2 g K kg^–1 when harvested monthly, with values ranging from 16.2 to 25.0 g K kg^–1. In the present study, K concentration increased (P < 0.01) by 9 to 40% in 2002–2003 and 6 to 29% in 2003–2004 from the lowest to highest rate of compost application (Table 2).

The amount of K removed ranged from 96 to 262 kg K ha^–1 in 2002–2003 and 73 to 149 kg K ha^–1 in 2003–2004 (Table 2), which fit a curvilinear response curve (r² = 0.76 and 0.58, respectively). The increase (P < 0.001) in K removal was due to greater yields during the 2002–2003 growing season. Several cool-season grasses have been reported to be luxury consumers of K (Cherney et al., 1998). Tall wheatgrass could also be considered a luxury consumer since K concentrations increased as soil K increased with compost rates despite adequate soil K in the untreated plots. Inorganic N fertilizer had no effect (P > 0.05) on K concentrations or the amount of K removed from the soil.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In a Windthorst soil, composted dairy manure can be applied at very high rates if incorporated before the maximum threshold for soil P is reached. The maximum threshold was not reached with the greatest compost rate in this study; therefore higher compost rates need to be reevaluated to determine rates that can be applied before the environmental maximum (200 mg P kg^–1) threshold is reached, especially when the composted manure is incorporated. The Mehlich-3 extractant is suitable for this acid soil receiving large amounts of composted dairy manure. Composted dairy manure increased soil OM, pH, infiltration rate, P, and K levels. The improved soil properties increased tall wheatgrass DM yields, P, and K concentrations in the forage. Tall wheatgrass is similar to other cool-season grasses in that it responds to improved soil fertility, especially to soil P, and could be used to provide forage of relatively high nutritive value with similar DM yields and P removal rates. It could be considered as an alternative to cool-season annual forage grasses, since it does not have to be replanted each year.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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