ALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION

ALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION
DO FOUR CUTTINGS OF ALFALFA USE MORE WATER THAN
EFECTOS_DE_LA_CALIDAD_DE_LA_ALFALFA_SOBRE_EL_ENGORDE_DE_NOVILLOS_HOLSTEIN

LAS CLAVES EN LOS CULTIVOS DE ALFALFA Y SOJA
PASTURAS DE ALFALFA IMPORTANCIA DE UNA ADECUADA INOCULACIÓN ING
TG65 ALFALFA 20050406 7 S TG65 ORIGINAL

ALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION ON LOW K-TESTING SOILS*

ALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION ON LOW K-TESTING SOILS*


Richard T. Koenig, James V. Barnhill and Jody A. Gale

Plants, Soils and Biometeorology Department and Cooperative Extension

Utah State University, Logan, Utah 84322-4820


ABSTRACT

Alfalfa (Medicago sativa) is an important forage and cash crop in Utah and the Western U.S. In the past 20 years, incidences of potassium (K) deficiency in Utah alfalfa have increased. Building on previous K research, a K fertilizer rate study was conducted in 1999 and 2000 to determine alfalfa yield and soil and tissue test responses to fertilization on low K-testing soils. Potassium (as potassium chloride, 0-0-60) was applied at rates of 0, 200, 400, and 600 lb K2O/acre in early spring to established alfalfa at one site in 1999, and applied at rates of 0, 100, 200, 400, and 600 lb K2O/acre in early spring 2000 at three sites. A split application treatment consisting of 300 lb K2O/acre applied in early spring followed by 300 lb K2O/acre after the first or second cutting was also included at all sites. The effect of fertilizer application rate on alfalfa yield and soil and tissue K concentrations was measured each year. Soil test K increased 1 part per million (ppm) for each 5 lb K2O/acre applied at the Cache site in both years, and each 12.5 lb K2O/acre applied at the Weber and Sevier sites in 2000. Alfalfa responded to K fertilization in all site-years, with yield responses ranging from 1.0 to 3.2 tons/acre. Yield was reduced with the 600 lb K2O/acre single application rate at two of the three sites, while split application of the 600 K2O/acre rate did not reduce yield. There were relationships between soil test potassium and a) relative yield and b) tissue potassium concentration at these sites, while the relationship between tissue potassium concentration and relative yield was poor. These results show that alfalfa may respond to high rates of potassium fertilizer on low K-testing soils. Relatively high rates of potassium are necessary to increase soil test potassium to adequate levels on low K-testing soils. Rates of potassium chloride fertilizer exceeding 400 lb K2O/acre, however, may need to be applied in split treatments to prevent yield reductions.


INTRODUCTION

Alfalfa is an important forage and cash crop for many producers in Utah and throughout the Western U.S. With a tissue concentration of 2%, approximately 48 lb K2O is removed with each ton of hay produced. More K is removed than any other nutrient except nitrogen in high-yielding alfalfa production systems. In the mid-1950s, Utah fertilizer guides stated no known potassium deficiencies existed in the State due to the high native levels of potassium in Utah soils and high potassium levels in many irrigation water sources (Nielson et al., 1955). Yields at that time, however, were less than 2 tons/acre. Today, many growers are achieving irrigated alfalfa yields in excess of 8 tons/acre. The incidence of potassium deficiency has also increased due to these high yields and the long history of K removal under intensive alfalfa production (Lindstrom, 1983; James, 1988). During 1995-1999, an average of 12% of the soil samples submitted to the Utah Sate University Soil Testing Lab report soil test potassium levels below 100 ppm.

Current K fertilizer recommendations for alfalfa in Utah are based on a 0.5 M sodium bicarbonate extract soil test conducted on the 0-12 inch soil depth (Topper et al., 1989; Koenig et al., 1999). The critical soil test value above which no potassium fertilizer recommendation is made was 100 mg K/kg soil (or parts per million, ppm) prior to 1999 (Topper et al., 1989; Tindall, 1986). The

*Proceedings of the Western Nutrient Management Conference, Salt Lake City, Utah, March 8-9, 2001. This research was supported by the Potash and Phosphate Institute and IMC Kalium.

critical soil test K value was increased to 150 ppm, and fertilizer recommendations for low soil test K classes also increased based on results reported earlier (Koenig et al., 1997, 1999a, 1999b). Applications of the revised K fertilizer rates have frequently not resulted in soil test K increases to critical levels on low K-testing sites according to research (Koenig et al., 1997, 1999) and reports from alfalfa producers. Furthermore, the results of previous research did not accurately establish or verify a critical soil test K level for alfalfa (Koenig et al., 1997, 1999). The objective of the current research was to determine alfalfa yield and soil and tissue test responses to K fertilization on low K-testing soils.


METHODS

Experiments were conducted at one location in 1999 and three locations in 2000 (Table 1). At the Cache location (Utah State University Greenville Research Farm), K fertilizer was applied at rates of 0, 200, 400, and 600 lb K2O/acre in early April 1999 to established alfalfa. An additional treatment consisting of 200 lb K2O/acre applied in April followed by 200 lb K2O/acre after the first and second cuttings was also included. At the Cache, Weber, and Sevier locations, K fertilizer was applied at rates of 0, 100, 200, 400, and 600 lb K2O/acre in early April 2000 to established alfalfa. A split application treatment consisting of 300 lb K2O/acre in early April followed by 300 lb K2O/acre after the first cutting was also included at these sites in 2000. Additional P was applied at all three sites, and sulfate-sulfur was applied at the Weber location, according to soil test recommendations. The Cache experiments had four replications while the other locations had three in a randomized complete block design. Plot dimensions were five feet wide by 20 feet long at Cache, and ten feet wide by 30 feet long at Weber and 50 feet long at Sevier.


Table 1. Select soil properties for the surface 12 inch soil layer at each site.




Cache County


Weber County


Sevier County


Soil series


Millville


Crooked Creek


Redfield


Texture class


silt loam


silty clay loam


clay loam


%clay


25


28


29


%Calc. Carb. Eq.


37


0


54


pH


7.8


6.7


8.1


Soil test K, ppm


72


88


73


K recommendation_


120-160 lb K2O/ac


120-160 lb K2O/ac


120-160 lb K2O/ac

_Based on Utah State University Extension recommendations (Koenig et al., 1999).


Soil samples were collected after the first harvest at each location, and after each harvest at the Cache location in 2000. Samples were collected at the 0 to 12 inch depth by compositing four cores/plot. Soil samples were air-dried, ground to <2 mm, extracted with 0.5 M sodium bicarbonate and analyzed for K by atomic absorption spectrometry. Fresh weight yield was measured at the early bloom stage by cutting a 2.8 by 15 (Cache), 25 (Weber), or 45 (Sevier) foot area from the center of each plot. Four cuttings were made at the Cache site each year, three at Weber, and four at Sevier. Subsamples of the tissue were oven-dried at 60 oC to determine moisture content, ground to < 0.2 mm, digested, and analyzed by ICP to determine potassium concentration.


RESULTS AND DISCUSSION

Soil test potassium.

Soil test potassium increased slightly at all locations between pre-fertilization (Table 1) and the first harvest (Figure 1) for the unfertilized treatment. Release of nonexchangeable forms of K has been shown to be temperature dependent, increasing as temperature increases (Bertsch and Thomas, 1985). This may explain the increase in soil test K between early spring and early summer. The amount of K fertilizer required to induce a unit change in soil test K was similar between the Weber and Sevier sites (12.5 lb K2O per ppm K increase), but much higher than for the Cache site (5 lb K2O per ppm K increase) (Figure 1). The lower response of soil test K to K fertilization at the Weber and Sevier sites meant that, even at the 600 lb K2O/acre application rate, soil test K did not increase to current critical levels of 150 ppm at these sites. Apparently, soils at the Weber and Sevier sites have similar K sorption potentials, and these potentials are much greater than for the Cache soil.

ALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION

Figure 1. The effect of K fertilizer rate on soil test K at the Cache, Weber and Sevier sites.


Yield.

Yield ranged from 12.5 to 15.7 tons/acre at the Cache site in 1999, and 6.6 to 8.0 tons/acre in 2000 (Figure 2). The lower yields at Cache in 2000 were due in part to drought conditions and water limitations imposed in 2000. Yields were generally lower at the Weber (4.2 to 5.4 tons/acre) and Sevier (4.3 to 5.3 tons/acre) sites than at Cache in 2000 (Figure 2). These latter sites are located in producer fields and, consequently, some delay occurs between field harvest and subsequent irrigation/regrowth as hay is allowed to dry and bales are made and removed. The Cache experiment farm site allows hay to be harvested and removed from the plot in one day, allowing irrigation to resume the following day. Relative to nearby producer fields, one additional cutting is normally obtained with the management system at the Cache site.

FALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION
igure 2.
The effect of K fertilizer rate on alfalfa yield at the Cache site in 1999 and 2000, and the Weber and Sevier sites in 2000.



Maximum responses to potassium fertilization ranged from 1.0 to 3.2 tons/acre among sites and years. Considering the current price of K fertilizer ($0.14/lb K2O) and value of alfalfa hay ($80/ton), the application of at least 400 lb K2O/acre would be economical at these locations. Some yield depression was evident at the 600 lb K2O/acre, single application rate for the Cache and Weber sites (Figure 2). Response to potassium fertilization was linear at the Sevier site. Split application of 600 lb K2O/acre resulted in significantly higher yields compared to the single application at the Cache site in 1999 and Weber site in 2000 (Figure 2). The application of 600 lb K2O/acre equates to 1000 lb 0-0-60 fertilizer/acre and apparently caused a salt-induced yield reduction at two of the three locations. Rates of potassium chloride fertilizer exceeding 400 lb K2O/acre may need to be applied in split treatments to prevent salt-induced yield reductions. Alternatively, applying high rates of a K fertilizer with lower salt index such as potassium sulfate may also prevent yield reductions.

Soil and tissue test K and yield relationships.

There was a relationship between soil test K and relative yield for these sites (Figure 3). The optimum soil test K was at or near the critical level of 150 ppm currently used in alfalfa fertilizer recommendations from Utah (Koenig et al., 1999b). It is interesting to note that maximum yield at the Cache site was achieved at higher soil test K levels than at either the Weber or Sevier sites. It is not clear whether this was due to the inability to reach soil test K values above 150 ppm with the range of fertilizer rates used at the Weber and Sevier sites, or whether a higher critical soil test K value was necessary at the Cache site because of the higher yields achieved at this location. Since yield increased linearly with K rate at the Sevier site (Figure 2) and the highest average soil test K was 121 ppm at the 600 lb K2O/acre rate (Figure 1), even higher K rates and soil test K levels may have produced higher yields at Sevier. This also appears to be the case at Weber where the split 600 lb K2O/acre application treatment produced significantly higher yield than single applications of 400 lb K2O/acre (Figure 2).

FALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION
igure 3.
The relationship between soil test K and relative alfalfa yield. Data are for all sites and each point represents a single observation.



The complete tissue potassium data set is not available at this time. Among available data there was a poor relationship between tissue potassium concentration and relative alfalfa yield (Figure 4a). One explanation for this is that tissue potassium concentration varied with cutting throughout the growing season (data not presented). At the Cache and Sevier locations, tissue potassium concentration was as much as 0.5% lower in the first cutting than in subsequent cuttings, and generally tended to increase throughout the growing season. This means that, in the relationship described in Figure 4a, maximum yield at a location was associated with a lower tissue potassium concentration in the first cutting than in subsequent cuttings. The same problem may have contributed to the variability in the relationship between soil test and tissue potassium (Figure 4b). For a given treatment and soil test potassium level, tissue potassium varied with cutting throughout the growing season.



FALFALFA YIELD AND SOIL TEST RESPONSE TO POTASSIUM FERTILIZATION
igure 4.
The relationship between (a) tissue potassium concentration and relative alfalfa yield and (b) soil test potassium and tissue potassium concentration.




REFERENCES

Bertsch, P.M. and G.W. Thomas. 1985. Potassium status of temperate region soils. Chapter 7 in R.D. Munson (Ed) Potassium in Agriculture. American Society of Agronomy, Madison, WI.

James, D.W. 1988. Leaf margin chlorosis in alfalfa: A potassium deficiency symptom associated with high concentrations of sodium in the leaf. Soil Sci. 145:374-380.

Koenig R., J. Barnhill and C. Hurst. 1997. Phosphorus and potassium management for irrigated alfalfa production in Utah. Pp 160-165 in T.L. Tindall (Ed) Proceedings of the Western Nutrient Management Conference, March 6-7, Salt Lake City, UT.

Koenig, R., B. Kitchen, C. Hurst and J. Barnhill. 1999a. Phosphorus and potassium fertilization of irrigated alfalfa in Utah. Pp 57-62 in T.L. Tindall (Ed) Proceedings of the Western Nutrient Management Conference, March 4-5, Salt Lake City, UT.

Koenig, R.T., C.J. Hurst, J.V. Barnhill, B. Kitchen, M. Winger and M. Johnson.1999b. Fertilizer management for alfalfa. Utah State Univ. Extension Electronic Publication AG-FG-01, 5p.

Lindstrom, T. 1983. Alfalfa production and quality as influenced by fertilizer potassium where soils and irrigation waters are low in potassium. MSc thesis, Utah State University, Logan.

Nielson, R.F., J.P. Thorne and G.T. Baird. 1955. Fertilizer requirements of alfalfa hay in Utah. Utah Agricultural Experiment Station Bulletin # 374. Utah State Agricultural College.

Tindall, T.A. 1986. Soil fertilizer management of alfalfa in Utah for optimum yields. Extension Circular No. 418. Utah State University Cooperative Extension Service.

Topper, K.F., T.A. Tindall and D.W. James. 1989. Field Crops. In Utah Fertilizer Guide, D.W. James and K.F. Topper (eds.). Extension Circular No. 431. Utah State University Cooperative Extension Service.

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Tags: alfalfa yield, of alfalfa, potassium, alfalfa, yield, response, fertilization