© 2004 Plant Management Network.
Accepted for publication 5 October 2004. Published 28 October 2004.

Responses of Alfalfa, Red Clover, and White Clover to Soil pH and Lime Treatments

John L. Caddel, Hailin Zhang, and Kendra Wise, Department of Plant and Soil Sciences, Oklahoma State University, Stillwater 74078

Corresponding author: John L. Caddel. john.caddel@okstate.edu

Caddel, J. L., Zhang, H., and Wise, K. 2004. Responses of alfalfa, red clover, and white clover to soil pH and lime treatments. Online. Forage and Grazinglands doi:10.1094/FG-2004-1028-01-RS.

Abstract

Many legumes are important for livestock production as components of pastures and in pure stands. This study was conducted to measure the effects of five liming rates to an acidic soil on yield and stand persistence of alfalfa (Medicago sativa), red clover (Trifolium pratense), and white clover (T. repens) and on soil pH. Soil samples, collected and analyzed twice a year for three years, documented that lime increased soil pH for about 14 months and then pH declined slowly. Lime application did not affect legume seedling density, but suppressed certain weed populations. Red clover and alfalfa yields were increased significantly with increasing pH, but white clover yield did not increase above a pH of 5.2. Soil pH did not affect the alfalfa and red clover plant density after 3.5 years, but higher pH resulted in plants with heavier crowns.

Introduction

Soil pH decreases with time due to nitrogen fertilization, rainfall, leaching, organic matter decay, and harvest of crops. Optimum pH for plant growth varies with crop species and with soil physical and chemical properties (8). Benefits to maintaining proper soil pH for crop production by applying lime (Figs. 1 and 2) include improved N fixation by legumes, increased availability of essential nutrients (e.g., phosphorus, molybdenum, and calcium), increased percent base saturation, and decreased solubility of toxic elements (e.g., aluminum and manganese) (5,6,8). Lack of available Mo causes a decrease in Rhizobium activity at low pH (4,8). Legume yield response under different levels of soil acidity has not been well quantified, and small-seeded legumes differ in their relative tolerance or resistance to low soil pH (14).

Fig. 1. Soil in the dead strip has a pH of about 5.0 while soil where alfalfa is green and growing has a pH of more than 6.0. The strip was caused when the lime application truck failed to start distributing lime where the previous load ran out.		Fig. 2. Agricultural lime is applied with trucks before alfalfa is established.

Soils with higher than 20% Al saturation on soil exchangeable sites should receive special management to optimize yields since soluble Al impairs plant growth (7). Aluminum toxicity limits root growth, thus reducing nutrient uptake, and hinders plant growth. Manganese toxicity is a problem in areas where Mn oxide is high in the parent material. Visual symptoms of Mn toxicity include stunting and stiffness of leaf tissue, purpling of leaves, white flecking, tip burn, and chlorosis (3). Elevated Mn level in low pH soils may result in deficiencies of Ca, Mg, and Fe (12). Deficiencies in Ca can lead to a lower critical level for Mn toxicity than usually found, which will hinder growth of legumes (10).

Alfalfa is widely adapted and probably the most popular forage legume for hay and is also used in pasture for grazing. Low soil pH and nutrient deficiencies enhanced in acid soils decrease yields (Fig. 3), shorten stand life of the crop, and reduce alfalfa competitiveness against weeds. Alfalfa harvested as hay removes larger amounts of nutrients, such as Ca, K, and P, from soil than most other forage crops and corps harvested for grain.

Fig. 3. A typical symptom of low pH in alfalfa is short plants (center).

Red clover (Fig. 4) is one of the most important legumes in the world because of its adaptation to a wide range of soil types, pH levels, and environmental conditions (11). Red clover grows well at a pH of 5.0 to 6.0 if all nutrient needs are satisfied, but a pH above 6.0 and adequate Ca are needed for maximum yields. It is sensitive to Mn toxicity, which is a concern when pH is below 5.7 in some soils (13).

White clover (Fig. 5) is sown on more acres of pasture throughout the world than any other legume, and pasture pH is likely to be neglected. Research shows root growth, shoot growth, and N fixation of white clover increased as pH increased to 6.0 (1). Increasing lime application increased N concentration and nodulation of white clover (6). White clover needs less lime than alfalfa (10), and white clover is more tolerant to Mn toxicity than alfalfa because white clover does not retain Mn in roots (2).

Fig. 4. Red clover is frequently grown with forage grasses.		Fig. 5. White clover has little stem material for grazing. Animals consume primarily leaves and petioles.

Our objectives were to quantify the effects of soil pH and lime rates on stand establishment, yield, and persistence of alfalfa, red clover, and white clover grown on a limed acid soil.

Research Design

This field study was conducted on a Taloka silt loam (fine, mixed, active, thermic Mollic Albaqualfs) with a mean initial pH ranging from 4.1 to 4.7 at the Eastern Research Station near Haskell, OK. Lime treatments of 0.4, 0.7, 1.2, 2.0, and 3.7 tons/acre effective calcium carbonate equivalent (ECCE) were applied in April 2000 to 24-×-100-ft plots and incorporated to a depth of about 6 inches. These lime treatments were the main plots of a split-plot design experiment with four replications.

In September 2000, ‘Kenland’ red clover, ‘Regal’ Ladino white clover, and ‘Cimarron VR’ alfalfa were planted as subplots (24 × 33 ft) in each main plot at 12, 5, and 18 lbs of seed per acre, respectively. Soil samples consisting of 20 6-inch cores were collected from each plot every spring and fall starting before lime was applied in April 2000 until October 2003. Soil samples were oven-dried at 150°F and ground to pass a 2-mm (approximately 9-mesh) sieve. Before establishment and each autumn thereafter, K and P fertilizers were applied to the entire study area according to soil tests results.

Soil pH was determined by using a pH meter and combination electrode in a 1:1 soil-to-water suspension. Plant-available K and P were analyzed using Mehlich 3 extractant (9). All soil samples had reactive Al concentration less than 20 ppm after lime was applied.

In March 2001, weeds and legume seedlings were counted in each plot. Weeds were identified in three 8-×-18-inch quadrats per plot. Legume seedlings were collected from three 8-×-18-inch areas in each plot by removing the top 1 inch of soil with the plants, and seedlings were counted, cleaned, and weighed.

Red clover and white clover were harvested three times in 2001 and four times in 2002, but not harvested in 2003 because stands of all treatments had declined excessively. Alfalfa was harvested four times in 2001 and five times in 2002 and 2003. Forage yields were measured by cutting a 3-×-24-ft area within each sub plot to a 2.5-inch stubble height. Wet forage weights were adjusted to dry weight by drying a 1-lb subsample from each plot at 140°F to determine dry matter concentration.

In March 2004, 1.5-×-12- ft areas from each alfalfa subplot and from the highest and lowest lime treatments of the red clover were undercut to a depth of 6 inches, and plants were counted to provide a measure of stand persistence. Samples of 25 plants were cleaned free of soil, trimmed to 5-inch root length and 0.5-inch stubble height, dried at 140°F, and weighed as a measure of remaining plant size.

Data were subjected to analysis of variance using SAS (SAS Institute, Cary, NC). When F tests indicated a difference (P < 0.05), LSD was used as mean separation tests. The relation between soil pH and forage yield was graphically examined, and linear regression equations were generated by using MS Excel.

Soil pH Changes

Soil pH increased as lime rates increased and continued for about a year after lime was applied and then declined slowly over the course of the study for all lime treatments (Fig. 6). Different treatments had a slightly different initial pH due to residual prior liming effects. The 0.4 ton/acre treatment had an initial pH of 4.1. Three treatments (0.7, 1.2, and 2.0 tons/acre) had initial pH of 4.3; while the 3.7-tons/acre treatment had an initial pH of 4.7. The two treatments with the highest lime rates (2.0 and 3.7 tons/acre) increased most rapidly, while the lowest treatment (0.4 ton/acre) raised pH slowest.

Fig. 6. Soil pH changes over time for five lime treatments (tons of ECCE per acre). LSD (P < 0.05) = 0.32 for comparing pH levels resulting from different lime treatments on the same date, and LSD (P < 0.05) = 0.15 for comparing pH levels resulting from different dates within the same lime treatment.

Effects of Soil pH on Seedling Legumes

Seedlings collected and weighed 6 months after sowing indicate that species had a significant (P < 0.05) difference on the number of seedlings per ft², seedling weight per ft², and weight per legume seedling, but neither lime rate nor the species by lime rate interaction was significant (P > 0.05). Alfalfa and red clover seedlings were more numerous and heavier than white clover (Table 1). Liming had only a small, inconsistent influence on the number of legume seedlings (Table 2).

Table 1. Number of legume and weed plants per ft² and weight of legume seedlings as affected by legume species, averaged over five lime treatments, March 2001.

 ^x Includes Italian ryegrass (Lolium multiflorum), rattail fescue (Vulpia myuros), henbit (Lamium amplexicaule), mustards (Brassica spp.), cutleaf eveningprimrose (Oenothera laciniata), sticky chickweed (Cerastium glomeratum), and miscellaneous weeds.

 ^y LSD = least significant difference at the P < 0.05 level.

Table 2. Number of legume and weed plants per ft² as affected by lime treatments, averaged over three legume species, March 2001.

 ^x Includes Italian ryegrass, rattail fescue, henbit, mustards, cutleaf evening primrose, sticky chickweed, and miscellaneous weeds.

 ^y LSD = least significant difference at the P < 0.05 level.

 ^z NS = Means are not different, P = 0.05 level.

Relationship between Seedling Weeds and Soil pH

Cutleaf eveningprimrose (Oenothera laciniata) seedlings were more numerous in the clovers than in alfalfa, although the number of seedlings per ft² was too low to cause serious competition for legume establishment (Table 1). Rattail fescue (Vulpia myuros) seedlings were more prevalent than all other weeds and tended to be more numerous in plots with low pH (Table 2). As pH increased from 5.1 to 7.0 during establishment, rattail fescue weed decreased from 18.7 to 5.5 plants per ft². Cutleaf eveningprimrose had a similar trend but was much less numerous (averaged 0.8 versus 9.8 seedlings per ft²). Increased prevalence of rattail fescue at a low pH may have been due to the decrease in legume competitiveness; thus, its presence could be used to indicate where pH is too low for good forage legume production.

Effects of Soil pH on Forage Yield and Quality

Legume species and lime treatments strongly influenced forage yield during the first two years, and alfalfa yield also was significantly affected during the third year (Figs. 7, 8, and 9). Alfalfa had highest yields followed by red clover and then white clover. In several cases forage yield was positively related to lime application rates and pH even when the pH was above 6.5. The three legumes responded differently to lime and variable pH as indicated by significant (P < 0.05) species × lime treatments interaction. This is an important finding because lime recommendations to correct low pH are sometimes the same regardless of the legume species.

Fig. 7. Relationship between annual mean alfalfa forage yields (tons/acre) and pH for 2001, 2002, and 2003, Haskell, OK. Soil pH is the mean of spring and fall pH measurements. Dry matter yield is the annual total averaged over four replications.		Fig. 8. Relationship between annual mean red clover forage yields (tons/acre) and pH for 2001 and 2002, Haskell, OK. Soil pH is the mean of spring and fall pH measurements. Dry matter yield is the annual total averaged over four replications.

Fig. 9. Relationship between annual mean white clover forage yields (tons/acre) and pH for 2001 and 2002, Haskell, OK. Soil pH is the mean of spring and fall pH measurements. Dry matter yield is the annual total averaged over four replications.

Forage yield and response to lime changed from year to year, and year affected the species differently (Figs. 7, 8, and 9). The generally low first-year yield for all three species is somewhat typical because the plants had not fully developed their root systems. The higher yields in 2002 also can be attributed to better distribution of rainfall during the growing season. The relatively low yield for alfalfa in plots with the highest pH in 2002 (Fig. 7) may be attributed to lower available soil P and K levels because the 2001 yields had used larger amounts of these nutrients than in plots with lower pH and lower yields. In spite of our efforts, P and K levels dropped during 2002 to yield-limiting levels. No explanation for the low forage yields for the mid range pH for white clover in 2002 (Fig. 9) is apparent.

Red clover responded to increased pH in both years, showing a yield increase of about 0.5 and 0.7 ton/acre per pH unit in 2001 and 2002, respectively (Fig. 8). Alfalfa was somewhat more responsive in 2001 and 2003 to lime rates. Its yield increased by almost 0.9 and 0.8 tons/acre per pH unit (Fig. 7). White clover, however, showed no substantial increase in yield (Fig. 8); therefore, a pH of 5.2 was probably satisfactory for white clover production.

The optimum pH range for legumes is generally reported to be between 6.6 and 7.5 (2,8,13). Our data indicate that white clover was not responsive to increased pH when pH was above 5.2. Reports from some soil testing laboratories for alfalfa production do not recommend lime application when soil pH is above 6.1; however, our data show increased yield for alfalfa and red clover to pH 7 when other factors are not limiting.

Stand Persistence and pH Relationship

Alfalfa stand density, as measured by the number of plants per ft², was not statistically affected by pH after 3.5 years but higher lime treatments tended to have somewhat thinner stands (3.9 versus 4.1 plants per ft²) (Table 3). Plants grown in the lowest pH weighed only about 59% of those grown in the high pH treatments. Heavier root/crown is consistent with the higher above-ground yields. Plants with larger crowns and upper roots may indicate a better ability to secure nutrients needed for higher forage yield.

Table 3. Roots per ft², mean root weight, and root weight per ft² for alfalfa and red clover 3.5 years after establishment in variable soil pH, Haskell, OK.

 ^x LSD = least significant difference at the P < 0.05 level.

 ^y NS = Means are not different, P = 0.05.

Conclusion

Alfalfa and red clover forage yields were adversely affected by low soil pH although reactive Al concentration was low. Alfalfa and red clover responded positively to pH increase caused by lime treatments; however, white clover yields did not increase significantly above a pH of about 5.2. Lime application did not affect legume stand density and stand persistence. More seedlings of certain weeds were observed at low pH levels during establishment, but their numbers were low enough to not create problems for legume germination and development. Of the weeds found at this site, rattail fescue was the predominant weed, and it was most prevalent in areas of low pH. The population of rattail fescue decreased as pH increased. This weed could be used as an indicator plant, which could aid producers in identifying areas of low pH. Alfalfa plants that grew in the high lime treatments were larger than those in low lime treatments.

This information can be useful in defining economically sound liming practice for soils with low Al saturation and Mn concentration. The Oklahoma Cooperative Extension Service (15) currently recommends lime for legume production when soil pH is below 6.1, but this study showed white clover did not benefit from liming above a 5.2 pH, when all other nutrient needs were satisfied and Al saturation and Mn concentrations were low. Alfalfa yields were highest at pH levels about 7, strongly suggesting the need for different liming recommendations for different forage legumes.

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