Responses of Legume Forage Crops to Liming an Acid Soil

About 350,000 acres of alfalfa (Medicago sativa L.) are planted in Oklahoma. Currently, nearly 30% of Oklahoma farmland soils have a pH less than 5.5, which is critical for legume crops. Soil acidity is damaging to legume crop production because it causes a decrease in plant growth and deficiencies in calcium, magnesium, and molybdenum.

Legume crop production requires optimum pH and good soil fertility. Nutrient deficiencies and low pH decrease yields and shorten the stand life of the crop. An increased solubility of toxic elements, such as, aluminum and manganese, reduces nitrogen fixation by the legume, and decreased availability of essential nutrients can occur with moderately acidic soils.

The effects of molybdenum and calcium fertilizers on forage legume crop yields have not been thoroughly studied due to the difficulty of distinguishing between Mo and Ca deficiencies and other soil acidity related problems. Legume crop yield reduction under different soil acidity has not been well documented. Therefore, the economical benefit of lime needs to be thoroughly studied.

Objectives: The purpose of this study is to determine the effect of lime rates on alfalfa, white clover, and red clover production on an acid soil. Also, to study the effects of calcium and molybdenum fertilizer on alfalfa yields on an acidic soil (pH below 5.5).

Approach and Procedures: Research was conducted on a soil with an acid surface pH (4.1 to 5.4). On April 14, 2000 the following rates of lime: 0.4, 0.72, 1.2, 2.04, and 3.68 tons effective calcium carbonate equivalent (ECCE) per acre were applied. On September 29, 2000, red clover (Kenland), white clover (Regal Ladino), and alfalfa (Cimarron VR) were planted. For the

lowest lime treatment (0.4 tons ECCE/A) of alfalfa crop, plots were subdivided into three more treatments: Sodium molybdate (1 lbs/acre Mo), calcium chloride (50 lbs/acre Ca), and no additional fertilizer to study the effect of Mo and Ca on alfalfa growth under low pH soil.

Soil samples were taken from individual plots. The pH and buffer index was measured along with soil nitrate nitrogen, plant available potassium and phosphorus. Exchangeable aluminum, calcium, potassium, magnesium, and sodium were analyzed to find out aluminum saturation. Manganese concentrations and percent organic matter were also measured. Forage was harvested for yields and tested for quality.

Results: Much progress has been made so far in this study. All results from soil and forage tests have been compiled and analyzed for possible relationships. Results from the soil analysis for pH, buffer index, nitrogen, phosphorous, and potassium are shown in Table 1. For the past two years, the change in pH was monitored for each treatment. The trend is shown in Figure 1. The pH has increased for each plot, peaked in 2001, and started to decrease. The total yield for each crop was calculated for the year 2001. Figure 2 shows alfalfa, red clover, and white clover yields with respect to lime treatments. All three crops had a significant difference between the lowest and highest treatment. There were no other significant differences.

This experiment is continuing for the second year. We hope to be able to identify the economical threshold for liming legume crops at different soil pH conditions. Producers would be able to make better decisions in managing acid soils. But more importantly, we’ll learn more about legume crop production in acid soils to avoid yield loss.

Figure 1: Change in soil pH due to lime treatment from April 2000 to March 2002. The initial soil test in April 2000 was prior to liming. Lime was applied on April 14, 2000. The planting date was September 29, 2000.

Figure 2: Total forage legume dry matter yields from four lime treatments in 2001. Each point represents the average of four replicated plots.

Table 1: Soil analysis for the forage legume-pH investigations from April 2000 to March 2002.

	April 2000				November 2000				June 2001				November 2001				March 2002
Treatment (t/A)	pH	NO3	P	K	pH	NO3	P	K	pH	NO3	P	K	pH	NO3	P	K	pH	NO3	P	K
0.4	4.1	4	128	189	4.8	27	106	257	5.4	8	83	297	5.2	6	113	310	5.2	4	94	259
0.72	4.3	4	117	179	5.3	33	95	237	6.2	6	74	255	5.9	9	99	277	5.8	4	84	205
1.2	4.3	5	121	190	5.6	27	94	250	6.5	7	78	265	6.2	7	102	275	6	4	80	225
2.04	4.3	4	117	186	6.3	26	92	247	6.9	7	66	252	6.7	7	103	283	6.6	5	80	224
3.68	4.7	5	111	169	6.7	23	86	241	7.2	6	63	249	7.2	10	100	280	6.8	4	74	225

Kendra Wise, John Caddel, and Hailin Zhang