No-Till Alfalfa Production: Limestone Amendment for Acid Soil

Some soils are not suitable for alfalfa (Medicago sativa L.) production because of the erosion potential if plowed and because conventional clean tillage is traditionally recommended for incorporating limestone where needed. No-till planting can be used for alfalfa establishment on erodible soils but when the soil pH is low, limestone amendments cannot be incorporated. Studies with a Davidson soil (clayey, kaolinitic, thermic Rhodic Kandiudult) and a Tatum soil (clayey, mixed, thermic Typic Hapludult) were conducted to determine whether nonincorporated limestone was a suitable substitute for the traditional practice of incorporation. Several rates of limestone were either incorporated throughout the plow-layer during seedbed preparation or left nonincorporated on the surface after seedbed preparation. After limestone treatments were applied, two cover crops were grown and removed before alfalfa was planted (10 mo after limestone application) using no-till methods. Soil pH was measured at various depths 6 yr after limestone application for the Davidson soil and 4 yr after limestone application for the Tatum soil. Relative soil water content was measured at two depths in the Tatum soil study. Initial soil pH in the plow layer of the Davidson soil was 5.3 and 4.5 for the Tatum soil. Incorporated limestone increased soil pH throughout the plow layer in both soils but had little influence on pH in the upper 2 to 3 in. of the soil profile and thus supported yields in the first 2 yr that were equal to incorporated limestone. Without limestone amendments, most alfalfa died within 2 yr of planting. At the recommended limestone rate (5 tons/acre), yields in each year from the incorporated and nonincorporated limestone application methods were similar. Alfalfa yields after 2 yr decreased with the 1.25-ton/acre limestone rate when compared with the 5-ton/acre rate. High alfalfa yields were sustained longer from nonincorporated than incorporated limestone. Soil moisture was used at a depth of 22 in. in the Tatum soil, where pH was 4.4 for both incorporated and nonincorporated treatments. When no-till procedures are used for alfalfa establishment, nonincorporation of limestone is a satisfactory alternative to incorporation during conventional tillage of soil with low pH. This study points out the need to sample the 0- to 3-in. depth for soil testing when planning to use nonincorporated limestone in a no-till alfalfa establishment plan. Research QuestionAlfalfa production requires a soil pH of 6.5 to 6.8. Current recommendations involve incorporation of limestone during conventional tillage in order to increase soil pH if needed. Where erosion potential is too great for conventional seedbed preparation, no-till planting without soil disturbance is recommended. The primary objective of this research was to determine if alfalfa can be grown successfully in low-pH soils that have limestone applied without incorporation instead using plowdown methods. Literature SummaryThe downward movement of acid neutralizing influence from limestone is very slow. With conventional tillage, incorporated limestone will only modify the plow layer so that the N₂ fixation by alfalfa can occur. Soil below the plow layer is not modified and will remain acidic after conventional incorporation of limestone. The plow layer cannot store enough water for alfalfa growth, thus water is removed from below the plow layer by plants during the growing season. This indicates that alfalfa roots can penetrate and be active in nonlimed soil zones with low pH if some of their roots are growing in a zone with favorable pH. This layer of favorable pH may be as small as 2 in. and is needed for the N₂ fixation process. Nonincorporated limestone could extend alfalfa production into fields with low pH where erosion from steep slopes prohibits conventional tillage practices. Study DescriptionAlfalfa was grown on Davidson and Tatum soil types with an initial pH in the plow layer of less than 5.3. Soil pH below the plow layer was less than 5.0. Treatments included a nonlimed check, incorporation of limestone through the 6-in. plow layer during conventional tillage, and limestone applied on the surface after conventional tillage and final seedbed preparation. Two cover crops were grown during 10 mo following limestone application before no-till planting the alfalfa. Six years after limestone treatment (4 yr for the Tatum soil experiment) soil pH was measured in different layers of the upper soil profile. Root activity was indicated by change in plant available water at a depth of 22 in. of the Tatum soil experiment. Yields are reported for six harvest years for the Davidson soil experiment and five harvest years for the Tatum soil experiment. Applied QuestionWere alfalfa yields equal for incorporated and nonincorporated limestone treatments? Without limestone, few alfalfa plants survived more than 2 yr. Yes; annual yields during the first 4 yr (5 yr after limestone application) were the same, whether the limestone was incorporated or nonincorporated during seedbed preparation. No; in the fifth yr, yields were greater from nonincorporated than incorporated limestone where 1.25 tons/acre were applied (Table 1). Then, in the sixth year, yields were greater from nonincorporated than incorporated limestone for both the 1.25- and 2.50-ton/acre rates. If the stand does not decline due to other factors, such as disease or old age, then nonincorporated limestone may result in higher yields in future years than incorporated limestone at the 5-ton/acre rate also. Low pH soils can be amended for no-till alfalfa production using surface applied limestone without incorporation. Did the acid neutralizing influence of limestone modify soil pH below the zone of application? Yes and no. During 6 yr, very little movement of the neutralizing influence occurred below the plow layer where limestone was incorporated (Table 2). With nonincorporated limestone, soil pH was modified to a depth of 2 in. or greater, which was adequate to support maximum yields. These data imply that soil samples to be used for making limestone recommendations for no-till plantings or established stands should be taken no deeper than 3 in. These data also imply that surface applied limestone could be recommended to correct low pH that might occur in established alfalfa fields. What rate was ideal for incorporated and nonincorporated limestone? After six harvest years, yields from the 2.5-ton/acre rate of incorporated limestone were less than from 2.5 ton/acre of nonincorporated limestone. These trends indicate that limestone applied on the soil surface without incorporation may support high alfalfa yields longer than incorporating a similar amount of limestone. Limestone at one half the recommended rate, applied to the soil surface without incorporation, was adequate to maintain high alfalfa performance where stands had a 6-yr life. Will alfalfa roots extend into acid soil for water? Yes. Soil pH in the Tatum soil between 9 and 22 in. was less than 4.4. During two drying cycles, alfalfa removed water at 22 in. to low levels of availability. No difference was observed between incorporated and nonincorporated limestone treatments in regards to water usage at the 22-in. depth (Table 2). Table 1Total season yields produced on a Davidson soil in the fifth and sixth years of production after incorporation and nonincorporation of limestone at several rates. Limestone applied (tons/acre)†Year of productionLimestone incorporation01.252.505Yes Yield (tons/acre)FifthYes0.200.982.323.57No0.151.652.483.37SixthYes0.000.231.542.28No0.001.262.162.30†Limestone treatments were imposed in November before seeding the following August. Table 2Soil pH and relative water content of a Tatum soil as influenced by incorporated and nonincorporated limestone 4 yr after treatment. Limestone incorporationSample depthNo lime checkYesNoin.----------------Soil pH-------------------0 to 15.06.76.41 to 24.66.06.42 to 34.55.26.23 to 64.44.75.46 to 94.34.54.6---------Relative soil moisture, %†-----------123000229650†Measured on 19 September. Soil pH at 12 in. was 4.6 and at 22 in. pH was 4.5. A value of 100 was maximum and a value of 0 was considered to have no plant available soil water at that depth. No weeds or alfalfa were growing on the no-lime check treatment.