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Soil testing is the only way to determine the fertility status and pH of your soil. Soil test readings (in pounds per acre) of at least 65 for phosphorus (P) and 350 for potassium (K), along with a soil pH near seven, are considered essential for alfalfa production. Yield of alfalfa will be significantly decreased and productive stand life shortened if these essential nutrients are inadequate, or if soil is more than slightly acidic (<pH 6.5). Soil pH needs to be near neutral (pH 7) so Rhizobium bacteria can fix nitrogen for use by the alfalfa plant. See Tips on Soil Sampling to learn how to obtain a reliable soil sample. An evaluation of Oklahoma alfalfa production fields in1995 showed that 75 percent of the alfalfa fields had low pH or were deficient in P2O5 or K2O, or both. During 2000, 92 percent of the 434 soil samples analyzed for alfalfa production by the Soil, Water, and Forage Analytical Laboratory at OSU needed lime, P, or K for good production (Fig. 4-1). Only eight percent needed none of the three, and 65 percent needed one or two of them. To insure proper pH and adequate fertility, have a soil test performed and apply enough lime to neutralize the soil. Also apply enough phosphorous and potassium to satisfy the crop's needs before planting alfalfa. For best results, both lime and fertilizer should be incorporated into the top six inches of soil.
Figure 4-1. Percentage of alfalfa soil samples that were adequate or
deficient in P, High-yielding alfalfa removes large amounts of nutrients from the soil (Table 4-1). Through the normal process of soil weathering, soils are able to supply a certain amount of the required nutrients annually according to their chemical and physical makeup. Monitoring the nutrient status of your soil through testing is the best way to know what nutrients are being naturally supplied, and how much fertilizer supplementation is needed to keep alfalfa productive.
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Alfalfa is not as tolerant of acid soils as wheat and some other crops. Soil pH must be maintained above 6.2 to ensure an environment favorable for nitrogen-fixing bacteria. Wheat grows well at a soil pH of 5.5, but alfalfa production can be reduced by 50 percent at pH 5.5, and stand failure is likely at a soil pH of 5.0. Alfalfa yield and quality decrease when production is attempted in acid soil. Whenever the soil pH is below 6.2, a minimum of one ton ECCE (effective calcium carbonate equivalent) lime should be applied and incorporated 4-6 inches deep prior to planting. Depending on the level of acidity, loamy and clay-type soils may require several tons of aglime per acre. Since it takes several months for lime to react with the soil, it is best to apply lime at least a year before planting alfalfa. A soil reserve of about 3-5 tons of lime is required to meet the needs of a five-ton alfalfa yield for 6-10 years without a drastic decline in soil pH. Given today's high production costs, attempting to establish alfalfa without first having the surface soil (to six inches deep) tested is unsound and may be a costly mistake. Having a nonacid subsoil cannot substitute for the need to lime an acid surface soil, since most of the nitrogen-fixing bacterial activity is in the upper six inches of soil. Even though alfalfa draws heavily on the basic mineral elements (K, Ca, Mg) in the soil, applying the proper rate of lime before planting to adjust the pH to seven should provide an adequate supply of elements for the life of the stand.
Nitrogen (N) – Some nitrogen (20-30 pounds per acre) is required for establishment of seedling alfalfa. This amount of nitrogen is often available in September in fields that have lain fallow the summer after a June wheat harvest. Once alfalfa seedlings form nodules on their roots, they can fix their own nitrogen from the atmosphere, so no more nitrogen needs to be applied during the life of the stand. Nitrogen fixation is the result of a symbiotic activity of alfalfa and Rhizobium bacteria. For the symbiosis to occur, it is important that properly inoculated seed be used and that the nitrogen-fixing bacteria become active. Response to nitrogen fertilizer after alfalfa is established is a sign that the soil conditions are too acidic or that nitrogen-fixing bacteria are absent. A soil test can determine if acidity is the problem, but neither acidity nor inoculation failure can be corrected after the alfalfa is planted. Phosphorus (P) and Potassium (K)-- As shown in Table 4-1, harvesting five tons per acre of alfalfa removes more then 50 pounds per acre of P2O5 and 250 pounds per acre of K2O each year. Soils usually can supply some of these nutrients, but phosphorus or potassium fertilizer (or both) often needs to be applied before and during the life of the stand. The fertility levels of P and K and the needs for lime in sandy soil change more rapidly under alfalfa production than with other crops. The best way to determine how much phosphorus (P2O5) and potassium (K2O) to apply is to test the soil. Results of a soil test are calibrated to give a "Soil Test Index" that relates to the amount needed in pounds per acre of P2O5 or K2O (Table 4-2). Note that phosphorus and potassium fertilizers are not needed at soil test values of 65 or greater for phosphorus and 350 or greater for potassium. Because these nutrients (P and K) are considered immobile in the soil, that is, they react with the soil and do not migrate with the soil's moisture content, the most efficient way to get them into the rooting zone is to incorporate them before planting. Applying additional phosphorus and potassium fertilizers to the surface of existing stands is somewhat less efficient, but it is necessary in most fields for profitable alfalfa production. Phosphorus (P) deficiencies are best corrected by applying and incorporating a three year supply of fertilizer in the summer before fall planting. Soil fertility levels should then be monitored through soil testing, and any addition P should be added following the second or third year of production from November to January, before early spring growth. Best response to surface-applied P is usually obtained in the first cutting. Good surface moisture in the spring results in P uptake by surface roots. Starting the follow-up applications of P in the second year also allows time for some movement of P through soil disturbances caused by insects, cattle, machinery, freezing, etc. Potassium (K) deficiencies are best corrected by applying only enough for one year because alfalfa will take up more than needed when large amounts are available (luxury consumption). After alfalfa is established, soil should be tested annually after the second year, and K should be applied as needed from November to January, before early spring growth.
Secondary and Micronutrients—Deficiencies of the secondary elements (calcium, magnesium, and sulfur) and micronutrients (iron, zinc, manganese, copper, boron, molybdenum, and chlorine) are usually not a problem with alfalfa production in Oklahoma. Some magnesium, boron, sulfur, and zinc deficiencies have been reported in the extreme southeastern part of Oklahoma. Because irrigation waters in Oklahoma are high in sulfur, response to sulfur-containing fertilizers can only be expected under high-yielding dryland production. Special fertilizers containing secondary and micronutrients should not be applied to alfalfa unless there is strong evidence of a deficiency. Deficiencies may be confirmed by observation of stunted yellow plants, a reliable soil test, or application of a fertilizer containing a single nutrient to a small area of the field and observing the response. However, it is critical that soil pH and levels of phosphorus and potassium have been corrected before trying to confirm a secondary or micronutrient deficiency.
Alfalfa Yield Response to Methods and Rates of Applied Phosphorus The results of a six-year test illustrate the importance of phosphorus fertilization when alfalfa was established in a phosphorus-deficient soil, and weeds were controlled with herbicides (see Table 4-3). Based on OSU soil test recommendations, the soil at this site needed no nitrogen, about 80 pounds per acre of P2O5, no K2O, and no lime.
Alfalfa’s response to phosphorus fertilizer the first year increased as applied phosphorus increased, with maximum yield of seven tons per acre at the 600 pounds per acre of P2O5 rate (Table 4-4). In the sixth year of the trial, yield response to the initial 600 pounds per acre of P2O5 treatment was lower (6.55 tons per acre) in relation to plots that had received annual and biennial phosphorus fertilization (Table 4-4). Despite the drop in yield response late in the experiment to the 600 pounds per acre of P2O5, this treatment still yielded the highest of all broadcast treatments of 18-46-0 over the six years of the experiment (Table 4-5). Every year, plots receiving no phosphorus produced the lowest yield.
Additionally, subsurface banding (knifing) of liquid phosphorus stabilized alfalfa yields over the length of the trial, resulting in the highest yield over the six years with these treatments (Table 4-5). These responses support the theory that banding of P2O5 increases availability by placing the nutrient in closer proximity to plant roots and minimizing soil-fertilizer reactions, maintaining availability for a longer period of time. Supplying a large amount (600 pounds per acre) of incorporated phosphorus before alfalfa establishment in a high-yielding (e.g., irrigated) environment provides maximum response because plant density is high. As stands age and plant density decreases, availability of fertilizer phosphorus decreases by reactions with soil, removal by crop uptake, and poorer extraction by a less dense root system. Smaller rates applied more frequently were better able to sustain a phosphorus-rich environment that supported higher yields in the sixth year.
Alfalfa Yield Response to Potassium and Sulfur Three additional fertility treatments were included in the six year study described above to evaluate the effect of 500 pounds per acre per year of K2O, 500 pounds per acre per two years of K2O, and 50 pounds per acre per year of sulfur, each applied along with 200 pounds per acre per two years of P2O5 broadcast as DAP. The potassium treatment was included to identify when blanket applications should be made in order to eliminate or minimize available potassium as a yield-limiting variable. Accordingly, the entire test site received blanket applications of 500 pounds per acre of K2O at establishment and in years three and five.
Potassium fertilization resulted in increased yields over the length of the experiment (Table 4-6). The 200 pounds per acre per two years of P2O5 in conjunction with 500 pounds per acre of K2O yielded the highest of all treatments over the six years. This response was somewhat surprising since the initial soil test of 326 pounds per acre was near the calibrated adequate level (K>350 pounds per acre). Apparently alfalfa responds to higher levels of available soil potassium in a high-yield environment. This statistically significant response of about four tons per acre (value about $320) was from an input of an additional 1500 pounds of K2O (cost about $165) and would merit economic consideration. It is possible that lower annual rates (e.g., 250-300 pounds K2O per acre) might have also supported this maximum yield and that the dollar difference would have been even larger compared to the alfalfa that did not receive potassium. Sulfur fertilization only slightly affected yield over the six-year trial period.
Final phosphorus soil test levels in alfalfa that received a single application of 600 pounds per acre of P2O5 (both broadcast and injected) were significantly lower than treatments receiving annual or biennial phosphorus applications (Table 4-7). Soil test phosphorus was significantly lower in the unfertilized check than for all other treatments. As expected, the treatment receiving 500 pounds per acre per year of K2O had the highest potassium soil test value. It should be noted that the potassium increased from 326 pounds per acre in the beginning to 650 to more than 700 in other plots that only received the initial and two subsequent 500 pounds per acre of K2O blanket treatments (Table 4-7). This difference illustrates that K2O builds up when applied in excess of the amount needed by the alfalfa. Table 4-7 also confirms the tendency for pH to decrease most where yields are highest. The pH in the check plots (lowest yielding) increased from the initial 6.6-7.2 during this time. Average NO3-N (nitrate nitrogen) was 27.2 pounds per acre in the beginning and ranged from 4.6-7.0 (approximations of zero) at the end of the six years. Observing rates of NO3-N that low should not be of concern; in fact, most alfalfa fields have NO3-N readings between four and ten pounds per acre after a few months of production. The alfalfa is using primarily nitrogen fixed in symbiosis with Rhizobium bacteria.
The positive significant response of alfalfa to both P and K at higher than currently recommended rates based on soil testing has important economic implications. If a producer is able to maximize yields over a six-year period by supplying the P fertilizer as a single event, additional profit may be realized because equipment and labor costs are decreased due to fewer fertilizer applications. However, some of the savings in fewer applications would be offset by lost interest on money used to purchase all the phosphorus at the beginning of the six-year period.
Response of Alfalfa to P Fertilizers in Thinning Stand Seven of the phosphorus treatments in the previous experiment were compared with and without weeds (Table 4-8). To exclude weeds, herbicides were applied to control weeds in years six through eight when weeds in hay at first harvest were 5 percent or greater of the total forage. Weeds started to compete with alfalfa in some of the plots by the fifth year, so the fertility study was continued for three more years to obtain weed interference data.
Data collected during years six through eight of the study indicated that weeds must be controlled in thinning alfalfa stands to obtain a favorable alfalfa yield response to fertilizer. By the sixth year, increased weed production and decreased alfalfa production resulted with some of the fertilizer treatments containing nitrogen. The fertilizer treatment having the greatest impact on weed interference in the sixth year was an annual application of 100 pounds per acre of 18-46-0. This treatment had the greatest weed production and lowest alfalfa production when weeds were not controlled, but had the highest alfalfa yield when weeds were controlled. In the seventh and eighth year, increased weed production and decreased alfalfa production resulted with all fertility treatments. Total alfalfa production from fertilized treatments in the seventh year averaged only two tons per acre when weeds were not controlled compared to six tons per acre when herbicides were used to control weeds. By the seventh year, the stem densities of alfalfa had decreased to the point that there was growing space for weeds in all plots. In conclusion, it appears that phosphorus fertilizer can have a negative effect on alfalfa hay production in thinning stands (< 25 alfalfa stems per square foot) if weeds are not controlled, especially if the phosphorus fertilizer contains nitrogen (For weed control recommendation, see Chapter 2, "Weed Management in Established Stands with less than 20 stems/sq. ft."). When fertilizer is applied and weeds are not controlled, weeds respond to the fertilizer and become more competitive, thus yield of alfalfa is reduced. To maintain a productive level of alfalfa with fertilizer, it is critical that weeds are controlled as stands thin and particularly when weeds start to comprise up to 5 percent of the hay at first harvest.
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Alfalfa Production Guide for
the Southern Great Plains, 2001
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No P2O5
added during the study