Critical Soil Fertility Issues in Alfalfa Production
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CRITICAL son... FERTll.ITY ISSUES IN ALFALFA PRODUCnON Roland Do Meyerl ABSTRACT Alfalfa yields are often limited by a lack of adequate amounts of essential plant nutrients. This presentation is intended to assist growers and advisors in assessing the soil fertility and plant nutrient status for the production of high yielding alfalfa. Strategies for understanding the quantities of essential plant nutrients, determining what portion the soils are able to supply and the amount, timing and method of application of necessary fertilizer nutrients will be discussed. High yielding, profitable alfalfa production involves determining when to apply the necessary fertilizer nutrients and soil amendments to insure that adequate nutrients will be available to meet plant growth demand. Developing an appreciation for the large amounts of plant nutrients that are removed by alfalfa and the ability of various soils on the acreage being farmed to supply these quantities will provide the background information of when to apply the needed fertilizers and amendments such as lime or gypsum. Laboratory testing of soil and plant tissue samples can be used as valuable tools to aid in assessing the nutrient status and soil conditions favorable for plant growth. Implementing these strategies will insure that plant nutrient supplies are adequate for producing profitable high yielding alfalfa of good quality for top animal performance. Key Words: alfalfa, soil fertility, fertilizer needs, fertilization of alfalfa INTRODUCnON Alfalfa forage crops contain large quantities of essential plant nutrients. -This characteristic makes alfalfa one of the best forages to feed many different animals. It has ~gh protein, calcium, phosphorus and many other digestible nutrients which lead to high animal performance. The first critical issue or step to providing and maintaining an adequate fertility level for alfalfa is to develop an understanding of the amounts of essential nutrients contained in alfalfa. Table 1 lists the nutrient content of alfalfa hay at several annual yield levels. More nitrogen is contained in the crop than any other nutrient. This would be expected given the high protein (18-20% or more) that alfalfa usually has. The major source is nitrogen from the atmosphere as fixation occurs through the mutually Roland Do Meyer, Extension Soils Specialist, University of California Cooperative Extension, Land Air Water Resources Department, Davis, CA 95616; Published in Proceedings, 27th National Alfalfa Symposium and 26th California Alfalfa Symposium, December 9-10, 1996, San Diego, CA. Certified Alfalfa Seed Council, Davis, CA., and University of California, Davis. 207 beneficial relationship between the alfalfa plant and the nitrogen fixing Rhizobia bacteria located in the nodules on the roots. Potassium and calcium are next in their relative amounts removed by alfalfa. In many situations the potassium supplied by soils is adequate to insure necessary quantities for maximum alfalfa yield production. However, there are several areas in California known to be low in potassium as wen as some sandy soils where response is likely. Calcium is adequately supplied by nearly all soils in California, but lime, which contains mostly calcium carbonate, is often applied when the soil pH is below 6.3 throughout a major portion of the soil profile. Table I. Nutrient removal eld levels. LroQ removal. IDSIA Nutrient Symbol Four Tons Eight Tons Twelve Tons Nitrogen N 240 480 720 Phosphorus P20S 48 95 143 Potassium K2O 173 346 519 Calcium Ca 128 256 384 Magnesium Mg 27 53 80 Sulfur S 16 32 48 Iron Fe 1.5 3 4. 5 Manganese Mn 1 2 3 Chlorine CI 1 2 3 Boron B 0.2 0.5 0. 7 Zinc Zn 0.2 0.4 0. 6 Copper Cu 0.06 0.13 0. 19 Molyb~!!~- Mo 0.008 0.016 0. 024 The second critical soil fertility issue in alfalfa production is detennining which soils are unable to supply the necessary plant nutrients for attaining high profitable yields. Table 2 lists the seventeen essential nutrients for plant growth plus an 18th nutrient, cobalt which is required by alfalfa and other legumes for nitrogen fixation. The table also gives the frequency of occuITence of fertilizer nutrient needs for alfalfa in California where soils are unable to meet plant needs. The nutrients that are most commonly needed by alfalfa are phosphorus, followed by sulfur, potassium and in some cases boron and molybdenum. DIAGNOSING NUTRIENT DEFICIENCIES A systematic approach is necessary to insure that proper diagnostic procedures are used to maintain rapid growth of high quality alfalfa. Fertilizing alfalfa may involve considerable cost, thus it is important to properly identify which nutrients are needed. Four methods can be utilized to diagnose the nutrient status of alfalfa: I) visual observation of deficiency symptoms, 2) soil testing, 3) plant tissue testing, and 4) applying strips of single nutrient fertilizers alone or in combination. Visual observation of deficiency symptoms. This is perhaps the easiest and most striking method to detect deficiencies, but often the most difficult to positively identify which nutrient or combination of nutrients may be the cause. Table 3 gives the nutrient deficiency symptoms that may be seen in alfalfa. 208 Table 2. Relative alfalfa fertilizer needs of the essential plant nutrients in Co Nutrient Symbol Frequency of Occurrence Nitrogen N Seldom3 Phosphorus P20S Frequently Potassium K2O Less Frequently Calcium Ca Never Magnesium Mg Never Sulfur S Less Frequentl~ Iron Fe Very Rare Manganese Mn Never Chlorine Cl Never Boron B Seldom3 Zinc Zn Never Copper Cu Never Molybdenum Ma Seldom3 Nickel Ni Never Cobalt Ca Never4 1Frequently =Over 25% of the acreage shows needfur fertiliZation with tI1iS nutrient 2 Less Frequently -Less than 25% of the acreage shows need. 3 Seldom -Less than 1% of the acreage shows need. 4 Necessary for nitrogen fixation only. Nitrogen, sulfur and molybdenum an show deficiency in the same manner, that of yellow, stunted growth of plants which most be confinned by the use of a plant tissue test. The symptoms shown by alfalfa that are listed for phosphorus are not specific and may indicate a number of growth limiting factors including cool temperatures, inadequate water, or high salt levels in the soil. If more severe deficiencies of potassium or boron occur, the symptoms can be specific and identify quite positively the need for one of these nutrients. More detailed descriptions of deficiency symptoms are given in the discussion of each nutrient. Table 3. Nutrient deficiency symptoms in alfalfa. Nutrient Deficiency Symptoms Nitrogen Generally yellow, stunted plants. Phosphorus Stunted plants with small leaves, sometimes leaves are dark blue-green. Potassium Pinhead yellow or white spots on margins of upper leaves. Yellow turning to brown leaf tips and margins of more mature leaves. Sulfur Generally yellow, stunted plants. Boron Upper part ofplant has yellow top side ofleaves with reddish- purple undersides and short internodes. Multiple stem tops of 3-5 internodes length may develop on a single plant stem. Molybdenum Generally yellow, stunted plants. 209 Soil and Plant testing. When a new planting is being considered, it is best to learn if the field has had any history of alfalfa, whether there were parts of the field that produced lower yields and, if so, sample the soil from each area separately. In addition to using soil testing to estimate the nutrient availability, sampling can be used to observe soil compaction, note layers that may impede root growth or water movement and collect soil at different depths for laboratory analysis of potential salt or other growth limiting conditions. Soil and plant sampling procedures are critical, since samples should adequately reflect the nutrient status or growing conditions of the whole field. Because a representative sample of an entire field gives an average of all the variation in that field, it may not be the best for developing recommendations for certain parts of the field which are less productive and need specific treatment. Since most fields have some variation in crop growth and productivity, the best technique to use in sampling is to divide each field into two or three areas representing good, medium and poorer alfalfa or previous crop growth. Within each area, permanent benchmark locations approximately 50 x 50 feet in size should be established. These permanent benchmark areas should be identified with measured distances to specific landmark points on the edge of the field so they can be sampled repeatedly over a ten to fifteen year period. Additional benchmark locations can be established as the crop productivity levels change over the years. Initially, both soil and plant tissue samples should be taken from these benchmark areas annually for 2 to 3 years. Once historical trends have been established, they can be sampled every 2 to 3 years or when alfalfa is replanted. Plant sampling at least annually should be used to more effectively guide fertilization practices. Another important point to remember is that soil and plant tissue testing differ in there ability to evaluate the nutrient status of alfalfa (Table 4). Since plants integrate a number of soil, environmental and management factors, plant tissue testing indicates directly which nutrients are at inadequate or excessively high concentrations. Soil testing has the advantage of measuring each nutrient or characteristic independently while plant tissue testing indicates only the most limiting nutrient. Fertilization to supply an adequate amount of the most deficient nutrient most be done before other plant nutrient levels can be evaluated. The sampling approach described provides a more comprehensive manner in which to compare areas of the field with regard to yields, fertilizer inputs as well as soil and plant tissue test level changes over the years. Phosphorus Good Excellent Potassium Good Excellent Sulfur Very poor Excellent Boron Poor Excellent Molybdenum Not recommended Excellent Soil Testing. The best time to sample soil is soon after an irrigation or rainfall when the soil is moist and the probe will easily penetrate the soil.