Each ton of alfalfa hay contains approximately 50 pounds of nitrogen (N), 10 pounds of phosphorus (P), 60 pounds of potassium (K) and 4 pounds of sulfur (S). These and other micronutrients are removed from fields with each cutting.

Plant deficiencies of other nutrients, while rare, can occur in some fields. Managing proper levels of plant nutrients begins with assessing nutrient levels available in soil and through plant tissue testing.

Test soils for nutrient availability prior to planting and each year afterward. In the West, phosphorus is needed more often and in much greater amounts than any other nutrient element. In addition, sulfur, potassium, zinc (Zn), boron (B) and molybdenum (Mo) are sometimes required. Lab soil testing results provide accurate information for determining nutrient availability and potential for plant deficiencies.

Properly sampled, plant tissue testing is used to assess nutrients taken up by the plants and is useful in determining in-season plant nutrition status. Soil and tissue testing are both useful for determining nutrient needs on established alfalfa. Tables 1* to 5* give Colorado State University’s (CSU) soil and plant testing recommendation levels for alfalfa.

Closely follow the lab’s procedures for taking and handling soil or plant tissue samples. The depth of the surface soil samples varies by labs as does the timing and way they suggest taking plant samples for tissue testing. Each lab has calibrated their testing procedures for providing accurate results to their customers. Taking and handling samples differently may introduce errors in the lab tests and reduce the consistency in their recommendation.

Take many soil or plant sub-samples to be combined into a composite sample. It is important to randomly collect soil or plant samples across several areas of the field or field partition. In taking samples from a field or portion of a field, ten samples are the minimum number needed and fifteen to twenty are recommended for gathering a representative composite sample for lab testing (see Figure 1*). Make sure to take samples well within the field, including areas around the center. Avoid sampling close to field edges where field traffic is greatest and where equipment slowing may result in greater fertilizer applications.

CSU recommends splitting large fields for sampling. CSU also recommends taking additional samples from field areas with different plant growth or appearance or a history of varying crop yield. Sample a minimum every forty acres for irrigated fields and a minimum of every eighty acres for dryland fields.

Based on the lab’s test recommendations, it is important to apply and incorporate two or three year’s supply of soil-immobile nutrients, such as P, K and zinc, prior to planting. When P or other soil-immobile nutrients are called for on established alfalfa, they can be applied by top-dress or chemigated fertilizer applications. Alfalfa roots can pick up the immobile nutrients near the soil surface readily enough to justify these in-crop applications.

Soil and plant testing laboratories use different soil phosphorus extraction methods. As a result, each lab uses different test thresholds for representing P availability and conversely fertility needed. For this reason, it is best to send samples to the same lab and use their fertility recommendations for obtaining consistent results and maintaining comparable records.

High-pH fields usually contain high levels of excess lime which can react with phosphorus, reducing its availability to plants over time. This chemical reaction is slow in alkaline soils (above 7.6) or in acidic soils (below 5.5) and is fairly stable in soils with pH levels near neutral (7.0). Even in alkaline soils phosphorus applications are generally available in the first season after application. For this reason, phosphorus should be evaluated each year until the seasonal P nutrient availability pattern of a field has been established.

Alfalfa, being a legume, has a symbiotic relationship with nitrogen-fixing soil bacteria. When active and present, these bacteria fix atmospheric nitrogen and supply all the nitrogen needs of alfalfa plants. Healthy alfalfa will develop pink nodules on the plant roots to facilitate good populations of these bacteria. Always inoculate alfalfa seeds with an alfalfa bacterial culture in fields with no history of alfalfa production. A small application of N (20 to 40 pounds per acre) at planting may be beneficial as well. Adding too much N can suppress the bacterial symbiosis and can reduce alfalfa growth and enhance weed establishment and competition.

Potassium and sulfur deficiencies most commonly occur in sandy soils with low organic matter. Irrigation water from groundwater wells, as well as irrigation ditches on rivers downstream from cities, may have enough sulfur and boron to supply alfalfa fertility needs. Sulfur deficiencies may also occur in rain-fed or very pure mountain stream-irrigated fields.

It is helpful to be able to recognize phosphorus deficiency symptoms because P deficiency is common in many fields. Phosphorus deficiency in alfalfa shows up as thin, weak stands with stunted and grey-green foliage. Deficient areas will appear drought-stressed even when the field is wet. Stems may appear red to purple in color in warm weather. Purpling in stems also can occur when alfalfa grows in cold soils or long periods of cold weather. Leaves are frequently folded, and not fully expanded. Compare these plant symptoms with vigorous plants taken from field areas with good growth characteristics. PD

*References, tables and figures omitted but are available upon request at editor@progressivedairy.com

—Excerpts from Colorado State University Agronomy News, Vol. 27, No. 4

Bruce Bosley
Colorado State University
bruce.bosley@colostate.edu

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