Sampling Technique and Maturity Effects On Nutrient Concentrations in Alfalfa.

Although plant tissue analysis is generally believed to be more accurate than soil analysis, it is rarely used by alfalfa growers to predict crop fertilization needs. The recommended plant sampling methodology for alfalfa varies between states, but all guidelines are based on alfalfa at one-tenth bloom growth stage.  However, the crop often doesn’t reach one-tenth bloom stage because growers harvest alfalfa at bud stage to meet the forage quality demands of the dairy industry.  Research was needed to compare different sampling methods and to evaluate the change in nutrient concentration with advancing maturity.  Plant tissue samples were collected from five different alfalfa fields over all three cuttings in 2009 in the intermountain area of Northern California.  Samples were collected at early bud, late bud and one-tenth bloom.  Whole plant samples, top 6-inch samples and fractionated plant samples (the current University of California protocol whereby plants are divided into thirds and specific plant parts are analyzed for certain nutrients) were collected at each sampling time.  The phosphorus, potassium, sulfur, boron and molybdenum concentration of the alfalfa was determined for each sampling method and alfalfa maturity stage. Phosphorus concentration declined markedly with advancing maturity. The fractionated plant samples analyzed for PO4-P appeared more affected by maturity than did whole top or top 6-inch samples analyzed for total P.  Potassium and sulfur also declined with advancing maturity but to a lesser degree than did the P concentration.  This confirms that plant maturity must be considered when interpreting plant tissue test results.  If critical values are not adjusted for maturity, a sample collected at bud stage may erroneously appear to have adequate nutrient levels.  The concentration of the micronutrients boron and molybdenum decreased slightly with advancing maturity, but the degree of decline was not considered to be sufficient to warrant adjusting critical values.