Rapid sap tests for better fertilizer management
1994
Handson, P.D. | Sheridan, J.
Most farming systems require fertilizer inputs to ensure an adequate supply of N,P,K and S.Determining the correct rate of fertilizer to apply is difficult.While laboratory analyses provide accurate results which maybe correlated against crop yield, their adoption for assessing fertilizer requirement is limited due to the associated costs and time delays.Simple, low cost, field based tests which provide on the spot assessment of crop nutritional status are a valuable tool in optimising fertilizer management.Researchers at the State Chemistry Laboratory (SCL) have been developing and calibrating simple field tests for NO3, PO4 K and SO4, for cereal oilseed and horticultural crops in Southeastern Australia.Various quick tests were evaluated in the laboratory to identify the most appropriate extraction method and analytical procedure and the best method was selected for field evaluation and calibration.One to two centimetres of the stem base or petiole is cut from 30-40 randomly selected plants and the sap is squeezed out using a garlic press.For determination of NO3 and K, undiluted sap is squeezed onto a pocket sized Horiba Cardy Ion Selective Meter.For PO4 and SO4 determinations sap is diluted, filtered, reagents added and the nutrient concentration read using hand-held comparators.Determination of all four nutrients in a single sample takes 30-40 minutes.The NO3 test is fully calibrated against yield for four crops and is now available as a commercial testing service.Good Correlations have been obtained between sap nutrient levels and total plant nutrient levels for SO4 and PO4.Critical crop PO4 values have been obtained for one crop.No significant correlation was found between sap K and K levels.
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