Evaluation of petiole nitrate measurement as an aid for N fertilisation of potatoes (Solanum tuberosum L.) on sandy soils
Potatoes are produced in the Sandveld of the Western Cape on soils with extremely low clay and organic material contents. A low nitrogen mineralisation potential and high leaching hazard complicate the management of the nitrogen supply to the plant. This study investigated the viability of a rapid field measurement of plant nitrate content as a decision support tool in three field trials. In the first two trials the standard practice of weekly N and K fertigation was used at four different N-levels on two cultivars. The nitrate levels were monitored in petioles during two seasons, using two different techniques. The procedure in which dried petioles were used for nitrate extraction and laboratory readings taken with an auto-analyser was less accurate than that where petiole sap was squeezed from petioles and indicator paper strips used for nitrate determination. The highest petiole nitrate levels in these trials corresponded with the highest tuber yields for both cultivars and were found at an application of 400 kg N ha-1. Increasing N levels (100 - 400 kg N ha-1) significantly reduced the specific gravity (SG) of Hertha but had no effect on the SG of Up-to-Date. With three irrigation treatments and three fertigation frequencies in another trial, the petiole nitrate concentrations were the highest when no leaching occured. The highest petiole nitrate concentration was obtained with two fertigations per week. The petiole nitrate levels differed between cultivars and peaked at five to six weeks after planting. These factors should be kept in mind when petiole nitrate readings are used as an aid for fertiliser programs. The Merck reflectoquant proved to be a viable decision support tool in the N management of potatoes under these conditions.