Rapid Determination of Nitrogen in Cane Leaves

An evaluation of a new non-destructive technique for the determination of total nitrogen in cane leaves using a Technicon infra-red reflectance analyzer is described. More than 200 leaf samples were analysed and the results indicated that a nitrogen content between 1 and 3% can be accurately determined in less than a minute. Regression analysis indicated that the N values determined by infra-red analysis were well correlated with the Kjeldahl N values (r = 0,89). Furthermore, reproducibility was acceptable with co-efficients of variation ranging from 0,8 to 4,5O/o. The advantages of the new technique are that it is much faster than the present method of weighing, digesting, diluting and dispensing, which is followed by a colorimetric determination using the indophenol blue method, and that the instrument can be handled by an unskilled operator. This method will simplify the determination of nitrogen in plant tissue analysis of a wide range of crops. Introduction The determination of the total nitrogen content of sugarcane third leaf samples is one of the most important analyses undertaken by the Fertilizer Advisory Service (FAS) of the SA Sugar Association Experiment Station. Of the 16 elements considered to be essential for sugarcane, nitrogen has the greatest effect on cane growth and juice quality. Early assessment of how effective nitrogen fertilization has been is important to the grower if timely remedial action is to be taken. Until about seven years ago the nitrogen content of leaf Kjeldahl digestates was estimated by a microdistillation/titration procedure (Bremner'). The method, although very reliable, was very time consuming and the maximum laboratory output was between 125 and 150 samples per day, leading to long delays in sample analysis at peak periods. The introduction of a semiautomated LKB sample processor into the laboratory for determining P in leaf digests and A1 and P in soil extracts (Burrows & Meyer2), led to an improved method for determining N in leaf digests based on the colorimetric indophenol blue reaction (Burrows'). The time to analyse a batch of 150 digests was reduced from eight hours to a little over an hour. Despite this considerable improvement in methodology, the system was still subject to the hazardous, unpleasant and time consuming procedure of wet digestion of leaf material with concentrated selenised sulphuric acid. There has been considerable interest recently in the USA and Europe in applying Near Infra-red Spectroscopy to determine the nitrogen and moisture content of plant material. This technique is well established for the determination of protein, oil, carbohydrate and moisture contents of raw materials used in the baking industry. Preliminary investigations conducted by the soil and plant analysis laboratory at the University of Georgia (Wesley et a14) have shown that the nitrogen contents of maize leaf samples may be determined to within 0,2 units of the nitrogen contents determined by the Kjeldahl procedure. Investigations conducted on crops such as soyabeans, cotton and peanuts have also produced promising results. The method has not yet been evaluated for sugarcane. The main advantage of this method is the elimination of the time-consuming digestion, so it was decided that its applicability to the determination of nitrogen in sugarcane leaf samples on a routine basis should be investigated. The equipment, the principle of the method and the procedure used to evaluate the accuracy and precision of the method when used under routine operating conditions, are described in this paper.