AFRIKAANS: Die vraag onstaan of insette by aartappelproduksie deurentyds ekonomies optimaal gebruik word. Hierdie vraag word al hoe meer deur produsente gevra, veral waar die prysknyptang al hoe sterker gevoel word. Uit ‘n opbrengs oogpunt was stikstof nog altyd ‘n baie belangrike inset. Verliese aan opbrengste kan egter baie groot wees as stikstof ondoelmatig bespaar word. Om optimum stikstoftoedienings te kwantifiseer bly problematies. Die stikstofinhoud van die grond kan gemeet word, maar in hierdie studie is bewys dat die inligting uit hierdie ontledings dikwels misleidend en kan sinvolle aanbevelings nie altyd daarvolgens gemaak word nie. Aan die ander kant, kan die aartappelplant as indikator gebruik word. Die afgelope dertig jaar was baie aandag aan petioolsapekstraksie reg oor die wéreld gegee. Meeste navorsers versap die petiool en gebruik duur laboratorium toerusting om die NO3--konsentrasie te bepaal en maak dan daaruit afleidings. Met hedendaagse tegnologiese ontwikkelings, het vinniger ontledingstegnieke beskikbaar geword. Die Reflectoquant (Merck) is ‘n draagbare instrument wat gebruik word om die NO3--konsentrasie in die aartappelpetioolsap direk te meet. Met kalibrasie-proewe wat deur Omnia gedoen was, is ‘n optimale NO3--konsentrasie reeks (OPTI-reeks) opgestel waarmee stikstof bo-bemestings geskeduleer kon word. Die bepaalde OPTI-reeks is oor twee persele getoets. Die twee persele het verskil in grond tekstuur en ligging. Die Delmas perseel het ‘n hoër klei-inhoud gehad as die Bronkhorstspruit perseel. By die Bronkhorstspruit perseel het verskille tussen die behandelings gouer na vore gekom. Die persele wat laer vlakke van stikstof bemes was, en eerste stikstof bemes is het die laagste NO3--konsentrasie lesings gehad. By die Delmas perseel het hierdie tendens nie herhaal nie. In van die lesingtye het die laer stikstofbemestingsbehandelings hoër ontledings gehad. NO3--konsentrasie lesings, soos vermeld is in die meeste literatuur, het afgeneem oor tyd en gedui op ‘n verdunnings-effek van NO3--konsentrasie in die aartappelplant. Die verdunnings-effek kan toegeskryf word aan die fisiese verdunning van NO3- oor ‘n groter wordende plante, knolle en die gebruik van die NO3- in plantmetaboliese aktiwiteite. Opbrengste en bakkwaliteit van die onderskeie behandelings, was nie verlaag deur die OPTI-reeks te gebruik nie. As in ag geneem word dat ten minste, 45 kg ha-1 stikstof bespaar kon word deur gebruik te maak van die OPTI-reeks, is die metode ‘n baie kragtige ontledingstegniek om stikstof doelmatig te skeduleer. Stikstof sal nie die beperkende faktor in produksie wees, indien hierdie tegniek te gebruik word nie. ENGLISH: The question arises whether inputs are used optimally in an economical way for potato production. This is something that is asked more and more by producers. Nitrogen plays a dominating role in determining yield. Yield losses will be considerable if nitrogen is saved in an unnecessary way. To quantify optimum nitrogen applications can be difficult. Determining the nitrogen content of the soil can be one option, but as was shown in this thesis, this method can lead to incorrect nitrogen applications. The potato plant itself can be used as an indicator for nitrogen applications. For the past thirty years, a lot of emphasis has been placed on petiole sap analyses. Petioles were liquified and then expensive laboratory equipment were used to determine the NO3--concentration, whereafter the recommendations for nitrogen application were made. Analyses are made easier today with modern technological methods and equipment. Direct measurement of the NO3--concentrations in the potato petiole are possible by using the Reflectoquant (of Merck). Omnia Fertilizers conducted many experiments to determine an optimal NO3--concentration range (OPTI-range) to schedule nitrogen applications. In this study the OPTI-range was tested at two sites. The two sites differed in soil texture and district. The Delmas site has a higher clay percentage than the Bronkhorstpruit site. Quicker reactions on the treatments were found at the Bronkhorstspruit site. The plots that received lower nitrogen treatments, were fertilized first and had the lowest NO3--concentrations in the sap. This tendency was not replicated at the Delmas site. The lower nitrogen treatment plots sometimes had higher NO3--concentrations than the higher nitrogen treatments. NO3--concentration readings, as mentioned in most literature, declined over time and is explained by the diluting effect of nitrates in the plant. Not only is there an increase in the potato plants above ground growth, but tuber growth also constitute a big proportion of the biomass at the end of the growth period. Nitrogen is used in all these processes, which contribute to the diluting effect. Yield and tuber quality were not lowered by applying the OPTI-range. The OPTI-range is a good method to manage nitrogen requirements of the potato plant. At least 45 kg ha-1 nitrogen could be saved by using the OPTI-range. Therefore this is a powerful method to determine nitrogen scheduling. Nitrogen will not be the limiting factor in potato fertilization if the OPTI-range is used. / Dissertation (MSc(Agric))--University of Pretoria, 2010. / Plant Production and Soil Science / unrestricted