Interaksie tussen stikstof- en wateropname deur koring en mielies

English: The main aim of this study was to optimise nitrogen and water application for irrigated wheat and maize, whereby a procedure could be developed for estimating the nitrogen application requirement (NAR) of the two crops by means of the irrigation water management model BEW AB. For this purpose a field experiment with wheat and maize was done on 3 m deep Bainsvlei Amalia (3200) fine sandy soil at Bloemfontein. Five water treatments (Wl to WS levels) were combined with six nitrogen treatments (NI to N6 levels), each of which was repeated four times. A commercial centre pivot was used for irrigating. The sprinkler package was designed in such a way that water applications from the W4 to the WI level were 75, 50, 25 and 0% respectively of the WS level. The irrigations at the WS level were scheduled by means of BEWAB. Before an estimation procedure for the NAR of wheat and maize could be developed, a study was made of the effect of nitrogen application on plant growth parameters such as grain yield, dry matter yield and the nitrogen concentration in it, as well as the uptake afwater and nitrogen at the five water levels. The most important findings were that both the grain yield and the dry matter yield of wheat increased linearly by the square root of nitrogen application at four of the five water levels (W2 to WS). In the case of maize the two parameters increased exponentially with nitrogen application. The fact that grain yield and dry matter yield responded similarly to nitrogen application can be ascribed to the even distribution of water application over the growing season. Where the water was distributed unevenly over the growing season, as in the case of the Wl level (dryland) of wheat in particular, the crop yield index was lower than in the rest of the applications. From the difference between wheat and maize in the response curves, it can be deduced that the growth parameters did not reach a biological maximum in wheat, but that they did in maize. This is also reflected in the curves of nitrogen application versus nitrogen uptake by the dry matter, which was linear in wheat and exponential in maize. Although all the measured growth parameters, including the leaf area index during the peak growth phase, which reacted positively to nitrogen application at all five water levels, no meaningful relation between nitrogen application and evapotranspiration could be deducted at these water levels from either crops. A regression fit technique was used to identify the plots that received optimum water and nitrogen applications. The data for these plots were used to develop a procedure whereby the NAR of wheat and maize could be estimated. This procedure is a function of the total nitrogen requirement of the crop (TNR), the nitrogen supply capacity of the soil-crop combination (NSC), and the nitrogen recovery efficiency of the applied nitrogen (NRE). The TNR depends on the quantity of available irrigation water whereby a planning or target yield of grain is calculated. This calculated grain yield is converted by an optimum crop yield index, which was determined as 0.52 for wheat and 0.51 for maize, to total dry matter yield. The optimum nitrogen -1 -1 concentration, which was determined as 0.015 kg kg for wheat and 0.009 kg kg for maize, must then be calculated. The NSC can be calculated from the clay content (%) of the top soil, while the NRE has been determined as a constant value of 64 % for wheat and 75 % for maize. Verification of the proposed procedure showed that the NAR of wheat and maize can be estimated successfully. However, it is clear that the estimation ofNSC and NRE will have to be refined further.