Growth, yield and quality response of beet (beta vulgaris L.) to nitrogen
To study the quality response of beetroot to nitrogen fertilizers, a pot trial was conducted in the glasshouse facility of the Department of Soil, Crop and Climate Sciences, Faculty of Natural and Agricultural Sciences, University of the Free State, during the 2011 season. The effect of five nitrogen sources (limestone ammonium nitrate, ammonium nitrate, urea, ammonium sulphate and urea ammonium nitrate) at five nitrogen levels (0, 50, 100, 150 and 200 kg N ha-1) on beetroot (Detroit Dark Red) on a Bainsvlei soil type was investigated. The data collected was analyzed using Tukey’s Least Significant Difference test, at 5% level of significance to determine statistically significant differences between means. The results showed that all fertilizers used resulted in a reduction in plant height for the first six weeks of growth. Nitrogen application only increased plant height significantly from week 8 where the height of plants that received nitrogen, irrespective of the fertilizer used, were significantly taller than control plants. At week 8 no significant differences in height were noted between various nitrogen application rates, but by week 10 significant differences in plant height were noted between the 50 kg N ha-1 and 150 kg N ha-1 or 200 kg N ha-1 application rates. The findings showed that beet plants reacted better to N-fertilization using ammonium sulphate nitrate and urea ammonium nitrate than other nitrogen sources, although limestone ammonium nitrate and ammonium nitrate also produced improvements in plant growth, whereas plants that received urea showed no improvements. Nitrogen at 100 kg ha-1 resulted in more leaves per plant than its application at other levels. Urea ammonium nitrate as a nitrogen source significantly improved plant leaf area, leaf fresh mass, total fresh mass and root diameter. Application of nitrogen at 200 kg ha-1 also increased leaf area, leaf fresh mass, total fresh mass, beet diameter and beet volume. Urea ammonium nitrate increased leaf dry mass by an average of 397% while the lowest leaf dry mass by (139.42% of control) was observed with the use of limestone ammonium nitrate as a nitrogen source. The greatest leaf dry mass was obtained at the highest rate of nitrogen application (200 kg ha-1) and the lowest leaf dry mass was observed at the control level. Beet yields were found to increase as the nitrogen application rate increased, from 2.99 t ha- 1 in the control treatments to 14.37 t ha-1 in the treatments that received 200 kg N ha-1. Fertilizing with urea ammonium nitrate gave the highest yields (12.17 t ha-1), while using limestone ammonium nitrate gave the lowest yields (9.00 t ha-1). Application of nitrogen at 50 kg ha-1 resulted in firmer beets than nitrogen application at other levels. Beets from plants that did not receive any nitrogen were significantly softer than those that received nitrogen at higher levels. The darkening of beet colour (decrease of L*) was experienced at the control level while the highest changes of colour (increase of L*) was obtained at the highest nitrogen level. Nitrogen at 100 kg ha-1 influenced the lowest change of coefficient a from red to green while the control level resulted in more intensive change. The results showed that nitrogen at the control level led to more intensive changes of coefficient b colours from yellow to blue and its application at the highest level resulted in less intensive changes of coefficient b colours from yellow to blue. Neither nitrogen source nor nitrogen level had any effect on the pH, sucrose or fructose contents of the roots. Application of nitrogen at 150 kg ha-1 resulted in greater total soluble solids content in the roots, while the starch content of plants that received no nitrogen was significantly greater than that of plants receiving nitrogen. Nitrogen application at 100 kg ha-1 and at the control level influenced the glucose content, which was significantly higher in these plants than in those that received 50, 150 and 200 kg N ha-1, however, the highest glucose content of the roots was observed at the control level. Nitrogen application at 200 kg ha-1 resulted in higher nitrogen content in the leaves as compared to application of other nitrogen sources at different levels. Limestone ammonium nitrate influenced potassium content of the leaves more than other nitrogen sources. Nitrogen application at 200 kg ha-1 resulted in a greater calcium content in the leaves than other nitrogen sources. The highest sodium content of the leaves was observed at 150 kg N ha-1 while the lowest sodium content was observed at 50 kg N ha-1. Urea ammonium nitrate had a greater positive influence on the manganese content of the leaves than other nitrogen sources. Plants that received no nitrogen had significantly greater levels of iron in the leaves than at all nitrogen levels. Ammonium nitrate as a nitrogen source influenced the calcium content of the beets more than other nitrogen sources. Other root minerals such as phosphorus, potassium, sodium, magnesium, manganese, copper, iron and zinc were not significantly influenced by nitrogen source or nitrogen level, or the interaction between these factors.