ots to develop rapidly. The holes were left open for rain to fall, after which seedlings were transplanted the following day in the morning and at sunset to avoid sun stress. Five kilograms (kg) of cow manure was thoroughly mixed with topsoil and used for transplanting the seedlings in the field  . The seedlings were about 60 days old with average heights of 19 cm for Moringa oleifera and 13 cm for Moringa ovalifolia at transplanting. Transplanting was completed within 2 weeks.
Watering was done once a week during the first growing season (2014/2015 summer), and once fortnightly during the second growing season (2015/2016 summer). Watering was done weekly for the first growing season because the trees were younger with shadow roots and needed water at closer intervals compared to the second growing season when the trees were bigger with established tap roots, thus, needing less water to survive. In Namibia, the winter season starts from April and ends September, which is followed by summer. During winter, watering was done biweekly for both first and second seasons (2014 winter and 2015 winter) to avoid freezing of the trees and subsequent mortality. Fuglie and Sreeja  discussed that Moringa oleifera trees do not need much watering, except in very dry conditions when it must be done regularly for the first two months and afterwards only when the trees are obviously suffering.
Superphosphate fertilizer with a P content of 83 g/kg (Wonder superphosphate granular) and nitrogen fertilizer with an N content of 280 g/kg (Limestone Ammonium Nitrate―LAN) were applied at the rate of 0 g, 100 g, 200 g and 300 g a month after transplanting to boost the roots system development as well as the leaves  since winter was to begin two months ahead. Both Moringa species were cultivated in the same field of 0.21 hectares (ha) but divided into two parts in which Moringa oleifera site had dimensions of 73 m × 15 m = 1096 m2 (0.11 ha), while Moringa ovalifolia area was measuring 60.4 m × 15 m = 906 m2 (0.10 ha).
2.4. Soil Composition of the Field
The results of soil analysis for the determination of the amount of nutrients and their properties are found in Table 1. The result shows the soil pH, electrical conductivity or soluble salts (EC), organic matter (OM), Phosphorus (P), Potassium (K), Calcium (Ca), Magnesium (Mg), Sodium (Na) and soil texture in percent (%) and parts per million (ppm). The 30 to 60 cm soil depth range had higher nutrient contents than 0 to 30 cm soil depth range, except OM and P were higher (0.87% and 24.60 ppm), respectively. Johnston and Steen  said that phosphorus must be adequate, readily available reserves in the soil; however, most un-manured soils contain too little readily available phosphorus to meet the large demand of crops, particularly during certain growth periods such as root development of seedlings and flowering for yield. Therefore, fertilizers containing phosphorus must be added to the soil to boost plant growth, development and yields. The soil analysis result shows pH of 7.22 and 7.67 for 0 to 30 cm and 30 to 60 cm depths respectively, which were slightly basic. Fuglie and Sreeja  emphasized that Moringa tolerates a wide range of soil conditions, but prefers a neutral to slightly acidic pH of 6.3 - 7.0. The pH unit measures the degree of acidity or basicity of a solution. Most living things depend on a proper pH level to sustain life. Different crops need different pH levels but grow best if the soil they are planted in is maintained at an optimal pH  . The particle size analysis revealed that the soil is sandy (84.2% at 0 - 30 cm depth and 82.1% at 30 - 60 cm depth) with low silt and clay (<10% each).
2.5. Data Collection Procedures
Trees’ heights were measured for data collection. Heights were measured for 20 Moringa oleifera and 20 Moringa ovalifolia trees at 30-day and 60-day intervals to determine the growth rates (heights) during the two summer-growing seasons (October 2014 to April 2015 and October 2015 to April 2016). The 30-day intervals
Table 1. Soil nutrients and properties.
were used to measure the two Moringa-species trees during the first summer-growing season (October 2014 to April 2015), while 60-day intervals were used in the second season (October 2015 to April 2016) to evaluate the trees’ elongation. In Namibia, rainfall is strongly seasonal since more than 99% of rain falls occur in the summer months  , which support plants’ growth and development.
2.6. Data Analysis
Statistical Package for Social Sciences (SPSS® version 23) and Microsoft Excel® version 13 were used to carry on all the analyses. The generalized linear model (GLM) was used to analyse growth rates of Moringa oleifera and Moringa ovalifolia in which heights of trees were used as dependent variables while months and years were used as independent variables.
Figure 1 presents Moringa oleifera and Moringa ovalifolia growth rates as measured by heights of trees for the two summer seasons (2014/2015 and 2015/2016), October 2014 to April 2015 and October 2015 to April 2016. The results showed the average heights of trees for each summer growing season in the orchard. Moringa oleifera had greater maximum height of 224.9 cm than Moringa ovalifolia with 77.03 cm in the month of April, whereas the lowest heights were 36.75 cm and 5.40 cm, respectively, in November 2014. In 2015/2016 season, Moringa oleifera had an average best growth in height of 281.45 cm in March, compared to that of Moringa ovalifolia that had 113.2 cm in April. On the other hand, the minimum heights of 70.05 cm and 22.0 cm were recorded for Moringa oleifera and Moringa ovalifolia, respectively in October 2015.
The descriptive statistics of Moringa oleifera and Moringa ovalifolia monthly and bimonthly growth in heights (cm) for the 2014/2015 and 2015/2016 summer growing seasons, respectively is presented in Table 2. Moringa oleifera grew
Figure 1. Monthly average increase in heights (cm) of Moringa species in 2014/2015 and 2015/2016 summer seasons.
Table 2. Moringa species monthly heights (cm) for 2014/2015 and 2015/2016 seasons.
faster with mean heights of 130.21 cm in 2014/2015 season and 194.06 cm heights in 2015/2016 season. Moringa ovalifolia, as slow-growing trees had a mean height of 40.77 cm in 2014/2015 season and 66.747 cm in 2015/2016 season.
The independent sample t-test of monthly growth of Moringa species in heights for 2014/2015 and 2015/2016 summer seasons is shown in Table 3. The test results showed that mean monthly and bimonthly height increments were significantly different (P < 0.05) within the two Moringa species and between the summer-growing seasons of 2014/2015 and 2015/2016, which differences can be seen in Table 2.
The highest growth in heights for Moringa oleifera (281.45 cm) and Moringa ovalifolia (113.2 cm) were observed in 2015/2016 season (October 2015 to April 2016), compared to 2014/2015 season (October 2014 to April 2015) with heights of 224.9 cm and 77.03 cm, respectively (Figure 1). The results can be attributed to the more robust establishment of the root systems of the trees in 2015/2016 than 2014/2015 when the trees were younger with shallow root system establishments. However, the results of this study for both species had lower heights due to the shorter growing summer seasons in Namibia (September to April) as discussed by Pallett  . Nevertheless, Moringa oleifera grew faster and had higher heights compared to Moringa ovalifolia in both 2014/2015 and 2015/2016 summer-growing seasons (Table 2). Moringa oleifera is an extremely fast-growing tree and can reach a height of 400 cm in a year, and, eventually, 600 to 1500 cm   .
Central Namibia having both arid and semi-arid climates, plants grow more rapidly during summer seasons (October to April) after which winter (May to September) sets in and negatively affects the plant growth. This explains the maximum heights in March and April and the minimum heights in October and November for the two Moringa species. In 2014/2015 summer season, the lowest heights of 36.75 cm and 5.4 cm for Moringa oleifera and Moringa ovalifolia, respectively were observed in November 2014 because the trees sprouted late after the winter dormancy since they were younger. Also in 2015/2016 summer season, lowest heights of 70.05 cm for Moringa oleifera and 22 cm for Moringa ovalifolia were recorded in November 2015 since the trees were older and sprouted earlier. Korsor, Ntahonshikira, Bello, and Kwaambwa  discussed
Table 3. Independent sample test of Moringa species seasonal heights for 2014/2015 and 2015/2016.
Assumes unequal variance between groups; * = significant at 0.05; *** = significant at 0.001 alpha level.
that younger Moringa trees take long time to sprout after the winter dormancy compared to older trees, which is due to shallow and deep roots establishment, respectively. Billings  emphasized that all environments and all stressors are not equal with regard to their effects on plant growth and reproduction; nor do all kinds of plants equally adapt to environmental changes or have the same level of coping with stress factors. Favourably, the two Moringa species are drought-resistant trees and do better despite the scarcity of water when fully established as described by Morton  .
Within species, heights were lower in each species in the first growing season than in the second season. In the 2015/2016 growing season, Moringa oleifera had the highest average heights of 281.45 cm against 224.9 cm in the 2014/2015 growing season. Moringa ovalifolia likewise elongated faster in 2015/2016, with a maximum of 113.2 cm in the month of April, while 2014/2015 had a maximum of 77.025 cm in the same month. Just as in the highest, the lowest heights (cm) for Moringa oleifera were observed during the 2014/2015 growing season as 36.75 cm recorded in November 2014 while for the 2015/2016 season, a minimum height of 70.05 cm was recorded in October 2015. Similarly, the lowest heights for Moringa ovalifolia were 5.4 cm in November 2014 and 22 cm in October 2015 (Figure 1), which clearly indicates that the more the trees mature, the faster they sprout and elongate after the winter season.
These differences in heights between the two summer-growing seasons can be attributed to two factors; namely, the ages of the trees and the cutting and/or harvesting intervals. That is, in the first growing season of 2014/2015, trees were younger/smaller and measured at 30-day intervals after which they were harvested and cut back for regrowth. This may have affected the elongation of the young trees. In the second growing season (2015/2016), trees were older/bigger and measured and harvested and/or cut at 60-day intervals for regrowth. This may have contributed to the highest heights for 2015/2016. Besides the differences between the 30-day and 60-day harvesting and cutting intervals, the differences in heights between the two growing seasons can also be attributed to the root systems becoming better established; that is, roots of young trees are shallow in the soil compared to roots of older trees that are deeply rooted, which enables them to elongate faster, thus promoting more vegetative growth. Fuglie  reported that in the first year, Moringa oleifera will grow up to 500 cm in height and if left alone, it subsequently reaches 1200 cm in height, with a trunk of 30 cm circumference over the years. The analysis reveals that both Moringa oleifera was significantly higher (P < 0.001) than Moringa ovalifolia in mean monthly height increments over the two seasons. This implies that the two species had different heights in the two seasons. Based on the field observations, Moringa oleifera grew faster on average than Moringa ovalifolia, and this was statistically significant. The lower height of Moringa ovalifolia might be attributed to the first focus on underground growth of the roots into tubers to preserve energy for rough conditions (winter/drought) as described by Korsor et al.  . Thus, the hypothesis (H02) that Moringa oleifera does not differ in growth performance (heights) with Moringa ovalifolia was rejected because they differ significantly.
Moringa oleifera and Moringa ovalifolia are drought-resistant trees that withstood the harsh climate of Namibia. Although both trees grew well in this harsh environment, Moringa oleifera grew faster in heights compared Moringa ovalifolia that grew slowly. Moringa oleifera had the higher heights of 281.45 cm for March 2016 and 224.9 cm for April 2015, while Moringa ovalifolia was lower with 113.2 cm and 77.025 cm for April 2016 and 2015 respectively. As young trees for the two Moringa species, but especially for Moringa ovalifolia, these heights estimates will change as the trees grow older. Their characteristics to survive in water-scarce environments qualified them to grow in arid and semi-arid climates as the case with Namibia. More research studies need to be done in this respect to establish the heights of Moringa ovalifolia when fully grown alongside Moringa oleifera.
This study was financially supported by Namibia Students Financial Assistance Fund (NSFAF) and Namibia-South Africa Bilateral Research Agreement through the National Commission on Research Science and Technology (NCRST). This article was extracted from Morlu Korsor’s PhD Dissertation at the University of Namibia, Department of Animal Science.