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African Crop Science Journal
African Crop Science Society
ISSN: 1021-9730 EISSN: 2072-6589
Vol. 5, Num. 2, 1997, pp. 175-188
African Crop Science Journal
Vol.5. No.2, pp.175-188, 1997

Response of castor cultivar "Hale" to rate and method of nitrogen fertilizer application in different environments of Zimbabwe

D. HIKWA and L.M. MUGWIRA

Agronomy Institute and Chemistry and Soil Research Institute
Department of Research and Specialist Services, P. O. Box CY 594 Causeway, Harare, Zimbabwe

(Recieved 7 February, 1995; accepted 25 March, 1997)


Code Number: CS97023
Sizes of Files:
    Text: 48.6K
    Graphics: Line drawings (gif) - 9.7K 

ABSTRACT

The effects of different rates and methods of N fertilizer application were assessed at five sites over two seasons (1987/88 and 1988/89). Fertilizer rates of 0, 30, 60, 90 and 120 kg N ha^-1 were applied in a single dose at planting or in two splits, half applied at planting and the other half as topdressing at primary floral initiation. The sites at Makoholi, Mlezu and Matopos were on sandy soils in low rainfall areas while those at Kadoma and Panmure were in sandy clay loam soils in higher rainfall areas. There were significant interactions between N rate and season on seed yield in the sandy soils except at Makoholi. The effect of N rate on these parameters contrasted at each site between the two seasons, indicating that the amount of N fertilizer needed for castor (Ricinus communis L.) on the sandy soils depended on the amount of rainfall. Application of N in a single dose did not significantly increase seed yield when available soil N was medium to high (> 24 ppm) except at Kadoma where application of 30 kg N ha^-1 increased yield when soil N status was rated as medium. Yield responses to N applied at planting in 1987/88 at Matopos and Panmure and in 1988/89 at Makoholi and Mlezu indicated that 60 kg N ha^-1 was the optimum application rate for castor. When compared to a single dose, splitting N application enhanced its effectiveness in increasing yield in 1987/88 at Mlezu and Matopos, and at Kadoma in two seasons. These results suggest that split N applications are beneficial to castor when seasonal rainfall is greater than 700mm.

Key Words: Nitrogen rate, rainfall, Ricinus communis L., soil type

RESUME

Les effets de differentes proportions et methodes d'application d'engrais azote ont ete evalues dans cinq stations au cours des saisons 1987/88 et 1988/89. Les proportions du fertilisant de 0, 30, 60, 90 et 120 kg de N ha^-1 ont ete utilisees. Elles ont ete appliquees soit a dose unique aux plantes et l'autre a dose dedoublee dont une partie aux plantes et l'autre a l'initiation de l'epanouissement floral primaire. Les stations de Makoholi, Mlezu et Matopos etaient situees sur des sols sablonneux dans des regions a faibles precipations. Celles de Kadoma et de Panmure etaient, par contre, situees sur des sols sablo-angileux dans des regions a fortes precipitations. Les interactions entre la teneur en N et la sason sur la production des graines se sont sevelees hautement significatives dans des stations a sols sableux sauf a Makoholi. L'effet de la teneur en N sur ces parametres variait a chaque site en fonction de la saison. Cela monre que la quantite d'engrais azote souhaitable pour la poivriere (Ricinus communis) sur des sols sablonneux est fonction de la quantite de pluies. L'application de N a dose unique n'a pas augmente de facon significative la production des graines lorsque l'azote disponible du sol etait de moyenne a eleve (> 24 ppm) sauf a Kadoma. Dans ce dennier cas, l'application de 30 kg de N ha^-1 augmentait la production quand le statut de l'azote du sol etait considere comme moyenne. Les reponses de la production a l'azote applique aux plantes en 1987/88 a Matopos et Pammure et en 1988/89 a Makoholi et Mlezu a montre que 60 kg de N ha^-1 etait la proportion optimale pour la poivriere. Contrairement a la dose unique, le dedoublement de l'application de l'azote a accru son efficacite. La production a donc augmente a Mlezu et Matopos en 1987/88 et a Kadoma au cours des deux saisons. Ces resultats montrent bien que les applications de l'azote a dose dedoublee sont benefiques a la poivriere loisque les precipations saisonnieres sont superieures a 700mm.

Mots Cles: Proportion d'azote, precipitations, Ricinus communis, type de sol

INTRODUCTION

The castor plant (Ricinus communis L.) is native to the tropics, including Zimbabwe. Castor grows in soils of wide ranging fertility, but varieties of the species may respond differently in growth and yield to similar levels of nutrients (Weiss, 1983). This necessitates determination of soil nutritional levels for each variety that are needed to produce the maximum yield at economic levels of fertilizer input.

Nitrogen is one of the limiting nutrients for crop production in most soils. According to Weiss (1983), excessive nitrogen tends to promote vegetative growth of castor, while reducing resistance to drought. Insufficient nitrogen may culminate in stunted plants, firing of older leaves and premature senescence, thereby reducing yield. Research work on fertilizer requirements of castor in Zimbabwe conducted for two seasons in Matabeleland and Midlands Provinces showed overall seed yield increases of between 23 and 45% and between 30 and 60 %, when 14 and 29 kg N ha^-1 were applied, respectively (Donovan and Landsberg, 1963). Elsewhere, seed yield increases have been reported with applications of up to 90 kg N ha^-1 (Muthuvel et al., 1987; Saran and Giri, 1987; Rao and Ventateswarlu, 1988).

The information base on fertilizer requirements of castor under various Zimbabwe conditions is inadequate and needs to be broadened. Determination of optimum levels of specific fertilizer nutrients such as nitrogen and phosphorus is imperative. Exploratory field experiments of 1986/87 had shown that there were hardly any interactions between N and P applied, while responses to N application were apparent (Agronomy Institute, 1991). Since castor is a long season crop, there was need to compare effects of single dose to split N application on performance of the crop. Therefore, the objective of this study was to assess how spliting N application compared to a single dose application impacted on performance of castor, particularly seed yield.

MATERIALS AND METHODS

The experiment was conducted during the 1987/88 and 1988/89 seasons in fields at Matopos Research Station (Matopos), Mlezu Agricultural Institute (Mlezu), Cotton Research Institute (Kadoma), Panmure Experiment Station (Panmure) and Makoholi Experiment Station (Makoholi). The Matopos, Mlezu and Makoholi locations were on medium-grained granitic sands with 8% clay and more than 85% sand, while the Kadoma and Panmure locations were in medium-grained sandy clay loam soils with 20-40% clay and less than 45% sand (Chemistry and Soil Research Institute, 1989). Selected soil test values on these sites are shown in Table 1. Matopos and Makoholi are in Natural Region (NR) IV, with a long term mean annual rainfall of 570 and 650mm, respectively, Mlezu is in NR III with a range of annual rainfall between 675 and 700mm, whereas Kadoma and Panmure are in NR IIb, with a long term mean annual rainfall of 726 and 721 mm, respectively. The location elevations are 1204, 1200, 1338, 1157 and 881 metres above sea level for Makoholi, Mlezu, Matopos, Kadoma and Panmure, respectively (Whingwiri et al., 1987).

TABLE 1. Available nitrogen, phosphorus and potassium in soils at the different sites in 1987/88 and 1988/89

-------------------------------------------------------
 Location    pH^1   Mineral N^2    P^3        K 
                      (ppm)       (ppm) (me/100 g soil) 
--------------------------------------------------------  
                             1987/88
  
Makoholi    4.8       18 VL        19 A      0.08
Mlezu       4.9       32 M         16 A      0.23
Matopos     5.0       35 M         15 A      0.26
Kadoma      6.2       44 H         16 A      0.27
Panmure     5.5       21 L         16 A      0.36
  
                            1988/89
  
Makoholi    4.8       17 VL        22 A      0.07
Mlezu       4.7       15 VL         5 D      0.09
Matopos     4.9       21 L         11 MR     0.12
Kadoma      6.3       30 M         11 MR     0.19
Panmure     5.4       24 L         14 A      0.17
  
Legend:VL = Very low, D = Deficient, L = Low , MR = Marginal, M = Medium, A = Adequate , H = High (Chemistry and Soil Research Institute , 1989)
^1 A 0.01M CaCl2 solution (1:5 soil: solution ratio) was used in determination of pH.
^2 Soil N after laboratory incubation for 14 days
^3 Resin extractable P
---------------------------------------------------------------------------

The N rates studied were 0, 30, 60, 90 and 120 kg ha^-1, applied as ammonium nitrate (34.5% N). These rates were based on results of an exploratory field experiment on both heavy and light sandy soils (Agronomy Institute, 1991) which showed responses of N up to 120 N ha^-1 at the same locations. The nitrogen applied at each rate was either a single dose at planting or was split, with half applied at planting and the other half as top-dressing at primary floral initiation. Uniform dressings of 100 kg ha^-1 of single superphosphate (8% P, 12% S) and 50 kg ha^-1 potassium chloride (50% K) were applied at planting at all locations in both seasons. The open pollinated dwarf-internode cultivar "Hale," developed in the USA, was used.

A factorial combination of treatments in randomised complete blocks (RCB) design with four replicates was used. A gross plot consisted of five 10m long rows, spaced at one metre apart with an in-row spacing of 0.5 metres. The net plot, from which seed yield was estimated, comprised three middle rows of 8.5m long. The average number of racemes per plant was based on a mean of five plants randomly selected in the net plot. Days to maturity of the primary racemes were estimated at about 80% physiological maturity of the plants in the plot. Analysis of variance (ANOVA) was performed on the data.

Daily rainfall amounts were monitored and recorded using a rain gauge at each site (Figs. 1 and 2).

    Figure 1 - Cumulative precipitation (mm) in five day periods (pentads) from Nov. 1 - April 30 at the five sites in 1987/88 season.

    Figure 2 - Cumulative precipitation (mm) in five day periods (pentads) from Nov. 1 - April 30 at the five sites in 1988/89 season.

RESULTS

Overall, the 1987/88 season was wetter than the 1988/89 one at all sites. The cumulative rainfall amounts received in 1988/89 season were 44, 58, 46, 41 and 28 % less than those received in 1987/88 at Makoholi, Mlezu, Matopos, Kadoma and Panmure, respectively (Figs. 1 and 2). Table 2 shows the amount of rainfall accumulated at intervals from November 1 to planting date, flowering and crop maturity.

TABLE 2. Rainfall amounts accumulated between the periods November 1 and planting date, flowering and crop maturity in both seasons

------------------------------------------------------------------------  
Interval From Nov.1   Makoholi    Mlezu    Matopos   Kadoma    Panmure
------------------------------------------------------------------------  
                                   1987/88
  
Planting date          48.0        49.0      34.0      34.0      58.0
Flowering             261.0       292.0     289.0     481.0     341.0
Maturity              558.0       775.0     736.0     780.0     747.0
  
                                   1988/89    
  
Planting date          5.0         32.0      53.0      12.0      12.0
Flowering             84.0        131.0     332.0     153.0     202.0
Maturity             327.0        318.0     412.0     530.0     727.0
--------------------------------------------------------------------------

Effects of applied N rate and season. There were significant effects of interactions between the rate of N applied and season on number of days to physiological maturity and on seed yield at Mlezu and Matopos, and on days to physiological maturity at Makoholi (Tables 3, 4 and 5). Although there were no significant interaction effects, the average number of racemes/plant were 18, 15 and 10 % less in 1988/89 than in 1987/88 at Mlezu, Kadoma and Panmure, respectively. At Matopos, 20 % more racemes/plant were attained in 1988/89 season. At Makoholi, the difference in number of racemes between the two seasons was negligible (Tables 3, 4 and 5).

Makoholi. At Makoholi, plants took an average of 41 days longer to mature in the wetter 1987/88 than in the drier 1988/89 season. In 1987/88, no marked differences in duration to maturity among the N rates were observed at this site. However, in 1988/89, the plots with zero N rate had the shortest (P<0.01) duration to maturity when compared to the plots where N was applied (Table 3).

TABLE 3. Effects of interactions between N rate and season on average number of racemes/plant, days to maturity and seed yield (kg ha^-1) in 1987/88 and 1988/89 seasons at Makoholi

--------------------------------------------------------------------------- 
Nitrogen      Racemes/plant      Days to maturity     Seed yield, kg ha^-1
rate        -----------------   -------------------   -------------------- 
(kg ha^-1)  1987  1988   Mean   1987   1988   Mean     1987   1988   Mean   
             /88   /89           /88    /89             /88    /89
---------------------------------------------------------------------------

  0         4.9    4.9    4.9    202    151    176    1 338    615     977 
 30         6.0    6.7    6.4    199    158    178    1 218    729     973
 60         5.1    6.0    5.6    200    161    180    1 149    867    1008
 90         6.6    5.9    6.2    199    158    178    1 304    802    1053
120         8.5    6.5    7.5    198    164    181    1 516    867    1193
  
Mean        6.2    6.0    6.1    199    158    179    1 306    776    1041
  
                    SEm                 SEm                    SEm
Interaction means   0.7                 1.4                  127.4
Season means        0.3                 0.6                   57.0
N Means             0.5                 1.0                   90.1
  
SEm = Standard Error of mean
---------------------------------------------------------------------------

TABLE 4. Effects of interactions between N rate and season on average number of racemes/plant, days to maturity and seed yield (kg ha^-1) in 1987/88 and 1988/89 seasons at Mlezu

--------------------------------------------------------------------------- 
Nitrogen      Racemes/plant      Days to maturity     Seed yield, kg ha^-1
rate        -----------------   -------------------   -------------------- 
(kg ha^-1)  1987  1988   Mean   1987   1988   Mean     1987   1988   Mean   
             /88   /89           /88    /89             /88    /89
---------------------------------------------------------------------------

  0         10.1   6.9    8.5    144    119    132    1756    1411    1583
 30          9.6   8.1    8.9    147    115    131    1670    1662    1666
 60         10.2   9.4    9.8    147    114    130    1761    1825    1793
 90         11.0   8.9    9.9    146    109    128    2036    1873    1955
120         10.9   9.4   10.1    148    114    131    1714    1994    1854
Mean        10.4   8.5    9.4    146    114    130    1787    1753    1770
  
                     SEm                SEm                   SEm
Interaction means    0.7                1.4                  86.3
Season means         0.3                0.6                  38.6
N Means              0.5                1.0                  61.0
  
SEm  = Standard Error of mean
---------------------------------------------------------------------------

There were no effects of interactions between rate of N and season on castor grown at Makoholi (Table 3). However, at the 30 and 60 kg N ha^-1 rates, 36.7 and 22.2 % more racemes were attained than where no N was applied, in the drier season (1988/89), while 34.5 and 73.5 % more were realised with the 90 and 120 kg N ha^-1 rates in the wetter season (1987/88) at this site (Table 3).

Mlezu. At Mlezu, effects of interactions between rate of N applied and season on days to maturity were similar to those of Makoholi in that plants also took longer (mean of 32 days) to mature in the first than in second season (Table 4). In the wetter season (1987/88) application of N lengthened duration to maturity by three days when compared to the zero N rate. However, in the drier season (1988/89), the 60 to 120 kg N ha^-1 rates significantly (P<0.01) reduced the period to maturity when compared to the zero rate (Table 4).

Although no significant interaction effects on number of racemes/plant were observed at Mlezu, the number of racemes increased progressively from the zero rate up to 60 kg N ha^-1 in 1988/89. However, main effects of season were significant, with the 1988/89 season having 18 % less racemes than the 1987/88 season (Table 4).

The significant effects of interactions between N rate and season were mainly a result of seed yield response to applied N during 1988/89. In 1987/88, there was no yield advantage in applying N to the crop (Table 4).

Matopos. In 1987/88, no significant differences were observed on duration to maturity among the various N rates applied. However, in 1988/89, the period to maturity decreased significantly (P<0.05) when 30 kg N ha^-1 was applied to the crop (Table 5). Differences in days to maturity among the 30, 60 and 90 kg N ha^-1 rates were insignificant (Table 5).

TABLE 5. Effects of interactions between N rate and season on number of racemes/plant, days to maturity and seed yield (kg ha^-1) in 1987/88 and 1988/89 seasons at Matopos

--------------------------------------------------------------------------- 
Nitrogen      Racemes/plant      Days to maturity     Seed yield, kg ha^-1
rate        -----------------   -------------------   -------------------- 
(kg ha^-1)  1987  1988   Mean   1987   1988   Mean     1987   1988   Mean   
             /88   /89           /88    /89             /88    /89
---------------------------------------------------------------------------

  0         4.5    6.7    5.6    149    149    149     845    1174    1010
 30         5.7    7.1    6.4    151    144    148    1300    1497    1399
 60         6.7    7.2    7.0    150    146    148    1746    1493    1619
 90         5.9    7.1    6.5    147    147    147    1456    1401    1428
120         7.2    7.7    7.5    149    148    149    1752    1425    1588
Mean        6.0    7.2    6.6    149    147    148    1420    1398    1409
  
                     SEm                SEm                    SEm
Interaction means    0.5                1.2                  120.7
Season means         0.2                0.5                   54.0
N Means              0.3                0.8                   85.4
  
SEm = Standard Error of mean
---------------------------------------------------------------------------

Even though interactions between rate of N applied and season on number of racemes/plant were not significant, main effects of season and those of N were apparent at Matopos. At each N rate applied, higher numbers of racemes were attained in the drier 1988/89 than in the wetter 1987/88 season.

When compared to the zero rate, all rates of N applied in the 1987/88 season enhanced seed yield. The seed yield increased progressively and significantly from zero N up to 60 kg N ha^-1, after which the trend was broken. When compared with the seed yield obtained at 60 kg N ha^-1, seed yield was depressed at 90 kg N ha^-1 and remained the same at 120 kg N ha^- 1. In the seacond year, the effects of aplying N on seed yield were not significant (Table 5).

Kadoma. At Kadoma, there were no significant interactions, but significant effects of season were observed on the measured parameters. On average there were 25 % less racemes in the second than in the first season. The plants also matured earlier by some 22 days in 1987/88 compared to 1988/89 (Table 6). Generally, yield differences between the two seasons were highly significant at each N rate applied, with an average of 137 % more being produced in the wetter 1987/88 than in the drier 1988/89 (Table 6).

TABLE 6. Effects of interactions between N rate and reason on average number of racemes plant, days to maturity and seed yield (kg ha^-1) in 1987/88 and 1988/89 seasons Kadoma

--------------------------------------------------------------------------- 
Nitrogen      Racemes/plant      Days to maturity     Seed yield, kg ha^-1
rate        -----------------   -------------------   -------------------- 
(kg ha^-1)  1987  1988   Mean   1987   1988   Mean     1987   1988   Mean   
             /88   /89           /88    /89             /88    /89
---------------------------------------------------------------------------

  0         7.1    5.5    6.3    112    132    122     1798    669   1234
 30         6.6    4.9    5.7    111    133    122     1785    933   1359
 60         6.7    4.9    5.8    113    138    126     1850    674   1262
 90         6.7    5.2    6.0    110    133    121     1577    760   1168
120         6.4    4.6    5.5    113    134    123     1745    661   1203
Mean        6.7    5.0    5.9    112    134    123     1751    739   1245
  
                   SEm                  SEm                    SEm
Interaction mean   0.4                  1.6                   124.6
Season means       0.2                  0.7                    55.7
N Means            0.3                  1.2                    88.1
  
SEm = Standard Error of  mean
---------------------------------------------------------------------------

Panmure. Just as for Kadoma, interaction effects between N rate and season on the measured parameters were also not significant at Panmure. However, the seasonal effects were such that more racemes were produced in the wetter 1987/88 than in the drier 1988/89 season (Table 7). In both seasons, fertilizer application resulted in early maturity of the crop. Seed yield was neither influenced by season nor by the N rate applied (Table 7).

TABLE 7. Effects of interactions between N rate and season on average number of racemes/plant, days to maturity and seed yield (kg ha^-1) in 1987/88 and 1988/89 seasons at Panmure

--------------------------------------------------------------------------- 
Nitrogen      Racemes/plant      Days to maturity     Seed yield, kg ha^-1
rate        -----------------   -------------------   -------------------- 
(kg ha^-1)  1987  1988   Mean   1987   1988   Mean    1987   1988   Mean    
             /88   /89           /88    /89            /88    /89
---------------------------------------------------------------------------

  0         7.7    6.7    7.2     99    149    124    1312    1174    1243
 30         8.5    7.1    7.8     97    144    121    1286    1497    1392
 60         7.2    7.1    7.2     95    147    121    1357    1401    1379
 90         7.2    7.1    7.2     95    147    121    1357    1401    1379
120         8.1    7.7    7.9     96    148    122    1422    1425    1423
Mean        8.0    7.2    7.6     96    147    122    1376    1398    1387
  
                    SEm                 SEm                   SEm
Interaction means   0.6                 1.0                  99.4
Season means        0.3                 0.5                  44.5
N Means             0.4                 0.7                  70.3
  
SEm = Standard Error of mean
---------------------------------------------------------------------------

Effects of method of N application and season. At two (Makoholi and Matopos) of the three sites on sandy soils, effects of method of N application on number of racemes/plant were affected by season only (Table 8). However, duration to maturity and seed yield were not affected by interactions between method of N application and season.

TABLE 8. Effects of interactions between method of N application and season on average number of racemes/plant at the sites with sandy soils

--------------------------------------------------------------------------- 
Method of N        Makoholi             Mlezu               Matopos
application -------------------   ------------------   --------------------
            1987   1988    Mean   1987   1988   Mean    1987   1988   Mean  
             /88    /89            /88    /89            /88    /89  
--------------------------------------------------------------------------- 
Sole         7.1    5.9    6.5    10.2    8.2    9.2     5.3    7.5    6.4
Split        5.3    6.1    5.7    10.5    8.8    9.7     6.7    6.9    6.8
Mean         6.2    6.0    6.1    10.4    8.5    9.4     6.0    7.2    6.6
  
                    SEm                   SEm                   SEm
Interaction means   0.5                   0.4                   0.3
Season means        0.2                   0.3                   0.3
Method means        0.3                   0.3                   0.2
  
SEm = Standard Error of mean
---------------------------------------------------------------------------

In 1987/88, split application of N at Makoholi resulted in significantly (P<0.05) fewer racemes/plant than when N was applied in a sole (single) dose (Table 8). However, in drier 1988/89 season, the method of N application did not affect the number of racemes/plant.

At Matopos, split N application produced a significantly (P<0.01) greater number of racemes/plant in 1987/88, while in 1988/89 the difference in the number of racemes/plant between the two methods was insignificant. In 1988/89, when N was applied as a sole dose, the number of racemes/plant was higher than that of 1987/88 by 42 % (Table 8).

At Mlezu, one of the three sites on sandy soils, neither sole nor split N application affected raceme production in both seasons (Table 8). Not withstanding this, there were 24 and 19 % more racemes with sole and split N application, respectively, in the wetter 1987/88 than in the drier 1988/89 season.

The interactions between method of N application and season were not significant for the measured parameters over the two seasons on the heavy soil sites of Kadoma and Panmure (data not presented).

Interactions between rate and method of N application. Effects of interactions between rate and method of N application on days to maturity were realised only at Matopos and on yield at Mlezu and Matopos (Tables 9 -14).

In both seasons, crop maturity period at Makoholi and Mlezu were not affected by the interactions between rate and method of N application (Tables 9 and 10) and neither was seed yield at Makoholi (Table 12).

TABLE 9. Effects of interactions between rate and method of N application on days to physiological maturity at Makoholi

--------------------------------------------------------------              
                         1987/88                1988/89   
N rate(kg ha^-1) ---------------------    --------------------  
                  Sole   Split   Mean     Sole   Split   Mean
--------------------------------------------------------------  
  0                 -      -     202        -      -     151
 30                200    198    199       160    157    158
 60                201    198    200       164    159    161
 90                199    198    199       160    156    158
120                198    198    198       164    164    164
Mean               200    198    199       159    157    158
  
                          SEm                     SEm
Interaction means         1.0                     2.6
N means                   0.7                     1.9
Method means              0.5                     1.2
  
SEm = Standard Error of mean
---------------------------------------------------------------

TABLE 10. Effects of interactions between rate and method of N application on days to physiological maturity at Mlezu

----------------------------------------------------------  
N rate                 1987/88              1988/89   
(kg ha^-1)        -------------------  -------------------
                  Sole   Split  Mean   Sole   Split  Mean
----------------------------------------------------------  
  0                 -      -     144     -      -     119
 30                149    146    148    117    114    226
 60                147    147    147    113    115    114
 90                146    146    146    109    110    109
120                146    151    148    116    113    115
Mean               146    147    146    114    114    115
  
                          SEm                  SEm
Interaction means         1.6                  2.4
N means                   1.1                  1.7
Method means              0.7                  1.1
  
SEm = Standard Error of mean
---------------------------------------------------------------------------

Table 11. Effects interactions between rate and method of N application on days to physiological maturity at Matopos

---------------------------------------------------------  
N rate                 1987/88              1988/89   
(kg ha^-1)        -------------------  -------------------
                  Sole   Split  Mean   Sole   Split  Mean
----------------------------------------------------------  
  0                 -      -     149     -      -     149
 30                151    151    151    146    143    144
 60                148    151    150    147    145    146
 90                153    141    147    147    147    147
120                154    145    149    148    148    148
Mean               146    152    149    147    146    147
  
                          SEm                  SEm
Interaction means         1.7                  1.6
N means                   1.2                  1.1
Method means              0.8                  0.7
  
SEm = Standard Error of mean
-----------------------------------------------------------  

TABLE 12. Effects of interactions between rate and method of N application on seed yield (kg ha^-1) at Makoholi

-----------------------------------------------------------  
N rate                 1987/88              1988/89   
(kg ha^-1)        -------------------  -------------------
                 Sole   Split  Mean    Sole   Split  Mean
----------------------------------------------------------  
  0               -       -     1338     -      -     615
 30             1282    1155    1218    849    608    729
 60             1177    1120    1149    918    816    867
 90             1214    1394    1304    729    875    802 
120             1697    1340    1519    797    938    867
Mean            1380    1231    1305    775    778    776
  
                          SEm                  SEm
Interaction means        237.5                 92.4
Nitrogen means           167.9                 65.3
Method means             106.2                 41.3
  
SEm = Standard Error of mean
----------------------------------------------------------  

TABLE 13. Effects of interactions between rate and method of N application on seed yield (kg ha^-1) at Mlezu

------------------------------------------------------------------  
N rate                 1987/88              1988/89   
(kg ha^-1)        ----------------------  ------------------------
                 Sole    Split     Mean    Sole     Split     Mean
------------------------------------------------------------------  
  0                -        -     1 756       -        -     1 411
 30             1 528    1 812    1 670    1 575    1 748    1 662
 60             1 424    2 099    1 761    1 861    1 788    1 825
 90             1 860    2 212    2 036    1 864    1 882    1 873
120             1 758    1 671    1 714    1 977    2 011    1 994
Mean            1 702    1 873    1 787    1 724    1 782    1 753
  
                          SEm                        SEm
Interaction means        139.5                      101.8
Nitrogen means            98.6                       71.9
Method means              62.4                       45.5
  
SEm = Standard Error of mean
--------------------------------------------------------------------------

TABLE 14. Effects of interactions between N rate and method of N application on seed yield (kg ha^-1) at Matopos

---------------------------------------------------------  
N rate            1987/1988               1988/89
(kg ha^-1)  ---------------------  ----------------------   
             Sole   Split   Mean    Sole   Split    Mean
---------------------------------------------------------  
  0           -       -      845      -       -     1174
 30         1451    1150    1300    1439    1555    1492
 60         1683    1809    1746    1332    1654    1493
 90         1219    1693    1456    1591    1212    1401 
120         1362    2141    1752    1468    1382    1425
Mean        1429    1698    1420    1457    1451    1398
  
                       SEm                   SEm
Interaction means     164.8                 176.5
Nitrogen means        116.5                 124.8
Method means           73.7                  78.9
  
SEm = Standard Error of mean
----------------------------------------------------------

Significant (P<0.01) effects of interactions between rate and method of N application were observed for period to crop maturity at Matopos in 1987/88. At the 30 and 60 kg N ha^-1 rates, there were no differences between the sole and split N application. However, at the 90 and 120 kg N ha^-1, when N was split, the delay in crop maturity was up to 12 days (Table 11). In 1988/89, the period to maturity was not influenced by method of N application.

At Mlezu and Matopos, the rate and method of N application interacted positively to influence yield performance in 1987/88 season (Tables 13 and 14). However, the differences between sole and split N application were only significant at the 60 and 120 kg N ha^-1 rates at Mlezu and Matopos, respectively, while in 1988/89, the seed yields at both sites were not influenced by these interactions (Tables 13 and 14).

Main effects of method of N application on yield were only realised at the heavy soil site of Kadoma, but there was no effect at the other heavy soil site at Panmure (Table 15). At the Kadoma site, splitting N application was highly favourable, in both seasons, compared to applying all the N at planting. Yield advantages of split over sole N application were 18 and 17% in 1987/88 and 1988/89, respectively, at the latter site (Table 15).

TABLE 15. Effects of method of N application on seed yield (kg ha^-1) at the heavy soil sites

--------------------------------------------------------------------- 
Method of N                Kadoma                       Panmure
application   --------------------------   --------------------------
               1987/88   1988/89   Mean     1987/88  1988/89   Mean
---------------------------------------------------------------------  
Sole            1 580      673    1 126      1 342    1 409    1 376 
Split           1 922      806    1 364      1 410    1 387    1 398
  
Mean            1 751      739    1 245      1 376    1 398    1 387
  
SEm             106.9     31.2     55.7       41.0    78.9    44.5
  
SEm = Standard Error of mean
---------------------------------------------------------------------  

DISCUSSION

The performance of castor at the five sites was greatly influenced by amount of rainfall. In the wet 1987/88 season, there was a higher number of racemes/plant than in the dry 1988/89 season and this was positively related to seed yield at Mlezu and Kadoma, but the same relationship between the two paramenters was not apparent at the other sites. However, since seed yield at Mlezu (364 mm rainfall) and Matopos (436 mm rainfall) in 1988/89 was not depressed by rainfall which was lower than that of 1987/88, while being depressed at Makoholi (300 mm), differences in the yield performance could also be attributed to seasonal precipitation. However, in terms of rainfall distribution, the results of this study indicated that seed yield in a low rainfall season was negatively related to the amount of rainfall between flowering and maturity, suggesting an earlier critical rainfall requirement for castor cv "Hale" (Table 2). The 364 mm rainfall (Fig. 2) which maintained seed yield at Mlezu in 1988/89 (Table 4) is comparable to the minimum of 375 mm reported by Van der Merwe and Clarke (1977), to be able to support a castor crop.

Besides the differences between the two seasons in the performance of castor noted above, there were significant interactions between season and N rate on maturity period and seed yield only at the sandy soil site at Makoholi, Mlezu and Matopos. At Makoholi, there was no seed yield response to N rate in both seasons. In the case of maturity period, however, responses to N rate at each of the three sandy soil sites in the second season was directly the opposite of that obtained in the first season. These contrasting responses to N between seasons were also obtained in the response of seed yield at Mlezu and Matopos where there was response only in 1988/89 at the former site but only in 1987/88 at the latter site. These trends suggested that the amount of N fertilizer needed for the production of castor on sandy soils was influenced by the amount of rainfall. Increasing N fertilizer rate significantly increased the number of racemes at Makoholi and Matopos, delayed maturity at Makoholi, while shortening it at Matopos and Panmure, and increased seed yield at Mlezu and Matopos.

The yield responses of 54 and 28 % due to applications of 30 kg N ha^-1 obtained at Matopos were within the range previously reported by Donovan and Landsberg (1963) in similar environments of the country. These researchers had reported yield responses of 30 to 60 % from application of 27 kg N ha^-1 at various sites in 1958/59.

There was little interaction between method of N application and season on the performance of castor. The only significant interactions showed that split N application reduced the number of racemes at Makoholi while increasing them at Matopos in 1987/88, when compared to sole N application. The only significant interactions between rate and method of N application were obtained on seed yield at Mlezu and maturity and seed yield at Matopos in 1987/88. Split applications of 60 kg N ha^-1 at Mlezu and 120 kg N ha^- 1 at Matopos had yield advantage over sole N application. Split application of 90 and 120 kg N ha^-1 delayed crop maturity at Matopos. However, split N application had overall yield advantage at Kadoma in each of the two seasons.

The responses to split N application appeared to be primarily due to the status of available N in soils and the amount of rainfall. These responses coincided with medium status of available N in the soils at Mlezu and Matopos or high N status at Kadoma (1987/88) and medium N status in the case of Kadoma in 1988/89 (Table 1). Furthermore, the response of castor to split N in the wet 1987/88 occurred only when cumulative rainfall, up to crop maturity, was greater than 700 mm as was the case at Matopos, Kadoma and Mlezu, respectively (Fig. 1), but not at lower rainfall at Makoholi (580 mm) and Panmure (480 mm). However, in 1988/89 split N had yield advantage over single N application at Kadoma under medium N status in the soil but a cumulative rainfall of 530 mm. The results of the two seasons suggest that thresh-holds for yield benefit from split N application were a minimum of medium N status in the soil and about 700mm rainfall to crop maturity in a good rainfall season such as 1987/88 or medium to high N status in the soil, supported by at least 500 mm of rainfall in a relatively dry season such as experienced in 1988/89.

Yield increases from split application at two of the sandy soil sites tended to improve with N rate, reaching a maximum of 47 % at 60 kg N ha^-1 at Mlezu in 1987/88 (Table 9), while the largest increments at Matopos were 57 % at 120 kg N ha^-1 in 1987/88 and 24 % at 60 kg N ha^-1 in 1988/89 (Table 12). In a similar study carried out in Andhra Pradesh (India), Sarma (1985) reported that spliting 40 kg N ha^-1 resulted in a 25.8 % yield increase of castor compared to a single application, on sandy loam soils. The results obtained in this study are within a similar range or above.

Comparisons of the performance of castor "Hale" at the different sites indicated that the highest yields (>1700 kg ha^-1) were obtained at Mlezu in both seasons and at Kadoma in 1987/88. The lowest yields were obtained at Makoholi and at Kadoma in 1988/89. Otherwise yields were fairly uniform across the five sites during the two seasons and ranged between 1300 and 1400 kg ha^-1. These values suggest that comparable yields of castor may be expected on sandy and clay soils provided seasonal changes do not severely limit yield.

The average number of racemes/plant and duration to maturity of the primary raceme were measured in consideration that they could be used as good indicators for seed yield. Generally, no set pattern between these parameters and seed yield were observed in this study.

The observations mentioned above suggest three possibilities; firstly; that the number of racemes per plant on its own, is not a good indicator of the plant's yield potential. In future work, additional information may be generated if an assessment is done on how number of racemes, size of raceme (length and number of capsules on it) and weight of capsules interact to influence overall yield performance. Secondly, duration to maturity of only the primary raceme does not indicate overall yield potential of a castor plant. At the same time, it is rather difficult to establish the cut off point in maturity days in castor due to the sequential ripening of racemes from primary, at the bottom, through secondary to tertiary at the top. In the variety "Hale", sequential sets can vary from three to five depending on the season length and rainfall situation.

The results obtained in this study, suggest that out of the several factors that affected yield, the amount of rainfall exerted the greatest effect. This was shown by yield reductions of 41 % at Makoholi and 58 % at Kadoma as rainfall decreased by 53 and 32 %, respectively, from the wet 1987/88 to the drier 1988/89 season. Furthermore, the yield advantage of split over sole N application was dependent on the amount of rainfall, particularly in a good season.

CONCLUSION

A major difficulty in assessing castor performance across the different sites was the fact that the sandy soils were all located in the drier agro-ecological zones, Natural Region III (Mlezu) and Natural Region IV (Makoholi and Matopos) while the fine-textured soils were located in a more humid region, Natural Region II. Neverthless, there were several important trends that were observed in this study.

In a good rainfall season the mean yields (1306, 1787 and 1420 kg ha^-1) of castor "Hale" obtained on sandy soils at makoholi, Mlezu and Matopos, respectively, were comparable to those obtained on clay soils (1751 kg ha^- 1 at Kadoma and 1376 kg ha^-1 at Panmure). Furthermore, in a drier season, the lowest yields obtained on sandy soils (776 kg ha^-1 at Makoholi) were comparable to those obtained on clay soils (739 kg ha^-1 at Kadoma). The yields remained remarkably constant at the other sites during the two seasons indicating a rather high drought tolerance ability by castor, particularly at Mlezu and Matopos which had cumulative rainfall of 330 mm and 420 mm, respectively, at maturity in 1988/89.

There was an indication that castor can tolerate "low" levels of available N. Splitting N application had seed yield advantage on the sands over single application in a good rainfall season under medium to high initial N in soils. The increased efficiency of such split N application was shown by the highest yields which were achieved at 60 kg N ha^-1 at Mlezu instead of a minimum of 90 kg N ha^-1 in single N application in 1987/88, while split N application increased average yield above that achieved from single N application at Matopos in 1987/88 and at Kadoma in both seasons.

ACKNOWLEDGEMENTS

The authors would like to express their gratitude to the Crop Productivity Units (CPUs) at Matopos, Makoholi, Kadoma/Mlezu and Panmure Experiment Station (under the leadership of Messrs C. Ndebele, H. H. Dhliwayo, M. Chisenga and J. Karambwe, respectively), for meticulously recording the field data. Many thanks to T.P. Kachidza for drawing the final graphs and to Sarah Musariri for typing the script.

REFERENCES

Agronomy Institute. 1991. Annual Report, Summer 1986/87 and Winter 1986. Department of Research and Specialist Services, Harare, Zimbabwe, pp. 83-85.

Chemistry and Soil Research Institute. 1989. A Guide to the Meaning of Soil Analysis. Department of Research and Specialist Services, Harare, Zimbabwe, pp. 4-5.

Donovan, P.A. and Landsberg, J.L. 1963. Castorbean investigations at Matopos Research Station. Rhodesia Agricultural Journal 60:53-56.

Muthuvel, P., Sivasamy, R. and Subramanian, V. 1987. Studies on nitrogen and phosphorus requirements of rainfed castor. Madras Agricultural Journal 74:26-28.

Rao, C.M. and Ventateswarlu, M.S. 1988. Effect of irrigation, nitrogen and plant density on yield attributes and yield of castor varieties. Indian Journal of Research, Apau 16:37-39.

Saran, G. and Giri, G. 1987. Effects of seeding time and nitrogen on summer castor. Indian Journal of Agronomy 32:155-157.

Sarma, D.A. 1985. Effects of varieties and levels of nitrogen on the yield of castor. Research Note. The Andhra Pradesh Agriculrutal Journal 32:133-134.

Van der Merwe, J.J. and Clarke, P.F. 1977. Growth requirements of the castor oil plant. Bulletin 2, Farming in South Africa.

Weiss, E.A. 1983. Castor. Chapter 3:53-62 in Oilseed Crops. Tropical Agriculture Services. Longman, London and New York.

Whingwiri, E.E., Natarajan, M., Kanyanda, C.W. and van Lindert, H.J.A. 1987. Agroclimate and soil characteristics of the research centres in the Department of Research and Specialist Services. Zimbabwe Agricultural Journal 84:37-53.

Copyright 1997 The African Crop Science Society


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