South African Avocado Growers’ Association Yearbook 1987. 10:49-51.

Proceedings of the First World Avocado Congress

 

 

Comparison of different calcium sources on avocado production

 

SF DU PLESSIS and TJ KOEN

Citrus and Subtropical Fruit Research Institute,

Private Bag X11208, Nelspruit 1200, RSA

 

 

INTRODUCTION

Avocados are mainly grown in high rainfall areas on relatively heavy soils in the Transvaal. Acidification is a serious problem in most orchards and the consequences not always appreciated by growers.

 

Beneficial effects due to liming have been claimed by several researchers. According to Snyman & Darvas (1982), root rot on young trees was reduced by liming. Broadbent & Baker (1974) claimed that Phytophthora cinnamomi did not occur on soils with a Ca status of 2 000 mg kg^-1. Koen & Smart (1973) showed that optimal growth and root development for Duke seedlings were obtained at a soil pH (water) of 6,0 to 7,0. Fouché (1981) stated that optimum production occurred in the pH range of 5,8 to 6,5. He also claimed that imbalances of K, Ca and Mg in the soil will enhance pulpspot. Kotzé & Joubert (1978) claimed that calcium silicate was not as efficient as agricultural lime in alleviating soil acidity, and that slaked lime moved more readily in the soil than both calcitic and dolomitic lime.

 

The purpose of this investigation was to compare the effect of different calcium sources on the production of mature avocado trees grown on a fairly acid soil, and to evaluate these sources in terms of their effects on soil acidity and nutritional value.

 

MATERIALS AND METHODS

Mature trees, cultivar Edrariol on Guatemalan seedling rootstock, grown on a red clayey soil (Hutton form, Farningham series) in the Burgershall area were used for this investigation. The treatments were as follows: four calcium sources viz dolomitic lime, calcium hydroxide, gypsum and calcium silicate, with three levels of each were compared with each other, as well as with a control (no application). The levels were comparable on either neutralisation value or calcium content. The treatments were applied annually from July 1979 to July 1982. The experiment therefore consisted of four calcium sources with three levels each and a control, replicated four times. Plots consisted of three trees each. The treatments are shown in Table 1. The trees were irrigated with dragline sprinklers, with a cycle length of 10 days. No visible symptoms of Phytophthora occurred during the period of investigation.

 

The trees were fertilised annually according to leaf analysis, to maintain an optimum nutritional status. Yield data were taken annually from 1982 to 1984 as well as soil, leaf and fruit samples for analysis.

 

 

 

 

RESULTS

 

Yield data

In Table 2 the effect of three levels of each of the calcium sources is compared mutually and with the control (no Ca treatment). The final applications were made in 1982; therefore the 1983 data indicate the immediate results of the applications, and 1984 the residual effects.

 

From the data in Table 2 it is obvious that the highest yields for 1983 were obtained with the middle levels of dolomitic lime, Ca silicate and gypsum. The hydroxide was not significantly better than the control. It is also noteworthy that a decrease in yield occurred with the highest levels of all treatments. The control plot showed a drastic decrease in yield from 64 kg tree^-1 in 1982 to 16 kg tree^-1 in 1984.

 

The residual effect of calcium hydroxide and gypsum were relatively poor, whereas good residual effects were obtained with the highest level of calcium silicate and the middle level of dolomitic lime. The second level of Ca silicate had a very poor residual effect, despite the fact that this was one of the best treatments in both 1982 and 1983. The reason is unknown, although it could be due to alternate bearing after two heavy crops.

 

In order to explain these differences, soil, leaf and fruit analyses were done.

 

 

 

 

Soil analysis

Table 3 shows the initial soil composition at the start of the experiment. It is obvious that the soil pH was very low for avocado production (Fouché, 1981), with a very high extractable Al content of 90 mg kg^-1 in the topsoil and 135 mg kg^-1 in the subsoil.

 

 

 

 

In Table 4 the soil composition for 1983 is shown. These data indicate the final treatment effects after four years of applications (1979 to 1982), as well as the condition of the control plot. The control showed very little difference from the original analysis, indicated in Table 3.