SUMMARY

Many farmers in Seychelles cultivate the acidic red soil which is a tropical laterite.

pH ranges from 4.3 - 6.5 and many areas, especially those subject to erosion where low pH is a limiting factor would benefit from the application of cheap agricultural lime.

Presently farmers apply dolomite and also guano to correct soils where a low pH is detrimental to plant growth (Guano being extracted from alkaline calcareous soils has liming potential). Dolomite sales for 1997 was around 2 tons (Source M.A.M.R). Most farmers apply dolomite as a soil ameliorant, that is to induce flocculation in clay soils and render it more manageable rather than for its liming properties.

Liming an acidic soil means that the farmer can grow a wider range of crops, phosphorus availability is improved, nitrification is stimulated, cation exchange capacity is increased, the soil structure is rendered more friable and changes in pH promotes nitrogen fixation in leguminous plants. Over liming however reduces potassium availability and also many micro nutrients. (Efficient fertilizer use in acidic upland soils of the humid tropics - F.A.O Fertilizer and Plant Nutrition Bulletin, 10 by H.R. Von Vexkull).

The trial had as the main aim and objective to compare the liming potential of a locally available by product, coral waste to dolomite, an imported item being retailed at Agricultural Requisite Stores. pH was gauged over time in the various sampling sites which had received increasing amounts of both dolomite and coral waste.

Results indicate a higher liming potential for the coral waste at a rate of 1500 Kg/ha and also has a prolonged liming effect at higher rates of application (2000 Kg/ha).

INTRODUCTION

The United Concrete Products Seychelles (UCPS) has a vast stock of crushed coral waste which is a by-product after the dredged up coral has been washed and sieved. The sand is sold to the public for construction works and the coral waste is stockpiled.

Laboratory analysis of this coral waste indicates that it has potential as a liming agent for the acidic ferralsols. (Table 1) The coral waste has 0% salinity as it has been stockpiled for over 10 years and most salts have been leached.

MATERIALS AND METHOD

The trial plot was at Beoliere, a site on the Research Farm with a slightly acid soil with a high gravel content. Ideally an acid soil of 5.0 would have been ideal but in this instance pH ranged from 5.9 - 6.4 and the plot was the only one available at the time of the trial.

The trial was a Randomised Complete Block Resign (RCBD) with 4 replicates for each treatment. Treatment for both dolomite and calcium carbonate were 0, 500, 1000, 1500, 2000 Kg/ha.

Each replicate was 1 m², which meant that each treatment received 0, 50, 100, 150, 200 gms/ m² of either dolomite or calcium carbonate.

The coral waste when stockpiled is in the form of a heavy sledge. A sample was air dried, passed through a grinder and a fine powder like material was seived.

Dolomite was bought from the requisite stores and has a more granular texture.

40 soil samples were taken in each replicate at the start of the trial and the average pH recorded for each treatment. Every week after, pH was again tested to gauge the liming potential and changes in pH of both Calcium Carbonate and dolomite treated plots.

Rainfall was also recorded over the trial period to see whether a certain rainfall pattern has an influence on the liming capacity and to what extent the liming ability is reduced.

Results and Discussions

1.4 Laboratory Analysis

Results of preliminary laboratory analysis gave the following; all of the ground

material passed though a 1mm seive, 29% is in the 0.25mm range 29.5% is in the

0.08mm range,7% is in the 0.0029mm range and 34.5% is in the 0.0017mm

range.

This compares favourably with the specifications in the United States for liming material, where 90% should passed through a 1.9mm seive, 50% through a 0.35mm seive and 25% through a 0.14mm seive. (Jones, 1982)

This fine powder-like material can quickly saturate exchange sites and its capacity to alter the pH is greater than dolomite which has more of a granular handling characteristic. (See graph 1 & graph 2).

The coral waste has a pH of 9 (1:2.5 H₂0), salinity of 0% and conductivity of 120 m mHos/cm. The material tends to be cement like when wet which gives it inferior handling characteristics.