Cocoa: Fertilizer Requirements

Introduction

In modern cocoa cultivation, the aim is to maximize early growth and obtain high early yields and sustained peak yields subsequently. To achieve this, it is necessary to have a good understanding of the factors affecting growth and yield of cocoa and to put in the necessary management practices on time as required by the cocoa plant. An essential ingredient in most cocoa growing situations is high fertilizer input. However, the agronomy of cocoa is much more complex than with other crops eg. oil palm and there is very strong interaction between nutrition and other agronomic factors in cocoa such as shade, planting density, pests and diseases etc. To achieve high yields therefore, these factors should not be limiting.

In this paper, the technique for drawing up fertilizer recommendations for the crop is outlined and the interrelationship of the agronomic factors in cocoa plantations with nutrition is discussed. It is hoped that this will enable managers to understand the need for ensuring that planting practices are correct in order to obtain the maximum yield responses from fertilizer applications made.

Nutrient requirement of cocoa

The cocoa plant in its native habitat on the ground storey of the Amazon jungle is very heavily shaded and grows slowly with very low yields. Under these circumstances, nutrient requirements are probably very low.

Nutrients are taken up by cocoa in growth and for yield. The nutrients may be immobilized in the plant or recycled as leaf litter when the plant are mature and the nutrients are exported in the pods that are removed from the field.

The growth and yield potential of cocoa is primarily determined by management factors, soil, climate and the availability of nutrients for growth and yield. The former factors determine the potentials largely and to assess nutrient requirements accurately, a good assessment of the potential growth and yield under the prevailing soil and climate conditions and expected management inputs is necessary.

The most important management factors are probably the types of planting materials affecting the yield potential and growth and nutrient uptake characteristics, nursery practices affecting the quality of seedlings, early growth and yields, shade regimes which interact very strongly with most other factors and determine the growth and yield that will be obtained, weed control practices determining the extent of competition for nutrients and soil moisture, plant density determining the overall yield and nutrients immobilized as well as pests and diseases which affect the total dry matter production and growth rates and thereby nutrient requirements.

The shade regime and the types of shade trees used are important as growth rates will be affected markedly. The extent of competition by the shade tree could be very significant e.g. in the cocoa/coconut cropping system as compared to monococoa with a nitrogen fixing shade tree such as Gliricidia.

The main climatic factors affecting nutrient requirements are probably the rainfall affecting soil moisture, temperatures affecting overall growth and also degree of moisture stress and light available determining the energy available to the plant for dry matter production and also having a bearing on the soil moisture requirement by the plants.

The third important factor is the soil. Although individual soil properties are measured and determined, they are finally intergrated to determine its ability to supply and store moisture and nutrients for the plant and its properties as a medium of growth for the roots and root activity which determine in turn its ability to exploit the soil moisture and nutrient contained therein.

These factors discussed so far determine the growth and yield potentials for cocoa in an area. An estimate is now required of the nutrients required by the cocoa plant for their growth and yield. Several estimates have been made of the nutrient requirements of cocoa e.g. Thong and Ng (1978), Teo and Chew (1984). These estimates obtained depend on the growth and yield expected under the growth conditions prevailing. It is necessary however to make these estimates to try and quatify the amounts of nutrients that are required and the important nutrients taken up by the plant.

For the purpose of this paper, the data provided by Thong and Ng (1978) i.e. where the nutrient requirements for growth and yield in monoculture cocoa on an inland soil are discussed. Table 8 shows dry matter production and distribution in cocoa plants of different ages. Dry matter accumulation in cocoa increases very rapidly up to the fifth year after planting and stabilizes after that.

The nutrients immobilized in the plant are shown in Figure 4 where the early very rapid growth rate is reflected in the very rapid nutrient uptake especially of potassium and nitrogen in the first 5 years.

Table 8 : Dry matter production and distribution in cocoa plants

(Thong and Ng, 1978)

Figure 4 : Nutrient uptake of cocoa (Ling, 1983)

The total nutrients immobilized in 5 year old cocoa in 2 inland soils in Malaysia when the cocoa canopies have reached the plateau in growth rate are given in Table 9. The very high accumulation in the cocoa plants of potassium and nitrogen is clearly seen. Taking the mean values of 250 kg/ha N and 300 kg/ha K, this is equivalent to 1.1 kg ammonium sulphate and 0.6 kg muriate of potash taken up by each cocoa bush. These nutrients are locked up within the bushes and only become available again to the plant through recycling of the leaf litter and from leaching particularly of K from the leaves in rainfall.

Cocoa leaf litter production and nutrient contents is shown in Table 10. Very high amounts of nutrients are recycled in the leaf litter particularly for N, K, Mg, Ca and after the sixth year. Nutrient cycling is still not high in the fourth year.

Beside nutrients removed from the soil and immobilized in the bushes, nutrients are exported as yield is removed in the pods harvested. The pods have very high nutrient contents equivalent to 31 kg N, 54 kg K, 4.9 kg P, 5.2 kg Mg and 4.9 kg Ca at a yield level of 1 ton dry beans/ha/year (Table 11). The beans are particularly high in N and P and the pods in K contents. The nutrients removed vary with age and yield levels. If the husks are recycled, considerable quantities of nutrients especially K and N, are returned and they have a marked effect on the nutrient availability of the soil as seen in Table 12. Nutrient requirements may then be markedly reduced.

In the monocropping system therefore, the cocoa bushes have very high nutrient requirement for growth and yield, increasing very rapidly in the first 5 years and then reaching a plateau after that with subsequent increases depending mainly on export of nutrients in increased yield. Even then, the nutrient requirements are reduced if husks of the pods which form a very substantial portion of the nutrients removed are recycled to the fields. In addition, in mature areas, nutrient recycling in the litter is significant. Overall nutrient requirements in mature cocoa may therefore not be high.

The above discussion deals with the basic factors involved in estimating nutrient requirements in cocoa. However, as the variables cannot always be assessed accurately, it is useful to employ additional checks to determine if nutrition of the cocoa trees is good. Commonly, the techniques adopted are to check for nutrient deficiency symptoms (Loue, 1962) to sample the leaves and check leaf nutrient levels, to take growth measurements of the cocoa to ascertain the growth rates and also to determine if the yield levels are up to expectation for the agronomic conditions involved.

Nutrient deficiency symptoms are described and illustrated in Wood (1975) and examples of interpretations of leaf nutrient levels are given in Tables 13 and 14 for major nutrients and minor nutrients respectively.

Table 9 : Nutrients immobilized in 5 year old cocoa (Ling, 1983 )

Table 10 : Cocoa litter production and nutrient contents (Ling, 1983 )

Table 11 : Nutrients in pod production (Ling, 1983 )

Ages (years)	Dry bean yield (kg/ha)	Component	(kg)
			N	P	K	Mg	Ca
3	450	Beans	9.2	1.6	4.7	1.2	0.5
		Husk	4.9	0.6	20.3	1.2	1.8
		Total	14.1	2.2	25.0	2.4	2.3
5	1000	Beans	20.4	3.6	10.5	2.7	1.1
		Husk	10.6	1.3	43.3	2.5	3.8
		Total	31.0	4.9	53.8	5.2	4.9
7	1400	Beans	28.6	5.0	14.7	3.8	1.5
		Husk	15.4	2.0	63.1	3.6	5.6
		Total	44.0	7.0	77.8	7.4	7.1

Table 12 : Effects of pod husks on soil chemical properties (0-15 cm) (Ling, 1983 )

Nutrient	% on D.M.
	Deficient	Low	Normal
N	< 1.80	1.8 – 2.0	> 2.00
P	< 0.13 – 1.15	0.15 – 0.20	> 0.20
K	< 1.20 – 1.40	1.40 – 2.00	> 2.00
Ca	< 0.30	0.30 – 0.50	> 0.50
Mg	< 0.20	0.20 – 0.45	> 0.45

 

Nutrient	ppm on D.M.
	Critical level	Deficient level
Mn	30	15
Fe	50	30
Zn	30	20
Cu	6	4
B	25	15

Nutrient deficiencies may be indicated from the analysis results especially in gross deficiencies. However non-nutritional factors eg. pests and diseases or drainage may sometimes affect interpretation. Interpretation of leaf analysis in cocoa is not as advanced as in other crops due to difficulties in correlating leaf nutrient levels with yields, probably a result of the high number of variables which affect leaf nutrient levels in the crop.

After consideration of the above factors, in particular, the growth and yield potential of the cocoa, the amount of nutrients required for the expected growth and yield and the nutrient supplying powers of the soil, it is possible to estimate the nutrients required by the cocoa to attain the growth and yield aimed for and to formulate fertilizer recommendations.

Fertiliser recommendations for cocoa

To draw up the fertilizer recommendations for cocoa, the choice of fertilizers to be used, methods and areas of fertilizer applications and factors affecting efficiency of fertilizers used have to be considered.

There is usually a wide choice of fertilizers available. It is necessary particularly to consider the availability and expected efficiency of uptake of the nutrients in the fertilizers, any possible effects on the soil and finally the cost/unit nutrient uptake which is expected. An example may be urea which is the cheapest N fertilizer. However, volatilization losses of N from this fertilizer may be 30 – 50% and in situations where leaf litter is very thick and the fertilizer applied not to be in contact with the soil, N blow-off may be very nearly complete so that no response is obtained from the fertilizer (HRU unpublished report). Ammonium sulphate as another example is usually considered to be an efficient source of N and leaching losses are lower. However, of the nitrogenous fertilizers, it has the highest acidifying effect on the soil and may therefore not be as suitable as less acidifying nitrogenous such as ammonium nitrate in more acid soil conditions. A final example on the importance of choosing the correct fertilizers is the use of the slowly available rock phosphate. In deficient conditions, high rates are required and the rock phosphate used must be finely ground to ensure efficacy.

The method of fertilizer application may determine the efficiency of nutrient uptake especially in marginal soil conditions. Also the fertilizers should be applied over the areas of active root activities to ensure good uptake. Poor application techniques are detrimental in view of the scorching and damage caused to the superficial root system especially in immature cocoa. Frequent use is made of alkaline materials eg. limestone dust. Interaction between fertilizers may be caused if areas overlap with nitrogenous fertilizer applications. Allowance should be made for this in the areas of application. Recommendations on areas of fertilizer application are given in Table 15. All fertilizers should be broadcast evenly over the areas of application to ensure good uptake by the plant.

After the choice of fertilizers for use and the methods and areas of fertilizer application, it is still necessary to schedule applications to take into account the crop nutrient requirements eg. peak cropping period, competition from weeds, rainfall patterns and soil factors such as topography and susceptibility to erosion to draw up the schedule of fertilizer applications that should be made to ensure maximum nutrient uptake for maximum growth and yield by the plants.

A final step is still required to check the estimated fertilizer requirements against fertilizer trial results available, in particular those that have similar factors affecting growth and yield and similar soil conditions. If these results differ significantly from the recommendations, then a thorough reexamination of the factors assumed should be carried out. Unfortunately, many fertilizer trials published have inadequate details to carry this out meaningfully and therefore they are of limited value. In such cases, in view of the systematic approach carried out in the method drawn up for estimating the fertilizer requirements of cocoa, it is often preferred to stick to the estimates drawn up by the system rather than on doubtful information, the basis of which are poorly known.

Economics of fertilizer application

As fertilizers are costly materials, and as nutrient excess or deficiency affect yields and costs of production, it is necessary to ensure that fertilizer applications are beneficial and economic. This is more easily carried out in mature cocoa areas by checking the results of fertilizer trials and responses obtained in these trials and working out the profitability. Strict economic computations are however not always possible in view of the long term nature of cocoa cultivation and the uncertainty of predicting price levels for the product. For immature cocoa, it is not possible to estimate accurately the economic benefits in view of the lack of crop but the very high nutrient requirements of the plant at this stage and the need to ensure good establishment and a uniform stand justify the high application rates of fertilizers at immaturity.

It is timely here to emphasise again the importance of correct assessment of fertilizer requirements in relation to the other growth factors affecting cocoa and the correct usage of fertilizers as otherwise little or no benefits may accrue from the expenditure on fertilizers.

Conclusion

Cocoa will respond well to fertilizer applications if the management factors, soil and climatic conditions are favourable for good growth and yield and if the soils cannot supply the nutrients required on time. This is in view of the strong interaction between nutrients and the management factors to produce good growth and yields. Fertiliser programmes drawn up consider many factors which are assessed on experience and from trial data and the schedules drawn up should be followed for best results to be obtained.

Table 16 : Recommended areas of fertilizer application in cocoa