Chapter 5. Crop response to fertilizers

Yam

The response of two varieties of yam to fertilizer application in different locations in the Transition Zone of Ghana is given in Table 32. Compared with the control, yam responded positively to fertilizer application in all the locations. However, there was a considerable degree of inconsistency in the response with respect to treatments with missing nutrients and treatments with complete fertilization. Yam is now an exportable commodity. In 2002 Ghana exported 8 248 tonnes of yam (ISSER, 2003).

Sweet potato

Effect of imbalanced fertilization on the yield of sweet potato

Sweet potato has become an important food crop in Ghana. It is also gaining importance as an export crop in the Bawku East District of the Upper East Region. The farmers in the District normally export the crop to Burkina Faso, where good prices for the crop are obtained. The effect of imbalanced fertilization on yields and on the number of tubers in the Sudan Savannah Zone is shown in Table 33.

The yield of sweet potato is significantly depressed if potassium is missing. However, eliminating phosphorus does not affect the yield.

TABLE 32
Response of yam to fertilization in the Transition Zone, yield (tonnes/ha)

Source: SRI - CSIR, 2003.

TABLE 33
Sweet potato: response to nutrient balances

Source: SRI - CSIR, 2003.

The yield is also depressed slightly when nitrogen is missing. Balanced fertilization gives large tubers while the number of tubers harvested decreases.

a. Nitrogen

When nitrogen levels are increased above zero, yield levels of sweet potato increase, but when nitrogen levels are increased above the optimum level of 30 kg/ha to 60 or 90 kg/ha, the yield decreases, relative to the optimum rate of 30-30-30 (columns 1 and 2, Table 34).

b. Phosphorus

Increasing the rate of phosphorus fertilization does not increase the yield of sweet potato. In columns 3 and 4 of Table 34, the increase in the yield of the crop may be due to the increase in the level of potassium to 60 kg/ha.

c. Potassium

Increasing the rate of potassium fertilization results in a significant increase in tuber yield (columns 5 and 6, Table 34). Potassium appears to be the most important nutrient in the production of sweet potato.

TABLE 34
Sweet potato: response to increasing nutrient rates

Source: SRI - CSIR, 2003.

Cassava

Cassava is grown in the Forest, the Transition and the Guinea Savannah Zones. Under rainfed conditions in tonnes/ha can be obtained (MOFA, 2003). According to a report on field crops prepared by SRI, cassava responds to fertilization in all the producing areas. Application rates of 68 kg N/ha, 45 kg P₂O₅/ha and 68 kg K₂O/ha are the recommended average rate of fertilizers required by the crop. Using the recommended technology an achievable yield of 28 tonnes/ha may be obtained (MOFA, 2003).

Maize

Phosphorus fertilization

Phosphorus is a major limiting nutrient in Ghanaian soils. In a trial on the yield of grain maize, in the Semi-deciduous Rain Forest Zone at Kwadaso, during the main growing season in 1999, triple super phosphate (TSP) at a rate of 200 kg/ha is compared with Togo rock phosphate (RP) applied at rates of 400, 600, 800 and 1 200 kg/ha, using two different methods of application (Table 35). As is well known in most tropical soils, incorporation of phosphorus decreases its availability compared to surface application, because of phosphorus fixation with the former method of application. However, in the case of RP, phosphorus availability improves with incorporation, at high rates of application. For low rates of application, surface application is more effective than incorporation.

TABLE 35
Grain maize: response to TSP and RP

Source: SRI - CSIR, 2000.

For TSP, surface application is favoured. For high application rates of RP with incorporation, it is likely that, once the fixation capacity of the soil is satisfied, excess RP present may interact with soil organic matter. This should improve the solubility of the RP, thus making the phosphorus easily available.

TABLE 36
Grain maize: on-farm response to cattle manure and fertilizer

Source: SRI - CSIR, 1999.

Effect of cattle manure and mineral fertilizer on grain maize yield

Soil fertility management in the soils of the Sudan Savannah Zone is critical because of the inherently poor fertility status of the soils, especially in organic matter. However, the yield of maize is far too low in the forest zones where organic carbon and rainfall are higher. The yield of maize in the Bawku East District in the Sudan Savannah Zone at different levels of fertilization is given in Table 36. The addition of 4 tonnes/ha of cattle manure increases the yield by more than 100 percent over the control. The use of compound fertilizer increases the yield of maize by more than 160 percent compared to 4 tonnes/ha cattle manure. The combination of cattle manure and the compound fertilizer improves the yield of maize by 22 percent over fertilizer alone. This is important in view of the improvement of soil structure as well which is likely to be associated with the addition of cattle manure.

Pepper

This study was carried out at Kwadaso in 1997 and 1998, on a sandy loam Ferric Acrisol. In addition to the nitrogen treatment, basal applications of P₂O₅ as TSP and K₂O as MOP were made at 45 kg/ha. The results of the study for 1997 and 1998 are given in Table 37. Ammonium nitrate (AN) and potassium nitrate (KNO₃) gave higher yields than urea and AS. In terms of the value cost ratio (VCR), AN gave the best result followed by urea.

TABLE 37
Pepper: response to different sources of nitrogen

Source: SRI - CSIR.

Cocoa

It has been found that low soil fertility is among the major causes of decline in yield of cocoa on has been confirmed through fertilizer use trials on cocoa by CRIG. Fertilizer use trials on cocoa by CRIG have identified the following fertilizer combination as ideal for cocoa fertilization: 0-18-22 plus calcium, sulphur and magnesium. This formulation has been tried on-farm and has been adopted by some farmers. It has more than doubled cocoa yields on their farms.

Table 38 shows the mean yield of cocoa for 20 farms, selected across the cocoa growing areas in the Semi-deciduous Rain Forest Zone of the Ashanti Region. The age of the plantations ranged from 9 to 27 years. The study was conducted between the 1991/92 and 1994/95 growing seasons. The fertilized plots received 129 kg/ha of P₂O₅ per year and 76.5 kg/ha of K₂O per year. Compared with the unfertilized plots, the average gross yields of the fertilized plots exceeded those of the unfertilized plots by 61.7 percent in the first year, 99.8 percent in the second year, 116 percent in the third year and 106.6 percent in the fourth year.

TABLE 38
Cocoa: on-farm response to fertilizer, Ashanti region

Source: Appiah et al., 2000.