Phosphorus Nutrient Management in Rainfed Lowland Rice Production in Some Acid Soils of Togo

ABSTRACT

In Togo, rice (Oryza sativa L.) is mainly cultivated on highly weathered acid soils where plant-available phosphorus is generally deficient due to the inherent low P availability of acid soils. Suboptimal and unbalanced P fertilizer rates used by rainfed rice farmers in Togo is often the main cause for low yields. The blanket P fertilizer recommendation applied to the rainfed lowland rice acid soils is outdated and thus, does not take into account the current fertility status of the soils. Moreover, information on P sorption characteristics and standard phosphorus requirements (SPR) of rainfed lowland rice acid soils in Togo is lacking. The present research focused on improving upon P fertilizer recommendations in rainfed lowland rice soils in Togo based on an appropriate SPR for the high P use efficiency by rice.

Phosphorus adsorption study was carried out on four rainfed lowland rice growing acid soils of Togo. These soils were collected from Akata Adame, Tchangaide, Kaniamboua and Tchitchao. Phosphorus sorption characteristics and standard P requirements (SPR) of the four soils were evaluated in the laboratory and were related to some chemical properties of the soils. A greenhouse experiment was also conducted using these four lowland soils to determine the response of rice (variety IR 841) to the different phosphorus fertilizer rates and to assess the phosphorus uptake and P use efficiency. Five phosphorus treatments namely, control: 0 mg P kg−1, P recommended rate (RR): 5 mg P kg−1, 4 × RR: 20 mg P kg−1, ½ SPR and SPR where the amounts of P needed to maintain the SPR were applied to the different soils as 62.70, 65.43, 70.40 and 74.85 mg P kg-1 for Kaniamboua, Tchangaide, Tchitchao and Akata Adame, respectively. A researcher-farmer-managed field trials were conducted at two sites (Akata Adame and Kaniamboua) to validate results obtained from the greenhouse study. Five treatments including control (0 kg P ha-1), RR (13 kg P ha-1), 2 × RR (26 kg P ha-1), 4 × RR (52 kg P ha-1) and ½ SPR (92 and 88 kg P ha-1 for Akata Adame and Kaniamboua, respectively) were applied.

The study showed that P sorption of the four rainfed lowland paddy soils fitted well into the Langmuir isotherm model and the maximum adsorption followed the order Akata Adame (200 mg kg-1) > Tchitchao (167 mg kg-1) > Tchangaide (154 mg kg-1) > Kaniamboua (143 mg kg-1). The standard phosphorus requirements (SPR) were 62.70, 65.43, 70.40 and 74.85 mg P kg-1

iii

soil for Kaniamboua, Tchangaide, Tchitchao and Akata Adame, respectively. Sorption maximum was positively correlated with the SPR (r = 0.629) and clay fraction (r = 0.712) (P < 0.05), but negatively correlated with soil pH (r = - 0.921), organic carbon (r = - 0.959) and available P (Bray-1) (r = - 0.863) (P < 0.01).

From the greenhouse experiment, no significant difference (P > 0.05) between the control treatment and the P recommended rate (RR) and between the ½ SPR and the SPR was observed for the rice shoot dry matter yield.

In the field experiment, the maximum rice grain yield was obtained at the two sites with the highest P application rates. At Akata Adame as well as Kaniamboua, the recommended P fertilizer rate (13 kg P ha-1) did not significantly (P > 0.05) increase the rice grain yield as compared to the control (without P application). However, higher P rates showed significant (P < 0.05) increase in grain yields. Phosphorus application at 26, 52 and 92 or 88 kg P ha-1 increased the rice grain yields by 2.5, 3.2 and 3.99 folds, respectively at Akata Adame and by 2.07, 2.83 and 3.45 folds respectively at Kaniamboua, as compared to the control. Significant (P < 0.01) linear relationships with r = 0.992 and 0.995 were observed between rice grain yield and total P uptake at Akata Adame and Kaniamboua, respectively.

Farmers from Akata Adame obtained only 1.13 t ha-1 of rice grain when no P fertilizer was applied compared with increases in grain yield when P fertilizer was applied. At ½ SPR, farmers could obtain more than 4 tons per ha (4.26 t ha-1). Compared with the familiar P recommended rate in Togo, farmers could only obtain 1.46 t ha-1 of grain yield. At Kaniamboua, farmers could obtain only 1.33 t ha-1 of grains without P fertilizer application. With ½ SPR, they could obtain up to 4.41 t ha-1 of grain yield which is much higher than that obtained with the P recommended rate (1.61 t ha-1). Economic analysis of the grain yield data obtained from the field experiments showed that with the P fertilizer recommended rate to rice in Togo, a farmer would obtain only 62.60 and 47.10 US$ per ha at Akata Adame and Kaniamboua, respectively. The application of the ½ SPR will enable a rice farmer to realise higher returns of 753.23 US$ per ha at Akata Adame and 749.38 US$ per ha at Kaniamboua.

The conclusion from the study is that the P fertilizer rate of ½ SPR may be recommended for rainfed lowland rice production in the study areas of Togo