Assessment of plant-available phosphorus in moist savanna soils of Nigeria and impact on the shoot dry matter yield of soybean (Glycine max (L.) Merr.)

Abstract

Phosphorus is an essential element in plant nutrition and must be available to the crop at the early stage of growth for proper development and meaningful productivity. However, most soils in the moist savanna region of West Africa are characterized by low phosphorus availability leading to low crop yields. In view of this, P fertilizer application is necessary to achieve optimum crop yields but most farmers in the region are resource-poor and unable to apply adequate amounts to their crops. Soils from 40 soybean-based farms in the northern Guinea savanna (NGS) region were evaluated for available P content using the Bray-1 and Olsen extraction methods. Another set of 43 soils from cocoa and cassava-maize farms in the derived savanna region we real so evaluated for available P content using the Olsen extraction method only. Twenty of the soils taken from soybean fields in the NGS were used to assess the effect of inherent soil Pon he growth and dry matter yield of soybean (Glycine max (L.) Merr.) under greenhouse conditions. The results showed that plant-available P content was low in nearly all the soils withonly20% of the soils having values greater than 10 mg kg -1irrespective of the method used for the assessment which indicated that phosphorus can limit crop production in these savanna soils. On average, the soils from the cocoa farms had relatively higher Olsen P than the soils from the other cropping systems. The shoot dry matter yield (DMY) of soybean obtained with inherent soil P was about 50% of the ‘potential’ yield (i.e., DMY achievable when grown without any nutrient limitation) in soils where the available P content was ≤ 15 mg kg -1. However, the growth of soybean was significantly improved by the application of P fertilizer. In conclusion, the low available P levels in most of the soils indicate that P deficiency may be a major constraint to crop production; appropriate soil fertility management options are necessary to enhance productivity. Furthermore, the potential yield of soybean can only be realized in these NGS soils if farmers are encouraged to practice adequate soil P management.