Effect of sulfur- and zinc-containing fertilizers on soybean yield and analysis of spatial and seasonal yield variability in Ghana, West Africa

Context

Soybean [Glycine max (L.) Merr.] is an important crop in Ghana. However, the variability of yields throughout the season and in space limits its potential to improve the lives of farmers.

Objective

A two-year study was conducted in eight regions of Ghana from 2020 to 2021 to evaluate the effect of 10 fertilization strategies that combined nitrogen (N; 0, 18 kg ha⁻¹), phosphorus (P; 0, 20 kg ha⁻¹), potassium (K; 0, 25 kg ha⁻¹), sulfur (S; 0, 10 kg ha⁻¹), zinc (Zn; 0, 2.5 kg ha⁻¹), and inoculant (I; Bradyrhizobium diazoefficiens) – NPS_I, PK_I, NPKS_I, PKS_I, NPKZn_I, PS_I, NPK, NKS_I, NPK_I, and control. The study also characterized the factors driving the spatial and temporal variability of yield.

Methods

Randomize complete block design was used to set up experiments in eight regions. The effects of the fertilizer treatments were analyzed using a linear mixed effects model, and a conditional random forest model was employed to predict yield spatially and seasonally, with the objective of characterizing the effect sizes of biophysical variables on yields.

Results

In 2020, the highest average yield was achieved through the additive effect of NPS_I, which resulted in an increase in yield of 381 kg ha⁻¹ over the control yield and 217 kg ha⁻¹ over the NPK_I treatment. Similarly, in 2021, NPS_I significantly increased yield by 261 kg ha⁻¹, although the highest average yield increase of 516 kg ha⁻¹ was achieved through the additive effect of NPKZn_I relative to the control yield. The yield’s coefficient of variation was significant in both years at ∼50 %. The random forest model only accounted for 20–24 % of the yield variability and predicted that the partial yield gain ranged from 7 kg ha⁻¹ to 70 kg ha⁻¹ with S and 14 kg ha⁻¹ to 26 kg ha⁻¹ with P. Increasing solar radiation and minimum temperature resulted in average yield losses of 35 kg ha⁻¹ and 71 kg ha⁻¹ respectively. Conversely, increasing the depth of the soil root zone and pH contributed to yield increases of 35 kg ha⁻¹ and 73 kg ha⁻¹, respectively.

Conclusion and significance

The results imply that the use of S in combination with P is an effective strategy for increasing soybean yields in Ghana. Spatial and temporal yield prediction maps indicate that soybean yield gain can increase significantly with fertilizer application in the northern part of the country, a stronghold of soybean production, but this depends on the growing period. Our study provides valuable information on the contribution of climatic and soil variables to soybean yield variability and recommends the use of S in combination with NPK for soybean planting in the month of May in Ghana.