The Effect of Organic and Inorganic Fertilizers on the Yield of Two Contrasting Soybean Varieties and Residual Nutrient Effects on a Subsequent Finger Millet Crop

The problems of low soil fertility resulting from continuous monocropping, crop residue removal and limited fertilizer use represent key challenges to produce surplus food for the ever increasing population of Ethiopia. However, the practices of crop rotation and integrated sources of fertilizer uses could potentially improve soil fertility and productivity. In 2012 and 2014, soybean with different trials consisting of two soybean varieties (Boshe and Ethio-ugozilavia), three levels of farm yard manure (FYM) (3, 6 and 9 t/ha) and three phosphorus levels (8, 16 and 24 kg P ha−1) were combinedin2×3×3factorialarrangements. Twosoybeanvarietiesreceivingnofertilizerapplication followed by ﬁnger millet receiving a recommended rate (20 kg P/ha) were included. The experiment was laid out in a randomized complete block design with three replications. In 2013 and 2015, ﬁnger millet was planted on each soybean plot as per previous treatment arrangements to evaluate the effect of the precursor crop (soybean) and integrated fertilizer application on yield performance of the subsequent ﬁnger millet. Soil pH, organic carbon, total nitrogen and available phosphorus before planting and after crop harvest of soybean in each year showed treatment differences. Both precursor crop and fertilizer application had a positive effect on soil fertility status and, hence, improved the performance of the subsequent ﬁnger millet. On the other hand, since the rainfall amount and distribution were different in the 2012 and 2014 seasons, the response of soybean varieties to applied fertilizers was signiﬁcantly affected, and the correlation between soybean yield and annual rainfall was strongly positive. Use of an early maturing soybean variety (Boshe) with the lowest rates of organicandinorganicfertilizersgavesigniﬁcantlyhigheryieldin2012(shortrainyseason)compared with other treatment combinations. In the 2014 cropping season, however, ‘Ethio-ugozilavia’ showed greateryield performancewith the combinedapplication of3 t FYM/haand 1616kg PP/ha followed by 3 t FYM and 88 kg P/ha. Hence, it is recommended to use the ‘Boshe’ variety under a short rainy season and under a low soil fertility status, while variety the ‘Ethio-ugozilavia’ can be used under goodrainyandsoilfertilitymanagementconditions. Consideringresidualeffects,theuseoftheearly maturing soybean variety as a precursor with 3 t FYM/ha and 8–16,816 kg P/ha during the short rainy season could enhance the yield of the subsequent ﬁnger millet. On the other hand, the use of the late maturing soybean variety as a precursor with higher organic fertilizer rates (6–9 t FYM/ha) resulted in a signiﬁcant yield increase of the subsequent ﬁnger millet. The use of a late maturing variety of soybean with lower rates of organic manure resulted in a ﬁnger millet yield comparable to farmers’ practice, indicating that this option can be adopted by smallholder farmers who cannot produce sufﬁcient organic manure. This study showed that planting of ﬁnger millet after a soybean precursor crop even without fertilizer application could give better yield and economic beneﬁts as it saves 70–85% of chemical fertilizer costs compared to the farmers’ practice.