Effect of inorganic and organic nitrogen sources on the yield, agronomic efficiency and land equivalent ratio of sorghum - groundnut cropping systems in Migori County, Kenya

Abstract

Persistent droughts, soil infertility and reduced land sizes are major limitations to agricultural productivity in Migori County. Intercropping drought tolerant legumes with cereals such as sorghum and groundnuts is one of the coping strategies that have been proposed but the effectiveness of these systems particularly under various sources of nitrogen (N) is not clear. A study was therefore conducted at Nyabisawa and Piny Oyie in Migori County during the long rains of 2019 to establish the effect of inorganic and organic N sources on the yield and yield components, land equivalent ratio (LER) and agronomic efficiency (AE) of sorghum - groundnut cropping systems. A Randomised Complete Block Design (RCBD) replicated thrice was used. Treatments included: sole sorghum, sole groundnut and sorghum - groundnut intercrop supplied with N from two sources, that is, organic (farmyard manure (FYM)) and inorganic (urea). Crops in the sole sorghum and sorghum - groundnut intercrop received N at rates of 40 and 80 kg/ha from each source and 40 kg N/ha provided by urea combined with FYM in equal amounts (20 kg N/ha from each source). Sole groundnuts received at 20 kg N/ha from each source and 20 kg N/ha provided by urea combined with FYM in equal proportions (10 kg N/ha from each source). There was a zero N input treatment for each cropping system. Sowing was at the onset of the rains and harvesting done at physiological maturity. The effectiveness of intercropping was determined using LER while the agronomic efficiency was determined using AE. All data were subjected to analysis of variance and the significant treatment means were separated using LSD (p ≤ 0.05.) Synergism was observed where use of 40 kg N/ha from combined sources significantly increased the sole sorghum plant heights above the unfertilized sole sorghum and also gave the tallest plants at 4, 6 and 8 WAP and the highest sorghum grain and stover yields at Nyabisawa and Piny Oyie. In the intercrop however, the use of N from urea at 40 and 80 kg N/ha from urea and 80 kg N/ha from FYM became important considering the higher demand for minimal resources available in the intercrop. The unfertilized intercropped sorghum had the shortest plants and lowest grain and stover yields in both sites due to the N deficiency which had not been managed thus the need to supply N to the crops so as to improve their yields. In groundnuts, the yields of sole groundnuts to which 20 kg N/ha from urea was applied were significantly superior compared to those supplied with 20 kg N/ha from FYM. Only FYM applied at 20 kg N/ha on sole groundnuts significantly increased the groundnut pods per plant and the biomass yields above the unfertilized sole groundnuts at Nyabisawa. Intercropping was beneficial (LER>1) where 0 kg N/ha, 80 kg N/ha from urea and 80 kg N/ha from FYM were applied. The AEs decreased with increasing N rates in the sole sorghum, while in the intercrop, they increased with increasing N rates. This was observed where sole sorghum that received 40 kg N/ha from urea and the combined sources had significantly higher AEs than those that received 80 kg N/ha from urea and FYM at Nyabisawa and those that received 80 kg N/ha from FYM at Piny Oyie. In the intercrop, it was noted that where 80 kg N/ha from urea and FYM was supplied, the AEs were significantly higher than those which received 40 kg N/ha from combined sources at Nyabisawa and Piny Oyie. In sole groundnuts however 20 kg N/ha from urea had a significantly higher agronomic efficiency compared to all the other treatments since the crops needed N before they were well established to start fixing their own N. In conclusion, the application of 40 kg N ha–1 from combined sources in sole sorghum, 20 kg N ha–1 from urea in sole groundnuts and either 80 kg N ha–1 from urea or FYM on the intercropped sorghum and groundnuts gave the best yields and could be recommended to farmers for adoption especially when production is done under similar conditions