In blueberry plantings, nitrification can result in losses of mineral nitrogen (N) through leaching because blueberries prefer ammonium (NH4+) over nitrate (NO3−). The objective of this study was to assess the effects of two rates of N fertilizer, mixed or not with nitrification inhibitors (NI) and applied through two fertigation systems, on berry yield and the concentrations of NH4+-N and NO3−-N along the soil profile. Thus, nine combinations of treatments including two N fertilizer rates (60 and 120 kg N ha−1), two NI (with DCD + Nitra-pyrin or without) and two fertigation application methods (buried and suspended drip lines) and a control (0 kg N ha−1) were tested over three years (2016–2018) in a long-term blueberry planting. Berry yield was on average 47.32 Mg ha−1 in 2016 and 26.86 Mg ha−1 in 2018. In 2017, berry yield varied between 8.60 Mg ha−1 under the control and 11.66 Mg ha−1 with 120 kg N ha−1 applied through suspended drip lines. Low berry yield in 2017 was due to a heavy pruning to rejuvenate the plants. In 2016, the concentration of NH4+-N in the sawdust mulch layer varied between 13.1 and 27.1 mg kg−1 in the spring, 11.4 and 32.1 mg kg−1 in the summer, and 7.9 and 72.9 mg kg−1 in the fall; the concentration of high NH4+-N along the soil profile did not exceed 5 mg kg−1. High concentrations of NH4+-N in the sawdust mulch layer were associated with NI, but did not translate to high berry yields. The concentration of NO3−-N in the soil profile reached 42.6 mg kg−1 in the summer and 39.0 mg kg−1 in the fall and these high concentrations were associated with NI. In 2017 and 2018, there was no effect of NI on NH4+-N concentrations even in the layer of sawdust mulch which was not consistent with the results obtained in 2016. High concentrations of NO3−-N were measured beneath the sawdust mulch layer with treatments including N fertilizer alone or mixed with NI. It is possible that NH4+ retained in the layer of sawdust mulch and not taken up by plant roots was subsequently oxidized to NO3− at the end of the residence time of NI in the soil. Our results showed that high concentrations of NO3−-N along the soil profile occurred mainly during the summer which could be explained by irrigation water driving NO3− leaching.

We studied the effects of liming to increase soil pH from acidic to near-neutral on i) degree of P saturation (DPS), P saturation index (PSI), Mehlich-3 P (PM3) and water extractable P (Pw). Soils collected from a long-term highbush blueberry experiment were incubated at 22.5 °C for 93 days after CaCO3 amendment to increase pH values from 4.1, 4.8, and 5.5 to 6 and from 5.8 to 6.5. Liming decreased PM3 by 8, 6, 10, and 11% with increasing initial soil pH. The PM3 concentrations of all the studied soils belonged to the very high class with critical DPS > 25%, which are associated with increased environmental risk of P loss with runoff. For soils with initial pH values of 5.8, Pw was 3.65 mg kg–1, in line with critical DPS > 25%, but decreased to 2.74 mg kg–1 with CaCO3 addition. In contrast, soils with initial pH < 5.5 had lower Pw concentrations and CaCO3 did not significantly decrease Pw at the end of the incubation averaging 1.02, 1.11 and 1.43 mg kg–1 for initial pH 4.1, 4.8 and 5.5, respectively. The low Pw concentrations of soils with initial pH < 5.5 were in line with low PSI (5.2 to 6.1%), but did not reflect DPS values > 25%. It is possible that high exchangeable Al (AlM3 > 2500 mg kg–1) enhanced the fixation of phosphate ions from the soil solution, thus reducing Pw. Our results suggest that using PM3 as a sole indicator of environmental risk likely underestimates potential P losses compared to Pw.

Cowpea (Vigna unguiculata) is an important legume crop in the tropics and subtropical regions of the world. It is mainly grown for its leaves and grains, and to a lesser extent as a fodder crop. Cowpea is considered as the most important food grain legume in the dry savannas of tropical Africa. This study compared the yield of local (Agondire) and improved (SECOW 2W) cowpea varieties grown on an Oxisol. Inorganic P at levels of 0, 10, 20, and 40 kg•ha −1 was tested on each variety with or without rhizobia inoculation. The experiments were set up in a randomized complete block design and replicated thrice during the short and long rains of the 2015/2016 seasons on fifteen fields in Arua district, northern Uganda. Agondire responded significantly (P < 0.05) better than SECOW 2W when high rates of inorganic phosphorus (40 kg P ha −1) were applied. A significant increase of 26.4% and 28.4% in grain yield of Agondire and SECOW 2W, respectively was obtained after inoculation with rhizobia. We concluded that inoculation and P fertilizer application increased the yield of both varieties, but with inoculation, SECOW 2W performs much better at lower P fertilizer rates than Agondire. Therefore, we recommend growing of SECOW 2W under inoculation with 20 kg P ha −1 and an application of 40 kg P ha −1 for Agondire local cowpea variety in northern Uganda.