Potential of Manure and Urea Fertilizer on Maize (<i>Zea mays</i> L.) Productivity and Soil Quality in the Northern Highlands of Tanzania

Many agricultural fields are no longer sustainable due to inadequate replenishment of soil nutrients through organic and inorganic inputs, particularly in smallholder farming systems. As a result, achieving potential crop yields in these systems has proven to be difficult. Field trials were conducted in two long rainy growing seasons in 2021 and 2023 to assess the effects of urea fertilizer and cattle manure as sources of nitrogen (N) on (i) maize crop yields and (ii) soil chemical properties at two sites (Kwa Sadala and Mungushi) located in Hai district, northern Tanzania. The trials employed a randomized complete block design with three replicates, including eight treatments. The treatments were: 0 fertilizer (control), 25, 50, 75 kg N ha⁻¹ (sole urea), 12.5 kg N (urea) + 12.5 kg N (cattle manure), 25 kg N (urea) + 25 kg N (cattle manure), and 50 and 75 kg N (sole cattle manure). Results show that the highest application rate of urea (75 kg N ha⁻¹) produced the highest grain yields of 4.21 and 4.09 t ha⁻¹ in the 2021 season and 4.32 and 4.04 t ha⁻¹ in the 2023 season at Kwa Sadala and Mungushi, respectively. The application of cattle manure at the highest rates increased the soil pH by 3.15 and 2.26% at Kwa Sadala and Mungushi, respectively. Similarly, soil total N, OC, available/extractable P, and exchangeable K increased by 100%, 56.3%, 52.36%, and 19.67%, respectively, at Kwa Sadala and by 16.67%, 18.13%, 20.95%, and 6.76%, respectively, at Mungushi. The use of urea alone at the higher rates or in combination with cattle manure at 50% each resulted in the highest net benefit (NB) in all sites. The findings from this study suggest that a comprehensive approach to managing soil nutrients, such as combining inorganic and organic inputs, may improve crop yields while maintaining soil health.