Validating SoilDoc kit for site-specific fertilizer recommendations for maize production in Tanzania

SoilDoc is a soil testing kit designed to determine site-specific fertility status and enhance the accessibility of soil test services for smallholder farmers in the tropics. This study aimed to validate the use of the kit in the Southern Highlands (Mbeya and Njombe) and the Eastern Zone (Mvomero) of Tanzania. Composite soil samples collected from 144 smallholder farm fields were analyzed using 0.01 M CaCl2 extraction solution as per SoilDoc protocols. Out of 144 sites, 24 sites were randomly selected for nutrient response trials. The treatments were (i) absolute control, (ii) N alone, (iii) P Low N (Recommended P and half of recommended N), (iv) recommended NP, (v) NPK, (vi) NPS, (vii) NKS, and (viii) NPKS, replicated three times. Maize yields were subjected to analysis of variance. Relative yield for each nutrient applied was subjected to Cate and Nelson procedure to determine critical ranges. Across all sites, the control yielded least (0.55-5.20 Mg ha-1). Application of NPS, NPK, NP, or NPKS produced the highest yield (2.5-6.9 Mg ha-1). The SoilDoc critical ranges were 0.30-0.48 mg P kg-1; 0.15-0.27 cmolc K kg-1, and 4.1-4.8 mg S kg-1. Mbeya and Njombe soils appeared deficient in either NS, NP, NPK, or NPK, while in Mvomero 53% of soils were deficient in N alone and 31% of soils were deficient in NS. In the absence of field-specific soil tests, applying N, P, S, and K in Mbeya and Njombe and N and S in Mvomero would be best bets for maximizing yield and return on fertilizer investment. Maize (Zea mays L.) response to applied N, P, K, and S varied with SoilDoc test results.Application of NPS and NPKS resulted in highest maize yield of 2.5-8.8 Mg ha-1.The SoilDoc critical values are 0.30-0.48 mg P kg-1; 0.15-0.27 cmolc K kg-1, and 4.0-4.8 mg S kg-1.Nutrient-deficiency groups consist of N alone, S alone, NS, NK, NP, NPK, NPS, NKS, and NPKS.