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Lower soil fertility was identified as a major problem affecting crop yield in Burundi, especially at Bihunge, a hill of Matongo commune Kayanza province. An experiment was therefore carried out with five treatments to assess the effective and economically affordable treatment on maze growth parameters improvement. The experimental has considered five treatments: the control (T1); Compost from maize residues + mineral adjuvant (T2); Compost from maize residues + organic adjuvant based on Calliandra calothyrsus Meisn (T3); ISABU formula for maize fertilization (T4); the maize compost + Calliandra + recommended mineral fertilization for maize (T5)), in randomized complete blocks with three replications for each. The recorded parameters were the leaves number, plant height, stem girth as well as the leaf area. After analysis with SPSS and Advanced Excel, results showed a significant difference between treatments. The treatment T4 was the most effective in improving all growth parameters than others and showed a significant difference from treatments T1, T2, and T3. However, it did not differ to treatment T5 which also significantly differed from other treatments with P<0.05. Treatment T5 has also effectively enhanced the leaves number, plant height, stem girth, and leaf area as treatment T4, and was in the same variation range as this treatment T4. As the effectiveness of T5 was based on the combination of calliandra which has more potential in improving soil fertility and nutrient followed by better fostering of nutrients to the plant leading to better-improved plant growth, this treatment was considered as the effective and farmer accessible treatment for maize cultivation.

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An 8-week incubation study was conducted under controlled laboratory conditions to evaluate the potential acidifying effect of urea and Di-Ammonium Phosphate (DAP) on three Burundi representative soils, namely two acidic high altitude soils (Nyabisindu and Kajondi) and a saline sodic soil from the Imbo plain (Gihanga). Urea was applied at equivalent rates of 40 and 80 kg ha -1 , while Di-Ammonium Phosphate (DAP) was applied at 100 and 200 kg ha -1 . At the completion of the study, the acidifying effect of urea and DAP increased with the increasing original soil pH values. In comparison with the unfertilised control treatments, soil pH decreased by -0.18 to -0.217 pH units in Nyabisindu soil (pH = 5.07), whereas in Kajondi soil (pH = 5.2) and Gihanga (pH = 7.40), it decreased by -0.073 to -0.286 and -0.367 to -0.470 pH units, respectively. Furthermore, results hilighted that each soil-applied kg of N as urea and each soil-applied kg of N as DAP requires 3.6 kg and 5.4 kg of CaCO 3 to neutralize their respective residual acidity. The study revealed a helpful approach of using CaCO 3 as a rational N fertilizers management that the agriculture extension services and farmers can undertake in order to reduce soil acidity, improve soil fertility and increase crop production in Burundi and beyond under similar soil conditions.

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The present investigation highlights and compares the direct and residual fertilizer values of maize (Zea mays L.) stover composted with Calliandra calothyrsus Meisn and Tithonia diversifolia (Hemsl.) A. Gray green manure on maize (Zea mays L.) and successive potato (Solanum tuberosum L.). Two field studies were installed with maize and a successive potato crops in a completely randomized block design (CRBD) with three replicates. Statistical analyses were performed using Rcommander, version 4.0.2. Grain yields, root biomass, above ground biomass and calculated parameters (Root/Shoot ratio, Harvest Index and GinningRate) were evaluated for maize. Total potato yields were evaluated and categorized into small size tubers (SST < 35 mm) medium size tubers (MST: 35-65 mm) and big size tubers (BST > 65 mm). Treatments under evaluation were: T 1 =Control, T 2 =Farm (cow) manure alone, T 3 =Maize stover co-composted with Calliandra calothyrsus Meisn green manure; T 4 = Maize stover co-composted with Tithonia diversifolia (Hemsl.) A. Gray green manure; T 5 = Farm (cow) manure+45-60-30; T 6 = Maize stover co-composted with Calliandra calothyrsus Meisn green manure+45-60-30; T 7 =Maize stover co-composted with Tithonia diversifolia (Hemsl.) A. Gray green manure+45-60-30; and T 8 =Farm (cow) manure+FOMI Imbura+FOMI Totahaza. Obtained results indicated highest maize yields with FOMI organo-mineral fertilizers (2.7 T/ha) seconded by the recommended fertilizer practice T 5 (2 T/ha). In the same line, highest R/S value (=0.38) was registered with the same FOMI treatment (T 8 ), while the highest HI value (0.21) was observed with the maize stover co-composted with Tithonia diversifolia (Hemsl.) A. Gray (T 4 ). Highest potato total yields were observed with the maize stover compost enriched with Tithonia diversifolia (Hemsl.) A. Gray and supplemented with mineral fertilizer (7.61 T/ha), followed by the same compost without mineral fertilizer addition (6.35 T/ha). We conclude our study with the following statements: i) the highest direct fertilizer value was observed with the FOMI treatment; ii) the residual fertilizer effects were more expressed with the maize stover co-composted with Tithonia diversifolia A. Gray green manure with or without mineral additive, followed by farm manure and maize stover enriched with Calliandra calothyrsus Meisn green manure. Consequently, we restate that the effects of compost-based organo-mineral fertilizers on crop yields should be definitely evaluated on a multiseasonal basis so as to fully capture their residual fertilizer potentials.

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