Table 1.
Variables and factors used in the analysis of the on-farm fertilizer experiments. (n = number of observations (paired treatments of control and fertilizer application), for SSA excluding Kenya)
| Variables | Description | Units | n | |
|---|---|---|---|---|
| Kenya | SSA | |||
| Yield | ||||
| Control | Mean maize yield for control plot | kg ha−1 | 202 | 256 |
| Treatment | Mean maize yield for fertilized plot | kg ha−1 | 202 | 256 |
| Environmental factors (continuous) | ||||
| Soil properties analysed prior to fertilizer trial | Soil pH, | – | 194 (4) | 230 (28) |
| Total carbon (C) | g kg−1 | 192 (5) | 181 (38) | |
| Total nitrogen (N) | g kg−1 | 155 | 216 | |
| P-Olsen | mg kg−1 | 105 (19) | 122 (22) | |
| P-Bray 2 | mg kg−1 | 112 | 132 | |
| P-Bray 1 | mg kg−1 | 23 | 41 | |
| Exchangeable K (Exch. K) | cmol kg−1 | 104 (34) | 102 (48) | |
| Exch. Ca | cmol kg−1 | 78 (37) | 71 (93) | |
| Exch. Mg | cmol kg−1 | 67 | 198 | |
| Clay | % | 109 (36) | 152 (77) | |
| Sand | % | 86 (38) | 143 (89) | |
| Silt | % | 96 (41) | 161 (67) | |
| Rainfall | Average per growing season | mm | 198 | 220 |
| Altitude | Height above sea level | m | 186 | 85 |
| Environmental factors (categorical) | ||||
| Soil orders (World Resource Base Reference Soil Groups) | Cambisols | 22 | – | |
| Nitisols | 68 | – | ||
| Vertisols | 6 | – | ||
| Ferralsols | 81 | 24 | ||
| Luvisols | 4 | – | ||
| Lixisols | 41 | |||
| Arcrisols | 37 | 22 | ||
| Alfisols | 12 | 4 | ||
| Phaeozems | – | 8 | ||
| Alisols | 8 | – | ||
| Soil textural classes (USDA) | Clay | 28 | 14 | |
| Clay loam | 21 | – | ||
| Loamy sand | 2 | 8 | ||
| Sand | – | 64 | ||
| Sandy clay | 4 | 6 | ||
| Sandy clay loam | 6 | 10 | ||
| Sandy loamy | 31 | 55 | ||
| Agro-ecological zone for Africa (Dudal 1980) and Kenya (Jätzold and Kutsch 2000) | Sub-humid (SSA) | 18 | – | |
| Humid (SSA) | 6 | – | ||
| Lowlands | – | 4 | ||
| Lower midlands | – | 6 | ||
| Upper midlands | – | 38 | ||
| Lower highlands (sub-humid) | – | 6 | ||
| Lower highlands humid | – | 6 | ||
| Management factors (continuous) | ||||
| Fertilization rate | Amount of N applied | kg N ha−1 | 254 | |
| Management factors (categorical) | ||||
| Nutrient applied | N only | 149 | 235 | |
| NPK | 49 | 14 | ||
| Manager | Farmer | 159 | 234 | |
| Researcher | 81 | 63 | ||
n, number of observations; N, nitrogen; P, phosphorus; K, Potassium, the numbers in brackets are the number of observations that data imputation was performed for soil properties