Steps in Conducting Soil Test Calibration and Fertilizer Recommendation
Find the soil that has a low level of nutrient so that it is used as a starting point for nutrient additions.
Select at least 5 points on the curve so as to define the shape of the curve. e.g. a soil may already have 5kg/ha from soil analyses we would add in this order giving this curve.
| 0 (5kg/ha) | 1000 |
| 5kg/ha | 1500 |
| 10kg/ha | 2000 |
| 20kg/ha | 2200 |
| 40kg/ha | 2300 |
For maximum yield, maybe we would need to add between 20 and 40kg/ha for that soil that fixes more P; therefore we carry out this type of experiment on different soils, on at least 4-5 locations.
After the maximum yield level, it takes a larger amount of uptake to have further increased yield. We may need to find out the toxic level.
The aim of soil calibration data is to know how such nutrient should be added to have maximum yield. We have to do a replicated experiment for 3-5 years in order to take account of variations of weather and other conditions.
We have to find out how much nutrient (e.g. P), should be added to raise level of available nutrient (soil test) to a certain amount because part of the added nutrient will become fixed.
Read Also: Soil Sampling Procedures
Fertilizer Recommendation
Soil test information should be extended to the farmer who should be instructed on the time, rate, and method of fertilizer application. The information should also include improved crop varieties available and other production management practices.
Means of disseminating the package of recommendation include radio, television, newspaper, field days, posters, agricultural shows, meetings, film shows, bulletins and leaflet, training courses, demonstrations, and personal contacts.
To make fertilize recommendation much more reliabl especially under multiple cropping systems, the use of fertilizer factor (ff) had been developed (Adeoye et al., 1998). Fertilizer factor is the amount of nutrient element required to raise the soil test level by 1mg/kg soil.
The means and ranges of fertilizer factors had been developed for some nutrients in the savanna regions of Nigeria. They are P = 3.1 (1-7), K = 4.5 (3-8), Mg = 3 (1-6), Zn – 6.4 (5-8), Fe = 2 (0.8-4), Cu = 2(0.8-5) and Mn = 0.8 (0.2 – 2.5).
The ff is used as a multiplier in calculating fertilizer requirement after determining the critical soil level. For example if Bray – I-P soil test of a field is 10mg/kg and the critical P-level is 15mg/kg, fertilizer recommendation for P will then be (15-10) x 3.1 = 15.50MG p/kg soil multiply by a factor of 2 to convert to kg P/ha = 15.5mgp/kg x 2 = 31.0kg p/ha.
Employing only the critical level concept (Cate and Nelson model) would have given (15-10) x 2 = 10kg p/ha as the fertilizer recommendation. This quantity of phosphorus will not be sufficient to saturate the exchange complex of the soil and release enough P for the crop.
Read Also: Soil Test Correlation and Calibration
