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The Interpretation of Soil Test Data

In order to obtain fertilizer recommendations from soil test data, the following information is required:

The functional curve (fertilizer response curve) between soil data (available nutrients) and yield data in order to deduce the optimum content of available nutrients required in the soil for maximum yield.

The amount of fertilizer required to cover the difference between a certain relative yield and the maximum yield, or (in practical terms) the amount of fertilizer required in order to reach the optimum soil nutrient content.

Interpretation can be made in form of mathematical curves or (in a less sophisticated way) in terms of fertility classes. The criteria for soil fertility classes in Nigeria are as follows:

Low- the value below the critical level which implies the application of a high dose.

Medium – the range above critical level variable response to fertilization is expected. Apply 1/3 – ½ dose.

High- The range where the response is unlikely and fertilization may not be necessary.

Nitrogen is defined in terms of total nitrogen, phosphorus in terms of Bray 1 or Bray 2 – P; potassium in terms of NH4 OAC exchangeable potassium, and organic matter in terms of Walkley and Black total organic matter.

Table: Rating for Soil Fertility Classes

Nitrogen (Total N) Low Medium High    –           0.15/N –           0.15 – 0.20/N –           >0.20%N 
PhosphorusBray 1-pBray 2-p
Low–           8mg kg -115mg kg –1
Medium–           8-20 kg-115-25
High–           20mg kg-12.5mg kg-1

Read Also: General Principles of Soil Analysis and Soil Testing

Potassium (Exch K)

Low     –           0.20cmo1 (+) kg-1

Medium          –           0.20 – 0.40 kg-1

High   –           0.40 kg

Organic Matter (%m)           Soil pH (H20)

Low2.0%Low–           4.5 – 5.0s
Medium2-3.0%Medium–           5.1 – 5.5
High3.0%High–           5.5 – 6.5

Magnesium (Exchangeable Mg)

Low     – 0.3 – 1.0 cmol kg-1 Medium     –      1.0 – 3.0 cmol kg-1 High  – 3. – 8.0 cmol kg-1

Read Also: Characteristics of Soil Analytical Methods for Practical Advisory Work

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