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Practical Use of Soil Analysis Results

Reading Soil Analysis Reports

All laboratories generate reports for each sample submitted for analysis. All reports will contain the same basic information although individual labs may present this information in their own unique format. Figure 3.4.1 is an example of the general layout of information on a soil analysis report.

The report will identify the client (# 1 in Figure 3.4.1) as well as the unique sample identification (#2 in Figure 3.4.1). When reviewing soil analysis reports, verify that the sample identification is correct. Although it may seem of minor significance, the legal land location is often used to identify agro-climatic regions that affect yield expectations and fertilizer recommendations.

The report will usually indicate when the sample was received and when it was processed (#3 in Figure 3.4.1). Review these handling dates to see if there were any unusual delays in shipping that might affect the accuracy of the results. Take note of the length of time the sample will be retained (#3 in Figure 3.4.1). Additional analysis or repeated tests to verify unusual results must be performed while the sample is still available.

The nutrient analysis (#4 in Figure 3.4.1) is the heart of the report but it is often overlooked compared to the fertilizer recommendation. The nutrient analysis is a measurement of the nutrients removed from soil using an extracting solution. These results form the basis for fertilizer recommendations.

Labs use diverse extraction methods so the nutrient analysis of one lab is not directly comparable to another lab unless both are using the same procedures. An individual lab may use various extracts for different nutrients in order to get the most reliable results. Find out what methods a lab follows since some extraction methods may not be suited to western Canadian soils.

For nutrient management purposes, it is useful to use the same lab every year or to use labs that follow the same extraction processes to track nutrient level changes with time.

Nutrient levels are reported in parts per million (ppm or mg/kg). For each 15 cm (6 in) sample depth, these values can be doubled to approximate the nutrient levels on a kilograms per hectare (kg/ha) or pounds per acre (lb/ac) basis (#5 in Figure 3.4.1).

Nutrient (kg/ha) =

Nutrient (ppm) x 2 x sample depth (cm) ÷ 15 cm

Nutrient (lb/ac) =

Nutrient (ppm) x 2 x sample depth (in) ÷ 6 in

A soil analysis report indicates there is 10 ppm N in a 0 to 6 in soil sample. This corresponds to 20 lb N/ac: Nutrient (lb/ac)

= nutrient (ppm) x 2 x sample depth (in) ÷ 6 in = 10 ppm x 2 x 6 in ÷ 6 in = 20 lb N/ac

There is 10 ppm N in a 0 to 12 in sample. This corresponds to 40 lb N/ac: Nutrient (lb/ac)

= Nutrient (ppm) x 2 x sample depth (in) ÷ 6 in = 10 ppm x 2 x 12 in ÷ 6 in = 40 lb N/ac

Examine reported nutrient levels for any unusual values. Soil N levels following average or above average crops should be low (i.e., below 15 ppm and often less than 10 ppm for 0 to 15 cm (6 in) depths). Phosphorus levels for fields that have not received manure should not vary

Nutrient levels are converted from ppm to lb/ac by multiplying by two because a one acre slice of soil, six inches deep weighs approximately two million pounds. In other words, lb/ac is essentially parts per two million. For a soil sample 12 inch deep, multiply ppm by 4.

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Soil test P and K levels are reported on an elemental basis (i.e., P or K) rather than oxide basis (i.e., P

2

O

5

or K

2

O).

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