Interpretation of Results
How to read and understand your test soil results
“Soil test results are only as good as the soil sample taken.” This is one of the most important steps in attaining reliable information regarding the soil fertility on your farm. Up to date soil test results are unique for the soils on your farm and will have a large influence on the productivity of your soils over the next 4 to 5 years. This information will form the basis to formulating fertiliser / lime advice and decisions regarding fertiliser types and formulations. It is important that soil samples are taken correctly, which includes sampling to the correct sampling depth of 10cm. Now is a good time of the year to take soil samples as the majority of fertiliser P and K is applied in the spring, so taking soil samples now will ensure that you have results back in time. Allow 3 to 6 months between fertiliser P and K applications and taking fresh soil samples.
Soil test results will reveal a lot about the soils on your farm and will help explain why some fields perform better than other fields on the farm. It is also a good exercise to compare old and new soil test results for individual fields to assess the effectiveness of the fertiliser programme on your farm over the last number of years. Recent trends show a decline in national soil fertility levels, so by not soil sampling, you may be missing out on knowing your soil fertility levels.
When soil test results return from the laboratory the results can be quite confusing to interpret, given that results appear that are measured in mg/ l, and then are converted into a soil P and K index. In this article I will look at soil test results and how fertiliser advice is formulated. I will look at a standard Teagasc soil test report and explain how it is put together and what the information means. Figure 1 shows an example soil test report. Each section of the report is given a number, and is explained in turn.
Sections 1 to 3 show the client, advisor and sample information describing the sample.
1&2) Client and Advisor Details
These sections show the names and addresses of the farmer who owns the sample and the farmer’s advisor.
3) Sample Details
This section shows the date when the sample was received and analysed by the soils laboratory. Each sample has a unique code and number so it can identified. The main farm enterprise practiced on the sampled area is required to calculate the nutrient advice shown further down. Field name and the land parcel number will appear here. However, the parcel number does not have to be included in the sample information when sending in the sample. It is best to use field names that you are familiar with as the land parcel number can change over time. On the example report the farming enterprise is beef and the field name is the hill field.
Sections numbered 4 to 6 show the soil test results for the sample
4) Soil pH
The soil pH result can be confusing because 2 different results are shown. The result for pH is the true soil pH result, and this is the result that is most useful. The SMP result is a separate test, and in its own right is no value. The SMP result is used to calculate the lime requirement for the soil sample. Only use the reading for pH as a guide for the pH of the soil. The optimum pH for grassland soil is pH 6.3 and 6.5 for tillage crops.
5) Soil test results
Depending on the soil analysis requested, the lab results for P, K and other elements will be shown. In this case, Mg is also included. The soil test results are used to classify the soils into a soil Index for each nutrient
6) Soil Index
The soil is categorised into a soil Index for each of the nutrients. The basis of this classification for P, K and Mg is shown in Table 1. Soil Index 1 and 2 are considered sub-optimal while Index 4 is considered to be high. Index 3 is considered the optimal level for all nutrients. Note that there are different ranges of soil test results that determine the soil Index for each nutrient. It is also worth noting that the usage of the field as either grassland or tillage can affect the soil P index, but has no effect on the K or Mg Index.
For this example, the soil test showed that P and K were both very low (Index 1), while Mg was high (Index 4).
|Table 1:- Soil nutrient Index, response to fertilisers and soil test range for P, K and Mg. (Source: Teagasc)|
|Soil Index||Response to |
|P (mg L1)|
|P (mg L1)|
|2||Likely||3.1- 5.0||3.1- 6.0||51-100||26-50|
Sections numbered 7 to 9 show the nutrient advice that is provided based on the soil test results and the information provided with the sample such as stocking rate, crop to be grown, organic manure application and levels of concentrate feeding.
7) Lime requirement
As mentioned earlier, the lime requirement shown in t/ha is the advice for lime application based on the SMP result. Sometimes the result can be shown as ‘xsl’, which means that no lime is required at all.
8) Fertiliser advice
9) Comments on the advice given
Comments are routinely printed in this section to help with interpreting the results and advice given. A comment on reducing lime advice for high molybdenum soils is often printed. This note is automatic if the sample is identified as being form an area where there has been instances of high molybdenum in herbage in the past. High molybdenum can result in copper deficiency in animals. In this example, the lime requirement is 10 t/ha (4 t/acre). Under laboratory comments it shows that this field is from a high molybdenum (Mo) area. The advice for high Mo areas is to deduct 5 t/ha from the lime advice as it is recommended to maintain a soil pH 6.2 in these areas to reduce the lock up of copper for grazing livestock. However, this advice is precautionary, and you should also consider your own experience in deciding on the final lime application rate.
For how long should you use a report?
The P and K advice on the report should be followed for the next 3-5 years, after which another sample should be taken. The lime advice relates to the total lime application to cover a number of years, while the P and K advice are annual application rates.
Cross check with nitrates limits
The advice given on the soil test report should always be cross-checked against an estimate of the total fertiliser allowance for the farm under the nitrates regulations. There are cases where the full P advice may not be permitted on the farm under the regulations, so it is important to check this before you purchase and/or apply the fertiliser.