Watch: how to take a soil sample

Barry says soil sampling should be done on an individual field basis. He says the sampling area should not exceed 4ha (10 acres). Except where soil types and cropping of lands were similar during the previous five years, a sample area of up to 8ha would be acceptable. Barry recommends taking separate samples from areas that are different in soil type, previous cropping history, slope, drainage or persistent poor yields. A field should not be sampled for phosphorus until three months after the last application of any fertiliser containing this nutrient, whether it is chemical or organic.

The main tool used for sampling is a soil corer. This consists of a steel implement with handle and a pointed tapered tube at the base which collects samples in a connected container. The corer allows you to take a number of accurate samples fast. The soil core should be pushed to the full (100mm) depth when taking each core. At least 20 cores should be taken from the sampling area. The sampling pattern should be carried out in the shape of a W by starting on one arm and following the pattern of the W, finishing on the opposite arm.

When 20 samples are taken, they should be placed in the soil container or bag to make up the sample. The farmer’s name, sample number and field description or plot number should be written on each soil sample bag. “Ensure the sample bag is properly sealed so none of sample spills out of the bags on the way to the laboratory,” said Barry.

The sample log book should now be filled out carefully, ensuring that all required details are accurately recorded. It is essential that the information entered in the sample log book matches exactly with the information recorded on each sample bag. All samples should be stored in a cool place. When sending to the laboratory, they should be placed in secure packaging and sent to the laboratory as soon as possible after sampling.

You can expect results from the soil test back by five working days. Most farmers look for lime, potassium and phosphorus levels. These do not change substantially in a soil over a period of several years. Therefore, sampling frequency every three or four years or when changing crop type should be sufficient on most grass farms. There are more detailed soil tests that can be carried out on farms which will show deficiencies of other major and minor elements too.

Ideally, samples should be taken just prior to reseeding. However, from a practical standpoint, this can be difficult because little time is left to plan a fertiliser programme and purchase fertiliser in time for seeding.

The best alternative is to lift soil samples in the autumn/early winter once soil microbial activity has declined.

The proper sample time is after the soil surface temperature drops to less than 7°C. At this temperature soil processes, such as mineralisation (breakdown of soil organic matter into plant available nutrients), that cause changes in soil nutrients proceed quite slowly and therefore, changes in plant available nutrient levels are normally not great, according to Barry.

Generally, it is safe to soil-sample in most areas after the beginning of October. By sampling at this time of the year, there is sufficient time to:

Reading the soil test result

The soil test results usually show the plant-available P and K in miligrams per litre of soil. For simplicity, this result is categorised into a soil index for each nutrient.

The index ranges are shown in Table 1 from 1 to 4. Soils in index 4 are considered very fertile, while soils in index 1 are considered poor. Grassland with good overall soil fertility has a pH of over 6.2 and is index 3 for phosphorus (P) and potassium (K).

GLAS

Farmers participating in GLAS should note the need to take soil samples now so that advisers can draw up nutrient management plans (NMP). According to the Department, a valid GLAS NMP should be in place by 31 December 2016 for tranche I participants and by 31 March 2017 for tranche II participants.