Soil Testing

Soil analysis is only as good as the sample taken.  A sample normally consists of about 1kg of soil which is representative of the whole field / area to be tested.

• Use a suitable tool (sampling auger / soil corer)
• Sample to a consistent depth, normal depth is 6″ (15cm) for arable soils, and 3″ (7.5cm) for grassland
• Walk the field in a W taking regular samples (for a regular shaped field, sample 7 cores per leg of the W)
• Mix all samples to form a single representative sample.  Thoroughly mix all cores and take a subsample from this to send to the lab
• Sample at the same time every year
• Avoid sampling under extreme conditions, eg waterlogged or very dry soil
• Don’t sample within 8 weeks of fertilising / 12 weeks of manure or slurry application for P, K and Mg analysis, or sooner than 12 months after liming for pH analysis
• If sampling to diagnose a crop problem, take multi cored samples (at best 16) from areas of poor growth and separate these from normal areas. The relative values between good and poor will be more informative that the actual values of the problem area.

It is an analysis of the soil to assess the adequacy and amounts of available nutrients for crop growth, and to monitor changes brought about by farming practice.

The information is needed for optimal production, to avoid transferring undesirable levels of some nutrients into the environment and to ensure a suitable nutrient content in crop products. If shortfalls are detected, they can be compensated for by applying manures, slurries, artificial fertilisers, or changes to land management. A soil test will help decide how many additional nutrients are required, allowing a more targeted approach to fertiliser use, saving time and money.

It can also help monitor changes in fertility, especially when there are uncertainties in the amount of nutrient removed (eg forage crops) and the amount of nutrients applied (eg manures and slurries).

A guide to frequency of sampling

Soil nutrient levels do not alter markedly over short periods of time unless major changes of supply (application) and demand (by the crop) are introduced. Additional sampling may be justified where there is a major change to husbandry practices (e.g. alteration of cropping or manure policy).

What do I need to know about soil pH?

• The pH determines the relative acidity or alkalinity of a soil. It is important to assess this in order to maximise crop growth, do this through a soil test at the lab
• Acidity in soil reduces bacterial and earthworm activity as well as nutrient uptake
• Correcting the pH status of the soil by applying lime to reduce acidity is a simple and effective way to increase crop productivity
• How much lime to apply depends on soil type and liming material. Remedying pH can take months and on very acidic soils may not be fully corrected in one season
• Take care as excessive lime can lock up minerals and prevent plants from using them
• The availability of trace elements is radically affected by pH. For more information on soil pH please click here.

Nutrient analysis guide

Indexes are used to predict the indication of a likely crop response and a guide to the need for additional nutrient supplementation.

Crop response and soil analysis (Defra index)

Soil analysis (while not perfect) is the most effective and practical method of assessing soil fertility in respect of pH and plant available P, K and Mg.

However results must be used, alongside knowledge such as soil type, structure, and crop yield to determine fertiliser and manure requirements.

Knowing what is going on in the soil both in terms of physical structure and chemical composition will allow for optimised management, reduced costs (through targeted use of fertiliser) and reduced risk of environmental issues.

Source: AHDB Beef and Lamb Better Returns Programme Manual 1 – Improving pastures for Better Returns, and Manual 3 – Improving Soils for Better Returns; Simply Sustainable Soils – LEAF and ASDA; The Potash Development Association.

A report looking into resource management on farms has asked whether farmers across the south west are testing their soils on a regular basis. The results are shown below.

This shows that just under half of all farmers that responded test their soils regularly. The graph below breaks down these responses to look at the differences in testing regimes depending on farm type.