Soil sampling methods may not be giving an accurate picture of the in-field nutrient status. Sarah Henly reports

For years we’ve been advised to sample soil for nutrients and pH by walking in a W pattern across fields, but next year the advice is set to change.

“We have known for some time that there are better ways to collect soil samples than on a W path through fields, and that alternative sample schemes may return more information for our effort, but we’ve stuck with the W because it is logistically simple,” says Murray Lark of Rothamsted Research. “Now we are assessing the economics of several different approaches to improve soil information and consequently crop management.”

The HGCA-funded desk study is using statistical models to optimise sampling schemes across the field. Patterns other than a W, but with the same number of sampling points, look promising, he says.

“The problem with a W route for field averages is that often, some of the sampling points are too close together so effort is wasted. Our provisional findings suggest that computer-generated, optimised schemes can give a better marginal return on the same effort and cost input.”

By the end of the project, we will know how the statistical and practical advantages of alternative sampling schemes compare. We will also have basic models that could be used to compare grid sampling and management-zone-based sampling for precision management of nutrients.

The outcome will help to ensure that nitrogen, phosphorus and potassium inputs meet crop requirements and that excess or inadequate nutrient applications are minimised. Being a seasonal requirement, nitrogen is the main focus and also the biggest challenge to address.

As project leader, Dr Lark has consulted with agronomists from NIAB TAG and the AICC, and precision farming company, SOYL, to ensure scientists develop a practical, as well as a rational strategy. All assumptions are consistent with the procedures adopted in the fertiliser manual, RB209, which has been updated this summer.

As our understanding of nutrient management improves, so more precise soil information will be needed, and these results will allow sampling practice to be sharpened up accordingly, he says.

A critical part of the project is a cost-benefit assessment. Costs that are being taken into account in the development of a model for decision-making include the costs of field sampling, usually done by a contractor or agronomist, and the costs of the laboratory analyses.

There is also the cost of uncertainty in a field average, which may lead to over or under-application of a fertiliser, suggests Dr Lark. The challenge for any soil sampling scheme is how the costs of doing it relate to the benefits from the reduced uncertainty.

“Sampling the soil to determine nutrient content has a high cost attached to it so it’s important to know that you have made a rational decision about how to collect such information. The statistical work in the project will also allow us to identify the best ways to collect soil information for variable rate management or precision farming.”

Until the project draws its conclusions, he advises growers to stick with their current soil sampling approach so as not to confuse the picture.

Crops perspective:

By identifying appropriate strategies for soil sampling and quantifying their cost-effectiveness, the project should allow growers to decide on a level of investment in soil information that is rationally justified. Provided these strategies are workable, they should ease management as well as optimise input costs.