Producers take soil sampling more seriously

High fertilizer prices and better than average crop returns are sending more producers to the field in search of soil samples this year.

Agrologist Tom Jensen of the International Plant Nutrition Institute, which represents fertilizer makers, said producers are approaching soil sampling more seriously than in recent years.

“There is more at stake this year, and producers are seeing the results of managing their nutrients in improved yields,” he said.

“This falls into risk management.”

John Mayko of the Canola Council of Canada said producers looking to increase their margins need to avoid estimating available nutrients.

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He said there is too much variability from field to field to rely on estimates. As well, producers often don’t have know the long-term history of rented land.

Jensen said only 10 to 15 percent of fields in the northern Great Plains are generally sampled annually, and only 25 percent receive occasional sampling.

Jensen said farmers often don’t have time to sample their fields, despite the increased returns that the practice makes possible.

Mayko said whether producers do it themselves or contract the work, soil sampling can pay off by avoiding overuse of fertilizer and by increasing yields.

Mayko said in the fall producers need to wait until soil temperatures fall below 7 C. At this point the microbial activity that releases nutrients from unavailable forms in the soil or causes their loss has all but stopped. As a result, the sample will be as close to what is available in spring as possible.

Soil phosphorus and potassium aren’t affected much by the timing of sampling after harvest. However, fields that held pulse crops, canola, corn and potatoes should be avoided in September and early October because they release nitrogen during the early fall.

Mayko recommended soil sampling down to 24 inches when possible. The deepest samples represent areas where nitrogen and sulfur will have been carried by moisture as it passes into the soil. The top six inches is where minerals that are immobile remain, such as potassium and phosphorus.

Agrologists say the value of a soil sample is only as good as the quality of the locations from which it is removed.

Samples of an entire field can be difficult to achieve because of changing topography or previous structures, such as fence lines and farmyards.

Samples should avoid low areas where mobile nutrients might collect and slopes where they may have been depleted by water movement.

Three types of sampling patterns are used to provide accurate samples:

Random sampling – Twenty to 30 cores are collected at random and mixed to form a composite sample of the field.
Benchmark sampling – A few small representative areas of the field are selected and 20 to 30 cores are taken from each area. The same areas are used every year to create a long-term soil nutrient profile.

Grid sampling – Samples are taken from a one to five acre grid pattern and each is analyzed. This method is useful for variable rate fertilizer applications but is time consuming and expensive.

Jensen said third party sampling services are becoming more common and may form the future of soil sampling.

“Realistically with farm sizes where they are today, producers don’t have the time to sample as much as they should,” he said.

“Even the agrologists that offer the service can find it too time consuming. That is where the specialized services are finding a niche.”

Soil sampling tips

How to sample:

Test 15 to 20 sites per field. Samples should be taken at zero-15 cm, 15-30 cm and 30-60 cm for each field.

Keep these tips in mind:

Soil samples taken from the wrong areas of a field can create an inaccurate picture of a field’s nutrient inventory.
Topography is not the only issue when it comes to sampling. History plays an important role in defining where to sample and what defines a field.
Fields that have grown several different crops in selected areas will leave those zones with unique nutrient profiles. Areas where manure was applied or that were part of a long-term forage rotation or the subject of a reduced tillage regime may have increased capacity to make nutrients available and may respond better to fertilizers than other areas.
Economically significant areas that are historically different from the rest of the land need to be sampled separately, say agronomists. This also applies to areas with different soil types and textures.
Once the samples are taken, the soils from zones that will be managed uniformly can be blended based on the depths from which they were acquired.
Samples should be segregated and unique areas recorded if variable rate management is anticipated.
Samples need to be kept in plastic containers or bags and producers should dry the dirt without excessive heat that might cause losses or transformation of nutrients.

Moist samples should be transferred to labs immediately.
Historical information should be provided on the soil sample information sheets so the soil lab can provide the most accurate nutrient recommendations.