Why Soil Sampling is So Important

Nutrient management decisions are often a key topic of discussion for growers. A frequently asked question is "why do I need to soil sample and when should I do it"? This article is here to help make effective and economic soil fertility management decisions.

Sampling and lab analysis

Soil sampling and soil lab analysis is the only method available to accurately assess and predict nutrient levels in agricultural soils. Soil sampling allows nutrient management plans to be tailored to individual fields and provides the ability to determine the optimum rates of fertilizer and/or lime that need to be applied.

Soil sampling timing

While soil sampling can be conducted at any time throughout the season, it is often conducted in the fall of the year following harvest. This is due to:

• Prime field conditions
• Ease of access
• Labor availability
• Timing of fertilizer and/or lime applications

For fields that are soil sampled regularly it is recommended to sample at the same time of year to manage seasonal nutrient availability fluctuations, allowing greater ability to track accurate historical changes.

Soil sampling decisions 

Soil sampling decisions begin by deciding where to sample. There are many ways to define sample areas, but three strategies dominate. 

Whole field sampling 

This approach utilizes one composite sample to represent a whole field. This approach can provide some insight but is generally not recommended in row crop agriculture as it is better suited for fields <25 acres. 

Zone and grid sampling

This approach divides a large field into smaller zones often based on soil texture, cropping history, and/or topography. It allows for management of differing zones throughout the field and accounts for some variability within fields. Zone sampling is often best suited for smaller fields with little variability or to provide insight into problem areas.  

Grid sampling provides the opportunity to identify variability across the field and can provide the greatest resolution describing nutrient levels throughout the field. This approach divides fields into uniform sections, utilizing precision equipment to determine sample points.

One of the most important decisions when grid sampling is determining grid sizes within a field. A smaller grid will provide a greater resolution into the field and give a more detailed assessment of the overall soil profile. 

Remember that the difference between a 2.5-acre grid versus a 10-acre grid will be a 4x increase in resolution. As technology has evolved, the benefits grid sampling can provide has continued to increase. With the increasing capability of variable rate nutrient applications, the use of grid sampling allows for economic optimum rates to be applied across the field. 

This capability decreases the risk of over or underapplying lime/fertilizer and provides a strong agronomic and economic foundation for a nutrient management plan. 

Why accurate soil samples are so important 

Accurate soil sample results require the use of proper soil sampling techniques. Proper technique include:

• Using the right equipment
• Sampling at the right time
• Maintaining consistent sample depth
• Collecting a proper composite sample

While a range of equipment can be utilized to take a soil sample, proper sampling equipment ensures variability between samples is minimized.

Particularly when grid or zone sampling, it is recommended that a soil probe be utilized so that each individual core is consistent in diameter and depth. A second point of emphasis should be placed on ensuring that sampling equipment (probe, bucket, etc.) be kept clean so that debris may not contaminate the sample. 

When to take a soil sample 

While a soil sample may be taken at any time throughout the year, it is recommended that samples not be taken when the soil is saturated. Fields that are sampled on a routine basis should be sampled at approximately the same calendar date year over year to minimize the influence of seasonal fluctuations due to crop removal, bacterial activity, soil moisture, etc. 

Sample depth 

Maintaining the proper sample depth is one of the most important steps to remember when taking a soil sample. Nutrient management recommendations are based on calculations made from the soil sample, considering sample depth and therefore, can be skewed if samples are collected from differing depths.

Sample cores 

Simplot Grower Solutions and most soil test labs suggest utilizing the 6-inch depth; it is strongly recommended that the lab where samples will be analyzed is contacted before samples are taken to ensure the proper depth.

Lastly, all these previous techniques combine to provide a proper composite sample. At each previously determined soil sampling point, multiple cores should be taken in random pattern at a reasonable distance apart. The number of cores taken at each point should be increased when a lesser number of samples will be taken throughout the field. 

When grid sampling 8 to 10 cores should combine for each composite sample, if zone sampling twenty cores should be taken for each composite sample. Fig 3. Taylor et al. (1997)[1] shows how increasing sample cores within an area reduces variability and creates an accurate result in the sample.

The takeaway

For further information, please consult any of your local Agronomic Sales Enablement Team members or your local Simplot Grower Solutions Crop Advisor to answer your application questions. 

Authors

Connor Ferguson,Texoma
Justin McCoy, Southwest
Ben Lawrence, Bayou
Jeffrey Mansour, Midsouth

Click to read more about the authors