Soil Testing Is Worth The Cost

Residual soil nitrogen may help reduce fertilizer costs.

An investment in a good soil sample from every field where nitrogen fertilizer or manure is applied will pay off this year! The cost of soil sampling ($1.00 to $2.50 /acre) can be offset by saving only three to six pounds of nitrogen (N) fertilizer per acre at current fertilizer prices. Several laboratories and consultants are reporting high residual nitrate levels in many fields this year. This is most likely due to an extended period of above normal soil temperatures last summer. This condition may have caused soil microorganisms to break down (mineralize) soil organic matter into ammonium and nitrate nitrogen at a higher rate. Take advantage of this situation by sampling all fields for residual soil nitrate.

Soil testing programs start with organizing planting areas into the sites that are similar in soil textures, slope, location or other characteristics that set them apart from other areas. The next step is to obtain a high-quality, representative soil sample. Generally taking 15-25 sub-samples from a uniform area (no more than 40 irrigated acres) will provide the most unbiased composite sample. More sub-samples taken within a designated area will give a better idea of the field’s fertility status and will provide a better comparison of data from year to year. Under-sampling may result in striking a fertilizer band that doesn’t represent the entire area, not striking any bands, or sampling areas where a heavy application of fertilizer occurred. Fields applied with manure or biosolids should have numerous sub-samples taken to avoid abnormally high or low levels of nutrients. Samples can be taken with coring devices, sampling tubes, augers or shovels. They should be rust free to avoid contamination of the samples with iron. Avoid zinc coated galvanized tools or buckets to prevent sample contamination with zinc.

Sampling depth for surface samples may vary with the type of cropping system used. For most row crops using conventional tillage methods, a surface sample of 8-12 inches is sufficient. For no-till or minimum tillage, a surface sample of 4-6 inches may be more representative, since more plant residues would be present at the surface and phosphorus levels may be much higher than phosphorus levels from 4-12 inches. Subsoil samples taken from 8-24 inches or 12-24 inches are necessary to fully evaluate nitrate levels. Laboratories can make a more accurate recommendation if you sample the subsoil. Subsoil samples are especially needed for corn, small grains and sugar beets. Additional subsoil samples of 24-36 inches and 36-48 inches are needed for sugar beets to measure nitrate levels throughout the soil profile.

Sub-samples should be well mixed and 2-3 cups removed, air dried, and packaged in an appropriate container. A plastic lined soil-sampling bag is ideal, however a plastic resealable bag can also be used. Bags can be obtained from laboratories, crop consultants or Cooperative Extension agents. Don’t use glass containers since they easily break when sent through the mail.

Soil sampling every year provides growers enough information to appraise how a particular cropping system is working and provides the best information for determining the most economical levels of fertilizer to apply.

by Jim Self
Manager
Soil, Water, and Plant Testing Lab

Fertilizer Recommendations

Lab recommendations are developed based on a fertilizer philosophy, good samples, and adequate history from the farmer.

Once a soil sample has been analyzed for its nutrient content, the laboratory can make a fertilizer suggestion. However, growers should be aware that contrasting philosophies exist between laboratories, which can result in different recommendations for the same field, soil, and crop.

These philosophies include the prescription method, the prescription plus buildup, and recommendations for a full rotation. The prescription method suggests only the amount of fertilizer needed for one season’s crop growth. The prescription plus buildup is used to increase nutrients such as phosphorus or potassium in the soil above those levels that are actually needed for one year’s growth. The buildup of nutrients ensures that a particular nutrient is always available for plant growth even if adverse growing conditions arise. Applying fertilizer for a full rotation requires the application of a large amount of nutrients the first year with little or no additional fertilizer application in following years.

During this time of higher fertilizer costs and low crop prices it is most economical to apply only those nutrients needed by the crop for one growing season. With the exception of alfalfa, the fertilizer suggestions at Colorado State University are designed to add only those nutrients necessary for one growing season.

While fertilizer suggestions are made using the nitrate-nitrogen (NO3-N), phosphorus and potassium levels in the soil, other information, such as yield goal, percent organic matter, previous crop, manure application rate and water usage are also needed. With the exception of alfalfa, parameters such as yield goal, percent organic matter and NO3-N in the soil determine the nitrogen (N) fertilizer suggestions from CSU. Growers should base yield expectations on a 5-year field average and provide this information to the laboratory.

If the grower does not provide a yield goal, the lab uses a default yield goal to determine a N fertilizer rate. However, the grower can later adjust the amount of nitrogen needed depending on their actual yield goal. For example, the standard yield goal for irrigated grasses is 4T/A, however the grower can adjust their yield goal to any reasonable level by adding or subtracting 40 lb. N per ton difference in yield goal. Past manure applications and previous crops, especially legumes may reduce the final N fertilizer recommendation. Laboratories use this information to estimate N credits and suggest the most economical fertilizer rate (see article titled Give Credit Where Due! ).

For most crops the NO3-N in the surface soil samples is used for N fertilizer suggestions. However, for corn and sugar beets the amount of total soil nitrate is determined from both the surface and subsoil levels of NO3-N, if subsoil is provided. If a subsoil sample is not provided, it is assumed that the subsoil contains one-half of the NO3-N found in the surface soil sample. If a soil sampling depth is not provided, it is assumed that the surface soil sample depth is to twelve inches and that the subsoil depth is 12 to 24 inches. For corn, a weighted average of surface and subsoil NO3-N is used where the amount of NO3-N in the sample is multiplied by depth.

When a subsoil sample is not provided, there could be a lower estimate of the subsoil nitrate level, resulting in less NO3-N in the two-foot profile and a greater N fertilizer suggested rate. Therefore, it is important to provide a subsoil sample to take advantage of the possibility of a greater nitrate level in the subsoil that will reduce the N fertilizer rate.

Fertilizer suggestions provide a guideline for growers to make management decisions regarding the addition of nutrients. Current fertilizer suggestions from CSU provide the most economical fertilizer rates to provide the best return on a crop yield within a given growing season.

by Jim Self
Manager
Soil, Water, and Plant Testing Lab