Soil Testing and
Fertilizer Recommendations for Nitrogen Use
Soil testing and sound recommendations are the
foundation of good nitrogen management.
Soil testing and development of a soil fertility management program can help save money, and with rising energy costs, will provide vital information to maximize dollars invested in fertilizer.
Soil testing programs begin with organizing planting areas into sites that are similar in soil texture, slope, location or other characteristics that set them apart from other areas. Generally, taking 15-25 sub-samples from a uniform area and compositing them will provide the most unbiased sample. Under-sampling may result in striking a fertilizer band or an over fertilized area that doesn’t represent the entire field. Fields applied with manure or biosolids should have more 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 sample iron contamination. Avoid zinc coated galvanized tools or buckets to prevent sample zinc contamination.
The sampling depth for most row crops using conventional tillage methods is usually 8-12 inches. For no-till or minimum tillage, a 4-6 inch surface sample may be more representative since 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. Subsoil samples are especially needed for corn, small grains and sugarbeets. Additional subsoil samples of 24-36 inches and 36-48 inches are needed for sugarbeets 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. Do not use glass containers since they easily break when sent through the mail.
Once a soil sample has been analyzed for its nutrient content, the laboratory can make a fertilizer suggestion. Growers should be aware that contrasting philosophies exist between laboratories, which can result in different recommendation for the same field, soil, and crop (Figure 2).
Recommendation philosophies include the sufficiency level approach, sufficiency plus buildup, and basic cation saturation ratio. The sufficiency level approach suggests only the amount of fertilizer needed for one season’s crop growth. The sufficiency plus buildup is used to increase nutrients such as phosphorus or potassium in the soil above those levels 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. The basic cation saturation ratio (BCSR) promotes the idea that maximum yields can only be achieved by creating an ideal ratio of calcium, magnesium and potassium. The BCSR is not concerned with recommendations for nitrogen, phosphorus, sulfur and micronutrients. Field evaluations however, have shown that cation ratios have no impact on the response of crops to calcium, magnesium and potassium. A major disadvantage of the BCSR is that even if the ratio of cations in the soil is considered optimum, a nutrient deficiency may still exist. During this time of higher fertilizer costs 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, thus following a sufficiency level approach.
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. Growers should base yield expectations on a 5-year field average and provide this information, along with the previously mentioned information, to the laboratory.
If a yield goal is not provided, the lab uses a default yield goal to determine the N fertilizer rate. However, the grower can later adjust the amount of fertilizer needed depending on their actual 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.
For most crops, the NO3-N in the surface soil samples is used for N fertilizer suggestions. However, for corn and sugarbeets the amount of total soil nitrate-nitrogen is determined from both the surface and subsoil levels, 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, and is obtained by multiplying the amount of NO3-N by depth.
When a subsoil sample is not provided, the subsoil nitrate level could be underestimated resulting in less NO3-N in the two-foot profile and a greater suggested N fertilizer rate. Therefore, it is important to provide a subsoil sample to more accurately ascertain subsoil nitrate levels, possibly reducing the suggested N fertilizer rate.
Fertilizer suggestions provide a guideline for growers to make management decisions regarding the addition of nutrients. Fertilizer suggestions from CSU provide the most economical fertilizer rates to provide the best return on a crop yield within a given growing season.
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Figure 2. Fertilizer
recommendations for grain corn from a field sample that was split and
sent to six different laboratories*. Differences result largely from
recommendation philosophy rather than analytical quality. |
