SoilOptix® Blog

Soil Test Fertilizer Recommendations – Sampling, Extraction and More 

Soil test fertilizer recommendations ensure the best and most efficient use of fertilizer. It helps to maintain soil fertility. Among the different methods of soil test fertilizer recommendations, the one based on yield targeting is unique because it indicates not only the soil test based on the fertilizer dosages but also the level of yield that can be obtained if the right practices are followed in raising the crop. The targeted yield approach also offers the scientific basis for balanced fertilization, not only between the nutrients from external sources but also with the soil’s available nutrients.

Balanced nutrition doesn’t mean the application of nitrogen, potassium, and phosphorous alone in different proportions through fertilizers. Still, it also ensures that the nutrients in available forms are in adequate quantity and required proportion in the soil to meet the requirements of the crops to obtain the desired level of production. Nutrients available in soil are rarely present in adequate amounts. They are not in balanced proportion to meet the nutrient requirement of the crops. Therefore, it needs the intervention by application of external sources of nutrients, i.e., fertilizers and manures.

Soil tests offer the necessary information about the amount of nutrients in the soil and the imbalances. The soil test fertilizer recommendations are aimed at correcting the imbalances in nutrients depending on the crop requirements. The fertilizer recommendations based on qualitative/semi-quantitative approaches or methods don’t give expected responses. Thus, a refined method of fertilizer recommendations for varying soil test values has to be developed.

Soil Sampling

soil test fertilizer recommendations

The soil testing program starts with collecting soil sampling from a field. The principle of soil testing is a field can be sampled in such a way that the chemical analysis of the soil sample will reflect the true nutrient status of the field. However, this doesn’t mean that the samples will show the same test results- but the test results should reflect true variations within the field. It is crucial to remember that the test recommendations for lime and fertilizer can never be more accurate than the accuracy of soil sampling. Nutrients are taken up by the roots when they are dissolved in water. Insoluble nutrients are not useful for plant nutrition.

Extraction and Chemical Analysis

The next step in soil test fertilizer recommendations includes extraction and chemical analysis. Once the soil samples are collected and prepared, the available nutrients in every sample should be determined. Many chemical methods are suggested in the measurement of essential plant nutrients. The criteria for chemical extraction and analysis of plant nutrients are the process must be rapid, reliable, and accurate. Most of the chemical extracting methods allow the extracting solution. The solution might include water, alkali, weak or strong acid or combinations of these chemicals, and all of these could react with the soil sample in a short time. The sample is filtered, and the solution is analyzed for the available nutrients.

Soil Test Parameters

Along with extracting solutions, there are different other parameters of each soil test. All of them are important to determine the final number that is printed on a soil test report for any one soil sample. The parameters include:

  • The ratio of soil to extractant
  • Shaking time, speed, and action
  • Method to express the results.
  • Cut-off levels for high test results
  • Overall techniques used in the lab.

soil test fertilizer recommendations

The extractants that include the dissolved plant nutrients are analyzed to determine the concentration of the plant nutrients. The results are reported as parts per million (PPM) or pounds per acre. For most nutrients, the PPM might be converted to lb/acre by multiplying by two. For nitrate, sulphate, and chloride, all the nutrient forms present in the soil are extracted, and the depth increments, other than the standard 6–7-inch surface layer, are sampled. For these measurements, ppm is converted to lb/acre.

The formula is:

Lb/acre = ppm*0.3*depth increment in inches. 

For instance, a 10-ppm nitrate N test on a soil sample taken to 24-inch depth would convert to 72 lb/acre (10 ppm * 0.3 * 24 inches). In this case, 72lb/acre of nitrate nitrogen was present in the top 24 inches of the soil sampled. Extracting the available plant nutrient helps to offer an estimate of the amount of plant nutrients that will be available to a specific crop during the growing season. The amount of plant nutrients extracted will depend on the strength of the extraction solution and different other parameters. Soil test values are a relative number, and they should be interpreted as low, medium, and high for a particular nutrient.

Calibration & Interpretation

This is one of the most crucial challenges in soil testing. It is crucial that the results of the soil tests are calibrated against the crop responses from applications of the plant nutrients in question. This information is obtained from the field and greenhouse fertility experiments that are conducted over a wide array of soils. The yield responses from rates applied to nutrients can be related to the quantity of the available nutrients in the soil.

The results of long-term soil test calibration research on different soil types are used to establish recommended amounts of plant nutrients to apply to a specific crop at a given soil test level. For instance, if the soil test level range (P) is in the range of 0-10 ppm, the P recommendation for a 150 lb/acre corn crop might be 100 lb/acre of P2O5. On the other hand, if the soil test P level is above 40 ppm, the recommendation might be 0 to 20 lb/acre.

The tools of site-specific precision management allow growers to manage homogeneous areas within the fields. There are some areas that have higher yield potentials than the database with which most of the soil these days are calibrated. This lack of calibration for high-yielding areas is one of the crucial factors that drive interest in using yield monitors and global positioning satellites. These help to conduct strip trials and determine whether the existing soil fertility programs are enough. The new precision tools have the ability to develop algorithms. Thus, anyone can manage multiple site-specific zones within individual fields. This means a balanced crop nutrition prescription can be delivered to each square foot of every field.

Factors to Keep in Mind While Interpreting Soil Test Results

  • The chance to get a profitable response to fertilization is much higher on soil that is low in a given nutrient than on one that tests high.
  • Soil tests predict the probability of a profitable response to nutrient application than predicting the quality of nutrients needed in any year.
  • A soil test that is low in nutrients will not yield as well as the soil test result at an optimum level, irrespective of how much fertilizer is applied that year.
  • Interpretation of soil test fertilizer recommendations is crucial only to improve the fertility status of soil that is less than optimum. How many nutrients will be needed to change the soil fertility from low to medium or high? What shall be the most economical level to maintain a balanced nutrition status of the soil?
  • With top-level management practices, the yield increases and the probability of a response at any given soil test increases.
  • Wise use of soil testing includes a long-term approach to fertility management. This includes site-specific soil test target levels for every field and nutrient management plan to reach and maintain the target levels.

Soil Test Fertilizer Recommendations

The objective of soil test fertilizer recommendations is to help farmers in achieving economically optimum yields while also protecting the environment. The primary philosophy of soil test fertilizer recommendation is:

  • Recommending the lower-testing soils be taken up to the higher-testing soil levels by adding extra nutrients/fertilizer
  • Applying the maintenance amounts of plant nutrients to higher-testing soils to keep them there and ensure higher productivity.
  • To not apply certain nutrients to soil that already test high on those nutrients.
  • Individual fertilizer recommendations use site-specific and grower-specific information instead of laboratory-generated recommendations, depending on the assumptions and generalizations. Computer programs are available that help in customizing the recommendations. However, the following are considered for custom soil test fertilizer recommendations:

Relevancy of Soil Test Calibration

How accurate and appropriate is the calibration used in the standard recommendation for the field? Unusual soil types, a different climate, a ridge-till or no-till, crop variety, field variability, and cropping history – all these factors can make a difference in the soil test results and soil test fertilizer recommendations.

Yield Potential

The yield potential will determine the economic value of every percentage change in the relative yield, and it might also influence the shape of the calibration curve.

Fertilizer Placement

Band placement will reduce the lost yield. Sub-optimal soil test levels are taken to optimum levels because the short-term recovery of applied fertilizer to the crop plants is improved. There are some recommendation systems that reduce the rate recommended when banding is used instead of broadcast. However, studies have shown the optimum rate when banding is sometimes equal to or greater than the optimum broadcast rate. It is a good idea to take the soil test levels to optimum, irrespective of the placement method used.

Land Tenure (The time period till the grower will farm the field)

Soil test phosphate and potassium are capital investments. The buildup costs must be amortized over the expected time of ownership or operation. The longer the period of time, the more benefits will be accrued from buildup, the lower will be the cost of the buildup, and the higher the optimum soil test level will be. Landowners and operators benefit from the development of agreements where the costs and returns of soil test buildup are equitably shared. Such agreements can help in increasing productivity, preventing soil erosion, and increasing soil fertility.

Soil Test Buffer Potential

This is the quantity of fertilizer needed to change the soil test level and is expressed as the pounds of P2O5 or K2O needed per ppm of soil test level change. There are some low-pH and some high-pH soils are fixed by applying phosphate. However, increasing phosphate is costly, and it reduces the optimum soil test levels. Also, soil test phosphate and potassium levels are easier to change in sandier soils than in medium or fine-tuned soils. With the sandy soils, potassium leaching becomes significant.

Soil Test Fertilizer Recommendations when the Levels are High

When the soil tests are interpreted, possible approaches to a nutrient management plan might include the following:

  • Sufficiency: Add necessary rates of deficient nutrients so that the yields are not limited in the present crop.
  • Build maintenance: Add enough nutrients to meet the present crop requirements and gradually increase the soil supply. Replace the crop harvest-removed nutrients to ensure the plant nutrient levels are at non-limiting levels.

If the soil test result shows that there is enough plant nutrient, applying more of that nutrient is not recommended, at least for the current crop. This is especially true when there is an abundance of nutrient present to the extent that the crop won’t be affected even when the nutrient was not applied for several years. However, there are some laboratories that assign high value to a level that shows little or no response to the applications of that nutrient in that year.

Failing to apply nutrients in the right proportion will result in soil depletion. Also, under specific conditions, the crops will respond rightly to a nutrient with a high test. For instance, on early-planted corn, and the addition of Nitrogen, Potassium, and Phosphorous. When the soil tests are in the high range, the fertilizer application is influenced by different factors. Specific maintenance strategies might be suitable for high soil test results that may not be appropriate for other growers.

Contact the experts at SoilOptix® to get the most detailed soil property maps of your field. Use it as a baseline dataset for your Variable Rate Applications. At SoilOptix® we offer a one-of-a-kind, premium top soil analysis system that sets the standard for accuracy and precision in agriculture. Get in touch with us today!