Soil Testing and Analysis

Soil sampling and testing provides an excellent inventory of plant available nutrients and other soil factors important for crop production.

 

Will Feenstra Soil Health Advisor collecting soil sample for analysis imageThis inventory serves as a basis for recommending additional nutrients to improve crop production on an individual field basis.  Soil nutrient levels vary from year to year, and frequently will vary within fields, even on fields that seem to be uniform. It is therefore necessary to follow recommended steps for soil sampling and testing to develop a sound soil fertility management program.

 

An understanding of general nutrient status can be obtained for a field when regular soil tests are conducted. Nitrogen soil testing is recommended annually as the available nitrogen can change considerably from year to year. Changes are dependent upon environmental conditions such as rainfall and temperature patterns during a growing season, type and yield of crop harvested, date of harvest, fall tillage, and amount of fertilizer applied to the previous crop. Potassium and phosphorus levels do not change substantially in a soil over a period of several years. Therefore, sampling for phosphorus and potassium may be conducted every 2 or 3 years, or when changing crop type. Sampling for sulphur should be done annually.


While it is recognized that soil testing is not an infallible guide to crop production and that other factors also come into play, soil tests help to reduce the guesswork in fertilizer practices. Generally speaking, farmers who soil test know more about their soil nutrient profile and can target their fertilizer purchases accordingly. They are able to make more informed buying decisions that save them money.  

 

Poor soil sampling technique is a problem which causes variation in fertilizer recommendations. Soil testing is only as good as the quality of the samples collected. Therefore, good soil collection procedures must include the following:

 

Determine where and how to soil sample each field.
Use proper equipment and supplies.
Sample at the proper time of the year.
Obtain samples from the correct soil depths.
Handle soil samples properly.

 

Being familiar with the proper soil sampling procedures is important whether you are doing the sampling yourself or it is being done by a custom operator.


WHERE DO I SAMPLE EACH FIELD?

Soil sampling should be done on an individual field basis. Samples from different fields should not be mixed. Begin by evaluating each field to determine representative areas. Sample hilly fields with knolls, slopes, or depressions, separately from mid slope positions to ensure any potential sulphur or other nutrient problem is detected. Major areas within fields having distinctly different soil properties, such as texture, should be sampled and fertilized as separate fields because of different nutrient requirements. Problem areas such as saline spots, poorly drained depressions, and eroded knolls should not be sampled unless they represent a significant portion of the field. If they do, separate samples should be obtained. In addition, other abnormal areas such as old manure piles, burn piles, haystacks, corrals, fence rows, or farmstead sites should be avoided. At least 15 to 20 sampling sites are required for each field to give a good representative sample. Samples taken from only four or five sites in a field are generally not representative and often result in incorrect fertilizer recommendations.

 

HOW DO I SAMPLE EACH FIELD?

There are four basic methods for taking soil samples:

 

Benchmark Soil Sampling - This method involves selecting uniquely different areas within a field and sampling each area separately. Unique areas are selected based on soil types, topography, and crop growth. Once sites are selected, soil samples should be collected from each specific area, each year, to use as a guide for fertilizing all similar areas within the field. This method is rapidly gaining popularity, particularly with farmers that are adopting precision farming techniques. This is the current recommended method which has the best chance to maximize your economic yield.

 

Grid Soil Sampling - With this method, a field is sampled in an organized grid pattern. Soil sample frequency may range from taking one sample in 0.5 acre units of the field to one sample for each 5.0 acre units of the field. The smaller the soil sampling unit, the greater the accuracy of the sample. The advantage of this method is that a field map can be prepared for each nutrient and be used for variable rate fertilization and precision farming. The cost of taking the soil samples and the soil analysis is very high and therefore is not economical for many farmers.

 

Topographic Soil Sampling - With this method, a producer selects the separate soil sampling sites based on topography. A set of soil samples is taken from each uniquely different topographic area within a field.

 

Random Soil Sampling - This involves taking soil samples in a random pattern across a field, generally avoiding unusual or problem soil areas within a field. Generally the field should not be more than 80 acres in size and has been cropped uniformly in the past. Normally, 15 to 20 sites must be sampled to obtain a representative soil sample of the field. This is the most common method of sampling presently used.

 

WHAT EQUIPMENT SHOULD I USE TO COLLECT MY SAMPLES?

A soil probe is best for taking samples to the 60 cm (24 in) sampling depth. Use clean, labeled, plastic pails for collecting samples. Metal pails should not be used if micro-nutrient testing is to be done. Soil sample augers can also be used but it can be difficult to accurately separate soil samples into 0-15, 15-30 and 30-60 cm depths. Soil probes can be purchased from Agriculture Solutions. We can also provide information sheets and soil sampling bags.

 

WHEN SHOULD I SAMPLE EACH FIELD?

Ideally, samples should be taken just prior to seeding. However, from a practical standpoint, this is difficult because little time is left to plan a fertilizer program and purchase fertilizer in time for seeding. The best alternative is to obtain soil samples in the fall once soil microbial activity has declined. The proper sample time in the fall is after the soil surface temperature drops to less than 7°C. At this temperature soil processes, such as mineralization (breakdown of soil organic matter into plant available nutrients), that cause changes in soil nutrients proceed quite slowly and therefore, changes in plant available nutrient levels are normally not great. Generally, it is safe to soil sample in most areas after the beginning of October. By sampling in the fall, there is sufficient time to properly process samples, provide test results and recommendations, and develop a fertilizer program for that fall or the next spring.

 

FROM WHAT DEPTH SHOULD I COLLECT MY SAMPLES?

Phosphorus and potassium recommendations are based on a measure of the amounts of the available forms of each of these nutrients contained in a 0 to 15 cm depth sample. Generally, most of the plant-available phosphorus in soil is confined to the plow layer as phosphorus is very immobile. Nitrate-nitrogen (NO3-N) and sulphate-sulphur (SO4-S) are both mobile nutrients and may be found in significant amounts in the 30 to 60 cm depth. For this reason, recommendations are based on a measure of the amount of nitrogen(N) and sulphur(S) from a 30 – 60 cm depth soil samples.

 

WHAT IS THE PROPER WAY TO HANDLE MY SAMPLES?

Soil from each depth should be placed in separate containers. Immediately after the samples have been taken:

 

  1. Mix the soil in each container thoroughly in order to obtain a homogeneous mixture.
  2. Remove the soil and spread on a piece of clean paper.
  3. Allow the soil to completely air dry at a temperature of not more than 30°C. Do not dry in an oven or at a high temperature since this can change the levels of some nutrients.
  4. Care should be taken to avoid contamination of the samples with fertilizer materials such as commercial fertilizers, manure, salt, water, and dust. Samples should not be dried on old fertilizer or feedbags or areas where fertilizers have been handled.
  5. A fan may be used to ensure constant air flow over samples to enhance drying.
  6. Once the samples are thoroughly dry, fill the soil sample cartons. Label each carton with correct field number and sample depth.
  7. Complete an information sheet on cropping and fertilizer history and mail to a reputable soil testing laboratory. The services of soil testing and fertilizer recommendations are not free and not all labs offer the same testing services. Consult your Agriculture Solutions Advisor for details of soil testing laboratories that are available.

 

To order a single soil sample or to have us come to your farm to collect samples of all your fields, contact us today.

To access our Guide to Soil Sampling, click here.