Soil Fertility Guide

Fertilizer Recommendation Guidelines

Soil testing is the only way to determine the available nutrient status of a field and receive specific fertilizer recommendations. General recommendations for those without a soil test are outlined in the Appendix of this guide. These recommendations can only provide "ball park" fertilizer requirements and are estimated for average conditions that may not occur in individual fields. As a result, these recommendations may lead to under-fertilization where optimum yield potentials and maximum economic returns will not be achieved. Conversely, these recommendations may lead to over-fertilization resulting in unnecessary costs, excessive vegetative growth, delayed maturity, lodging, reduced quality factors (e.g. protein, oil, etc.) and soil and water contamination problems.

Sound fertilizer recommendations for Manitoba are based on soil fertility analysis and fertilizer response. Research is conducted in the province, or under similar soil, climatic and cropping conditions as occur throughout the other parts of the Prairie region. Fertilizer recommendations based on soil testing are also included in the Appendix of this guide.

Soil testing

Yield and economic return from fertilizer can be optimized and potential soil and water pollution minimized, when nutrient application is geared to the needs of a particular crop grown on a specific field. An effective on-farm soil testing program is one in which every field is properly sampled and tested every year. This gives the producer an inventory of the nutrient levels in each field, plus specific recommendations as to the kinds and rates of fertilizer nutrients to apply for each crop. Recommendations may be based on specific times and methods of application and may provide information to modify application rates for different times and methods of application.

Reliable soil test results and recommendations depend upon:

  • proper soil sampling and sample processing procedures
  • proper soil analysis techniques
  • sound fertilizer recommendation guidelines

Soil sampling and sample processing

Soil sampling is the key to a sound soil testing program and the one step over which producers have complete control. Generally, it is important to follow the procedures recommended by the soil testing lab that is analyzing the sample. The following general procedures are usually recommended to ensure representative samples are provided for laboratory analysis.

  • Samples should be taken prior to seeding in spring, or in the preceding fall after soil temperatures drop. Soils that have cooled to 5°C have minimal microbial activity and hence little change in soil nitrate levels. 
  • Samples should be taken to the full 24" depth to get a proper and complete measure of the amounts of nutrients (particularly nitrogen and sulphur) available. All crops usually extract nutrients and water to at least the 24" depth over the course of a growing season.
  • Samples should be kept cool and shipped immediately to the soil lab for analysis. Alternatively, samples should be laid out to dry completely within 24 hours at a temperature less than 35°C or samples should be frozen immediately until they can be dried or analyzed. High temperature drying, or use of a microwave oven, will invalidate test results and fertilizer recommendations.
  • Samples should be kept clean. Substances such as fertilizer dust, salted sunflower seeds, cigarette ashes and manure can contaminate samples and result in erroneous test results and fertilizer recommendations.
  • If measuring zinc levels in the soil galvanized pails should not be used.
  • Samplers may wish to use lubricants to prevent soils from sticking to sampling equipment. WD-40 is preferred over vegetable oil-based lubricants.
  • The use of latex gloves will prevent contamination from hands.

Sampling Patterns

Traditional Composite Random Sampling

  • 15-20 cores are randomly taken throughout a field, thoroughly mixed, subsampled and sent to the lab as a single sample.
  • Representative sampling areas should be sampled when using traditional composite random sampling (Figure 4). For hilly fields with knolls, slopes or depressions, take samples from mid-slope positions to get average results. Level fields appear relatively easy to sample.
  • Avoid sampling obvious areas of unusual variability, such as saline areas, eroded knolls, old manure piles, burnpiles, haystacks, corrals, fence rows or old farmsteads, on headlands, within 50 feet of field borders or shelterbelts and within 150 feet of built-up roads.

Figure 4. Traditional Composite Random Sampling

Traditional Composite Random Sampling 

X = single soil probe sites

Benchmark Soil Sampling

  • A small ¼ acre area is selected as typifying the field or majority soil type within the field. In this benchmark area, 15-20 samples are randomly collected and mixed together.
  • This technique (Figure 5) assumes that the benchmark area is less variable than the entire field because it is smaller. This same area will be sampled year after year which should minimize sampling errors.
  • Selection of the benchmark area is critical. Representative sites may be selected through close crop observation (particularly during early growth stages when fertility differences are most evident), past grower experience, yield maps, soil surveys and/or remote sensed images.

Figure 5. Benchmark Soil Sampling

Benchmark Soil Sampling

15-20 soil cores in each bench-mark area

Grid Soil Sampling

  • This technique (Figure 6) uses a systematic method to reveal fertility patterns and assumes there is no logical reason for fertility patterns to vary within a field.
  • The field is divided into small areas or blocks. A sample location within the block, often at the point in the centre or grid point, is sampled 3-10 times. Modifications to the grid point sample may be done to avoid repeat sampling of regular spaced patterns within fields, such as fertilizer overlaps, tillage or tile drainage.
  • Grid sampling may be costly depending on the grid size selected. Experience in the United States indicates that a sampling density of one sample per acre is required to provide accurate information for variable rate fertilization. Sampling of larger areas may still provide useful information on the magnitude of field variability.

Figure 6. Grid Soil sampling

 Grid Soil Sampling

8-10 cores taken in each grid cell

Landscape Directed or Zone Soil Sampling

  • This technique (Figure 7) is used when major areas within fields have distinctly different soil properties, such as texture or landscape features. These areas should be sampled, and possibly fertilized separately.
  • Fields need to be delineated into different polygons or soil management zones. These patterns may be detected by soil survey, detailed elevation mapping, aerial black and white photographs, yield maps or remote sensed images.

Figure 7. Landscape Directed or Zone Soil Sampling Landscape Directed or Zone Soil Sampling

O = probe sites from low, saline areas X = probe sites from sloping areas * = probe sites from high sand ridge

A popular option with soil samplers is to georeference (i.e. GPS) selected sample sites so that soil samples can be taken from the same point during future samplings.

Proper soil analysis techniques

Soil analysis techniques that provide meaningful test results should be used.  For nitrogen (N) and sulphur (S), analysis results are typically reported in lb/ac. In the case of phosphorus (P), potassium (K) and micronutrients, analysis results are reported in parts per million (ppm). For Manitoba, the following are the recommended and approved procedures for the four major nutrients:

Nitrogen (N) - Water soluble nitrate-nitrogen measured to the 24" depth. When samples are taken to less than the 24" depth, a conversion value is commonly used to approximate the amount that is not measured 42 . This approximation may be affected by weather conditions and soil zone. It is recommended that samples be taken to the full 24" depth.

Phosphorus (P) - "Olsen" (sodium bicarbonate) technique measures extractable P in the top 6" depth and is well-suited to alkaline soils. Some laboratories (Bodycote Norwest Labs and ALS Laboratory Group (former Enviro-Test Labs)) use the acetic fluoride or modified Kelowna test. Evaluations in other Prairie Provinces indicate these methods perform satisfactorily in assessing P responsiveness of the soil. However, since the amount of P extracted is different than the Olsen (sodium bicarbonate) method, the Manitoba provincial recommendations in Appendix Table 17 cannot be used directly. The following conversions can be performed to approximate the Olsen P equivalent amount 43 .

Olsen P test (ppm) = Bodycote Norwest P test (ppm) x 0.9

Olsen P test (ppm) = ALS Laboratory Group P test (ppm) x 0.9

Olsen P test (ppm) = Mehlich-3 P test (ppm) x 0.5

Potassium (K) - The "Ammonium Acetate Exchange" technique measures exchangeable K in the top 6" depth. The acetic fluoride or modified Kelowna test also contains ammonium acetate and is a suitable technique.

Sulphur (S) - Water soluble sulphate-sulphur measured to the 24" depth.

Copper, Zinc, Iron, Manganese – Diethylene triamine pentaacetic acid (DTPA) extractable in the top 6" depth.

Boron - Commonly extracted by commercial labs using hot water.

Soil pH - Measurements of soil pH can vary based on analytical methods used. Using a 1:2 soil to calcium chloride solution will reduce interference from soil salts and is used in scientific and soil survey soil characterization. Most commercial labs use the 1:1 or 2:1 soil to water ratio, which tends to increase pH readings of Manitoba soils by 0.5 units.

Salinity or Electrical Conductivity (E.C.) - Salinity measurements for research and soil survey characterization are determined by the saturated paste method where enough water is added to the sample to saturate it without leaving any free water. This best reflects the salinity that occurs at the root surface. Most commercial labs use a 1:1 or 2:1 soil:water ratio method and salinity levels will be approximately half that of the saturated paste method. E.C. values determined in a 1:1 soil to water ratio are generally multiplied by a factor of 2 to approximate the saturated paste measure. This conversion is soil texture specific and can vary. EC is expressed in dS/m, mS/cm, or mmho/cm (all equal).

Use of recommendation guidelines or application of Manitoba guidelines to different analytical techniques may not provide sound fertilizer recommendations.

Other techniques exist to estimate nutrient supply (e.g. ion exchange resins), however these have not been calibrated for fertilizer recommendations printed in this guide.

Plant Tissue Analysis

Plant tissue analysis is a tool that can be used to fine-tune fertilizer management practices. Plant tissue analysis measures the nutrient levels in growing crops. Test values are compared with established values for inadequate, adequate and excess levels for each element and plant species. In this way, the nutritional health of the plant sample and the crop it represents can be assessed and the supply and availability of nutrients to crops during the growing season can be evaluated.

Plant tissue analysis is useful in evaluating fertilizer management programs and practices (including a soil testing program), diagnosing nutrient-related crop production problems and identifying nutrient levels in crops that may limit top yield achievement, including potential micronutrient problems.

Like soil testing, the validity and usefulness of plant tissue analysis depends on proper plant sampling and sample handling procedures. These include:

  • Sampling crops from individual fields separately.
  • Sampling the proper plant part at the proper growth stage. This is specific to each individual crop and lab. Sampling guidelines should be obtained from a reliable laboratory providing the service.
  • Sampling an adequate number of representative plants from a large number of "average" locations in a field. Abnormal plants from non-representative field locations should not be included unless the "comparative sampling" approach is used. Here, samples are taken separately from both normal and abnormal areas to determine if plant nutrition is the cause of the apparent difference.
  • Dry samples as soon as possible after removal at normal room temperatures that do not exceed 35°C.
  • Avoiding contamination of sample with fertilizer dust, cigarette ashes and other substances.

Like soil testing, analytical results must be assessed using standards developed specifically for crops and cropping conditions in Manitoba. Interpreting the results of plant tissue analysis often requires the assistance of a professional agronomist.

Table 18 provides the sufficiency levels of nutrients for many Manitoba crops at specific growth stages 44 . Nutrient levels below these sufficiency levels are considered deficient.

Other methods of assessing nutrient sufficiency of crops have been developed, but are less commonly used than traditional plant analysis. Such methods include:

  • High N reference plots in the field and the SPAD chlorophyll meter for in-field assessment of N sufficiency for oats 45 , winter wheat 46 , corn and spring wheat.
  • Final grain protein content for N sufficiency in hard red spring wheat and winter wheat (page 5)
  • Fall stalk nitrate test for N sufficiency in corn
  • Forage feed analysis, taken for balancing feed rations, may identify nutrient deficiencies of forage crops 47

Many potato fields are routinely sampled to assess nutrient sufficiency through the season. The 4th fully developed leaf from the tip of a main stem is sampled and leaflets are removed exposing the petiole. Some 25-40 petioles are collected per field, usually from marked areas. Repeat sampling is done at these same locations at intervals through the season, as critical levels for N, P and K decline with crop development 48 . Sampling should be done in mornings using the established sampling pattern for most consistent results. In-season soil sampling for N may help in interpreting petiole results and making decisions for supplemental N applications. Contact your soil and plant analysis laboratory for further sampling and handling instructions.

Crop nutrient sufficiency levels.

For further information, contact your MAFRD GO Representative.