Soil sampling and analysis for managing crop nutrients

Overview

Soil testing plays an important role in crop production and nutrient management. If you use commercial fertilizer as your main nutrient source, soil testing is the best way to plan profitable applications. On livestock farms, knowing how much nutrient is present in the soil to start with is critical. With that information, you can develop a nutrient management plan. This helps you manage both the nutrients that have been generated on-farm and any nutrients that are being imported to the property, such as biosolids or commercial fertilizer.

Soil testing involves 3 steps:

1. Collect a representative sample from each field or section.
2. Analyze the sample to measure available nutrient levels.
3. Use the results to determine optimum fertilizer rates.

Keeping records is also an important part of the soil-testing process. Over time, your records will help determine if soil test levels are increasing, decreasing or being maintained.

Soil sampling

Soil sampling is an important step in managing crops and nutrients. A typical lab sample weighs about 400 g (1 lb.), but this sample must accurately represent up to 20,000 tonnes of soil — the amount of soil in the top 15 cm (6 in.) of a 10 hectare (25 acre) sample area. That’s why careful sampling is necessary.

Sampling area

The area you sample can have a large impact on the accuracy of your soil test. For small fields, you can collect one sample for each field. For larger fields, you should divide them into smaller, uniform sampling areas. Avoid combining soil from areas that are obviously different.

Soil fertility can vary due to:

• native fertility of the parent material
• soil texture
• nutrient removal by crop growth
• landscape position
• past applications of fertilizer or manure (this is the largest source of variation)

When the variation is small, include several cores in each sample. When the variation is large, sample areas separately. If you know past field boundaries, use them to divide large fields into smaller units. Base further subdivision, or divisions where past field boundaries are not known, on soil type or topography. The maximum area included in a single sample should be 10 ha (25 ac).

There is no minimum size for the area that can be represented by a single sample. Precision, site specific or grid sampling is allowed but not required for nutrient management.

Any areas with different nutrient levels from the balance of the field should not be included in the composite sample. This could include dead furrows, eroded areas, laneways or areas where manure or lime has been piled. If these areas are large enough to be managed separately, sample and analyze them separately.

Sample depth

The normal sampling depth for nutrients is about 15 cm (6 in.). Most plant roots grow, and tillage mixes most nutrients into the soil to that depth. Subsoil is normally much lower in nutrient content. Sampling too deep will produce a sample that is not representative of the field. When sampling for soil nitrates, sample down to a depth of 30 cm (1 ft). This provides a more accurate indication of available nitrate.

Sample depth is not changed in a no-till system, even though the nutrients are no longer being mechanically mixed into the soil, For pH samples, collect a shallow sample (5 cm or 2 in.) to check for acidification in the surface layer if nitrogen is being surface applied. Do not use these samples for nutrient analysis, since they will overestimate the nutrient availability from the soil.

Sample collection

A representative sample from a field must include enough cores, collected randomly from across the entire area. Too few cores increase the risk that a non-representative core could skew the result for the whole field. Non-random sampling increases the risk that a bias could be introduced into the sample. The most efficient way to achieve random sampling is to follow a zig-zag pattern around the field. Collect at least 20 cores to produce the composite sample and one additional core per 0.4 ha (1 acre) for fields larger than 8 ha (20 acres).

Often the most overlooked step in collecting a soil sample is the thorough mixing of soil cores before the sub-sample is collected. Sampled soil cores should be mixed in the bucket until no evidence of soil cores exist. Heavy clay soil cores sometimes need to be dried before they can be sufficiently mixed to allow for a suitable sub-sample. The sub-sample should be 400 g or about 1 cup of soil. Store collected samples at room temperature before sending to the lab. Soil nitrate-nitrogen samples require more careful handling.

Soil nitrate-nitrogen collection

Soil nitrate-nitrogen (N) sampling is often done before sidedress timing (V3-V4 stage for corn) and helps determine if additional nitrogen fertilizer is needed. Soil nitrate-N samples are collected from the top 30 cm (12 in.) of soil because nitrates move easily with soil water and may be present deeper in the soil profile. For the most accurate nitrate-N sample results, refrigerate samples immediately (4 °C or 49°F) and deliver to the lab within one day for analysis. If not possible, air-dry for 1 to 2 days at room temperature before sending. Do not freeze nitrate samples, as this can skew the results.

Sampling equipment

While it is possible to collect samples using a shovel or spade, it is more efficient to use a sampling probe or auger. These should be constructed of stainless steel, particularly if the samples are going to be used for micronutrient testing. Many agricultural retailers will lend sampling probes for soil sample collection.

Collect soil cores in a clean plastic pail. Galvanized pails contaminate samples with zinc, making micronutrient results unusable. Avoid pails that have contained sanitizers or detergents, as phosphates can contaminate the samples.

A sturdy stainless steel or aluminum trowel works well for mixing the cores before collecting a sub-sample. A screwdriver is also useful for dislodging any soil cores that might get stuck in the sampling tube.

Sample frequency

Collect samples frequently enough to detect changes in the soil before they become large enough to affect crop yields or fertilizer requirements. For most farms, sampling once every three years is adequate. This often works out to once in the rotation, at the same crop point.

Rapid changes in soil test values can occur where the soil has a low capacity to hold nutrients or when crops that extract large amounts of a particular nutrient are grown. More frequent sampling is necessary on coarse-textured soils or where crops that remove large quantities of potassium are grown (for example, alfalfa, corn silage or processing tomatoes).

Ontario Regulation 267/03 and sampling frequency

Ontario Regulation 267/03 states that a sample must be collected and analyzed from each field area prior to the completion of each nutrient management plan, and that the results from this sample must be used in the preparation of the plan. This normally means the maximum sampling interval would be 5 years unless a change in the operation made the preparation of a new plan necessary. For new operations, default values are used that apply the maximum restrictions on nutrient application.

Sampling time

There is some variation through the year of soil pH and nutrient content, particularly related to soil moisture, but these differences are not large enough or consistent enough to impact a nutrient management plan. Taking soil samples at the same time each year eliminates seasonal variation as a factor when comparing soil test results over time. If samples are taken immediately after harvest, the results will be back in time for planning the fertilizer program for the next crop.

Sample analysis

Samples for a nutrient management plan must be analyzed at an OMAFA-accredited lab, using OMAFA-accredited tests. The accreditation process assures quality analysis using Ontario-proven methodology. A list of OMAFA-accredited labs can be found at Soil, leaf and petiole tissue, and forages and feed testing labs.

Soil testing for available nutrients involves extracting and analyzing a portion of the nutrient from the soil. The value measured by this process is not the exact physical quantity that is available to the plant but is the amount of nutrient the plant root can extract. The complexity of soil chemistry and plant uptake is too great to make an exact physical quantity measurement possible. These values can vary widely with different tests. You cannot use the results from different tests with the OMAFA fertilizer recommendation tables. The accredited tests have been chosen to provide accurate results in the range of soil conditions found across the province.

The soil test value used in the OMAFA fertilizer recommendation tables are calibrated to relate the extractable nutrient with the amount of fertilizer required to achieve optimum crop yields.

The OMAFA fertilizer recommendation tables can be found in Publication 811: Agronomy Guide for Field Crops and Publication 839: Guide to Vegetable Production in Ontario.

Phosphorus

The OMAFA-accredited test for phosphorus uses a sodium bicarbonate solution for the extraction. This method, often referred to as the Olsen method, has been found to provide accurate results across the wide range of soil pH found in Ontario. Other methods used in neighbouring states or provinces, such as the Mehlich-3 or Bray methods, provide inconsistent results in alkaline soils and are not accredited for use in Ontario. These methods give results on a different scale, so they cannot be used with the OMAFA fertilizer recommendation tables.

Potassium

Available potassium is measured using an ammonium acetate extract. The ammonium displaces cations such as potassium, from the negatively charged soil particles so they can be measured in solution. The same extract can be used to measure the quantity of available magnesium, calcium and sodium.

Soil pH

Another important parameter measured in a soil test is pH. This is a measure of the acidity or alkalinity of the soil. Soil pH influences:

• the availability of nutrients
• the ability of crops to grow
• the activity of herbicides

Measure pH in a soil-water paste with just enough water to saturate the soil pores. More dilute suspensions will provide readings that are higher than the actual soil pH, particularly in coarse soils. A soil pH test is required where biosolids are to be applied.

Zinc and magnesium

OMAFA-accredited tests are also available for zinc and magnesium. These are useful for predicting the need for supplemental nutrients but are not required for a nutrient management plan.

Regulated metals

Nutrient management plans for biosolid application require testing for regulated metals. These quantities are measured in an acid digest of the soil sample, where all the soil minerals and organic compounds are dissolved. Labs that perform this analysis must be separately accredited to meet ISO/IEC 17025 standards.

Ontario Regulation 267/03 and required soil analyses

Soil samples for a nutrient management plan that includes manure application must be analyzed at an OMAFA-accredited lab for phosphorus and potassium. Plans that include biosolid application must have soil analyzed for the same parameters, plus soil pH and regulated metals.

Using the results

Soil fertility involves more than just a soil test report. Soil health, including tilth, soil structure and crop rotation, also affects nutrient cycling. Soil sample results should be used to prepare nutrient management plans. This means comparing the results to the OMAFA fertilizer recommendation tables to determine fertilizer rates, or inserting the test results into nutrient management software, such as Agrisuite, where nutrients from all sources can be used to calculate application rates.

Soil sample results are also useful in recordkeeping for comparing the analysis data to results from previous years. Determining soil fertility levels can be used to evaluate the effectiveness of your fertilizer program or nutrient management plan.

Using results becomes more complex when multiple samples have been collected for a field or single management zone. Multiple sample results can come from fields that have been grid sampled. If the field size is larger than 10 ha (25 ac), you may want to fertilize the entire field as one block. Table 1 outlines options for dealing with multiple sample results.

Averaging results

In averaging, results must be weighted to reflect the area included in each sample. This is done by multiplying the sample result for each parameter by the number of hectares (or acres) represented by that sample, and then adding the products of that multiplication for each sample in the field. This total is then divided by the total hectares (or acres) in the field to give a weighted average for the entire field. This prevents a single sample from a small area skewing the results if it is widely different from the rest of the field.

In table 2, the weighted average soil phosphorus test is 9 (810 weighting ÷ 90 ha). If the soil test values were simply averaged, the high values for the old barnyard and the front east field would skew the number upwards and give a result of 31. On a farm that is using commercial fertilizer, this represents a difference in phosphate fertilizer recommendations from 0, for the simple average, to 70 kg/ha, for the weighted average.

When combining sample results for a nutrient management plan, clearly note the method used so reviewers can understand how the numbers were calculated.