Ward Laboratories uses soil test methods developed and calibrated by land grant universities. Standard methods are published in several manuals. We prefer to use standard methods that have performed well for many years.
Soil pH & EC:
We use a 1:1 water pH. This means we measure 10 grams of soil and 10 mL of water. The soil and water react for 30 minutes and then we read EC to measure soluble salts and measure soil pH to determine if soil is alkaline, neutral or acid. If sample shows acid pH (<6.5), a buffer solution is added to measure total acidity of the soil so we can predict the amount of lime to neutralize the acidity.
Soil Organic Matter:
Soil samples, in crucibles, are dried for 2 hours at 105° C to drive off hydroscopic water. The samples are cooled to room temperature and weighed. Then the samples are heated to 360° C for 2 hours, cooled and weighed. The difference in the weights is calculated as percent organic matter by LOI (loss on ignition).
Nitrate is very soluble in soils, so it is easy to extract and measure the actual pounds of N per acre in the soil depth provided on the submittal sheet. We extract nitrate with KCl (potassium chloride) solution. Flow injection analysis (FIA) analyzes nitrate.
Our normal extractant for phosphorus (P) is Mehlich 3 solution. Phosphorus is attracted to soil particles and leaches very slowly. Research has found P moves about ¾ inch per year in loamy soil and up to 2 inches per year in sandy soils. Mehlich 3 extractant is similar to Bray P-1. Mehlich 3 is buffer with acetic acid, so the extract can measure available P in alkaline and calcareous soils. Other P tests we provide are Bray P-1, Bray P-2, and Olsen P (for calcareous soils).
Potassium and other cations:
Our extractant for cations is ammonium acetate (pH 7.0). This extract floods the soil with ammonium that replaces potassium (K,) calcium (Ca), magnesium (Mg), and sodium (Na) on the cation exchange sites (CEC), as depicted in this video. Inductively Coupled Argon Plasma (ICAP) measures the cations in the extract. High temperature of the ICAP makes elements give off light. Each element has its own wavelength for detection. Sum of cations (estimated CEC) are calculated from the 4 cations and buffer pH. With sum of cations, we can report base saturation for each cation. Sodium base saturation is not a problem when less than 5% Na.
Mehlich 3 solution extracts soluble and available sulfur. ICAP analyzes sulfur. Sulfur in Mehlich 3 extract is mainly sulfate, which is soluble like nitrate.
Zinc, Iron, Manganese, and Copper:
Our extractant is DTPA (pH 7.3). DTPA is a chelate that simulates uptake of the micronutrients by plants. Over a 2-hour shaking time, DTPA absorbs the micronutrients held on soil particles. This is a good estimate of their availability to plants. ICAP detects the 4 micronutrients.
A dilute calcium chloride hot water solution extracts boron. Boron is more soluble than phosphorus and less soluble than sulfate and nitrate. The hot water is a good measure of boron availability. ICAP measures boron in the extract.
Chloride is a soluble anion, like nitrate and sulfate. Calcium nitrate solution extracts chloride that FIA measures. Chloride is in low supply in the Great Plains. Potash fertilizer is potassium chloride, so there is no shortage of chloride where potash is applied.
In conclusion, we use soil test methods that are found in the literature and proven by land grant universities. This ensure accurate quality data on laboratory reports.
Originally Printed in April 2021 WardLetter.