Lachat Provides A Complete Solution for Soil Analysis 

For labs performing soil analysis, it is understood that it can be a monumental task. In order to accurately estimate the amount of nutrients available during the growing season, a large number of samples (thousands per day for some labs) must be prepared and analyzed. For this testing to be of use, the cost per test must be low.

This is possible with procedures that allow for the automated testing of multiple analytes. Lachat Instruments provides complete soil analysis solutions, from sample preparation to analyte analysis for labs running at any throughput requirement. A brief description of the total Lachat soils solution is given in this article:

Sample Preparation

Analytes of interest in soils often require preparation before analysis. Methods that require digestions and distillations are very time consuming, and can require expensive equipment. These methods are also prone to errors, as sample preparation can be difficult for even a skilled lab technician.

The Lachat MicroDist system quantitatively scales down the sample and reagent volumes (distilled samples are diluted to 6 mL). The key benefit to this system is a patented disposable (or recyclable) polypropylene tube assembly. MicroDIST eliminates the need for bulky, expensive, and often time-consuming glassware-based distillation units, while accommodating solid samples associated with soil analysis.

Sample Analysis

Once the sample has been prepped, Lachat can provide the solution for analysis with its large number of Soil Analysis Methods, for use with the QuikChem FIA analyzer. For labs with high throughput requirements, Lachat also offers a suite of Ultra High Throughput methods for Soil Analysis.

A brief look at common analytes for soils analysis, and the solution that Lachat offers is given below:

Nitrogen:

Most nitrogen present in soils is in the organic form. Plants use mainly inorganic nitrogen. Nitrate can be assayed in soils at any time, but the value obtained may not accurately reflect the available amount. Through the process of mineralization, inorganic nitrogen is released from organic forms. However, the rate of release cannot be predicted. Post harvest analysis can be used to assess the effectiveness of application.

Ammonium — N: Kjeldahl nitrogen test is used to determine the total N. This test is not normally performed on soils. Ammonium N may be requested where manure has been applied and a determination of whether fertilizer is needed as a supplement.

Lachat Method:

12-107-06-2-F, 0.1 –20 mg N/L, 2M KCI

Nitrate — N: a common lab test found on most all result forms. Available N test may underestimate available N where organic N has been applied but has not had time for mineralization.

Lachat Methods:

12-107-04-1-H, 0.05-10 mg N/L; 0.5M K2SO4
12-107-04-1-G
, 1-20 mg N/L; 0.0125M CaCl2
12-107-04-1-J
, 0.025-20 mg N/L, 2M KCl

Total Nitrogen:

12-107-04-3-B; 0.2-30 mg N/L; persulfate digestion in-line;
                       0.5M K2SO4 Extracts of soil.

Potassium:

Soil potassium maybe a structural component of clay soils or fixed in structural voids of some types of clay. K is adsorbed to negative charges on the surface of clay minerals, sesquioxides (a.k.a. amorphous aluminum and iron materials), and organic matter. A small amount is in the soil solution, in equilibrium with exchangeable fraction.

Exchangeable K is usually extracted by ammonium acetate, and is measured by Flame emission or absorption. Ammonium is especially effective in displacing K from exchange sites, because both ions are similar in charge and hydrated size. Most soil K is in inorganic form.

Potassium is not mobile in the soil, and the test should be done on soil from the top 15 cm. Manure application can lead to build up.

The following Lachat methods use Flame A.A. for detection:

Lachat Methods:

12-119-03-1-A, 0.2-10 mg K/L in ammonium acetate extracts
12-119-03-1-B, 1-50 mg K2O/L in calcium lactate/calcium acetate extracts
12-119-03-1-C, 1-20 mg K2O/L in calcium acetate extracts of soil
12-119-03-1-D, 5-200 mg K/L in Morgan’s Extract

Sulfur:

considered a secondary plant nutrient, required in smaller quantities than N or K, but in larger quantities than Boron or Zinc. Relatively little work has been done to develop soil tests for sulfur. It is present in the soil in both organic and inorganic forms, with organic S predominant.

In the Northeast US, surface soils are not usually tested, as it is assumed there is enough sulfur available for plant growth. Sulfate is primarily adsorbed to clays and Fe/Al oxides. Adsorption increases as pH decreases. Other inorganic forms of S are pyrhite, marcasite and pyrrhotite. Sulfur minerals in soils are difficultly soluble in water, but can be solubilized by chemical or biological mineralization. Organic and sulfide forms may contribute to plant nutrition, plants primarily absorb S in the form of sulfate. Estimation of soil S is done mainly by extraction for sulfate. The most widely used extract agents are water, 0.1M LiCl, Ca(H2PO4)2, or KH2PO43.

Sulfur is mobile in the soil, and so sampling below 30cm may be appropriate.

Lachat Method:

12-116-10-1-D, 3-60 mg SO42- /L in mono calcium phosphate extracts of soil. Turbidimetric.

Micronutrients:

(includes Ca, Na, Mg, Fe, Cu, Zn, B, Mn and Cl). Inadequate amounts will reduce crop growth. Chloride is considered a micronutrient, but is needed in greater quantities. In limiting conditions, plants will respond to these as they would to application of fertilizer. Excessive application however is detrimental.

Magnesium:

12-112-26-1-A, 5-200 mg Mg/L in Morgans extract of soils

Manganese:

12-131-35-1-A, 0.5-2.0 mg Mn/L in 0.1M HCl extracts of soils

Molybdenum:

12-123-23-1-B, 0.1-0.4 mg Mo/L in 0.1M HCl extracts of soils

Zinc:

12-130-18-2-A, 1.25-5.0 mg Zn/L in 0.1M HCl extracts of soils

Phosphorus:

Most soils also contain significant amounts of organic P; soil tests are focused on availability of inorganic P. The amount of P extracted can be correlated with the extractant, although direct comparisons are difficult. Different extractants remove different amounts of phosphorus from the same soils. Dissolved P is about 10-40% of the P transported through run-off.

There are a large number of extractants that can be used with Phosphorus:

Bray-1:

12-115-01-1-N, 0.4-20 mg P/L, High Throughput method
12-115-01-1-A, 0.4-20 mg P/L

Bray 2:

12-115-01-1-N, 0.4-20 mg P/L, High Throughput method

Olsen:

12-115-01-1-B, 0.01-1.0 mg P/L

Mehlich I:

12-115-01-1-N, 0.4-20 mg P/L, High Throughput method
12-115-01-1-A, 0.4-20 mg P/L

Mehlich III:

12-115-01-1-N, 0.4-20 mg P/L, High Throughput method

Morgans:

12-115-01-1-K, 1-30 mg P/L
12-115-01-1-L, 0.05-6.0 mg P/L

Phosphorus Extraction Methods by Soil Type

Soil pH Minerals Methods
Acidic < 6.0 Al-P, Fe-P, Mn-P Bray I, Mehlich I, Mehlich III, Water, IIP and AER
Slightly acid to slightly alkaline 6–7.2 Al-P, Fe-P, Mn-P, Mg-P, Ca-P Bray I, Mehlich I, Mehlich III, Olsen, Water, IIP and AER
Alkaline, calcareous > 7.2 Ca-P and Mg-P Olsen, Water, IIP and AER

Different results in different labs?

The main contributor to obtaining different results for the same soils tested in different labs is the use of different extracting solutions. Also, different instruments can give different results. Quality and technique of lab employees has an effect, as does method of reporting the results.

The quality of results is based upon the two main components of any analysis: bias and precision. Bias is the deviation of the measurement from the actual value - the accuracy of the results. Precision represents the reproducibility of a measurement. Keep this in mind when performing soils tests – quality counts!

Don’t see a method your lab needs? Contact us! Lachat Instruments can develop methods specifically for your lab.

Contact

Please contact Lachat Technical Support with any questions.
support@lachatinstruments.com

US:(800) 247-7613
International:(970) 669-3050