Agricultural soil analysis provides information on chemical, physical and biological properties. This knowledge of soil characteristics, especially in terms of soil nutrients, enables planning of fertilizer programs that are accurate, efficient, and environmentally responsible.

There are many possible soil tests, able to report on a variety of soil features and properties. As much of a farmer’s soil analysis costs are incurred during sample collection and transportation, they are keen to get the most out of every sample sent for analysis. Laboratories therefore often perform a range of soil tests on each sample to maximize the farmer’s return on investment.

Soil testing laboratories themselves also need to operate cost effectively while maintaining accuracy and reliability. Efficient filtration plays a key part in this as a first step in sample preparation before analysis. A laboratory that can make sure it is using optimum filtration practices and materials can simplify soil testing workflows, provide accurate and reliable results, and maximize its profitability.

Why is soil testing important?

Soil testing is carried out for various purposes:

  • Assessing land capability for different forms of agriculture.
  • Identifying and quantitating soil constraints.
  • Monitoring soil fertility.
  • Providing fertilizer guidelines to optimize plant/crop growth.
  • Diagnosing reasons for poor performance.

In plant and crop production, soil testing plays an important role in building nutrient management plans and developing cost-effective fertilizer programs. By understanding the nutrient availability of soil in each location, a farmer can use the most appropriate type and amount of fertilizer to maximize crop production without unnecessary environmental effects or costs.

Types of soil analysis

Soil sample collection and transportation is the costliest part of soil analysis. Performing a range of analyses on each sample therefore helps maximize cost effectiveness.

Most soil analysis is performed by central soil testing laboratories that have the capacity and capability to offer different packages of soil tests depending on requirements. Each laboratory will use soil testing methods that are applicable to the region. Table 1 summarizes common types of soil analysis performed in these central facilities.

Basic soil analysis tends to examine the main macronutrients, which include nitrogen (nitrate), phosphorous (phosphate), potassium, and magnesium. As well as pH, these provide an insight into some of the most important aspects of soil health in terms of crop production.

A full broad-spectrum analysis, which is often the recommended course, covers many more important elements, including additional macronutrients and a range of micronutrients. Chemical, physical and biological analysis, as well as investigations into the presence of heavy metals, can also be useful.

Table 1. Different types of soil analysis

Type of soil analysis Examples of soil tests
Nutrient analysis
  • Nitrogen (N)
  • Phosphorous (P)
  • Potassium (K)
  • Magnesium (Mg)
  • Calcium (Ca)
  • Sulfur (S)
  • Manganese (Mn)
  • Copper (Cu)
  • Iron (Fe)
  • Zinc (Zn)
  • Molybdenum (Mo)
  • Boron (B)
  • Sodium (Na)
Chemical and physical analysis
  • pH
  • Cation exchange capacity
  • Organic matter
  • Soil texture
Biological analysis
  • Soil organisms
Heavy/toxic metal analysis
  • Arsenic (As)
  • Cadmium (Cd)
  • Lead (Pb)
  • Mercury (Hg)

Soil testing methods: the role of filtration

A selection of different soil testing methods are available for each element of interest, the choice of which varies depending on the properties of the geographical region of interest. Soil testing laboratories will choose an analytical method that meets a farmers specific needs, adjusting for crop type, geography, any known crop responses, and target yield.

The Food and Agriculture Organization of the United Nations (FAO) has published a guide to laboratory plant nutrient analysis, which includes a review of analytical methods available for different soil tests.

In general, determining available and/or total levels of specific nutrients present in a soil sample involves adding a prescribed amount of extractant to a fixed amount of soil. This is then shaken for a specified amount of time before filtering to recover the extractant, which now contains dissolved nutrients, for testing.

Different extractants, times, and analytical procedures are used for different nutrients or groups of nutrients. Instrumentation, such as spectrometers, are often used for quantitation, although manual methods are also possible.

As collecting and transporting soil samples is costly to the farmer, soil testing laboratories aim to make sample preparation and analysis as efficient as possible. Using the most appropriate filter helps to maximize their use of time and resources, and provide a cost-effective, accurate, and reliable service that is attractive to their customers.

The FAO recommends Grade 42 quantitative paper due to its retention capabilites, which is the finest in the Whatman range of ashless filter paper grades. Whatman Ashless Grade 42 retains particles as small as 2.5 µm, and is designed for gravimetric analysis and sample preparation for instrumental analysis, as needed for soil testing.

Soil nutrient analysis for rational fertilizer use

As mentioned, a critical part of agricultural soil testing is soil nutrient analysis, which determines the presence and quantities of essential plant nutrients in a given soil sample.

Several essential plant nutrients (Table 1) must be available to plants/crops in specific required quantities to achieve yield targets. Any limiting or deficient nutrients will affect crop growth, potentially restricting potential yield.

These nutrients come from various sources, including the atmosphere, soil, irrigation water, mineral fertilizers, manures, and biofertilizers. The combinations and quantities of nutrients, and the methods of integrating them, from various sources depend on factors such as the type of crop, soil, resource availability, and economic considerations (1).

As such, determining the amount of these nutrients in a relevant soil sample provides vital information to guide farmers on how best to incorporate any deficient nutrients, in the form of various types of fertilizer, to achieve optimum yield.

Specifically, NPK soil testing examines the amount of three important soil macronutrients, which are also three key fertilizer components: nitrogen (N), phosphorous (P) and potassium (K). The amount of total and/or available N, P, and K directly impacts on a farmer’s fertilizer requirements in each region, for a crop type, and yield.

Determining the nutrient quantities in a soil sample is crucial in developing a rational fertilizer program that makes sure the right amount of fertilizer is used to maximize profitability and minimize environmental impact.

Soil test crop response (STCR) fertilizer recommendations

Fertilizer is key for sustaining crop production throughout the world. However, with escalating demand and costs, as well as decreasing soil health, the safe and efficient application of nutrients through fertilizers is an important consideration for all farmers.

Through soil testing, it is possible to determine the nutrient quantities of a field or area, relate this information to crop response studies, and then determine the most appropriate amount and types of fertilizers for that location. This process is termed soil test crop response (STCR) fertilizer recommendations, or sometimes soil test-based fertilizer recommendations (2).

Accurate soil testing is the first step in generating reliable soil test-based fertilizer recommendations.

A recent nationwide study conducted in India involved extensive soil testing to determine the different soil climates throughout the country using an STCR approach. The result is a detailed online resource that allows farmers to search by geo-coordinates and find recommendations for optimum fertilizer usage on their land to enhance farm profitability.

While this current online resource for fertilizer recommendations is based on soil tests in India, the principles of an integrated process, which coordinates soil tests and research-led fertilizer trials in different agricultural and ecological regions to produce country-wide fertilizer recommendations, has wide applicability. It also demonstrates the power of soil testing and its importance in agriculture.

GE Healthcare Life Sciences provides a variety of quantitative ashless filter papers, including Grade 42, as well as Ready-To-Use, pre-folded soil analysis filter papers made from the same high-quality materials. The Whatman Filter Selector app provides a guide for identifying the most appropriate filter for an application, while the GE Healthcare Scientific Support team is available for all other support and enquiries.

References

  1. Guide to laboratory establishment for plant nutrient analysis, FAO Fertilizer and Plant Nutrition Bulletin 19, ISSN 0259-2495 (2008).
  2. Sahu, V. et al. Soil test based fertilizer recommendation for targeted yield of crops: A review Int. J. Chem. Stud. 5(5), 1298-1303 (2017)