What is soil analysis?

President Rodrigo R. Duterte emphasized the conduct of soil analysis in the country during his first State of the Nation Address (SONA) on July 15, 2016. He said that “we shall also conduct a nationwide soil analysis to determine areas most suitable for rice farming to optimize production with the use of effective soil rehabilitation and fertilization.”

As an effect of this presidential pronouncement, many people including professionals from various academic fields have been wondering what soil analysis is. Several readers of this blog suggested that I write about this topic hence, this article.

Soil analysis refers to the measurement of soil physical, chemical, and biological properties. It is done, depending on the type of soil analysis, for the following purposes: 1) to evaluate the origin and formation of the soil; 2) to assess the level of contamination of the soil; 3) to characterize the soil as a habitat of soil organisms; 4) to assess the soil fertility status; and 5) to evaluate the soil’s suitability for certain crops. Soil analysis is generally synonymous with soil testing. The major steps of soil analysis are soil sampling (and field soil examination) and laboratory analysis.

Soil profile examination and sampling to evaluate the origin of the soil

The first type of soil analysis is the most difficult and complex type. It is carried out by soil specialists called pedologists.  It involves detailed field description of the soil using standard procedures such as the Guidelines for Soil Description (4th edition by Jahn et al., 2006) published by FAO, Rome. Soil description is done on newly dug soil pit at least 1.5m deep or fresh road cuts. Collection of soil samples for intensive laboratory analysis is done on every soil layer (soil horizon) down to the bedrock. Laboratory analyses include the physical, chemical and mineralogical properties of the soil. Geochemical analysis of rock samples is also necessary.

The second type is conducted by soil scientists interested in soil pollution or contamination. Soil samples are usually collected in areas where soil contamination is suspected. Soil sampling is done at random or at fixed interval. Only the top soil layer (0-10 or 0-20 cm) is sampled using a soil auger or similar sampling tool. Soil samples are analyzed for their contents of soil pollutants (e.g. heavy metals) and are compared with published threshold values to know if the sample is contaminated or not.

Soil sampling to assess the contamination of Taft River in E. Samar

The third type of soil analysis is conducted to know if the soil is favorable for certain soil organisms of interest (e.g. earthworms). This is popular among soil ecologists. Soil samples are collected usually from the top soil layer and then they are analysed for soil physical and chemical properties. Correlation analysis is then done between the population of the soil organism and the different soil properties to know which among the soil properties influences the population of the organisms.

The fourth type is the most well-known and commonly done type of soil analysis to support crop production. The main purpose is to know if the soil is fertile or not. Specifically, it is performed to assess, using high-tech laboratory equipment,  if the soil contains sufficient amounts of the essential nutrients required by plants (crops) to grow well and produce good yield (grain, tubers). The essential nutrients that the pant takes up from the soil include: nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulphur (S), iron (Fe), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), boron (B), chlorine (Cl) and nickel (Ni). Since N, P and K are required by the plant in largest amounts, these three are usually the nutrients that are of limited supply in the soil. So farmers need to know how much of these nutrients must be applied to the soil through fertilizers. The final result of this type of soil analysis is a fertilizer recommendation.

Soil sampling for fertility evaluation

The fifth and last type of soil analysis is carried out to assess if a certain soil is suitable for crops such as rice, corn, vegetables, fruit trees, and others. This involves field soil examination to know the soil’s texture, structure, drainage, slope and depth using the methods of the first type of soil analysis. The soil samples are analysed for soil chemical properties such as soil pH, organic matter content, nutrient holding capacity as well as the amounts of the major nutrients (similar to the third type of soil analysis). The soil properties are then matched with the ecological requirement of the crop. The final result is a suitability map or table showing the suitable crops for each soil. It also indicates the soil constraints (or problems) if a crop is grown in a soil that is not suitable for that particular crop.

From the president’s pronouncement, it looks like that he meant the fifth type of soil analysis. Due to the tremendous amount of field and laboratory works, it cannot be done by the Bureau of Soil and Water Management alone. It needs the participation of universities with strong soil science program throughout the country such as Central Luzon State University, University of the Philippines Los Banos, Visayas State University, Central Mindanao State University, and University of Southern Mindanao.

Soil science students at VSU performing laboratory analysis of soil samples

The laboratory step of soil analysis or soil testing is tedious, time-consuming, and costly because the chemicals and apparatus required are very expensive. Although there are some rapid soil test kits available, they are not reliable. Also, the laboratory analysis has to be done according to accepted procedures and by trained personnel. Examples of accepted procedures of laboratory analysis of soils are:

Carter M.R. and E.G. Gregorich (Eds.). 2008. Soil Sampling and Methods of Analysis (2nd ed). CRC Press, Boca Raton.

International Soil Reference and Information Center (ISRIC). 1995. Procedures for Soil Analysis (L.P. Van Reuwijk, Editor). Wageningen, the Netherlands.

Jones J. B. Jr. 2001. Laboratory Guide for Conducting Soil Tests and Plant Analysis. CRC Press, Boca Raton.

Margesin R. and F. Schinner(Eds.). 2005. Manual for Soil Analysis – Monitoring and Assessing Soil Bioremediation. Springer Verlag, Berlin.

Pansu M. and J. Gautheyrou. 2006. Handbook  of Soil Analysis. Mineralogical, Organic  and Inorganic Methods. Springer Verlag, Berlin.

Schlichting E., H.P. Blume and K. Stahr. 1996. Bodenkundliches Praktikum (Soil Science Practicum). Blackwell Wissenschaftsverlag, Berlin.

Sparks D.L., A.L. Page, P.A. Helmke and R.H. Loeppert (Eds.). 1996. Methods of Soil Analysis Part 3—Chemical Methods. Soil Science Society of America, Madison, Wisconsin.

Westerman R.L. (Ed.). 1990. Soil Testing and Plant Analysis (3rd ed). Soil Science Society of America, Madison, Wisconsin.