Soil Survey, Mapping, & Classification Do we need to map soils? Why and for what?

Need for Soil Survey for Irrigation To ensure that productive soils are selected for irrigation To aid in locating irrigation infrastructure Determine irrigation needs of specific soil types Determination of soil reclamation needs Determination of erosion control needs Determination of land leveling needs Assist in determination of suitable crops Assist in devising farm management needs

Who is a “Soil Surveyor? A Soil surveyor is a natural resource scientist who needs to have a good grasp of geology, geomorphology, hydrology, … A soil scientist is an artist who paints the soil and landscape on his canvas, the soil map.

Basic Principles of Soil Survey Soil maps should be miniature and symbolic representation of reality. Landscape should be delineated to soil mapping units, presenting soil bodies with less variability than the total population of soils.

Basic Principles of Soil Survey (cont.) Soil survey should depict unique relationships of soil properties that can be observed in the nature having similar inherent characteristics. An experienced soil scientist should predict the location of different soil types on the landscape reasonably accurately and at affordable cost.

What to aim for! Remember! We should not make auger holes and soil profiles to find out what soil type is there, but to see whether it is there. First develop a hypothesis (model) about soil position on landscape based on initial visits, then check whether the hypothesis is correct or need changes.

How to develop the model Make a preliminary field visit Talk with the elders. They know much more than any of us about what soil types are there! Get back to office and study the aerial photos (API) Discuss with other technical staff in the institute (agronomist, geologist, hydrologist, etc.)

How to develop the model (cont.) Check the model based on API, field visits, discussion with farmers and peers, develop a working model of soil development on landscape. Check the working model in the field through soil observation and sampling. If field data do not conform with model, revisit and refine the model.

Qualities and Limitations of Soil Surveys We are responsible to inform the users of soil survey limitations. Soil survey tries to divide a continuum into individual, delineated units. Soil survey is not an abstract science: there is a lot of judgment and inference that goes in mapping soils.

Qualities and Limitations of Soil Surveys Just remember! Even in a detailed (1:20,000 scale) soil map (1 obs./10 ha) we only look at the 3 rd dimension (soil profile) in less than % of the soil continuum!

Qualities and Limitations of Soil Surveys Yet the association of soils with certain landscape characteristics possesses a degree of correlation that is high enough to allow for prediction of soil behavior to be made with certain (but unknown) confidence limit.

User’s Expectations! While soil scientists cannot record what the soil is at every point on the landscape, nevertheless, those who commission and/or use soil surveys often want or expect such information. (Webster, 1977)

Misunderstandings! Although the main objective of soil survey is the spatial portrayal of information on soils, the way the information is obtained and the way interpretations of soil behavior are inferred are a common source of misunderstanding.

Main Reason for Misunderstandings Failure of users to understand our mapping techniques and our failure to communicate in “plain” language (through our soil reports), is leading to the use of soil surveys beyond their confidence limits with highly undesirable results.

Soil map must show soils! Soil mapping description should be based on soil profile characteristics. A map that is developed based on soil-landform association is still a soil map as long as the mapping unit descriptions show soil description on the landscape.

Soil map must show soils!  A map that is only based on land form units, with soils being added to the legend as an “add-on”, is only a geomorphologic map, not a soil map.  Soil should be defined based on data obtained from a limited number of field observation (auger holes and profiles).

Soil map must show soils!  Therefore, soil surveyors have to depend on “external” clues that infer changes in soils on the landscape.  As long as correlation between soils and “external” indicators has been well established on the mapping legend (based on soil properties), the resulting map is a genuine soil map.

I hope you all are now looking forward to make your hands “dirty” and taste the soils very soon!