September 2005

Detailed Soil Survey:

The Foundation of Sustainable Resource Management

Author: Petra Loro, M.Sc. Soil Science Livestock Environment Specialist - Agri-Environment Branch - Manitoba Agriculture, Food & Rural Initiatives

Introduction

Soil is essential to human survival. We rely on it for the production of food, fibre, timber and energy crops. Together with climate, the soil determines which crops can be grown, where, and how much they will yield. In addition to supporting our agricultural needs, we rely on the soil to regulate the flow of rainwater and to act as a filter for drinking water. With such a tremendously important role, it is imperative that we manage our soils for their long-term productivity, sustainability and health.

The first step in sustainable soil management is ensuring that the soil will support the land use activity. For example, only the better agricultural soils in Manitoba will support grain and vegetable production, while more marginal agricultural soils will support forage and pasture-based production. For this reason, agricultural development should only occur in areas where the soil resource will support the agricultural activity. The only way to do this is to understand the soil resource that is available. Soil survey information is the key to understanding the soil resource.

A soil survey describes the characteristics of the soils in a given area, classifies the soils according to a standard system of classification, plots the boundaries of the soils on a map, and makes predictions about the behavior of soils. The different uses of the soils and how the response of management affects them are considered in designing and carrying out the survey. The information collected in a soil survey helps in the development of land-use plans and evaluates and predicts the effects of land use on the environment.
Adapted from the USDA definition of ‘soil survey’.

Soil survey is simply an inventory of the properties of the soil (such as texture, internal drainage, parent material, depth to groundwater, topography, degree of erosion, stoniness, pH, and salinity) and their spatial distribution over a landscape. Soils are grouped into similar types and their boundaries are delineated on a map. Each soil type has a unique set of physical, chemical and mineralogical characteristics and has similar reactions to use and management.

Soil interpretation refers to the behavior and response of soils to human activities. The information assembled in a soil survey can be used to predict or estimate the potentials and limitations of the soils’ behavior under different uses. As such, soil surveys can be used to plan the development of new lands or to evaluate the conversion of land to new uses. Soil surveys also provide insight into the kind and intensity of land management that will be needed. An example of soil survey interpretation is the evaluation of a given landbase for manure application. The type of soil and its associated characteristics determine the crops that can be grown, their yield potentials, the quantities of nutrients that are needed and the field practices that will be necessary to maintain optimum soil conditions for plant growth. As well, soil data on permeability (the rate at which water moves through a soil), depth to groundwater, flooding, slope gradient, soil texture and depth to bedrock can be useful in determining the risk of groundwater contamination due to leaching or surface water contamination due to runoff and erosion.

Soil Survey in Manitoba

Reconnaissance, or small-scale, soil surveys were started in 1926 in Manitoba as the first step in the development of a basic program of soil research, education, conservation and use for the province. The first reconnaissance soil survey for Manitoba was published in 1940 for the southwestern area. It was at a small-scale of 1:126,000 or ½ inch to 1 mile. Although reconnaissance soil surveys have improved with aerial photography and interpretation since the 1940s, these maps are still based on only a small number of site investigations per section of land (approximately 6 inspections per 640 acres) and the soil map units identify only the dominant soil groups.

The first large-scale, or detailed, soil survey in Manitoba was published in 1972 for the Portage-la-Prairie area at a scale of 1:20,000. Detailed soil surveys identify more of the variation in soil types across smaller landscapes. Detailed soil survey maps are much more accurate and reliable for making decisions at the farm-level (Table 1). They are created from much more intensive site investigations ranging from 16 to 30 per section of land.

Table 1. Number of inspections and intended uses for soil survey maps according to scale.

The Use of Detailed Soil Survey for Environmental Management

The recent intensification of agriculture, coupled with serious water quality issues throughout North America, has increased the public’s concern about agriculture’s management of chemicals, fertilizers and livestock manure. In response to the public’s concern, there are many new initiatives being developed in Manitoba that use soil survey information to direct on-farm management practices. These activities include:

• Environmental Farm Planning
• Manure Management Planning
• Nutrient Management Planning
• Precision Farming
• Watershed Management Planning
• Land Use Planning

The success of many of these activities will depend on the availability of reliable, detailed soil survey information upon which the on-farm decisions are made. Unfortunately, currently only about 30% of agro-Manitoba has detailed soil survey. Essentially, there is virtually no detailed soil survey information for the Northwest region and very little for the Eastern-Interlake Region. Approximately ¾ of the Central Region has detailed soil survey and approximately 1/3 of the Southwest Region.

In many instances, when detailed soil survey information is not available, it is substituted with reconnaissance level information. Unfortunately, reconnaissance soil survey data is not reliable enough for making on-farm management decisions.

The Limitations of Soil Survey Information

Soil survey maps are not without limitations. Although the map may say that a discrete area of land contains a certain soil type, it must be understood that the reliability of that information is a function of the map scale. All soil delineations (called polygons) contain small areas of dissimilar soils that are not identified (called inclusions). The smaller the scale of the map (or the more general the map), the more frequently this occurs. Small-scale, reconnaissance or general soil surveys give only a broad picture of the dominant types and distribution of soils that occur over relatively large areas. The landscape may actually include fairly significant areas of different soils that are not identified on the map. As such, reconnaissance soil surveys are best suited to making general comparisons of soil capabilities and limitations on a regional, national or even worldwide scale. They are not reliable for making on-farm decisions as they lack the detail necessary to describe the variation in the soil types on the farm (Table 1).

In addition to the limitations of map scale, the boundaries of the soil map polygons imply there are abrupt changes in soil types within the landscape. In reality, however, soil varies continuously across the landscape. It must be recognized that, although the map lines imply abrupt changes, the soil grades from one type to the next and the lines on the map are only approximations of where these transitions occur.

The limitations of soil survey were summarized very simply by Hubert Byrd (Soil Survey Horizons) when he wrote:

"It is nature which controls the areal variability of soils,
not soil scientists"

Similarly, it can be stated that it is nature which controls the areal variability of soils, not maps.

When Detailed Soil Survey Information is Not Available

When detailed soil survey data is needed but unavailable, on-site investigations are necessary. On-farm soil survey can be designed for a specific purpose or general purpose. A specific or single purpose survey may be appropriate when there is only one, well-defined objective (such as siting a livestock operation). In this instance, only the information required to meet the single objective may need to be collected. The major advantage to a single purpose survey is decreased cost. A general-purpose survey, on the other hand, contains a wider range of information. Although more information is more costly to collect, the general-purpose soil survey may have more value over the long-term as it can be interpreted in a variety of ways and can be reused for many purposes.

Whether single or general purpose, soil survey begins with field inspections to a depth of approx. 1 m and collecting information about the physical, chemical and biological properties of the soil. Differences in soil colour, texture and pH within the soil profile are then related to the surrounding landscape in order to determine soil type and predict behavior. The reliability of the field data and its interpretation is largely dependent on the experience and ability of the surveyor. As soil survey expertise is scarce, data collection by less experienced individuals should always follow standardized procedures and should only be carried out by those who have received training in soil survey.