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A Guide to the Practical Use of Aerial Color-infrared Photography in Agriculture

Agricultural Applications of Color-infrared Film


Traditionally, black-and-white aerial photographs have been used for soils mapping, especially by the U.S. Department of Agriculture Soil Conservation service. Panchromatic film and its processing are less expensive than natural-color and color-infrared films. However, color differences between vegetation and soil and subtle variations in soil moisture and vegetation vigor are more readily detected using color-infrared film, compared with black-and-white photography in the analysis of crop-soil relationships.

As discussed in preceding sections, variations in crop signature on color-infrared photography may be attributed to several causes, including moisture stress, blight, or stage of physiological development. However, these variations might also indicated differences in soil properties (for example, texture, salinity, or internal drainage).

In general, if we assume that we have a constant situation and a uniformly non-vegetated soil, light-toned areas on the air photos tend to be located on relatively high topographic positions, which tend to be low in moisture content, coarse in soil texture, and low in organic content. Dark-toned areas on CIR images tend to possess the opposite characteristics. Experience has shown, though, that these conditions are true only in an ideal environment. In the real world, these variables and others form a complex system that makes the interpretation process difficult. A few examples are given in the following paragraphs.


Rarely is a field uniform in soil texture and structure. When water reaches a field, either in the form of precipitation or irrigation, it infiltrates at a variable rate according to the texture and structure of the soil. A high infiltration rate in coarse-textured soils with lower infiltration rates will receive lesser amounts of water. The implication of these variations in infiltration rates is that moisture stress could occur in both cases, one from excessive internal drainage and the other from high surface runoff. If the field has received a sufficient amount of water and variations in crop signature occur, one of the possible caused for moisture stress may be traced to differences in soil texture and structure and the effects these two variables have on the infiltration rate (fig. 12).

Color-infrared Photograph Soils Map
Fig. 12 - Differences in soil texture may be indirectly observed through variations in crop vigor. On this late-August color-infrared photograph, the effects from the highly permeable Cass fine sandy loam soil (indicated by "Cc" on the map just above) can be seen beside the less permeable Colo and Kennebec soil (Ck). The bright red signature of the Colo and Kennebec soil indicates that it has a higher water-availability capacity relative to the Cass soils, which are underlying crops that have a less vigorous growth (purplish-blue color).

Differences in soil texture can be estimated by the sharpness of the boundary between the lighter and darker toned areas on an aerial photograph. Fine textured soils generally will have gradual gradations between light and dark tones while coarser textured soils usually will have sharper gradations. These variations in tonal transition are the result of differences in capillary action occurring in soils with different textures.


Besides the variations found in soil texture on the surface of the soil, there are differences in texture within the vertical profile of a soil. For example, a field may have a relatively homogeneous texture on the surface but have variable textures in different parts of the field below the surface. The infiltration rate is regulated by the least permeable layer in the vertical profile.

In the early stages of normal crop growth, little variation in crop signature may be noticed on color-infrared aerial photography, assuming that all other variables affecting crop development are equal. However, as the plant roots move deeper into the profile, where soil texture differences are present, variations in crop signature begin to develop. For example, in semi-arid regions such as western Nebraska, claypans or hardpans are found just below the surface. As root development progresses downward, the claypan or hardpan impedes root penetration and causes lateral root development. During drought, when the available moisture in the upper layers of the soil is below the wilting point of plants, moisture stress becomes evident in the vegetation (as indicated by a pale pink signature on the photographs of the affected vegetation). Plant roots that are able to penetrate deeper into the profile may find more available water and appear less stressed on the color-infrared photography (relatively brighter red than vegetation on the Claypan or hardpan soils).

A relatively impermeable layer may also cause too much moisture to be retained in the upper layers of the soil profile, resulting in a perched water table. This condition is more harmful to vegetation than situations where moisture stress occurs. As the air pores in the soil fill with water, less and less oxygen is available to plant roots. The anaerobic condition that develops encourages root diseases and molds (for example, Phytophthera root rot). An oxygen-free environment in the soil can kill plant roots in a few hours and the entire plant in a few days.

Detection of poorly drained soils on color-infrared aerial photographs depends on soil color and crop response. In poorly drained areas, soils are generally darker than the surrounding, better drained soils. The dark tones are indicative of relatively higher concentrations of soil moisture (fig. 13). In naturally poorly drained areas, the dark tones may also indicate the buildup of organic matter.

Fig. 13 - The dark green areas in the field indicate relatively moist conditions in the soil. The linear pattern is caused by tile drains located immediately below the soil's surface.


Soil compaction, generally a result of intensive agriculture, is also apparent on aerial CIR photos. With compaction, the natural structure of the soil breaks down through repeated cultivation and the pressure exerted by heavy farm equipment. Soil compaction impedes water infiltration and root penetration, reduces pore space and, ultimately, crop yields (fig. 14).

Fig. 14 - Compaction marks are identified by their ellipsoidal and circular shapes. Relatively poor plant vigor is reflected by the pink-gray tones in the compacted portions of the field.


The effects of soil salinity are observable on aerial photography by the presence of limited and poor vegetative growth in areas adjacent to healthy vegetation. Usually, white "slick" spots are also found throughout the affected area. In moderately saline environments, plants are not usually killed but will exhibit an abnormal, dull red or purple color on color-infrared photographs (fig. 15).

Fig. 15 - Vegetation on moderately saline soil exhibits a light purplish-red signature, while more vigorous vegetation on non-saline soils contrasts with the presence of "slick" spots among the low vigor vegetation.

Cut and Fill

Cut-and-fill areas, a result of land leveling, can cause problems resulting from compaction and a lack of nutrients. Heavy earth-moving equipment rearranges the soil profile and compacts the soil surface, reducing permeability and soil pore space. Fill spots create a greater problem than cut areas because of the accompanying deterioration of soil structure. (CIR signatures relating to permeability and compaction are described in previous paragraphs.)

In places where the topsoil is removed, the readily available source of non-mobile nutrients (for example, phosphorous) also is taken away. If nutrient compensation is not made on the cut area, crop yields will probably be lower than the yields from uncut areas due to stunted plant growth and a reduction in crop population. The signature on the CIR imagery will be a light red to pinkish-gray, a result of relatively low biomass per unit area of soil surface.

Rill and Inter-rill Erosion

Color-infrared photography also is used to analyze conditions influencing cropland erosion. Crop management (for example, crop selection and rotation method) and conservation practices (for example, placement of terraces and grass waterways on steep slopes) provide information that can be used in the estimation of potential soil loss attributed to rill and inter-rill erosion. Early season photography is useful in the identification of erosion-control structures (fig. 16), while sequential photography during the growing season assists in the interpretation of crop types and the subsequent type of plant residue on the fields during the fall.

Fig. 16 - Grass waterways form deep red "fingers" on the landscape in early spring. Slight variations in topography, such as terraces, may be delineated by the differences in soil moisture (light gray: dry; dark gray: wet

Season-to-season aerial coverage provides information relating to crop-rotation practices, an important element in estimating the potential for soil erosion (figs. 17 and 18).

Fig. 17 - Evidence of rill erosion on terraces can be observed on this photograph. The sharp contrast between vegetation and soil may be caused by many things, such as water erosion on slopes.
Fig. 18 - The effects of eroded soil on crop vigor are evident on this photograph. The deep red color of the center-pivot-irrigated corn is underlain by Holdrege silt loam soils (HoA and HoA2). The light pink areas belong to the eroded soil complex, Holdrege-Coly (HCC2) and rough, broken land (RB).

Through systematic interpretation of the various elements of the landscape (for example, topography, drainage patterns, vegetation, and photo tone and color), the aerial-photo analyst can identify different terrain conditions. In complex areas, though, the interpreter should not make unsupported decisions about terrain conditions and should consult other information sources, such as soil surveys, to supplement interpretations.