Precision Agriculture – Mapping a Path to Success

By Michael Princevalle

My previous editorial in this publication presented some background and perspectives on the subject of precision agriculture (PA); particularly pertaining to the use of data to a) improve effective use of resources, b) bolster yields and / or product quality, and c) ultimately increase profits. Additionally, the use of PA is an essential tool in sustainable farming practices and, in some instances, a valuable tool to achieve compliance with local or regional governmental regulations. The previous editorial also discussed the importance of collecting sound, reliable data as critical elements to successfully use PA in farming management. Data that is incomplete, misaligned or misused will likely derail both PA practices and the desirable goals it is intended to achieve.

This article will discuss the importance in spatially identifying where soils change or transition within a field or area, and methodologies commonly used to perform for this identification task. Future articles will discuss investigating and defining soil characteristics, and PA strategies to best use soil characteristics for maximum benefits.

Where to Begin Collecting, Vetting and Applying Data. . . Let’s Start with the Soils

Soils are the foundation for most of agriculture. As expressed by UN Secretary General Ban Ki-moon, “Without healthy soils, life on Earth would be unsustainable”. Identifying and considering boundaries of soil types are major components in successfully maintaining soil health, and the successful use and outcome of precision agriculture. Astute agriculturalists are aware that a given area or field can have different soil types, and that these differences can be appreciable.

In viticulture it is especially important to obtain an awareness of where soil types change as these (soil) differences can appreciably impact the outcome of management decisions and desired effects. For now, the discussion of plotting (or mapping) soil changes will pertain to its significance in obtaining consistent, reliable and meaningful data, and meaningful applications of that data for management. Parenthetically, defining and mapping soil variability can appreciably enhance the application of data obtained from aerial imagery; another commonly used tool in management decisions and PA. Discussions pertaining to aerial imagery will be discussed in future articles.

Ground Zero; Finding Soil’s Hidden Treasures

To iterate, soil variability can significantly impact PA. In particular, soil variability impacts a) data collection (sampling) strategies, b) effectual understanding and application of the data, and c) implementing the most appropriate management strategies to account for variable soil characteristics. In other words, management actions based on data acquired from one soil type may not be appropriate, or maybe detrimental, if the same actions are used on different soil type. Perceptive farmers, especially vineyard managers, are aware of this.

Soils types typically do not reliably show their transitions or variability through topical, visual inspection, or even aerial imagery. Investigating and delineating the boundaries of soil variabilities will necessitate using methodology(ies) that are effectual and practical for time or financial resources. And, probably most importantly, the method(s) must provide consistent, reliable data.

Methods Used to Delineate Soil Boundaries

Various methods have been tried to delineate soil types within a particular site in effort to select sample points and, in turn, to provide management advice on various soil–related topics to clientele for their farming operations. The key is to employ methods that can delineate soil variability with sufficient accuracy for use in precision agriculture, and at a competitive cost.

Various methods have been used to vet and delineate soil changes. For example, remote aerial sensing (infra-red, NDVI, LIDAR, Gamma-ray spectrometry, or other imagery techniques) delineate differences in soil topography, or soil surface colors, or differences in vegetation types, populations or vigor. In turn, the observed imagery from these methods are assumed to delineate (and map) changes in soils. However, it can be very misleading (even disadvantageous) to assume that the displayed variability in surficial soil colors, topography or vegetation can correctly or effectively delineate boundaries of soil variability. Variability in aerial imagery features can be due to a number of factors not necessarily related to changes in soil types. “Accurate soil maps cannot be produced solely by interpretation of aerial photographs. Time and place influence the clues visible on the photographs. Human activities have changed patterns of vegetation and confounded their relationships to soil patterns. The clues must be correlated with soil attributes and verified in the field”. Soil Mapping Concepts, Soil Science Division Staff, Kenneth Scheffe and Shawn McVey, USDA-NRCS.

Another method to identify soil variability includes using soil surveys by the USDA and / or Natural Resources Conservation Service (NRCS). These soil surveys are generally easily accessible and can provide some broad information on soil boundaries as well as soil chemistry, physical characteristics, etc. on the soils with in the survey area. Soil surveys can provide preliminary information for government agencies and others on regional land use planning matters, and can be a guide to develop more specific soil investigations. However, in general, the soil variability demarcations mapped in the surveys are too broad to support for an effective PA program.

A third method used to delineate soil variability is ‘grid’ pattern sampling, which consists of field sampling soils in a systematic pattern of equally spaced lines or cells. However, this method requires numerous samples points within an area, and is generally labor intensive, time consuming and may not provide adequate soil boundary definition to produce sound data for effective PA or management decisions.

Fortunately, through several years of research and development, field data verification, and use in a myriad of field conditions, Coastal Viticulture Consultants (CVC) has developed high confidence in a soil delineation methodology using soil resistivity. Soil resistivity is a measure of the soil’s resistance to carrying an electrical current. Resistivity is the inverse of conductance; the ability to carry an electrical current. The equipment used in the mapping consists of a GPS receiver, data logger, and an array of sensors. The sensor array has direct contact with the soil’s surface and is towed by an ATV back and forth in a pattern of mostly parallel lines across the study area, or down the ‘tractor row’ of an existing vineyard, or orchard. (See Figure 1.)

The recorded GPS and sensor data is filtered and processed using software programs. The result is a geo-referenced soil resistivity map (See Figure 2) that both identifies and locates soil variability within the study area, to a depth of approximately 1.9 meters (or 6 feet) and laterally within a couple of meters. Particularly note, in Figure 2, the soil variabilities within just a few acres and within the soil unit boundaries (colored with purple lines) mapped by the NRCS.

Figure 2: Soil Resistivity Map. Note the occurrence of soil variations within these fields and the NRCS mapped soil units.

In general, the lowest soil resistivity readings (reddish colors) reflect clay loam or clay-textured soils and/or wetter soil conditions. The more moderate resistivity readings (yellows / orange colors) typically represent loam-textured soils. The highest readings (blue in color; most resistive) are usually sandier or rocky soil conditions. Note that soil resistivity maps do not thoroughly evaluate soil properties (textures, compaction, total salts, fertility status, etc.). The maps show where the soil variability is located that is very important to identify sampling / data collection points in evaluating the soils. Again, understanding soils, which will be further discussed in future articles, is an integral part of PA and effectual management decisions. Parenthetically, particularly in wine grape vineyards, soil resistivity will also delineate soils derived from different parent materials (sandstone, shale, metamorphic rock, etc.), which can have a profound impact on vineyard performance, management decisions, and the flavors imparted to the fruit and wine.

As a side note, electromagnetic technologies (different from resistivity technologies) which are also used for soil mapping, are prone to significant interference issues (from metallic infrastructure) in existing vineyards with a tractor row that is 3 meters (+/-10 feet) wide or less. This can be a serious limitation when soil mapping in existing vineyards. In these situations using soil resistivity typically provides more reliable data.

Taking It Further – The Best Is Yet to Come

Once you have reasonably defined the soil changes within your site, the next steps are to procure a better understanding of the soil’s characteristics (i.e., chemistry and physical) with the (soil) boundaries you have delineated. This information is critical to PA and better provides better implementation of PA and other management actions. These matters will be discussed in subsequent articles.