June, 1999

 

ON THE ROAD TO PRECISION AGRICULTURE
Methods of Directed Sampling

Topography
Topography sampling is conducted by first measuring elevations. However, dividing the field into zones of elevation is not appropriate because hilltops may be the same elevation as depressions in rolling fields. It is the landscape structure or topographic structure that is important, not the elevation. However, it is difficult to draw lines with confidence around landscape features without other means of support. Other tools can help to define zone boundaries to direct sampling in addition to topography alone.

Soil conductivity sensors
Soil conductivity sensors can be used to identify important zones and for verifying the importance of topography zones. It is important to note that the readings do not correlate directly with nutrient levels. High conductivity may be related to high N, but low conductivity may more often be in areas of medium N content, while medium conductivity corresponded with low N levels. These devices reveal patterns and it is the responsibility of the sampling to reveal the significance of these patterns.

Aerial photography and satellite imagery
Satellite imagery can be effective in identifying fertility patterns if the patterns are larger than the resolution of the pictures. In fields where spatial patterns are small, aerial photography might be a better tool.

Soil surveys
Soil surveys in North Dakota or Illinois have not been helpful in determining management zones. Sometimes the boundaries are drawn incorrectly, other times, too many soil series are combined into one mapping unit. In all of the fields investigated by this researcher (Dr. Dave Franzen), soil series boundaries were not useful for direct sampling for precision farming.

Yield mapping 
Yield maps are the result of a number of soil and environmental factors that affect the growth and yield of crops. Yield maps appear to be most useful in defining areas of recurring patterns between years, such as areas with consistently low or no yield. However, those areas of the field that are only slightly different than other areas of the field in yield have not been useful in defining management zones. A great deal of interpretation is essential if yield maps are to be included in any management zone development.

Managing Directed Sampling
The concept of directed sampling is exciting because it usually will result in a reduction in sampling expenses, making it more practical to conduct precision farming activities in lower return crops. However, directed sampling may have additional demands on the quality of help necessary to carry out a directed sampling program.

In a directed approach, the sampler must make a decision prior to entering the field on where to define the zones. This requires some field interpretation skills and a higher level of sampler knowledge and experience than generally would be necessary in a grid program. So, although directed sampling appears less expensive on the surface, there is also attached to the system a need for more advanced employees.

Some progress is being made to automate the process of developing sampling zones. Differences in topography, conductivity, image values and/or yield levels are used to automate the zone development process. Developing zones automatically is in the beginning stages, but if perfected, could make sampling zones as easy to do as grid sampling is today.

Roger Brook

At the Precision Agriculture session during ANR week this year, Dr. Dave Franzen North Dakota State University, discussed this topic with those in attendance. This is the first of a two part summary of the handout. Copies of the handout are available to MSU Extension staff on request from: brook@msue.msu.edu