Good Fruit Grower / April-15th 2010 / Managing soil ­variability

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Managing soil ­variability

Detailed soil data can help growers manage practices such as irrigation and nutrition.

Soil is one of the biggest variables a grower must manage. Traditional methods of gathering soil information lack the detailed information to allow growers to manage soil zones individually, but new technology is now available to provide farmers with actionable, cost-effective soil information.

Dr. Robert Wample is the viticulturist for Soil and Topography Information LLC, a Madison, Wisconsin-based company founded in 2002, with offices in Portland, Oregon, and Fresno, California. The company develops soil and topography mapping technology for agriculture. Before joining STI, Wample was chair of the viticulture and enology department at California State University, Fresno, and taught for two decades at ­Washington State University.

Wample shared STI’s patented soil information technology during the Washington Association of Wine Grape Growers’s annual meeting in Kennewick, Washington. “This new technology allows us to analyze and make recommendations to solve problems that, in the past, we had a great deal of difficulty even understanding what the problem was,” he said. And it can be done in a cost-­effective manner, Wample added. “The key is knowing how to use information to be part of your decision ­support system.”

STI’s Soil Information System technology collects, processes, and interprets physical and chemical soil information, providing clients with information on elevation, soil makeup, moisture and drainage availability and potential, nutrient and mineral availability, and more. The information can help target nutrient sampling and identify where to place soil moisture sensors and precisely map where soil amendments are needed. It can be applied in a variety of ways, from designing and laying out a new vineyard or orchard to matching rootstock and variety to the site and designing and laying out irrigation systems. The information can also be used in existing vineyards and orchards to manage fertility and plant nutrition, remediate compaction, and even selective ­harvesting.

Variability

During Wample’s more than 30 years as a grape researcher, soil variability was often an issue in his field trials. “Most research results were not as conclusive as I would have liked. The reason being, these were done on large-scale blocks—40, 60, 80, 120 acres—and the variability across that acreage was enough to drive you crazy,” he said. “Soil variability is always an issue in field research. There’s variability in soil type, water-holding capacity, soil depth, and such. It’s a killer.”

In wine grapes, nonuniformity in the vineyard resulting from soil variability can negatively impact fruit quality and yield, and cause management problems in other areas like irrigation, nutrition, and canopy development.

In two San Joaquin Valley, California, vineyards about 100 miles apart, Wample reported that STI used information generated from its Soil Information System technology to find a relationship between grape quality and specific soil metric data. “What we found was that out of 65 different soil characteristics, six accounted for 95 percent of the variability in both vineyards.” Relationships of anthocyanin and Brix levels were correlated to six soil characteristics through principal component analysis, a multivariate, statistical computer program that can ­analyze multiple factors, he explained.

“We’re getting closer to understanding the soil char­acteristics that are driving the parameters of the fruit ­quality attributes that we can measure,” he said.

Collecting information

Wample described the information collection process in four steps. In the first step, a dual electromagnetic conductivity device, combined with survey-grade GPS (less than one-inch accuracy), is dragged across the field to collect spatial variability and topography. These data are combined with proprietary software to identify soil ­sampling locations for further analysis.

The second step uses the “diver” to identify vertical variability. Detailed information is collected from the soil profile with a hydraulic push machine that uses four different probe-based sensors to measure compaction, frictional resistance (an indicator of sand and clay ratio), electrical resistivity (a measure of salt content), and soil moisture, all at approximately one-inch increments. The probe, which usually goes about four feet deep, can be positioned near the root zone of trees or vines.

Step three analyzes data collected from the first two steps to develop a “smart sampling protocol” for additional soil core samples, which are processed using a commercial laboratory for standard soil analysis.

The last step processes all the data using proprietary correlation and geo-processing algorithms, transforming it into more than 60 physical and chemical soil properties. Data provided to the grower includes things like soil texture, compaction, topography, potential plant-­available water, root zone moisture, root limiting moisture, drainage potential, and more. Information is also provided about nutrient-holding capacity, salinity, and fertility management.

“Once mapped, the soil physical properties are relatively constant, assuming you do not use major earth-moving equipment, and serve as a basis for future smart sampling protocols for fertilizer, and other management decisions,” Wample said.

Costs

Cost of the Soil Information System technology is economically viable, according to Wample. Though the costs for field mapping and providing recommendations are higher than traditional soil sampling and mapping, he said that the real savings occur when applying soil amendments. Amendments like lime, urea, phosphate, boron, and zinc can be applied only where they are needed, and, with the use of precision agriculture variable rate technology, in the exact amounts needed to meet specific crop requirements. Irrigation can also be tailored for the specific zones instead of applying water using the same emitter output and spacing across the entire block. This allows the grower to compensate for soils with different infiltration rates and water-holding capacities.

In a 35-acre example he used, traditional field mapping and recommendation costs were $30 per acre compared to $120 per acre using the Soil Information System technology. Soil amendments for the traditionally mapped acreage cost $410 per acre, but only $259 per acre for the Soil Information System, with $20 per acre for variable rate technology application. With the Soil Information System, Wample said the grower saved $71 per acre in reduced soil amendment and fertilizer costs, resulting in a net overall savings of $1,500 on 35 acres.  The cost of providing this information can vary from $50 to $170 per acre depending upon the location, field size, and management needs identified during consultation with the grower/manager. ­