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Managing farm fertility: Precision or pretty good?

The other day I had breakfast with an agronomist who runs the fertility program for one of the largest carrot growing companies in the world. We bounced over the familiar terrain of setting application rates for fertilizers, greenwaste compost, animal manures, gypsum and sulfur in light of a mountain of laboratory analyses from different fields. But the bulk of conversation revolved around managing the public perception of growing “clean” fresh carrots and sustaining environmental quality.

Satisfying public demand for “quality” is a mercurial blend of science and politics. Private agencies like the California Compost Quality Council are stepping forward to recommend standards. In response to industry and public concerns the California Department of Food and Ag released a report on heavy metals concentrations in commercial fertilizers in the late 1990's. Revising animal manure management regulations is the latest endeavor of the U.S. Environmental Protection Agency and the Regional Water Quality Control Board in California. Commercial agriculture, especially in California, recognizes that there is room for improvement, but can we engineer an approach to the management of soil fertility/quality as a precise science?

Dreams came true

The advance of the super-fast desktop computer, coupled with satellite technology has spawned a high tech segment of the ag service industry that was only the stuff of research dreams 10 years ago. Using satellite global positioning systems (GPS), aerial imagery, ground sampling and sophisticated computer databases (GIS), large-scale production acreage can now be mapped for a variety of nutrients, soil types, crop characteristics and yield. This information is usually overlaid and woven together to come up with a precise fertilizer prescription that applies only the actual nutrients required for each acre of the field for the proposed crop; hence, the name “precision agriculture.”

And there's the rub! Crop root systems, field soils, and season-long weather and pests are anything but precise. When subsurface drip irrigation (SDI) made its debut in cotton and tomatoes in the San Joaquin Valley in the mid 1980s, there were a couple trials yielding 60-plus tons per acre of processing tomatoes and 4 bales per acre cotton using precise control of water and spoon feeding various fertilizers. However, consistent, production-scale results with this technology has not met these levels and, because of the cost of the systems in the 1990s, was often less profitable than furrow irrigation. This is now changing as water and energy costs climb and drip tape system prices have declined.

We know that soil organic matter as humus is important to crop growth. But corn fields in the Sacramento Valley on mineral soils with less than 1 percent organic matter can yield 12,000 pounds per acre grain, while about the same grain yields come off of Delta soils 100 miles to the southwest with 4-6 percent organic matter.

Three years of greenwaste compost trials in Kern County, with up to 20 tons per acre per year, gave a slight yield increase in one sandy field, but was not at all cost effective. Laboratory and “microplot” studies at UC Davis found mineralization of organic nitrogen (conversion to a mineral form that plants can use) varied from 10 percent to 29 percent, for a variety of materials like greenwaste, different composts, dairy manure and biosolids. Production scale biosolids field trials in Kern County in cotton and wheat found this rate to be as low as 3.9 percent to a high of 84.8 percent in a single year, with an average of 42.3 percent over seven tests. Field studies near Riverside, Calif., in the early ‘80s indicated 45 percent or more.

The point is that plain, good ol' dirt still has a way of humbling the best minds in the world. One of my favorite illustrations came from a 1994 soil microbiology workshop where Howard Ferris, nematology professor at UC Davis, described the complexity of the soil biotic community with a simple image: “The combined weight of all the soil organisms in an acre of actively growing crop rootzone is equal to about 49 sheep. They all have to eat something, but there's no way we can begin to tell you what they're all doing!”

The quality and source of fertilizers along with managing ercycled organic matter amendments such as urban green waste, biosolids and dairy manure are crucial issues to 21st century agriculture. There is no doubt that we must improve regional management of these materials to avoid further pollution of our groundwater resources. But does our current technology and scientific knowledge base mean that we're ready to jump from ‘pretty good’ to “precision?” You be the judge.

As we ponder new regulations, and the cost to the grower and support industry of implementing those regulations, let's make sure we drive those decisions with science, sound economics and not just public pressure.

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