Nitrate is the most common chemical contaminant in the world's groundwater, including in aquifers used for drinking water supply. Much of California's water supply is polluted with nitrates. One of the sources of this pollution is farming.

California is in the throes of creating a massive regulatory system that will mandate each farm adopt a fertilizer management plan. California’s Central Coast is the focal point of this process and regulators there are calling for Draconian nutrient management regulations.

Backman said N is the most problematic of the basic crop nutrients.

When too much N is applied, it goes below the plant’s root zone and eventually into the water table. “Once it is past the root zone, it is too late and out of reach of the root zone,” Backman said.

It may take it 5 years or 200 years, but it will eventually reach the water table, he noted.

To avoid this, professionals like Backman need more information than ever to make an N recommendation.

This starts with a uniform sampling of an entire field, divided into areas of 10 or 40 acres (or any appropriate size) based on the following:

  • Soil type, texture or color
  • Topography, slope, changes in slope, cuts and fills
  • Variation in crop history
  • Plant variety, rootstock, age, harvest quality, irrigation source or leaf coloration
  • Good areas vs. poor areas
  • Seasonal weather conditions
  • Water source and irrigation system
  • Is there a cover crop?
  • Are the grapes table, raisins or wine grapes?
  • Trellising system and pruning system
  • How nutrients are applied
  • Soil structure issues

Non-uniform fields should be sampled by taking a composite sample from areas with the same characteristics.

Equipment

  • Paper or plastic bags can be used. Care should be taken not to allow the sample to dry or mold. Be sure to keep plastic bagged samples cold en route to laboratory. Deliver to the laboratory as soon as possible.
  • Most samples can be gathered by hand, although hand clippers may be useful in some cases. Sampler should have clean hands. Do not use borax-containing hand soaps.
  • Samples collected for microbiology, pathogens and pesticide analyses generally need refrigeration. Have a cooler and coolant available.

Procedure

  • Select proper plant part and sampling time; timing is important.
  • Collect 50-80 petioles or leaves per sample.
  • Leaves: Collect only fully-expanded mature leaves.
  • Petioles: Take only petioles, removing the leaf blade in the field. (It only takes a few minutes for blades to absorb nutrients from petioles.)
  • Clearly print on each bag using an indelible marker or ballpoint pen your name/company, location (site) and sample description.
  • Complete work request form and return it with your sample.