The rules for nitrogen management have changed dramatically in California.

Old school N management for decades was basically making sure a crop did not stress for N. If you thought it would run short, you’d call the fertilizer salesman to bring more out.

Growers have long soil or tissue-sampled for N, but even with that all they wanted for years was a number from the lab that told them how much they needed to make sure they did not run out.

With growing nitrate monitoring in subsurface and surface water as part of water quality regulations, the line between too much and too little has become much more finite. Labs are asking for far more information these days to make sure the recommendations they issue fit, as closely as possible, the need and timing of the crop without any going to waste or harming the environment.

Don’t expect that recommendation to be one-number-fits-all advice, according to Keith Backman, consultant/manager for Dellavalle Laboratory, Fresno, Calif.

Might as well ask him how much gas it would take to drive from Paso Robles, Calif., to Los Angeles. That would depend on whether you are riding a motorcycle or driving an 18-wheeler and a lot more information.

A sample is only a start in getting a recommendation from Backman or his peers. The more information, the better, according to Backman. There are some general guidelines to point a farmer or consultant into a general direction of a plant’s nitrogen needs, Backman told about 250 people last fall at the Sustainable Ag Expo in Paso Robles, Calif., sponsored by the Central Coast Vineyard Team.

Backman, a Certified Horticulturist/Pomologist and Certified Crop Advisor, says it is a sure bet that vineyards struggling to make late season sugar at the end of last fall are likely to be short on nitrogen at bud break this season.

He commented in his presentation on nitrogen management that red grapes likely went into dormancy last fall short on N for next spring because they were held on the vine in an attempt to reach desirable sugar levels for wineries. Earlier harvested white wine grapes, he added, likely will be better off this spring.

While he offered those guidelines for grape growers, he did not make a specific recommendation and he won’t unless he has a lot more information about where the nitrogen is going; what form of fertilzer is used and how it will be supplied. That’s for starters.

Backman was on the program largely because of the growing problems of nitrates in surface and groundwater.

Nitrate pollution

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.

Soil sampling

For soil sampling, Backman recommends:

  • Soil probe or shovel; however, backhoe, soil auger or posthole digger may be used.
  • Clean moisture-proof quart-size bags. Use paper bags only if soil is very dry.
  • Clean plastic bucket to mix samples, optional.
  • Samples collected for microbiology, nematode and pesticide residue analysis generally need refrigeration, so have a cooler and coolant available.
  • Walk a zigzag course around or through the sampling area taking 20 to 30 cores, enough to make a quart of soil. Stay away from field edges.

— For furrow or field crops, take soil 12 inches below the surface or to the plowing depth.

— For tree and vine crops, soil samples should be taken one-foot increments to the depth of rooting which may require three to five samples from the same area.

  • Collect one quart of soil and put in a clean quart-size bag.
  • If collecting soils from multiple sites, composite by mixing in clean plastic bucket and taking one quart per sample.
  • Clearly print on each bag using an indelible marker or ballpoint pen your name/company, site and sample description.

— Complete work request form. Print clearly and return it with your sample.

Backman prefers a tissue sample on permanent crops. With sufficient information, professionals like Backman can provide nutrient application recommendations. Picking fertilizer is important in system used, he added.

He said “80 percent” of the nutrient problems he encounters are water related. Nitrogen is very mobile in water and how and when a crop is irrigated has a lot to do with where nitrogen ends up.

Weather also plays a key role in when a crop is irrigated and its response to N. In a wet spring year a grape growers may wait until June to irrigate; in a dry year, it may be April when the first irrigation is applied. If a vineyard is irrigated too early in a wet year, N will move below the root zone.

Nitrogen management should focus on a quick, uniform bud break in grapes with slow foliage growth through the growing season with enough N to finish the season, promoting sugar and color. There should be enough N in the vines going into dormancy to promote strong spring growth the following year.

Backman encouraged growers to utilize crop consultants to monitor plants and keep good year-to-year records, taking tissue samples at specific points within a season.

“You cannot step into a river twice. Agriculture is always changing,” he said.

Be “proactive” in improving yield and quality while managing the environment above and below the ground, he concluded.