Grapevine nutrient management is gaining far greater significance in the wake of the sharper California regulatory focus on preventing fertilizers and pesticides from getting into surface and groundwater.

California agriculture is being scrutinized like never before in the wake of new water quality monitoring rules that are expected to result in additional farm water management regulations.

There are nine regional water quality control boards in the state charged with monitoring and mitigating water quality issues.

None is more controversial than the Central Coast Regional Water Quality Control Board which is proposing draconian regulations on agriculture to regulate the use of fertilizers, particularly nitrogen.

Lowell Zelinski, owner of Precision Ag Consulting, a Paso Robles-based company, told those attending a VINE Symposium recently that some of the ideas proposed by the water board could well put agriculture out of business. Those regulations are not based on science or understanding of plant nutrient use.

Zelinski’s company sponsored the recent symposium.

Zelinski says the Water Quality Control Board is proposing 100 percent utilization of applied nitrogen in agriculture. Typically, vines efficiently utilize use 40 percent to 50 percent of the nitrogen applied. The rest can be lost through denitrification, mineral fixation, volatilization, removal at harvest with the crop and other outlets.

Zelinski noted that for the most part, nutrient use is low for grapevines. 50 pounds per acre per year is high. It is easy, therefore, to add too much fertilizer.

This is not only economically wasteful, it can lead to excessive vine growth, decreased berry set, delayed harvest, vegetative character in the grapes and reduced bud fruitfulness in subsequent years.

The key to nutrient management is nutrient budgeting, according to Zelinski, using information about the chemical, biological and physical properties of the soil’s fertility levels.

From that evolves the agronomic, economic and environmental implications of fertilizer applications.

There are hundreds of rootstock/scion combinations and each has a unique fertility profile. For example, rootstock 3309 has difficulty taking up zinc from the soil.

K use a significant issue

Use of potassium is less complex than nitrogen use, but is still a significant issue.

K uptake comes from fertilization as well as organic matter and mineral weathering. Potassium utilization can also be influenced by clay particles in the soil.

Zelinski said if K stays too long in the soil without being taken up by the vines, it becomes fixed to the clay particles. “What I like about drip irrigation is that K can be taken up more readily by the plant by applying water directly to the roots.”

Drip irrigation also plays a role in soil pH which impacts nutrient utilization. Sulfuric acid is often used to keep irrigation lines free from algae and this can dramatically change the soil pH around in the wetted emitter area compared to the rest of the field.

Zelinski said it is important to evaluate a fertilizer application based on:

It is also critical to evaluate sources of nutrients. They may not always come from applied fertilizer. They could come from bacteria in the soil, microbial fungi or even from irrigation water.

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