Water quality regulation and the vegetable industry

California has environmental water quality problems: Nitrate and phosphate in surface runoff causes ‘biostimulation’ - surface water in many watersheds exceed NO3-N and PO4-P goals

California has environmental water quality problems: Nitrate in drinking water considered a human health hazard - Federal standard is 10 PPM NO3-N

2008: SBX21 - mandated a report to ‘improve understanding of the causes of nitrate groundwater contamination, and identify potential remediation solutions’

2012: Report from UC Davis task force

.pdf files available at groundwaternitrate.ucdavis.edu

Nitrate in groundwater is now the hot topic …

Estimated nitrogen loading to groundwater:

Harter report suggested that a system of agricultural N use reporting would improve the estimation of a nitrogen ‘mass balance’ for impacted watersheds

Nitrogen mass balance is an appropriate way to evaluate environmental N loading

Reporting of agricultural N application is important to understanding watershed water quality trends

Nitrogen use reporting started October, 2014, in Region 3 WQCB

Basic assumption of a ‘nitrogen mass balance’ : N applied to a field but not removed in harvested products is at risk of eventually leaving the field through gaseous release (an air pollution hazard), surface runoff or leaching to groundwater

Fertilizer Organic amendments Irrigation water NO3-N

Harvested products

Volatilization Denitrification

Leaching or runoff

Volatilization losses can be significant for: Anhydrous ammonia, especially water-run Topdressed urea Animal manure

In most vegetable fields volatilization is a minor component of an N balance

Denitrification losses can be significant with: Frequent, saturating irrigation Heavy, slowly draining soils High soil nitrate level

Denitrification is typically a minor part of an N balance in the San Joaquin Valley, but may be substantial in coastal situations

What about long-term soil N storage? Short-term root zone NO3-N carryover is common Significant long-term soil N sequestration is uncommon,

and probably limited to conservation tillage management

Bottom line: 1) Within some level of uncertainty, evaluating agricultural N management on a mass

balance basis (inputs - outputs) does estimate potential environmental N loading (all forms of loss); in most vegetable cropping, leaching loss likely to dominate

Bottom line: 1) Within some level of uncertainty, evaluating agricultural N management on a mass

balance basis (inputs - outputs) does estimate potential environmental N loading (all forms of loss); in most vegetable cropping, leaching loss likely to dominate

2) Some crops will draw more regulatory scrutiny than others, based on acreage and current N practices

Bottom line: 1) Within some level of uncertainty, evaluating agricultural N management on a mass

balance basis (inputs - outputs) does estimate potential environmental N loading (all forms of loss); in most vegetable cropping, leaching loss likely to dominate

2) Some crops will draw more regulatory scrutiny than others, based on acreage and current N practices

3) At similar yield levels, a grower applying substantially more N than his neighbor is probably releasing more N to the environment over time

lb N / acre typical N

application crop N uptake

N removal in harvest

Lettuce 140-200 110-140 60-80 Broccoli 150-220 220-350 70-100 Processing tomato 180-220 220-280 140-180

Crops have different N balance profiles, therefore regulators likely to have different levels of interest:

Reliable California data also exists for cauliflower, cabbage, spinach; N balance data are insufficient for most other vegetables

Reliable N balance data requires monitoring of multiple fields, to understand field variability and yield effects

Means

253 lb N total

156 lb N fruit

97 lb N vine

So, what will have to change? ‘N use outliers’ have to be get better

Mean 56 tons/acre total fruit yield

Mean 190 lb N/acre in seasonal N application

Grower N fertilization practices vary widely, often unrelated to yield potential:

Processing tomato fields

N application – fruit N removal

Simplified tomato N balance:

more N application than removal

more N removal than application

So, what will have to change? Growers will need to make better use of non-fertilizer N - residual soil NO3-N - Irrigation water NO3-N

Currently, integrating soil residual NO3-N into N fertilizer management is not common:

Soil residual NO3-N varied from 23-219 lb/acre Seasonal N application did not reflect this difference

Data from Horwath et al., 2013

Processing tomato fields:

In most cases the majority of crop residue N mineralization occurs before the planting of the next crop

Therefore, post-planting evaluation of residual soil NO3-N picks up the majority of crop residue effects

What about ‘N credits’ for crop residues?

How to credit irrigation water NO3-N?

How to credit irrigation water NO3-N?

Fertilized control

Levels of irrigation water NO3-N

So, what will have to change? Irrigation practices must improve

Growers have radically different approaches to irrigation

Field Number1 3 5 7 9 11 13 15 17 19 21

App

lied

Wat

er (i

nche

s)

0

5

10

15

20

25

30

Total Applied WaterPre-Thinning Water

Total and Pre-thinning Water Applied to Lettuce

Data from Mike Cahn, 2009-2012

Mean ETc

average application = 176% of ETc

So, what can UC do? Get the basic concepts of N balance across to growers and consultants Steer them toward field-specific practices, and away from ‘program’ farming Help commodity groups work out their overall crop N balance profiles