Nitrogen BMPs for horticultural crop production Tim Hartz UC Davis
Historically, agronomic N fertilization has been practiced : whatever is necessary to guarantee the ideal crop
However, in the not too distant future environmental water quality regulation may require strategic N fertilization and irrigation management
Basic nitrogen management principle : N applied to a field but not removed in harvested products is at risk of eventually leaving the field in surface water or groundwater Therefore, water quality protection requires reasonable proportionality between N application and harvest removal (across a crop rotation)
Current N fertilization rates may have to be modified: seasonal N application Typical range (lb N / acre) Total crop N uptake N removal in harvest Lettuce 120-200 110-140 60-80 Broccoli 150-250 200-300 60-90 Celery 200-300 180-240 120-160 Spinach 120-180 80-110 60-80 Strawberry 160-260 180-200 70-100
Can N fertilization rates be reduced without loss of crop productivity? Be realistic in estimating crop N fertilizer requirements Adjust N application to fit field-specific needs Understand crop N uptake pattern, and supply N just in time Control irrigation efficiently Use monitoring tools appropriately
What is a reasonable N fertilization template? Growers have widely different fertility practices Summer lettuce Strawberry
Since N deficiency is uncommon in commercial fields seasonal N rates on the lower end of the typical range are adequate under most field conditions
Once you decide on a general crop N template, what factors justify field-specific adjustment? Soil contribution - soil organic matter content, crop residue or organic amendments
Contribution of soil N mineralization: Between 5-6% of soil organic matter is organic N You can count on at least 1-2% of soil organic N content to be mineralized in a vegetable crop season Example: Top 8 inches of soil weighs 2,500,000 lb/acre 1,400 lb organic N per % organic matter 20-25 lb N/acre per % soil organic matter
Contribution of prior crop residue: lettuce celery broccoli Typical residue N content (lb/acre) 70 80 200 Typical residue %N 2.5-3.5 2.5-3.5 3.0-4.0 2013 residue N mineralization trial
Once you decide on a general N template, what factors justify field-specific adjustment? - Residual soil nitrate In coastal vegetable crop production, the greatest opportunity to reduce overall N application is presidedress soil nitrate testing (PSNT)
Why is PSNT so important? It integrates the main factors influencing soil N mineralization (rapid soil organic matter N mineralization, and from prior crop residue and organic amendments, has already taken place) The measurement is taken after crop establishment, when additional leaching should be limited (at least with drip irrigation)
What is the common range of pre-sidedress soil NO 3 -N in vegetable fields? Survey of 50 lettuce and cauliflower fields, Salinas and Santa Maria : PPM x 4 = approximate pounds of NO 3 -N per acre, top foot of soil
Once you decide on a general N template, what factors justify field-specific adjustment? Irrigation water NO 3 -N PPM NO 3 -N x 0.23 = lb NO 3 -N per acre. inch But how efficiently is this N taken up by the crop?
2013 irrigation water NO 3 -N uptake efficiency trial: Step 1 - lower residual soil NO 3 -N by cover cropping Step 2 - grow a lettuce crop using only different levels of irrigation water NO 3 -N (10, 20, 40 PPM); measure yield and crop N uptake
Results: When irrigation was controlled to match crop evapotranspiration (ET c ), plant uptake of irrigation water NO 3 -N was nearly 100% How can that be? Every drop of irrigation water transpired by the crop brought the NO 3 -N to the root surface
So, how to account for irrigation water NO 3 -N? Calculate the fertilizer credit as the NO 3 -N content in the water transpired by the crop inches of ET c x PPM NO 3 -N x 0.23 = lb N fertilizer credit
Apply N on a just in time basis Crop biomass N (lb/acre) 300 200 100 0 celery lettuce 0 500 1,000 1,500 2,000 2,500 3,000 Growing Degree Days Establishment phase (lasts 2-5 weeks) < 1 lb N/acre/day Rapid N uptake phase (lasts until harvest) 3-5 lb N/acre/day, higher for Brassica crops and spinach
Strawberry N uptake is much slower: About 1 lb N/acre/day from spring summer Santa Maria Watsonville
Irrigation management can make or break fertilizer N efficiency
Leaching can carry large amounts of NO 3 -N : Salinas Valley tile drain monitoring: PPM NO 3 -N lb NO 3 -N / acre. inch 40-60 9-14 80-100 18-23
Plant N monitoring options : Leaf total N - reflects overall crop N status Petiole NO 3 -N - shows amount of NO 3 -N not yet incorporated into plant organic compounds
Leaf total N monitoring : provides a reliable indicator of overall plant N status changes relatively slowly over time is poorly correlated with soil NO 3 -N availability early in the season, so has limited value in estimate future fertilization requirement 2009-10 lettuce field trials, cupping stage
Petiole NO 3 -N monitoring : Rate of conversion of NO 3 -N to organic N compounds is affected by environmental factors, and therefore is not well correlated with current soil NO 3 -N availability 2009-10 lettuce field trials
How variable is petiole NO 3 -N over time? Six sprinkler-irrigated broccoli and cauliflower fields, sampled every 2 days over a sprinkler irrigation cycle : Petiole NO 3 -N (PPM) 25,000 20,000 15,000 10,000 5,000 0 2 day 4 day 6 day 1 2 3 4 5 6 Field
How variable is petiole NO 3 -N across strawberry fields? Graphs give leaf N and petiole NO 3 -N for high-yield, nutritionally balanced strawberry fields in a 2010-11 survey Bars indicate standard deviation
Bottom line on plant testing : whole leaf sampling gives a good snapshot of current crop N status but, if it is in the adequate range, does not project forward very far to predict if or when additional N application will be required maintaining high petiole NO 3 -N throughout the season should ensure crop nitrogen sufficiency; however, because environmental conditions can cause low petiole NO 3 -N even when soil NO 3 -N is adequate, using petiole analysis to determine fertigation requirements often leads to unnecessary fertilization
In summary : strategic N management will become essential in the future strategies exist to substantially reduce environmental N loading, but these strategies will require substantial modification of current practices, and more management time for implementation