Western Nutrient Management Conference Salt Lake City, UT Mar. 8-9, 2007

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1 Soil Testing to Improve Phosphorus Management in Intensive Vegetable Production Systems of Hawaii J. Deenik, S. Fukuda, R. Hamasaki, R. Shimabuko, R. Uchida Western Nutrient Management Conference Salt Lake City, UT Mar. 8-9, 2007

2 Outline Problem Project objectives/methods Results of field trials, head cabbage (2005) Basis for revising current soil P critical levels Alternative fertilizer strategies

3 Problem Phosphorus tends to accumulate in cultivated soils Many states show majority of soil samples testing high in P

4 Soil P high in Vegetable Farms in Hawaii 65% of soil samples sent to the UH testing lab in 2003 tested high in P

5 Extractable P > 700 ppm (MT)

6 Applying P to soils high in P is a waste of resources Adding P fertilizers to soils high in P reduce farmer profitability Soils high in P pose a threat to surface water quality Extractable P > 700 ppm

7 Objectives 1. Test whether crops grown on soils testing high in P respond to additional P fertilization. 2. Evaluate the suitability of the current soil P critical levels used by the University. 3. Increase farmer use of soil testing for efficient fertilizer use.

8 Cooperators 1. Oahu (smectitic, neutral soils) 2. Maui (oxic aridisol and neutral ash soils) 3. Big Island (neutral ash soil) Photos: J.L. Deenik

9 Grid soil sampling Approach Replicated field trials comparing farmer practice with 1 or more alternative fertilizer practices. 1. Farmer practice 2. Urea 3. Urea + Starter P Randomized complete blocks with 3 replicates Photos: J.L. Deenik

10 Baseline nutrient status in cabbage fields Farm N ph P P Sufficiency Range ppm (190) (22) (4.6) (10) (95) (3.4) (48) (19) (86) (36) (7.5) (1.6) (2.1) 10-15

11 Treatments at Selected Field Trials Farm Treatments P Rate N Rate Crop FP N+P lb acre -1 2 FP, N+P, N Cabbage 5 FP, N 68 NA 246 Cabbage 7 FP, N+P, N Cabbage 8 FP, N+P, N Bean 9 FP, N+K, N Onion

12 Head Cabbage Yields on Ash Soils Farm 2 Farm 5 Fresh Weight (lb/acre) No significant difference between N alone and NPK treatments

13 Head Cabbage Yields on Oxic Aridisol Farm 3a Farm 3b Fresh Weight (lb/acre) Initial P = 775 ppm Initial P = 314 ppm No significant difference between N alone and NPK treatments when soil P = 775 ppm but a significant response to P when soil P = 314 ppm

14 Head cabbage yield response to increasing rates of P fertilizer on an ash soil in Waimea Fresh Weight (lb/acre) Farmer rate P Added (lb/acre) 35

15 Treatment Effects on Oxic Aridisol Bush Bean Bulb Onion Fresh Bean Weight (lb acre -1 ) Bulb Yield (lb acre -1 ) FP Urea Urea + P 0 Urea FP N+K

16 New Soil P Critical Levels Needed Preliminary Results Head Cabbage Romaine Lettuce Fresh Wt (lb acre -1 ) Critical Level: 410 ppm Critical Level: 468 ppm Modified Truogg Extractable P (mg kg -1 ) Modified Truog Extractable P (ppm)

17 Potential Fertilizer Cost Savings 800 $772 FP Urea $ $ per acre 400 $ $197 $221 $151 0 Farm 2 Farm 7 Farm 5

18 In soils testing high in P, farmers can significantly reduce fertilizer costs, by either applying N alone or blends low in P Fertilizer Blend Quantity P Added Cost lb per acre $ per acre , , , , ,

19 Current Trials Lettuce and Chinese cabbage P trials. Evaluating Olsen P extractant as the standard for all Hawaii soils. Extension and outreach to assist more farmers reduce P fertilizer use. Photos: J.L. Deenik

20 Current Work Lettuce and Chinese cabbage P trials. Evaluating Olsen P extractant as the standard for all Hawaii soils. Extension and outreach to assist more farmers reduce P fertilizer use. Photos: J.L. Deenik

21 Summary In fields where soil P > 700 ppm cabbage showed no response to added P. Cabbage grown in soils with soil P < 400 ppm showed a response to added P. Revised soil P critical range is ppm Farmers can increase profitability and good stewardship by decreasing/omitting P fertilizers in soils high in P Farmers are beginning to explore fertilizer options

22 Acknowledgements NRCS, Cooperative Agreement 68-3A , Modification 2 R. Hamasaki (CES, Waimea), R. Shimabuko (CES, Kahului), and S. Fukuda (CES, Wahiawa) R. Uchida, ADSC, UH Manoa All the farmers on Oahu, Maui, Hawaii islands