Item 1: The validity of traditional plant tissue analyses for California lettuce and cauliflower is questionable.
Item 2: Conventional fertility programs for these crops are based more on habit than need and lack guidance from soil tests.
Tim Hartz, Cooperative Extension vegetable crops specialist at the University of California, Davis, built a case for these two “take home” statements during the recent Fertilizer Research and Education Program (FREP) conference in Salinas.
Hartz reached his conclusions after surveys in more than 100 commercial lettuce and cauliflower fields in Salinas and Santa Maria production areas from the spring of 2004 through the fall of 2005.
A main object of the project was to make a comprehensive review of tissue analysis for the two crops to revise currently suggested “sufficiency” levels (as given in fertility reference publications).
It also was designed to quantify the effects of environmental factors and re-evaluate sampling and handling techniques.
His research is only the latest of a series of investigations that point to shortcomings of tissue analysis as a viable management practice.
Collectively, he reported, the earlier studies found three things: “a) poor correlation between tissue nutrient concentration and concurrently measured soil nutrient availability; b) a high degree of variability in tissue nutrient concentration in adequately fertilized crops from different fields; and c) unrealistically high nutrient sufficiency standards for several crops.”
For example, he measured the midrib concentration of NO3-N at the same time as the concentration of NO3-N in the top foot depth of soil and found no useful correlation values for either crop.
Hartz went on to say the studies challenge “the practical value of tissue analysis and interpretation as currently performed. Commercial use of a flawed technique could result in excessive fertilization in some fields and possibly yield loss in others.”
He also said, “While tissue nutrient monitoring can detect potentially yield-limiting nutrient deficiencies, the value of tissue testing as a fertilizer management technique is questionable.”
For his trials, he obtained from cooperating growers the variety, planting and harvesting dates, seasonal fertilizer rates, and commercial yield of each field.
Growers rated crop quality as good, fair, or poor. Any fields that did not reflect the productivity of the crop due to a poor market, pest pressure, or other reasons were deleted from the data.
The numbers were processed by a system to compare nutrient concentration differences between high- and low-yielding crops. Then an optimum tissue nutrient concentration range was developed for each nutrient.
“These optimum ranges,” Hartz said, “should be more widely applicable than current tissue concentration guidelines, which in most cases were empirically derived from just a few fertilizer trials.”
Noting the ranges will be the first systemically developed micronutrient guidelines available for these crops in California, he said “they should be applicable to salad crops other than lettuce and cauliflower.”
Pending computation of results from 2005 season data, Hartz said some observations were made from the 2004 results.
“Head lettuce and romaine have very similar whole leaf macronutrient concentration ranges; however, romaine appeared to have lower midrib NO3-N and PO4-P values. Compared to lettuce, cauliflower had marginally higher whole leaf N and P, and substantially lower K.
“These typical nutrient concentration ranges from our 2004 sampling differed considerably for some nutrients and some growth stages from the sufficiency ranges given in the reference publications,” he said.
Most obvious for major nutrients were the relatively low whole leaf K, and the low midrib NO3-N and PO4-P found for lettuce.
“Regarding micronutrients, the most obvious difference between the reference sufficiency ranges and the values from our survey was for calcium. For lettuce the high end of our typical ranges was below the low threshold of both references. For all other micronutrients our ranges fell close to the reference values.”
The project's soil analysis for 2004 pointed to a wide range of physiochemical and nutrient characteristics, Hartz said. Soil micronutrients were plentiful with only the lowest fields approaching established agronomic threshold values for Zn and Fe.
High content of P and K in the soil, however, revealed the long-term effect of vegetable crop fertilization. “The very high P and K levels now observed in some fields suggest that many growers continue to ignore soil testing in the formulation of their fertility plans.”
Hartz said some fields having at least five times the agronomic threshold for lettuce were still receiving K fertilization.
“This,” he said, “not only represents a waste of money, but with regard to P it also represents a water-quality hazard.”
As a result of his research, Hartz suggested a few best management practices to reduce nutrient inputs and to reduce off-site nutrient movements.
They include using a realistic template for crop N requirements, basing P fertilizer applications on a soil test for P, and monitoring in-season nutrient status.
Twenty lettuce fields in the Salinas area having equal yields had various rates of N, from 180 to 325 pounds, applied, while others got less than 160 pounds.
‘Founded in habit’
This led Hartz to say grower nitrogen practices “are not founded in need but in habit, and growers should think in terms of applying 150 pounds rather than 200 to 250 pounds.”
His survey for P showed fields having an average preplant soil content of 60 to 75 ppm. However, he added, crop response to P is minimal once 50 to 55 ppm is available in the soil.
In 25 lettuce fields in the two survey areas, use of a pre-sidedressing soil nitrate test in most of them enabled skipping the first sidedressing for application of 40 percent less N without loss in yield or quality.
Another practice, Hartz added, would be to convert to drip irrigation to reduce tailwater and limit nutrient transport from the field.