- Author: Mercy Olmstead
- Author: Michael Cahn
Join us for the 2025 Irrigation and Nutrient Management Workshop and Cover Crop Field Day!
DAY/DATE: | Wednesday, February 19, 2025 |
TIME: |
7:55 am to 3:00 pm |
LOCATION: |
Agriculture Conference Room, 1432 Abbott St Salinas CA 93901 |
Free Workshop + Pizza
We are excited to invite you to our UCCE Annual Irrigation and Nutrient Management Meeting, designed to help growers like you optimize your crop production and sustainability practices for the upcoming season.
Whether you're a seasoned grower or new to irrigation and nutrient management, this meeting offers valuable insights and practical solutions for all. You'll also have the chance to network with fellow growers and industry experts.
Agenda:
7:30 AM |
Sign-in and Refreshments |
7:55 AM |
Introduction/Welcome |
8:00 AM |
Using Sudangrass as a Low Biomass Winter Cover Crop for Controlling Runoff and Erosion in Vegetable Systems |
8:25 AM |
Validating OpenET Satellite Measurements of Water Use of Broccoli and Lettuce |
8:50 AM |
Monterey County Water Resource Agency's Groundwater Monitoring Program: Data Collection and Reporting |
9:15 AM |
SGMA Perspectives on Water Use Efficiency |
9:40 AM |
Water and Nitrogen Management Field Trials in Broccoli |
10:05 AM |
BREAK |
10:20 AM |
Cover Crop Strategies for Complying with Ag Order 4.0 A-R targets |
10:45 AM |
Ag Order Progress and Requirements for the Upcoming Year |
11:10 AM |
UCANR Nitrogen and Irrigation Initiative: Helping Growers By Providing Technical Assistance |
11:25 AM |
Grower Panel: Practical Experiences in Using Cover Crops in Vegetable Systems |
12:10 PM |
Lunch - Enjoy some free pizza and salad! |
1:30 PM |
Cover Crop Field Day (Travel to USDA-ARS Spence Farm) |
3:00 PM | ADJOURN |
CCA continuing education credits have been requested.
For more information, contact Michael Cahn @ 831-759-7377, email: mdcahn@ucdavis.edu
/span>- Author: Richard Smith and Stuart Pettygrove, Farm Advisor, Monterey County and Soil Specialist, UC Davis
Potassium is a critical nutrient in vegetable production in the Salinas Valley. In nearly all key vegetable crops that are produced here, the amount of potassium removed in the harvested crop is similar to or exceeds that of nitrogen (Table 1). Potassium dynamics in the soil are distinctly different from nitrogen, and the need for it can be assessed by a soil test. Ammonium acetate extraction is the most common technique for assessing potassium availability in the soil. Some laboratories use the Mehlich-3 test, which gives numerically a similar result to the ammonium acetate method. In general crops growing in soils with test values above 200 ppm do not respond to potassium fertilization (Table 2). In general, the decomposed granite soils on the eastside tend to have the lowest potassium levels in the Salinas Valley, while clay loams and clays tend to have higher values (Table 3). However, due to fertilization practices there are some exceptions noted in the 2010 survey of Central Coast soils.
However, there are some details about potassium that need to be kept in mind; the standard soil tests for K (ammonium acetate and Mehlich-3 analytical methods) provide an index of available potassium and not a quantitative measure of soil potassium content. Other cations in the soil solution (calcium, magnesium, sodium, ammonium) can compete for plant uptake, and therefore it can be helpful to look at the percent potassium of exchangeable soil cations. Cation competition with potassium is not a problem if potassium makes up more than 3% of exchangeable cations. In soils where potassium makes up less than 2% of exchangeable cations, potassium uptake may be restricted. Table 3 shows examples of soils with relatively high potassium levels; in some of these soils potassium is relatively low as a percent of exchangeable cations and may be of concern.
Potassium uptake by plants is affected by the density of the root system of the plant. Factors that affect root development affect potassium uptake: rooting depth, irrigation system (drip vs sprinkler), soil compaction and root disease. In general, any production practice or unfavorable soil condition that reduces rooting density will reduce potassium uptake. An advantage of drip irrigation over sprinkler is the ability to deliver a concentrated dose of potassium by fertigation at the time of the season when demand is highest.
Certain soils have a tendency to fix potassium and make it unavailable or only slowly available to plants. Soils that contain large amounts of the silicate mineral vermiculite have the ability to trap the potassium ions between layers of the crystalline structure of the mineral, thereby making it unavailable to plants. Vermiculite is generally associated with soils derived from granite, but the type of granite affects the levels of vermiculite. In addition, the age of the alluvium and the degree of weathering impacts the quantity of vermiculite in the soil. Very young soils may not have had sufficient time for all the parent material potassium (in mica and hydrous mica) to weather out of it, and will not have a significant K fixation potential. On the other hand, in very old soils, the vermiculite may have weathered to a different type of clay (smectite) which does not have the proper crystalline structure to fix potassium. Soils with high levels of potassium (e.g., > 200 ppm) are likely low in vermiculite and have little or no potential to fix potassium.
Potassium fixation potential has been identified in some soils used for cotton and other crops on the east side of the San Joaquin Valley but has not been examined in other parts of California. In a recent survey of a small number of soil samples from Salinas Valley vegetable field, none were found to fix potassium in a UC Davis laboratory test for this. The lack of fixation capacity in Salinas Valley soils may reflect the impact of crop production and past fertilization practices. Crop production may affect the weathering of vermiculite on the surface layer of soil. Fertilization practices over the years result in higher soil potassium levels which can saturate potassium fixing sites in the soil minerals.
Potassium is also capable of leaching from soils. We conducted an evaluation of leaching during winter storms on a sandy loam soil on the eastside of the Salinas Valley. The amount of anions and cations leached was measured using suction lysimeters placed two feet deep in the soil; in addition, we measured the quantity of moisture moving through the soil during each rain event. The data indicates that there was movement of potassium and other ions deeper in the soil profile (Table 4). It is interesting to note that, the relative loss of potassium was much less than other cations. Potassium is bound to the negatively charged sites on clay and soil organic matter and losses of potassium due to leaching are generally assumed to be low rich in these materials.
In general, there appears to be robust levels of potassium in some Salinas Valley soils. This is particularly true for heavier soils. However, crop removal by many Salinas Valley crops is also quite robust. Fertilization with potassium does not have the environmental consequences that we observe with nitrogen and phosphorus and it seems prudent to utilize potassium fertilization programs that replace potassium that leaves the farm in harvested product and to use soil testing to monitor the situation.
Table 1. Nutrient content of Salinas Valley crops at harvest (lbs/acre).
Crop |
Nitrogen |
Potassium |
Lettuce |
90 – 1401 |
150 – 180 |
Broccoli |
180 – 220 |
160 – 240 |
Cauliflower |
180 – 220 |
160 – 240 |
Celery |
180 – 240 |
350 – 450 |
Spinach - clip |
60-1102 |
25-552 |
1 – higher nitrogen uptake occurs on 5-6 seedlines on 80 inch beds;
2 – these values are currently being evaluated by a research project
Table 2. Soil adequacy levels of potassium for Salinas
Valley cool season crops
Crop Response |
Potassium ppm |
|
Celery |
Other cool season vegetables |
|
Response unlikely |
>200 |
>150 |
May respond |
150 – 200 |
100-150 |
Response likely |
<150 |
<100 |
Table 3. Analyses of 14 Central Coast soils for available potassium
(ammonium acetate extraction top foot of soil) in 2010
No. |
Soil Type |
Potassium ppm |
Potassium percent of all cations |
Sand % |
Silt % |
Clay % |
1 |
Chualar Loam |
182 |
3.2 |
53 |
30 |
17 |
2 |
Metz loamy sand |
112 |
3.7 |
84 |
10 |
6 |
3 |
Metz loamy sand |
182 |
2.7 |
80 |
10 |
10 |
4 |
Gary sandy loam |
147 |
2.0 |
69 |
16 |
15 |
5 |
Cropley clay |
419 |
3.1 |
27 |
31 |
42 |
6 |
Mocho silty clay |
317 |
2.3 |
15 |
52 |
33 |
7 |
Salinas clay loam |
500 |
3.8 |
33 |
31 |
36 |
8 |
Sorrento clay loam |
424 |
3.7 |
21 |
50 |
29 |
9 |
Chualar sandy loam |
370 |
7.9 |
75 |
15 |
10 |
10 |
Clear lake clay |
496 |
3.6 |
18 |
41 |
41 |
11 |
Salinas loam |
217 |
2.6 |
35 |
41 |
24 |
12 |
Antioch sandy loam |
171 |
1.6 |
38 |
34 |
28 |
13 |
Sorrento clay loam |
346 |
3.5 |
26 |
45 |
29 |
14 |
Sorrento clay loam |
261 |
2.0 |
37 |
29 |
34 |
|
Mean |
296 |
3.3 |
44 |
31 |
25 |
Table 4. Estimate of cations and anions leached during winter
storm events (winter 2009-2010) under three cover crop treatments
Cover crop treatment |
Nutrient leached (lbs/A) |
|||||
K |
Ca |
Mg |
Na |
Cl |
SO4-S |
|
Bare Fallow |
9 |
133 |
32 |
88 |
158 |
36 |
Triticale |
18 |
216 |
55 |
178 |
275 |
60 |
Rye |
16 |
226 |
63 |
191 |
289 |
69 |
- Author: Richard Smith, Tim Hartz and Ryan Hayes Vegetable Crops and Weed Science Farm Advisor, Extension Vegetable Specialist and USDA Lettuce Breeder, respectively.
Tipburn of lettuce is a calcium-related disorder which causes the development of necrotic areas on the inner leaves of romaine and other leaf lettuce and on enclosed leaves of head lettuce (Photo 1). The necrotic areas likely develop due to a localized calcium deficiency that causes tissue collapse of the affected cells. There are two key factors that affect the development of localized calcium deficiency: 1) uptake of adequate calcium from the soil, and 2) calcium transport through the plant.
In a study conducted from 2005-2006 Tim Hartz, Michael Cahn and I showed that soil calcium levels in the lettuce growing areas of the Central Coast were optimal for plant growth. Soil calcium levels as measured in saturate paste extract was found to be the best indication of available soil calcium to the plant. The reason for this is that soil calcium levels from saturated paste extracts can be easily converted to concentration of calcium in the soil water, as well as pounds of calcium per acre in the soil water. In the two-year survey of a number of soils in the Salinas and Central Valleys, calcium concentrations in the soil water were found to average about 680 ppm; this value exceeds the calcium levels that are used in hydroponic vegetable production systems (100 – 200 ppm). Only one sandy soil in the survey had calcium levels below 200 ppm.
In the study, the effect of supplementing soil calcium with calcium thiosulfate, calcium nitrate and calcium chloride through the drip system was evaluated. Calcium from these materials was applied at the rate of 10-15 pounds per acre. No impact on tipburn symptoms was observed in these trials. To understand why this occurred, it is important to compare the amount of calcium that was applied to what was already in the soil. Calcium levels in the soil water can be converted to pounds of available calcium on a per acre basis; the amount of available calcium in the soil water in these evaluations was approximately 200 pounds/A, so the fertigated calcium supplied only a small fraction of what already existed in the soil.
Another factor in calcium nutrition of lettuce is the effectiveness of calcium transport through the plant. Calcium moves in the xylem by transpirational flow and is delivered to all parts of the plant this way. Factors that reduce transpiration also reduce calcium availability to leaf tissue and may lead to tipburn symptoms. Leaves enclosed in head lettuce and inner leaves of romaine are susceptible to tipburn because they are not transpiring as readily as leaves in the open which can transpire freely. In addition, root growth of lettuce decreases approximately two weeks prior to maturity which can further limit the supply to calcium to the transpirational stream. Various aspects of the growth pattern and physiology of lettuce varieties affect their susceptibility to tipburn (Tables 1-3). In a study conducted in 2005 and 2006, Sylvie Jenni and Ryan Hayes, showed that, in general, head lettuce types are less sensitive to tipburn than romaine types. This is primarily due to greater breeding efforts on tipburn resistance dedicated to date for head lettuce types.
An additional factor that influences the development of tipburn on lettuce is the weather. Foggy weather that reduces transpiration in the last 6-10 days before harvest is conducive to the development of tipburn in susceptible varieties.
One practice that may have a place in reducing tipburn is the use of foliar applications of calcium prior to and/or during the critical period of lettuce development. I currently have four trials out evaluating this technique. At the recommended application rates of the materials being tested, weekly applications of 0.15 to 0.20 lbs of calcium are applied. The trials are currently underway, but the question will be, can we get enough calcium into the plant at the right time? In addition, can the spray reach the tissue that may develop the symptoms, given that calcium does not readily move in the phloem of the plant?
Conclusions: Lettuce production soils of the Central Coast generally have high levels of readily available calcium. Most coastal soils have well over 200 ppm of calcium in the soil water which is adequate for optimal plant growth. The greater issue for the development of tipburn in lettuce is the variety. In addition, persistent foggy conditions that reduce transpirational flow of calcium to all parts of the leaves in the last 6-10 days prior to harvest will trigger this disorder in sensitive varieties.

Photo 1. Tipburn symptoms along the edge of an inner leaf or romaine lettuce.
Table 1. Head Lettuce: percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).
Variety |
California |
Arizona |
|||||
Salinas |
Yuma |
||||||
2005 |
2006 |
2005 |
2006 |
Mean |
|||
Calicel |
99.0 |
78.0 |
99.0 |
100.0 |
88.7 |
||
Calmar |
16.7 |
40.0 |
89.3 |
60.0 |
51.5 |
||
Cochise 47 |
43.3 |
13.3 |
79.3 |
66.7 |
50.7 |
||
Desert Spring |
40.0 |
76.7 |
89.7 |
80.0 |
71.6 |
||
Diamond |
1.0 |
36.7 |
86.0 |
13.3 |
34.3 |
||
Gabilan |
53.3 |
20.0 |
46.7 |
0.0 |
30.0 |
||
GLMesa659 |
37.0 |
40.0 |
99.0 |
33.3 |
52.3 |
||
Head Master |
10.3 |
39.6 |
76.3 |
33.3 |
39.9 |
||
Navajo |
16.7 |
66.7 |
73.0 |
20.0 |
44.1 |
||
Pacific |
1.0 |
8.1 |
49.7 |
40.0 |
24.7 |
||
Salinas |
14.0 |
46.7 |
76.3 |
13.3 |
37.6 |
||
Silverado |
7.0 |
26.7 |
66.3 |
0.0 |
25.0 |
||
Sniper |
26.7 |
26.7 |
76.3 |
26.7 |
39.1 |
||
Sundance |
30.3 |
80.0 |
69.7 |
20.0 |
50.0 |
||
Tiber |
1.0 |
53.3 |
69.7 |
13.3 |
34.3 |
||
Van 75 |
40.3 |
60.0 |
36.7 |
60.0 |
49.3 |
||
Vanmax |
43.3 |
90.0 |
79.3 |
26.7 |
59.8 |
||
Mean |
28.3 |
47.2 |
74.3 |
35.7 |
46.1 |
Table 2. Romaine: percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).
Variety |
California |
Arizona |
|||||
Salinas |
Yuma |
||||||
2005 |
2006 |
2005 |
2006 |
Mean |
|||
Avalanche |
88.9 |
86.7 |
99.0 |
46.7 |
77.7 |
||
Beretta |
53.3 |
80.0 |
79.7 |
93.3 |
76.6 |
||
Brave Heart |
70.0 |
86.7 |
76.3 |
33.3 |
66.6 |
||
Caesar |
49.7 |
80.5 |
73.0 |
9.1 |
57.2 |
||
Clemente |
37.0 |
50.0 |
59.7 |
33.3 |
45.0 |
||
Conquistador |
93.0 |
78.3 |
82.7 |
33.3 |
71.8 |
||
Darkland |
53.3 |
80.0 |
86.3 |
66.7 |
71.6 |
||
Fresh Heart |
99.0 |
46.7 |
76.3 |
66.7 |
72.2 |
||
Gladiator |
36.7 |
96.7 |
99.0 |
33.3 |
66.4 |
||
Gorilla |
53.3 |
90.0 |
86.3 |
40.0 |
67.4 |
||
Green Towers |
40.0 |
60.0 |
89.3 |
53.3 |
60.7 |
||
Heart's Delight |
56.3 |
50.0 |
92.7 |
13.3 |
53.1 |
||
King Henry |
63.3 |
80.0 |
33.3 |
26.7 |
50.8 |
||
Lobjoits |
73.0 |
86.7 |
92.7 |
20.0 |
68.1 |
||
PIC 454 |
40.0 |
84.7 |
86.3 |
26.7 |
59.4 |
||
Paris Island Cos |
66.3 |
70.2 |
73.2 |
73.3 |
71.4 |
||
Ruebens Red |
93.0 |
82.9 |
99.0 |
73.3 |
86.6 |
||
Siskyou |
16.7 |
66.7 |
66.3 |
0.0 |
37.4 |
||
Sunbelt |
10.3 |
90.0 |
96.0 |
33.3 |
57.4 |
||
Triton |
66.3 |
81.1 |
76.3 |
33.3 |
64.3 |
||
Valmaine |
76.7 |
70.0 |
80.0 |
53.3 |
70.0 |
||
Mean |
58.9 |
76.1 |
81.1 |
41.1 |
64.4 |
Table 3. Leaf Lettuce: percent of plants with tipburn tested in Salinas, California and Yuma, Arizona (adapted from Jenni and Hayes, 2010).
Variety |
California |
Arizona |
|||||
Salinas |
Yuma |
||||||
2005 |
2006 |
2005 |
2006 |
Mean |
|||
Green Leaf |
|||||||
Big Star |
1.0 |
13.3 |
82.7 |
40.0 |
34.3 |
||
Envy |
73.0 |
33.3 |
76.7 |
26.7 |
52.4 |
||
Genecorps Green |
56.7 |
50.0 |
92.7 |
93.3 |
73.2 |
||
Grand Rapids |
99.0 |
60.0 |
96.0 |
73.3 |
82.1 |
||
Green Vision |
36.7 |
66.7 |
89.3 |
80.0 |
68.2 |
||
Ocean Green |
43.3 |
40.0 |
79.7 |
53.3 |
54.1 |
||
Shining Star |
13.7 |
33.3 |
86.3 |
53.3 |
46.7 |
||
Tehama |
51.1 |
53.3 |
99.0 |
33.3 |
59.2 |
||
Two Star |
60.0 |
36.7 |
96.0 |
66.7 |
64.8 |
||
Xena |
60.0 |
75.0 |
63.3 |
6.7 |
58.0 |
||
Mean |
49.5 |
46.2 |
86.2 |
52.7 |
59.3 |
||
Red Leaf |
|||||||
Aragon Red |
63.3 |
100.0 |
99.0 |
93.3 |
88.9 |
||
Deep Red |
69.7 |
23.3 |
99.0 |
93.3 |
71.3 |
||
New Red |
59.7 |
36.7 |
79.7 |
86.7 |
65.7 |
||
Red Fox |
1.0 |
93.3 |
69.7 |
13.3 |
44.3 |
||
Red Line |
63.0 |
93.3 |
89.7 |
86.7 |
83.2 |
||
Red Tide |
20.3 |