Situation: The complaint we were invited to evaluate in this field was less typical than what one often finds with yellow plants in Salinas or Watsonville. These patches of yellow plants were typically dispersed in patches of various sizes in the field (photo 1 below), but on one side of the field this was particularly pronounced.  This area corresponded with a farm road from a previous artichoke plantation as well as being the end of the drip tapes installed by the strawberry grower.

The bed tops tended to be dry and moisture has been adequate but not excessive, so my running thesis of excess water actually did not fit so well in this situation. 

I had the good fortune on this particular call of being accompanied by Frank Shields from Soil Control Lab, who did a thorough evaluation of a bed and plant tissue (photo 2 below) and arrived at the results as depicted in photos 3 and 4 below.

Evaluation: As readers can assess from the attached analyses, this sampling was extraordinarily detailed.  The soil analysis breaks down the bed into ten zones and evaluates no less than 14 parameters of each zone.  Furthermore, in zone 7, Frank took a general sample of the soil texture and shrinkage, which gives an assessment of at what moisture percentage cracks form in the soil.

As a standard operating procedure, Frank sampled one bed containing symptomatic plants according to a pattern of ten different areas, the cross section of which can be seen in the soil report attached below.  Additionally, Frank uprooted several plants for evaluation of the roots and tissue mineral contents in the foliage.

This soil test strives to accomplish a number of things.  First, it is designed to monitor the movement and location of accumulated soluble salts for the purpose of determining the water pattern from the irrigation system. Similarly, since during the season strawberry fertilizer is supplemented via the water it is useful to know where the nutrients are ending in the bed to confirm that they are getting to the plant and not being leached out of range or being affected by adverse pH or salt conditions.

Plant available nutrients are both water soluble and exchangeable. This analysis is designed to monitor the water soluble fraction for the water soluble plant nutrients nitrate, nitrite, chloride, and sulfate as well as cations which accumulate with the water soluble fraction like calcium, potassium, sodium and ammonia. 

Interpretation of Results:

Moisture: All zones were very wet indicating recent irrigation of sufficient volume to completely fill the bed and should leach salts and nutrients down and out of the bed.   Additionally, there were no shrinkage cracks visible when observing the fifty foot section of row again indicating that the soil had been kept sufficiently moist.

The pH is on the high side and evenly distributed between 7.8 and 8.4 though all zones.  However, this pH is not unusually high and normally would not be said to be problematic for strawberries.

Soil potassium is this sample low.   It is typical for soils in Salinas and the Pajaro Valley to have potassium concentrations above 150 ppm, but with the exception of zone 10 of the bed this particular sample ranges around 50 ppm or below.  This is reflected in the leaf potassium concentration of 0.9% when a range of 1.3 – 1.8% is ideal for this time of year.

Soil nitrate appears to be substantially leached out, and indeed the percentage in the leaves of the sampled yellow plants is 2.0%, which is somewhat less than the optimum of 2.4- 3.0 % for this time of year.   Still, 2.0 % dry weight tissue nitrogen would not explain the substantial yellowing we observe in the field.

Phosphorous, calcium, magnesium and the other micronutrients appear to be sufficient according to the attached tissue analysis.

Nitrites, which are oxidized to plant available nitrate in the process of nitrification but toxic to plants in quantity are absent from the sample.  This is quite probably because of the well aerated soil and lack of packing. 

EC5: This is a measure of the amount of water soluble components in a zone. Zone six has the highest in the root zones (1 – 8) but all within values to support plant growth. With proper watering, salts should accumulate in zone six, nine and ten. In this sample, salt accumulation is highest at zone ten typical of a pattern of a bed having full mulch and plenty of water.

We can also look at the ratio of water soluble cations to get an idea on the ratio of plant available constituents like the SAR value and soluble Ca/Mg ratio. The very high sodium and chloride compared to the low calcium and magnesium in this sample is significant and indicates a problem.   Indeed, sodium and chloride are excessively high in the plant tissue and very likely to be interfering with normal plant function.

Outside of the carbonates, the soil minerals are an accumulation both from the irrigation water and nutrients added to the soil. If most of the salts accumulated in the bed are naturally found in the irrigation water (as is the case here), it means that the nutrients are being leached out of the root zone with excess water and additional fertility could be considered.  

Conclusion:   Rather than being a problem of the deficiencies of nitrogen or potassium, Frank maintains that at issue here is a buildup in the affected areas of chloride and accumulated salts as a result of irrigation.  Carrying this thesis further, perhaps if the amount of current irrigation water could be limited in the areas experiencing yellow plants, one would also be reducing the amounts of apparently damaging sodium and chloride.  That might be one way to address the problem.

The scattershot pattern of yellow plants across the field is the major confounding point of the problem being evaluated here.  On the one hand it would indicate that the toxicities and deficiencies described above are occurring in the same scattershot pattern across the field but it does beg the question why the differences are so dramatic, sometimes even from one plant to the next.

So, while this evaluation has given us a good look at what is going on around these yellow plants, we still don't conclusively know what exactly is the cause of this problem.  It absolutely merits further work. 

Thank you to the grower who invited us out.  I thank Frank Shields of Soil Control Lab for the contribution of his time and expertise to working on this problem. 

Here is an interesting case regarding a slight purpling of the newer leaves of raspberry.  While the case below involves ‘Polka’ variety red raspberry, I’ve seen it this year on ‘Josephine’ red raspberry in a different field as well.

The question posed is whether this purpling is meaningful from a plant health standpoint.   Will this problem get worse and detract from yield and cause problems with next years crop?

For starters, there is are no disease symptoms, for example necrotic spots or goo seeping out out of the leaves or stems, nor are there any signs of disease, such as spores or conidial structures visible.

We should consider also possible side effects of insecticide or fungicide sprays.  I do recall once in a trial on the strawberry variety ‘Diamante’ that repeated applications of a strobilurin fungicide such as Pristine or Quadris resulted in a similar pattern of purpling on the leaves.  However in this case on ‘Polka’, the PCA in charge of this field confirmed with me that Pristine had been applied after the symptoms appeared, and this is only one application of the material.  There was only one other pesticide application previous to this one, and it was more than a month ago.

What about nutrient deficiency?  A simple application of what we know from nutrient deficiency books would inform us that the purpling we see here refers to some sort of phosphorous deficiency, but other nutrients can cause this too.  Furthermore, those having more than a passing knowledge of the agricultural soils in the Pajaro and Salinas valleys know they are rarely phosphorous deficient, and more often than not actually have an excess of this nutrient. 

Which brings us to nitrogen.  Nitrogen, while commonly associated with yellowing rather than purpling of leaves, leaches out of the soil easily and as such can be deficient even in the rich soils of the California central coast.   As we know, nitrogen deficiencies can be manifested in plants as a reddening or purpling of the leaves stemming from an accumulation of the same carbohydrates resulting from phosphorous deficiency.

The only way we are going to know if the above has any truth to it at all however is to take some leaf samples.  The chart below is from a bulk sample consisting of at least twenty leaves, each taken from around the fifth leaf of the plant (note that this is a bit younger than the seventh leaf common for sampling, but the purpling was only found at this stage and younger). 

Samples were analyzed by the Soil Control Lab in Watsonville. 

The chart above shows us that for one there are no dramatic differences between in nutrient concentrations of green leaves compared to those which are purple.  On the other hand, concentrations of nitrogen, phosphorous and potassium are trending just a tad low, with perhaps the nitrogen being the most significant especially since the sampled leaves were on the young side.   

Some points to ponder on a rainy few days regarding phosphorous fertility in strawberries.

Since the work described below was conducted both years in a soil of or above 80 ppm phosphorous, we can’t define where the agronomic threshold for crop response from additional phosphorous lies and subsequently can’t publish in a peer reviewed journal.  Subsequently this is not meant to be a guide for phosphorous fertility in California strawberries.  Nevertheless, since many of the soils we farm are well within the concentrations of P described, this work could be instructive to those thinking about soil fertility in strawberries.

Introduction:  There is a significant body of work in lettuce and several other vegetable crops on the California Central which finds that above a certain soil test threshold concentration there is no longer any plant response to any added phosphorous (P).  The following two year study was designed to approach the hypothesis that this also holds true for strawberries.

Materials and Methods:

Year 1 (2008-2009): The field for the study in year one was located south of Salinas and had the following characteristics:

The area for experiment had 24-0-15 added on 9/23/08 at the rate of 362 lb/A applied.  Plots to contain phosphorous had super phosphate applied to them at the rate of 48 lb P/A.

Supplemental fertilizer applications to trial plots through the season were in total the equivalent of 54.7 lb N, 60 lb P, and 60 lb K and were achieved with applications of 3-18-18, 20-20-20, and ammonium sulfate.

Yield data for marketable and cull fruit was taken once a week beginning April 7 and ending September 16. As soil phosphorous levels do not change rapidly even in the presence of a crop, soil data for phosphorous concentration was taken once from the field before trial placement and then on August 24, 2009, and plant tissue data consisting of petioles and blades was taken once a month beginning in January, 2009.

Year 2 (2009-2010):

Field for year 2 was near to the field for year 1, so initial soil characteristics were similar.

The area for the experiment had 325 lb/A of 27-0-18 applied to the plots free of phosphorous addition and 24-8-18 at an equivalent rate to the above applied to plots with phosphorous.  The result for pre-plant fertilizer addition then was 78 lb N, 26 lb P, and 49 lb K applied to phosphorous addition plots and the same amount of nitrogen and potassium applied to plots without added phosphorous.

Supplemental fertilizer applications to trial plots through the beginning of September were in total the equivalent of 172 lb N, 0 lb P and 12 lb K and were achieved with applications of CN9, 0-0-25, and UN32.

Yield data for marketable and cull fruit was taken once a week beginning April 30 and ending September 2.  Soil data was taken once a month starting in October, 2009 and no samples taken in June or July of 2010.  Plant tissue data consisting of whole leaf blade nutrient concentrations was taken April, May, August and September.

Results- Year 1

Fruit Yield: There were no significant differences in marketable fruit yield or number in any month sampled, nor were there any significant differences in yields and numbers of cull fruit.

Blade and petiole information given below is a summary of the data sampled over the course of the first year of this study.  There are no significant differences by date between plots with P added and those with no P added.

Petioles by sampling date

Blades by sampling date

Field soil phosphorous was 90 ppm as per Olsen’s P before trial placement and average readings were 85 ppm from plots with additional phosphorous added,  and 87 ppm from plots where no phosphorous was added.  These readings are not significantly different from one another.

Results- Year 2

Fruit yield: There were no significant differences in marketable fruit yield or number in any month sampled, nor were there any significant differences in total yield of cull fruit or number.

Blade and soil nutrient information given below is a summary of the data sampled over the course of the second year of this study.  Unless indicated so, there are no significant differences between plots with P added and those with no P added by date.

Blades by sampling date

 Soil by sampling date

* Significantly differs (P=0.05, Student Newman Kuels)

Comments and Discussion: It is clear that the addition of phosphorous in this study covering two years of distinct strawberry crops did not result in yield differences, nor in plant tissue concentrations.

That soil P values did not decline very much, if at all, in both years should not be surprising.  For one, the soil test value is a measure of the soil P equilibrium, which means that when plants remove P from the soil solution, additional P comes into the solution from precipitated forms.  In other words, a soil with a high P test like the one in these tests simply backfills what is taken up by the plant from its ample reserve.  Secondly, according to sampling done by Tim Hartz and his lab over the past few years, we have no indication that strawberries are removing more than 40 lb P per acre per season, meaning that soil test P would not decline more than 15 ppm even if no more P were to come into solution as described above.  It simply will not fall very much.

Many thanks to Tim Hartz at UC Davis and the DANR Analytical Laboratory at Davis for their dedicated assistance with this work.

Finally, I want to thank the grower who collaborated with me on this work for two years.  You know who you are, and you helped me a lot- I wouldn’t have been able to even think about doing this study if you hadn’t helped me so much.  Thank you!

It has been more than 30 years since UC published strawberry leaf nutrient diagnostic guidelines (Publication 4098, ‘Strawberry deficiency symptoms: a visual and plant analysis guide to fertilization’, released in 1980).  In the years since that publication, varieties, production practices and yield expectations have changed considerably.  In 2010 we began a project, funded by the California Strawberry Commission, to reevaluate leaf and petiole nutrient sufficiency ranges for day-neutral strawberries.  With the cooperation of many berry growers in the Watsonville-Salinas and Santa Maria areas we collected leaf and petiole samples from more than 50 ‘Albion’ fields over the past two production seasons.  In each field samples were collected 5 times over the production season, from early spring through September, to document the nutrient concentration trends from pre-fruiting to post-peak production.  Leaf samples were analyzed for total concentration of nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), zinc (Zn), manganese (Mn), iron (Fe) and copper (Cu).  Petioles were analyzed for NO₃-N, PO₄-P and K concentration.

After the season cooperating growers provided yield information, which allowed us to categorize the fields as being ‘high yield’ or 'low yield’.  We then applied a process called DRIS (Diagnosis and Recommendation Integrated System) to mathematically evaluate the difference in nutrient concentrations as well as nutrient ratios between high yield and low yield fields.  This process allowed us to identify which of the high yield fields were ideally balanced nutritionally.  From this group of nutritionally balanced, high yield fields we were able to calculate a DRIS sufficiency range for each nutrient at each growth stage.

Fig. 1 shows that leaf N, P and K concentrations were highest before harvest began (stage 1, which was late February in Santa Maria and late March in Watsonville-Salinas), and declined to a reasonably stable level throughout the main harvest period (stages 3-5, May-July in Santa Maria, June-August in Watsonville-Salinas).  The decline in leaf macronutrient concentrations during the peak harvest period was expected; it happens in many fruiting crops because the leaves rapidly translocate nutrients to the developing fruit.  By contrast, micronutrient concentrations either increased from early vegetative growth to the main harvest period (as was the case for B, Ca and Fe), or remained reasonably stable over the entire season (all other micronutrients).  The vertical bars on each data point on Fig. 1 indicate the range of values typical of nutritionally balanced, high yield fields at each growth stage.  These are the DRIS sufficiency ranges; leaf nutrient concentrations within these ranges can safely be assumed to be adequate for high yield production.    

Table 1 lists the DRIS leaf nutrient sufficiency ranges for pre-harvest and main harvest growth stages.  For the sake of comparison, both the sufficiency ranges given in UC Publication 4098 and the current University of Florida guidelines have been included.  Although for most nutrients the ranges match pretty well, for others there are substantial differences.  Where the DRIS sufficiency range is substantially higher than the other sources (Ca, Mn and Fe, for example) it is because those nutrients are in such abundant supply in our coastal soils that plant uptake is far in excess of actual plant requirement; for those nutrients a lab test result marginally below the DRIS range would not be a matter of concern. 

For several nutrients (N, Zn and Cu) the DRIS sufficiency range fell below the other recommendations.  We are confident that the DRIS ranges represent nutrient sufficiency because they were determined by measuring the levels common in high yield fields.  The field survey approach used in this project ensured that a wide range of field conditions and grower practices were included, so the results are broadly representative of the coastal industry.  Also, for all three nutrients the average leaf concentrations of the high yield and low yield groups were essentially equal, suggesting that availability of these nutrients did not limit yields.

Fig. 2 shows the trends in petiole nutrient concentrations over the season.  Petiole NO₃-N was so highly variable as to be nearly worthless as a diagnostic technique; during peak fruit harvest (our sampling dates 3 and 4) petiole NO₃-N in high yield fields varied from < 200 PPM to 2,600 PPM.  While we believe that leaf total N is a more reliable measurement, this study suggests that maintaining petiole NO₃-N > 1,000 PPM pre-harvest, and > 400 PPM during peak harvest, is adequate to maintain high productivity.  Given the high variability of petiole NO₃-N it is possible that concentrations < 400 PPM would be adequate during the summer.   

Petiole PO₄-P and K were less variable than petiole NO₃-N.  Maintaining PO₄-P > 1,200 PPM throughout the season should ensure P sufficiency.  Given the high soil P availability in most coastal soils rotated with vegetable crops, this level is probably much higher than the ‘critical value’.  Maintaining petiole K > 2.5% preharvest, and > 1.5% during peak harvest, appears to be adequate.

Table 1.  Comparison of DRIS leaf nutrient sufficiency ranges with prior UC recommendations, and current University of Florida guidelines.