I was brought out to the situation in strawberry pictured below.  Yellowing leaves and very little flowering or fruiting.   For whatever reason, the street’s call on this was that it is iron, but to me the youngest leaves being as green as they are (Photo 2 below), is a dead giveaway that it’s not iron, because the youngest leaves in iron deficient plants are the first to yellow, not the last.

No sense standing around arguing about the problem, we just have to roll up our sleeves, get dirty and figure it out! 

The charts below are threefold replicates of sampled leaves and soil of affected plants in the field.

Table 1 : Tissue analysis

Table 2: Soil analysis

So, it looks like the main culprit here is indeed a lack of nitrogen, seeing that at an average of 1.4% it is well under the 2.4% tissue concentration threshold given in the UCCE nutrient guidelines.  Just to be sure though, we should check to see if any of the other nutrients are low, but they are not and everything else is within normal ranges.  I would have some concern about the high levels of sodium (197 ppm) and chloride (7930 ppm), but beyond some marginal burning of the leaves, these plants don’t seem to be exhibiting symptoms consistent with real salt poisoning.

Looking to the soil, sure enough we get confirmation of what we are seeing at the tissue level, and see that nitrates are pretty low here, running at a lean 4 ppm.  I would probably want to bump that up a bit.

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.

One of the less well understood issues in our industry on the Central Coast is the phenomenon of yellowing of strawberry plants in certain areas of the district, especially in a number of fields north of Salinas.  The following is meant to share what we have found out on this problem so far, and discuss some of my thoughts about the most probable cause.

While there are many causes of yellowing in strawberry plants, for example lack of nitrogen, iron or zinc, the yellowing of strawberry plants in the Salinas area seems to stem from something else and occurs in the same area, year after year.  In fact, some spots no more than a few meters square give the same symptoms every time strawberries are planted there.  Yet, subsequent plantings of other crops such as broccoli or lettuce do not show any yellowing. 

To address the thought that the yellowing comes from nutritional deficiency, I have taken many samples with colleagues of these yellow plants and never found anything exceptional nutrient wise.  Consider the table below which is an eightfold replicated comparison taken in a large strawberry field south of Castroville with large areas of yellow plants in a field of healthy green plants:

What one immediately sees from the table above is that the trend is actually for yellow plants to have HIGHER levels of essential nutrients than their apparently healthier counterparts. 

Interestingly, manganese is very much higher, and a t-test tells us significantly so, in the yellow plants than in green plants.

So, the assumption that nutrient deficiencies are leading to this yellowing of the plants is not backed by the evidence of a plant tissue test.  To be sure, yellowing from nitrogen tends to be stronger on the outer, older leaves as this mobile nutrient is transported to the younger leaves.  Deficiency of zinc generally has a green halo around the leaf edges.  Perhaps the symptoms are consistent with that of iron deficiency, and indeed the iron from the soil sample from around the plant itself is significantly higher around the green plant than the yellow.  However, the levels of iron in the evaluated plants are well above those described as critical by UC Publication 4098 and Tim Hartz’s strawberry fertility work in 2010.

One of the considerations though all of this research is that the yellowing is caused by waterlogging and a subsequent deficiency in the amount of oxygen available to the plant.  This is not necessarily water or saturation that is easily measurable at the surface and may be deeper down in the bed.  It is also possible that salinity, which has a slight inverse effect on the solubility of oxygen in water, is also playing a role. 

Plants respond to decreased oxygen levels, known as hypoxia, in different ways and some species are in fact quite sensitive to this condition.  Roots, as the plant organ which face the hypoxic condition in a waterlogged soil, respond to this stress by switching from respiration to a fermentative metabolism which in turn increases the demand for carbohydrates.   That this metabolic change in strawberry is the cause of the yellowing in our strawberries is something which yet remains to be explored.

As a final thought, consider the the fourth picture below in which the drip tape on the right was clogged and less water delivered to that bed for several weeks.  The result was a lessening in the yellowing of the plants in that bed, and only that bed.  Absolutely, this is not a very scientific evaluation, but it does strongly suggest that excessive water from the plant's perspective has something to do the yellowing we know from around Salinas.