3 Key Nutrients That Make or Break Your Harvest

Written by Chris Morgner - Consultant, Crop Advisor, and PCA at Agri-Valley Consulting

Harvest is in full swing for many crops in the San Joaquin Valley, with almonds and tomatoes in focus.

Almond leaf samples taken in July can serve as a “report card” for orchard nutrient status. These results can highlight areas where adjustments are needed for next year’s crop. Over the past two years, I’ve noticed a trend of lower nitrogen and potassium levels in many orchards. Rising fertilizer costs have led to reduced application rates. While trees can store nutrients and small reductions in fertilizer may be acceptable, some orchards have depleted these reserves and now require careful attention to fertilizer rates and application methods.

Once nutrient levels in the tree become low or deficient, it can take a year or two to replenish—especially for potassium. I’ve seen cases where it took three or four years of higher potassium applications to correct a deficiency.

So, what are the ideal leaf sample levels? Nitrogen should be around 2.4%, and potassium between 1.7–1.8%. Levels can also be too high: nitrogen above 2.8% is excessive, and potassium above 2% does not increase yields—spending extra to maintain these levels is simply wasteful. Post harvest applications are generally not effective, as nutrient demand is low at this time. Instead, plan fertilizer rates and application schedules for spring, when nutrient uptake is more efficient. Weekly applications at the right rates can improve both uptake and efficiency.

One final note on post harvest nutrients: boron levels, as determined from hull samples, can be addressed in the fall—typically via foliar application—to enhance flower bud development and increase nut set at bloom.

Tomatoes have high nitrogen and potassium demands. Leaf samples taken before bloom can indicate whether the field has adequate nutrient levels or may face deficiencies later. Nutrient demand spikes dramatically when fruit begins setting, which is when fertilizer rates are typically increased. Leaf samples taken at fruit set and sizing reflect current nutrient status, but if low levels are detected at that point, it’s usually too late to correct. Adjusting application timing and rates—starting earlier in the season—can prevent deficiencies.

Nitrogen applications can begin in the third week after transplanting, while potassium should start in the fourth or fifth week. Research shows that potassium can increase fruit numbers, highlighting the importance of earlier applications. Once deficiency symptoms appear, it’s too late to correct.

Finally, water management is critical for nutrient uptake. Proper irrigation is key to crop development and efficient nutrient utilization.

Wishing you a successful and safe harvest season!