Nutrient Management Research Database

https://www.cdfa.ca.gov/is/ffldrs/frep/pdfs/completedprojects/03-0652Johnson2007.pdf  

Mid summer is the recommended period of sampling leaves for nutrient analysis in fruit trees. This practice was first developed in the 1940s and was widely applied to orchards during the 1950s. In 1971 Leece compiled a list of 39 peach studies from seven countries and concluded, “with few exceptions, good agreement existed amongst the studies for any particular nutrient”. Thus, the practice became very widespread and is the standard to this date. The timing of about 100 to 140 days after bloom has been established because the concentration of most nutrients remains fairly stable during this time. This corresponds to the period from early June through late July in the central valleys of California. We see at least two problems with this timing, especially in light of our modern era of environmental concerns and an emphasis on good sustainable fertilization practices. First, the timing of mid summer for sampling does not fit well into a sustainable fertilizer program since many fertility decisions need to be made early in the spring. Growers are generally most interested in fertility management as they watch their trees flower, leaf out and set a crop in the spring. By summer their interest has waned. Second, this timing also does not make sense from a tree physiological perspective. Many critical processes such as flowering, fruit set, initial fruit growth, and shoot growth are all happening in the early spring and should be dependent on tree nutrient status at that time. A dormant or early spring sample should give a good indication of stored nutrients and whether any are limiting. This study will evaluate the nutritional status of several different plant parts in hopes of finding a reliable and consistent sampling procedure to help guide fertilization practices in the spring.

• While widespread practice is to sample stone fruit tree leaves during mid-summer, an early spring sampling date offers more information relevant to making informed fertilization decisions.
• A handful of nutrient level dependent processes occur before mid-summer, including flowering, fruit set, initial fruit growth, and shoot growth.
• This study tested the feasibility of measuring boron, zinc, and nitrogen in stone fruit tree leaves during dormancy or in the early spring.
• Based on these results, the authors developed deficiency threshold values to guide fertilization decisions and tested the usefulness of these threshold values in commercial orchards.

• Nitrogen content in dormant shoots was not a good measure of tree N levels.
• While P deficiency isn’t often observed in California orchards, a threshold value of 0.12% P in dormant shoots was found to reliably identify deficient trees.
• A threshold level of 14 ppm B was identified.
• While symptoms of leaf Mn deficiency were observed at 8-9 ppm, the authors state that fertilization is likely not required until levels drop even further.
• A tentative threshold level of 15 ppm was set for Zinc.