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Differential fertilization treatments were started in the summer of 2000 and continued through the 2001 and 2002 seasons. By 2001, nutrient deficiencies were apparent in some trees.

A small amount of fruit was harvested from most of the trees that year. By 2002, fruit production was substantial on the majority of the trees.

Big differences among trees were measured in tree size, flower production, fruit set, fruit size, and fruit quality parameters. Many of these differences were correlated with leaf nutrient levels. The research also suggests the conventional mid-summer timing of nutrient analyses may be too late for some nutrients. An early spring sampling may be more appropriate in a few cases.

Results Of The Study

* Leaf nutrient levels. Some of the leaf samples tested below the published deficiency thresholds for nitrogen (N), boron (B), zinc (Zn), and iron (Fe). The remaining nutrients also measured very low on some of the trees, often just above the deficiency threshold. The one exception to this is potassium (K), which had some very high values but no minimum values as low as those measured in 2001. The other two major cations, calcium (Ca) and magnesium (Mg), had minimum levels that were considerably lower than those achieved in 2001.

Despite the low leaf nutrient levels, there were few leaf deficiency symptoms observed on the trees. N deficiency was obvious on many trees, starting right after bloom. Also early in the spring, some Zn deficiency symptoms were apparent, especially on some of the plum trees, but these disappeared as the weather warmed up. By mid summer, other than the yellow and red leaf symptoms of N deficiency, the trees looked healthy and vigorous. However, there were other subtle symptoms such as fruit size, fruit color, and shoot vigor that were obviously caused by the nutrient treatments.

* Flowering and fruit set. Flower density varied about threefold for both peach and nectarine and much more for plum. The treatments with no N had distinctly lower flower densities than the other treatments, especially with plum. However, there was generally a very poor correlation between leaf nitrogen content and flower density for all the trees together. Instead, it appears other nutrients such as phosphorus (P), B, and Fe may have contributed to flower development as well. There was also moderate water stress in some of the trees during 2001 that may have affected flowering. Irrigation amounts and soil water status were monitored much more carefully in 2002 to make sure no stress occurred.

Fruit set was dramatically different from one tree to another. A few nectarine trees had good flowering but ended up with virtually no fruit even though some flowers started to develop initially. The peach trees were not quite as extreme but still had some trees with fruit set as low as 4%. On the other hand, some peach and nectarine trees had 200 to 300 fruit per tree before thinning.

* Fruit size. Several of the trees had excellent fruit size even with fairly heavy fruit loads. The analysis conducted to this point suggests that just about every nutrient measured had some effect on final fruit size.

The peach and nectarine trees tended to show similar results in their relationship of fruit size to leaf nutrients. However, fruit on the plum trees appeared to follow a somewhat different pattern. Most notably, Ca seemed to play a major role in fruit size with some of the "minus calcium" trees having noticeably larger fruit than many of the other treatments. These trees had leaf Ca levels around 2%, which is far from deficient and, in fact, is about the same level as that found in the peach and nectarine trees with the largest fruit. It may just be a case of many of the plum trees having excessive Ca levels, which could depress fruit size.

* Fruit quality. At harvest, several parameters of fruit quality were measured. These included firmness, red color, soluble solids content, and acidity. As with the other parameters measured, there tended to be at least a twofold difference from the lowest to the highest values. Firmness and red color correlated somewhat with leaf N but appeared to reflect maturity of the fruit more than nutritional status of the tree. Soluble solids content did not show a significant correlation with any nutrient. Fruit acidity showed a high correlation with many different nutrients suggesting it might be affected by P, K, Ca, Mg, and B.

Information for this article was based on 2002 research by Scott Johnson and his project team at the University of California's Kearney Agricultural Center Department of Pomology. For updates on this research, contact Johnson at . E-mail questions or comments about this article to .