Practical Application of Fruitlet Mineral Analysis
Introduction and Methods
Fruit quality characteristics such as color, firmness and storage potential have long been known to be related to the concentrations of certain fruit minerals (Boynton, 1954). Fruitlet analysis allows commercial fruit growing, packing and marketing operations to take advantage of these known relationships and increase profit margins by supplying information that can be used to increase fruit quality and reduce storage losses and market claims. This paper presents a brief overview of the Fruitlet Mineral Program practiced by the Okanagan Similkameen Cooperative in Oliver, BC.
Standard leaf samples are taken for mineral analysis in the third week of July from all Gala, Jonagold, Braeburn and Fuji blocks that ship to the house. Fruitlet samples are collected for mineral analysis from the same blocks six weeks before harvest. Storage samples are collected from the blocks at harvest. Fruit are evaluated for quality and storage disorders at harvest and again after two, four and six months of storage. Leaf and fruitlet mineral recommendations are then determined based on correlations of mineral data with fruit quality and storage results.
Once recommended mineral levels have been established, fruitlet analysis is used in two basic ways. First, storage predictions for individual blocks in combination with field observations are given to the packinghouse two to three weeks before the fruit is harvested. Management uses this information as an aid in making storage and marketing decisions. Second, it provides growers with target values for fruit mineral levels and allows them to see the effects of their own nutrition programs in their efforts to achieve these values. Thus, over a period of time, growers can develop feeding programs that are custom tailored to their own specific orchard blocks.
The concentrations of various minerals in the fruitlet are only a few of a very large number of interacting factors that ultimately affect overall fruit quality. As a result, the relationships between fruitlet minerals and fruit quality are far from perfect. This is illustrated in Figure 1 where the percent of Jonagold fruit with bitter pit after two months of storage is plotted against fruitlet Ca concentration measured six weeks before harvest. Each point represents data from one orchard. Orchards with high fruitlet Ca levels had very little or no bitter pit. As the average Ca level decreased, the chances of fruit developing bitter pit increased. The probability of bitter pit increased dramatically as fruitlet Ca levels fell below 5 mg/100 g FW even though fruit from a small number of orchards with extremely low Ca did not develop the disorder.
The real storage and marketing benefits that can be derived from fruitlet analysis are based on the probabilities inherent when dealing with large volumes of fruit. The packinghouse views the data in Figure 1 from the perspective of averages of orchards grouped on the basis of fruitlet Ca as presented in Figure 2. On average over three crop years, more than 21% of the fruit from orchards having less than 4 mg Ca/100 g FW had developed bitter pit after two months of storage. Although there were some blocks in this group with no bitter pit as seen in Figure 1, blocks falling into this group were generally at very high risk of developing bitter pit early in storage. There are very few storage and marketing options with fruit in the low Ca groups. Management must allocate fruit from these blocks for immediate sale before disorders develop and the fruit's full market value can still be realized. Likewise, the fruit with 4.0 to 4.9 mg Ca/100 g FW should also be sold early. On the other hand, fruit in groups with higher levels of Ca offers many options. These lots can be sold immediately, held for short term in air storage or held long term in CA with greater confidence knowing that weak fruit lots have already been removed.
The groups of orchards in Figure 2 do not contain equal volumes of fruit. In the three years' data presented in Figures 1 and 2 more than 70% of the blocks had at least 5.0 mg Ca/100 g FW. Thus, selling the weak, low Ca fruit early does not mean that the packinghouse has to sell a large portion of its total volume immediately after harvest because of low Ca. Furthermore, over a period of time after a fruitlet program has been initiated, the volume of fruit with high storage potential will increase as growers make adjustments in their nutrition programs in response to their fruitlet results.
In addition to bitter pit, fruitlet Ca is associated with watercore, internal breakdown, storage decay and, sometimes, fruit firmness.
High N is a problem in many blocks. For every one block that is low in N we see at least five or six blocks with excessive levels of N. Jonagold is especially sensitive to N and readily develops bitter pit when fruitlet levels exceed 50 mg N/100 g FW (Figure 3).
Growers often think that high N fruit matures less rapidly than low N fruit because high N fruit tend to remain green and growers associate green color with immature fruit. However, based on starch-iodine testing, higher N fruit generally mature more rapidly than lower N fruit as illustrated in Figure 4. On average, 1995 Gala growers with the lowest N blocks had a harvest window of twelve days for every seven days of harvest window open to growers with the highest N blocks.
Fruitlet N is also associated with internal breakdown, red color development, persistent green background color, storage decay and fruit firmness.
N/Ca and K/Ca Ratios
Whenever two fruitlet nutrients have opposing effects on a given fruit quality characteristic it is often useful to look at their ratio "balance". For example, high N in Jonagold results in higher levels of bitter pit whereas high Ca results in lower levels of bitter pit. The N/Ca ratio for Jonagold fruitlets is generally more highly correlated with bitter pit than either N or Ca alone (Figure 5). Similarly, where fruitlet K is often positively correlated with internal breakdown (Figure 7) and Ca negatively correlated with the disorder in Jonagold (data not shown), breakdown is more highly correlated with the K/Ca ratio than for either nutrient alone (Figure 6).
Fruitlet K and the N/K Ratio
Potassium is of special interest because of its positive association with both red color development and with storage disorders such as breakdown and bitter pit (Figure 7). In an effort to improve fruit color, growers sometimes include high rates of K in their feeding programs. However, when fruitlet K rises above a certain level, fruit storage potential can begin to decline rapidly. When Jonagold K levels exceed 135 mg/100 g FW, the incidence of storage disorders begins to become unacceptably high. Good color and good storage performance can be achieved together when fruitlet K is managed relative to fruitlet K using the N/K ratio (Figure 8).
Balancing Ca, N and K
As an example in using fruitlet mineral ratios, a fruitlet nutrient profile of N/Ca = 8, K/Ca = 22 with a maximum of 135 to 140 mg K/100 g FW would be a reasonable initial target for a Jonagold black with a history of poor fruit quality. Achieving these ratios would require a minimum of 6 mg Ca and a maximum of 48 mg N. This would give an N/K ratio of approximately 0.36. These values can be readily achieved through a balanced feeding program based on fruitlet results and a good Ca spray program. The grower would still have further room to improve the mineral profile in the future by lowering fruitlet N to 40 mg.
Fruitlet P is positively associated with fruit firmness (Figure 9). We have seen this relationship in all the varieties with which we have worked. While the effect of fruitlet P on firmness can be evident at harvest, it often becomes more pronounced after two or more months of storage.
As mentioned above, fruit quality characteristics are the result of many interacting influences. The combined effect of fruitlet P and harvest maturity on firmness is a good example of such an interaction. As fruit maturity advances beyond the storage recommended for long term storage it will begin to exert an ever increasing effect on fruit firmness, thereby reducing the effect of P on firmness. Eventually, any measurable effect of fruitlet P on firmness will be completely eliminated. The same holds true for most other relationships between fruitlet minerals and fruit quality. Fruit must be harvested at the proper maturity in order to assure reliability of storage predictions based on fruitlet mineral analyses. All of the fruit used to generate the storage data presented here were harvested at a maturity recommended for long term storage (Lau, 1985, 1996).
We have only recently begun to measure fruitlet B. In the 1995 Fuji crop, fruitlet B was positively correlated with watercore after two months of air storage, core flush after four months and internal breakdown after six months (Figure 10). Recommendations cannot be based on a single year's results and we will be looking to see if this relationship persists in future years. Based on recommended fruit B levels from other work (Peryea, 1994; Johnson and Luton, 1984; M. Banwell, Phosyn PLC, U.K.. personal communication), the optimum range for fruitlet B might vary quite widely from cultivar to cultivar.
Leaf N, P, K, B, Mn and Cu are somewhat correlated with fruitlet levels (R2 > 0.30) while leaf and fruitlet Mg, Ca, Fe and S are poorly correlated (R2 < 0.15). After recommendations for fruitlet N, P, K, and B have been established, leaf mineral recommendations for those nutrients can be fine tuned based on leaf and fruitlet correlations.
Leaf minerals are never used as a basis for storage potential predictions but are used to confirm N, P, B and K fruitlet results. This has proven to be especially useful in cases where fruitlet mineral results indicate an unusually high or low value. Leaf results are also used to assess the status of nutrients for which there are no fruitlet recommendations and for Mg and Ca, which are not correlated with fruitlet levels. Therefore, although fruit quality and storage potential predictions are based solely on fruitlet minerals, leaf minerals play an important role in the program by increasing the overall base of information available concerning the orchard's nutrient status.
Comments and Summary
Jonagold data were used in the majority of the examples in this paper because it was the original focus of our work and is supported by the largest database of the four cultivars currently in the program. The general principles of the approach apply to all varieties.
The economic value of a fruitlet mineral program may vary somewhat in relation to the value of the cultivar under consideration. In general, cultivars for which returns per pound to the grower are high or varieties that are more subject to developing mineral related storage disorders are the most likely candidates for a mineral program. It should be noted, however, that the costs of a well run program are not that great when calculated on a per acre basis and the application of this approach can be economically justified on most, if not all, commercial varieties.
Fruitlet mineral analysis is one part of a much larger whole that generally falls under the umbrella of "Orchard Management" which includes cultivar selection, planting, irrigation, tree training, pest control, leaf and soil analysis, maturity testing and harvesting. It is no more important than any of these individual aspects of orcharding but, instead, compliments them in a total package program aimed at producing and marketing the highest quality fruit possible.
We acknowledge the growers and management of the Okanagan Similkameen Cooperative in Oliver, B.C. whose support has made this research possible and Dr. W.J. McPhee whose insight and efforts helped bring the program into existence. We extend our thanks to J. Quaedvlieg, R. Dilcher, M. Watson, S. Cerqueira, R. Miranda and K. Butler for their hard work in making the program successful.
Boynton, D. 1954. Apple nutrition. In Childers, N.F. (ed.) Mineral Nutrition of Fruit Crops. Somerset Press, Somerville, NJ. pp.1-78.
Johnson, D.S. and M.T Luton. Excess boron can reduce storage life. Grower 101:31.
Lau, O.L. 1985. Harvest indices for B.C. apples. B.C. Orchardist 7: 1A-20A.
Lau, O.L. 1996. Maturity and storability of eight newer apple cultivars - quick reference. Industry Research Program. Okanagan Federated Shippers Association. Kelowna, B.C.
Peryea, F.J. 1994. Boron nutrition in deciduous tree fruit. In Peterson, A.B. and R.G. Stevens (eds.) Tree Fruit Nutrition. Good Fruit Grower, Yakima, WA. pp. 95-99.
William D. Wolk
Okanagan Similkameen Cooperative Growers Oliver. B.C.
13th Annual Postharvest Conference