Orchard Nutrition: How Do You Know What Fertilizers You Need?

No, this is the beginning of when you need to be checking up on how well the trees are growing; this is nutritional analysis time.

Back in the early spring in anticipation of the fruit crop, you applied fertilizers to the orchard. Most commonly you probably made ground applications of nitrogen, possibly some potassium. Some growers may also have made some foliar applications of boron to their apple trees as well. How effective were these soil and foliar applications? The only way to know for certain is to collect a sample of leaves from the trees and analyze their nutrient content. The Penn State Agricultural Analytical Services Lab can provide this service.

Mid-July through mid-August is the time to collect those leaves for chemical analysis. This is the time in tree fruit growing season when the mineral nutrient levels are the most stable and represent the overall nutritional status of the trees. Analysis at this time gives an objective guide to the grower to help choose the most appropriate fertilizers for the next season. Fertilizer applications based upon these results can reduce production costs and improve fruit quality. For example, applying excess nitrogen (N) based solely on tree age can result in reduced fruit quality and potentially increased bitter pit in apples.

Samples of mid-shoot leaves should be collected from a single variety/cultivar on one rootstock of the same age trees. Mid-shoot leaves are defined as fully expanded leaves from the mid-portion of the current season's terminal growth (See image). Shoots selected should be of average length and vigor, i.e., not overly vigorous water sprouts. They should be free of any disease, insect or mechanical damage. Shoots should be from about chest to shoulder height and fully exposed to sunlight. A sample should consist of 50 to 60 leaves.

Example of mid-shoot leaf to be collected for leaf sample. Photo: Rob Crassweller, Penn State

Make a note of tree age, crop load and any foliar nutrient sprays. To reduce contamination collect the samples immediately before any normal cover or foliar spray applications or as long as possible after a foliar cover spray.

If you suspect that there may be a nutritional problem, then two samples should be taken to compare results. Collect leaves from suspected problem trees and an equal sample from healthy or non-problem trees and have them analyzed separately.

Seasonal considerations

Growing conditions can also affect results, and you should consider the following:

• Dry Seasons: usually result in lower nutrient levels.
• Weeds: competition from weeds can result in lower levels of N in leaves.
• Rainfall: the high amounts of rain that we have had this season could show higher nitrogen (N) levels due to increased growth but may also show lower levels of calcium (Ca), especially in the fruit.
• Crop Load: heavy crop loads may be associated with higher levels of N and Ca in the leaves and lower levels of phosphorus (P) and potassium (K). On low cropping trees, N is lower due to greater vegetative growth that dilutes the quantity of N in relation to the amount of vegetative growth
• Cultivar: Different cultivars have different N requirements. For processing cultivars generally, it is desirable to have higher N levels. On the other hand, red-colored cultivars with high N may have poorer color and increased problems with bitter pit. A distinction can also be made for hard cultivars versus soft cultivars, the latter requiring higher N.
• Fungicides: Recent application of some fungicides can alter micronutrient levels with manganese (Mn) or zinc (Zn) being the most commonly affected.
• Other elements: high levels of one element may result in lower levels of another. The best example is the interaction between K, Ca and Mg. High levels of Mg or K can result in lower Ca and may impact bitter pit in fruit.