Ambrosia fruit. Photo: R. Crassweller, Penn State
However, in some cases the soils can be too fertile resulting in excessive growth or inducing physiological disorders such as bitter pit or corking. Sometimes it is necessary to supplement these soils with fertilizers to meet some of the special needs. The question is how do you determine what fertilizers are needed? There are several 'tools' you can use to help you make the decision on what you need to supply or what nutrient is in over supply. The correct tool depends upon the stage in the life of the orchard.
Prior to Planting the Orchard
The only choice you have is a soil test. Soil tests used before the orchard is planted can give you an idea of the nutritional potential of the site. The major information that you obtain from the soil test at this stage is soil pH, cation exchange capacity (CEC), phosphorus, potassium and calcium levels. You cannot really determine the nitrogen potential because nitrogen soil tests are not very indicative of the total amount because it can fluctuate during the season and largely exists in the soil solution. Levels are impacted by microbial mineralization of organic matter and the conversion of ammonium to nitrate (nitrification), nitrate uptake by microorganisms and plants and nitrate leaching by water from rainfall or irrigation. Nitrogen can also be added to the soil by microbial fixation of N2 from the atmosphere, dissolved in rainfall, or by inorganic fertilizer and organic amendments. Therefore, the nitrate level in the soil can vary throughout the year. Microbial activity can be affected by temperature, carbon source, oxygen and water supply resulting in changes in the nitrate levels.
One test that has not received much attention, or use, is the test for organic matter. We recommend that when taking a soil test prior to planting you should also have the soil tested for percent organic matter. Organic matter can provide multiple plant nutrients; improve soil structure resulting in improved soil aeration; improve water and nutrient retention; and increase biological activity in the soil. Think of organic matter as free fertilizer because it helps recycle and release nutrients in the soil.
Following the recommendations on pre-plant soil tests should be considered as putting money in the bank for a rainy day because it will establish a base upon which the trees can withdraw nutrients. The chief benefit is from establishing the soil pH and soil phosphorus levels. Soil pH ideally should be between 6.0 and 6.5. Phosphorus is important in helping young roots become established. Incorporating the lime and phosphorus into the soil is much easier before trees are planted. Later in the life of the orchard it is difficult to incorporate and both the effects of lime and phosphorus do not easily or rapidly move downward to the roots.
Soil tests after the orchard is established are primarily for following the soil pH and following soil CEC and phosphorus levels. As such, soil test samples should be collected on a three year rotational schedule.
The primary method of determining nutrient needs is from foliar samples for leaf analysis. These should be taken every 3 years and collected from mid-July to mid-August. The reason the samples are taken during this period is that the nutrient levels in the trees are the most stable. In the early spring nutrient levels will fluctuate as nutrients are mobilized from the tree reserves located in the branches, trunks and roots. These reserves are affected by the previous season's crop and growing conditions. Most of the early growth up until bloom is fueled by the reserves in the trees. After the reserves are spent then the roots start to draw in the nutrients within the soil. The soil nutrients can come from the natural reservoir of the soil and/or the spring applied fertilizer.
Trees were the first recycling plants in agriculture; long before humans learned the value of recycling. The leaves that drop each fall can recycle their contents back to the tree, dormant pruning wood that is ground up by a flail mower also contribute to the nutrient reservoir. The 'reservoir' levels are enhanced by the natural recycling of grass clippings from row middles, leaf drop in the fall, fruit drops, pruning waste (if ground and left in the orchard), natural nutrient percolation from rainfall or irrigation.
The major loss from the orchard system is through fruit harvest, some leaching of nutrients beyond the root zone and pruning wood (if removed from the orchard). Haynes & Goh estimated the net change which included applications of fertilizer in pounds/A/yr. was positive for N (20.5); and P (14.2) and negative for K (-53.5); Ca (-16.9); Mg (-26.8). More recently Cheng & Raba compared the net gain in nutrients (or the amount needed in one season) for Gala/M.26 trees with a yield of 1113 bushels/A. Net gain in lbs./A for the season from bud break to harvest was N (49.3); P (8.2); K (89.4); Ca (35.4); Mg (10.9). While these are valid measurements under the conditions of these two studies it should be noted that conditions vary from site to site and under different microclimatic differences. None the less it does show general trends and that recycling of nutrients does occur.
Leaf analysis results compared over time can show the general nutritional status of orchards and can indicate the need or lack of need for particular nutrients. Samples collected in the coming month will provide you with a guide for fall and/or spring fertilizer recommendations. Leaves should be collected from the midpoint of the current season's shoot growth. There are two ways to choose which trees to sample. The first is similar to how soil samples are collected by walking a zig-zag through the orchard and selecting one or two leaves from the same cultivar on the same rootstock. Collect leaves from shoots that are approximately shoulder height and on the exterior canopy of the tree. A total of 50 to 60 leaves will provide enough material for the sample.
Leaves should be collected from the midpoint of the current season's shoot growth, indicated in this photo by the yellow circle. Photo: R. Crassweller, Penn State
The second method is to choose 15 trees that represent the average tree size and crop load and collect 4 leaves from all compass points of each tree. Crop load has been identified as a major factor influencing fruit mineral concentration (Hansen, 1980). Volz et al. (1993) reported low crop trees produced fruit with decreased Ca and increased K concentration with more fruit disorders. If you know there is a difference in soil type within the block select trees equally from the different soil type.
It is important that you do not mix cultivars because different cultivars may have different nutrient requirements. The nutrient levels may also vary based upon the rootstock the trees are on. If you are regularly applying nutrient sprays to your orchard try to collect the leaf samples just before the next nutrient is applied. We are currently looking at a study comparing two cultivars on 4 different rootstocks in a tall spindle high density orchard to see if rootstock can influence nutrient uptake. Some research suggests that there may be some differences.
As part of our current study we are also collecting leaves for sap analysis. Sap analysis involves collecting leaves and removing the petioles and shipping them to a lab in The Netherlands. In the process has been used primarily for vegetables and mostly tomatoes. In vegetables they collect two types of leaves the newest leaves and older fully developed leaves. Since by mid-July there are very few new leaves we are only collecting the mid-shoot leaves. There is no published research data on sap analysis for use in commercial orchards. Given the cost of the samples and the lack of standards to compare; sap analysis of perennial fruit crops is still only experimental and not recommended as a proven analytical tool for orchards.
More recently, Tara Baugher and Rich Marini have been looking at fruit analysis by collecting fruit peel taken from the calyx end of apples. Their goal is trying to predict lots of fruit that may be more prone to the development of bitter pit. I am also collecting fruit skin samples in the above mentioned cultivar and rootstock experiment so that we can compare the potential correlation to traditional leaf analysis.
Penn State Agricultural Analytical Services Lab
Leaf analysis submittal form.
Cheng, L. & R. Raba. 2009. Nutrient requirements of Gala/M.26 apple trees for high yield and quality. NY Fruit Quart. 17(4):5-10.
Goh, K. & R. Haynes. Nutrient inpust and outputs in a commercial orchard and their practical implications. NZ J. Experimental Agriculture. 11(1):59-62.
Hansen, P. 1980. Crop load and nutrient translocation p.201-212. In: Mineral Nutrition of Fruit Trees. D. Atkinson et. al. (eds). Buttersworth, London, UK
Haynes, R. & K. Goh. 1980. Distribution and budget of nutrients in a commercial apple orchard. Plant and Soil. 56:445-457.
Nielsen, G. and D. Nielsen. 2003. Nutritional requirements of apple. In Apples: Botany, Production & Uses. D. C. Ferree & I. J. Warrington (eds). CABI Publishing
Volz. R., I. Ferguson, J. Bowen & C. Watkins. 1993. Crop load effects on fruit mineral nutrition, maturity, fruiting and tree growth of Cox's Orange Pippen apple. J. Hort. Sci. 68:127-137.