Introduction:

Many Michigan orchard sites contain multiple soil types that contribute considerable variability in soil nutrient levels and pH, and tree growth performance. Past fertilization practices tend to introduce addition variability in soil nutrient levels and pH. Fertilizer is applied to mature orchards by broadcasting over the orchard floor or banding in the weed spray strip, while lime is applied by broadcasting the orchard floor. Since the nitrogen fertilizers typically applied in orchards are acid-forming, soil pH often declines with time, and lime is required to maintain the pH at desired levels (6.5 to 7.0). However, banding fertilizer under the tree row can result in pH and nutrient gradients between the row middles and the weed free strip under the trees. Individual fruit growers in NW Michigan have found that the soil within the row (weed spray strip) is often 1.0 or more lower in pH than that between the rows (sod center). Therefore, most of the tree root system may be in a much more acidic environment than the average orchard pH would indicate.

Acidic soil conditions can reduce tree productivity, increase susceptibility to insects/diseases, and sometimes contribute to tree death. These effects may result from reduced availability of needed nutrients, or accumulation of toxic levels of other elements, such as aluminum (Melakeberham et al., 1993). The end result is a significant economic impact on the grower. Precision application of lime and fertilizers may help to maintain a uniformly appropriate pH throughout orchards, and avoid nutrient deficiencies and excesses in specific areas of orchards. Healthy trees are better able to utilize fertilizer inputs, which may reduce the risk of fertilizer-derived nitrate leaching into water sources.

Phase I of this project is a survey of nutrient, pH levels and soil types within existing orchards (within sod centers and weed sprayed strips), aimed a describing the variability in soil characteristics and ascertaining the value of precision application of lime and fertilizer. If precision application looks valuable, then research to develop improved application technology and verify the biological response (tree growth/yield) to precision application (Phase II) and utilization of site specific technology (Phase III) will be proposed.

Objective

To describe the variability in soil P, K, Ca, Mg, and pH levels within orchards, both between general areas within a block, between the sod centers and the weed-sprayed strips under the trees and across various soil types, to ascertain the value of precision application of lime and fertilizer.

Materials and Methods

Soils samples were collected from 18 Northwest Michigan orchard to assess differences in soil characteristics within orchards. Orchards were 15-30 years old, over 5 acres in size, and included apples and tart and sweet cherries. Orchards were split into 2 to 12 sub-blocks, based on aerial photographs, tree growth, soils maps, and grower input. Separate soil samples were collected from the row middle and from the weed-free strip beneath trees in each sub-block. Samples consisted on multiple cores extracted with a soil probe to a depth of 10 to12 inches. Soils were analyzed for pH, and P, K, Ca, and Mg levels by the MSU Soil and Nutrient Laboratory.

Results

Soil under trees is more acidic (pH 6.0) and higher in P (93 lb/acre) than that in the row middles (pH 6.8; 57 lb P/acre). This apparently reflects the long term effects of banding fertilizers beneath the trees. Fertilizers containing ammonium have an acidic reaction in soils, and tend to reduce pH over time. Phosphorus is very immobile in most soils, so soils tend to accumulate P where it is applied repeatedly. Differences in pH between the herbicide strip and the row middles present a dilemma for growers since liming the entire orchard floor in order to raise pH levels in the herbicide strip to 6.5 or 7.0 may result in excessively high pH values in the row middles. Application equipment is needed to efficiently spread lime only in the herbicide strip areas of most orchards. Apple blocks tended to be more acidic than cherry blocks.

The variability in pH and nutrient levels between locations within the same orchards was assessed by computing the coefficient of variation (CV) for the individual samples. The CV expresses the variability between individual samples in an orchard (standard deviation) as a percentage of the average value for the orchard. High CV’s indicate greater variability between blocks within each orchard. Soil Mg levels were the most variable between blocks within each orchard, whereas soil pH levels varied the least within an orchard. CV’s for pH measurements were less than 11% for all orchards. In contrast, the CV for soil Mg levels was greater than 50% in almost 40% of orchards. Variability in soil pH was greater in samples from under the dripline of trees than in samples from row middles. This may reflect the fact that pH beneath the trees is influenced by varying fertilizer use rates and distribution.

Conclusions

Soil pH was more uniform from block to block than soil P, K, Ca, or Mg levels. The greatest contrast in pH was observed when comparing the row middles to the area beneath the trees. Nutrient levels tended to vary considerably from block to block within an orchard. With the exception of P, nutrient levels were similar in the row middles and beneath trees.

We met with MSU agricultural engineers about these data, and it was generally agreed that the amount of variability in nutrient levels may not justify the expense of developing and using precision application equipment for orchards based on this data alone. However, given the generally low pH values and the importance of pH in tree health, several questions were raised about the tree to tree variability in soil pH, and the potential influence of trickle irrigation and fertigation on soil pH gradients with depth and distance from the emitters.

Literature Cited

Melakeberham, H., A.L. Jones, P. Sobiczewski, and G.W. Bird. 1993. Factors associated with the decline of sweet cherry trees in Michigan: nematodes, bacterial canker, nutrition, soil pH, and winter injury. Plant Dis. 77:266-271.