Acidification of the Soil in Northeastern Apple Orchards1

T. W. EMBLETON, DAMON BOYNTON, AND H. A. MAcDoNALD2

DURING the past 15 years sulfur fungicides havecome to be far more significant in causing acidi-

fication of the soil in northeastern orchards than theyused to be. There are two reasons for this. First, thevariety McIntosh, which is very susceptible to applescab, has become the most important apple of -thearea, replacing Baldwin which is less susceptible.Second, elemental sulfur sprays and dusts which havevalue only as protectants against scab have largelytaken the place of lime-sulfur sprays which havepowerful eradicative action and hence were not ap-plied so often. The common use of acid-forming ni-trogenous fertilizers has also caused appreciable de-crease in soil pH.3 Besides acidification by these twopractices and that resulting from normal leaching ofthe soil, there may be an average decrease in pHunder fruit trees due to the presence of roots inrather great concentration.4

The maximum possible effects of spray and ferti-lizer practices may be estimated with reasonableaccuracy. For instance, Table i shows the pounds ofsulfur uncompensated for by calcium that were ap-plied per tree for the last 5 years in a block of 35-year-old McIntosh apple tree,s in the Cornell Uni-versity orchard. The past 3 years have been difficultscab years. The average annual uncompensated sulfurapplied in this orchard would probably be- in theorder of 10 to 12 pounds per tree (270 to 324 poundsper acre) rather than the 12 to 16 pounds per tree(324 to 432 pounds per acre) that the table indicates.Ammonium sulfate is the most acidifying nitrogenousfertilizer commonly used, and no more than 8 poundswould be applied annually to the ground under suchtrees. The sulfur contribution of such a dose is 1.9pounds per tree or 51 pounds per acre, about one-sixth of the uncompensated sulfur applied in the 1943

TABLE I.—Sulfur spray and dust load on a 3^-year-oldMcIntosh apple orchard, 1943 to 1947.

Year

19431944194519461947

Number of sulfurapplications

Sprays87

HHii

Dusts

23oI3

Approximatetotal gallons

spray per tree

2IOI9O33°310240

Estimated sulfuruncompensated

by lime in spray,lbs. per tree*

12-315-516.116.816.1

"There are 27 trees per acre at 40 X4O feet. About two-thirds of theorchard floor is covered by branches. Approximately 3 pounds of calciumcarbonate are required, to neutralize the acidity cauesd by i pound ofsulfur.

•sprays. Actually, smaller doses are more common,and in recent years ammonium sulfate has been re-placed by ammonium nitrate as the main source ofnitrogen for northeastern orchards. Thus the acidify-ing effect of the fertilizer in most McIntosh orchardsmay be closer to one-tenth that of sulfur spray thanto one-sixth of it.

Most of the spray materials that reach the trees areweathered to the ground under the branches in thecourse of the growing season, a small proportion re-maining on the leaves until they fall. The fertilizer isusually broadcast under, the branches. Thus, exceptfor spray drift and blown leaves, the acidification re-sulting from these practices is concentrated under thespread of the branches. In a mature orchard, such asthe one involved in Table i, from one-half to two-thirds of the orchard floor is covered by branches, sothat the actual rate of acidification per unit area undermature trees may be 50 to 100% greater than thatindicated by multiplying the sulfur load per tree bythe number of trees per acre. That is, the uncompen-sated spray and dust sulfur reaching the ground underthe oldest McIntosh trees in the Cornell Universityorchard in 1943 may have been at a rate of 324pounds per acre, but under the trees the rate wasprobably closer to 500 pounds per acre. Limestone atthe rate of about 1,500 pounds per acre would havebeen required to counteract the potential acidity ofthe sulfur falling on that zone in the course of oneseason.

Thus, one would expect to find under mature treesgreater acidification at the surface and greater depthof acidification than in the zone beyond the spread oftheir branches. Fig. i shows that this has happenedunder one of the 35-year-old McIntosh trees situatedon Dunkirk silty clay loam, a soil of high exchangecapacity derived from calcareous materials, in theCornell University orchard. The arbitrary separationbetween soil above and below pH 5 in Fig. i revealsa bowl-shaped zone of acidification deepest close tothe trunk and becoming shallower as the perimeter ofthe tree canopy is passed. Data on which this figureis based indicate also that the soil has been markedlyacidified below a depth.of 3 feet under the branchesof the tree, but probably not deeper than 2 feet in theregion not covered by tree branches.

There are areas under the tree of Fig i. and underother trees in this same block, where the surface pHis considerably below 4 and where there has been nograss growth for several years, in spite of the factsthat the soil was not disturbed by cultivation and that

'Contribution from the Department of Pomology, Cornell University, Ithaca, N. Y.2Assistant in Pomology, Professor of Pomology, and Associate Professor of Agronomy, respectively.3SuDDS, R. H. Tests of four nitrogen carriers in a mature apple orchard at Martinsburg, West Virginia, W. V. Agr. Exp. Sta.

Bul. 315. 1944.4VoLK, GAYLORD M., and PEECH, MICHAEL. Factors affecting the soil sampling procedure. Proc. Soil Sci. Soc. Florida, 2:12-20.1940.

370

EMBLETON, ET AL. I ACIDIFICATION OF SOIL IN APPLE ORCHARDS 371

DISTANCE FROM TRUNK16 2 K) B 64 20 2468 K> 12

TABLE 2.—The pH and responses to lime and fertilizer bylegume and grass seedings on acid soil under apple trees.

48NORTH SOUTHWEST

FIG. I.— Soil pH in relation to distance from the trunk on twosides of a 35-year-old McIntosh apple tree situated onDunkirk silty clay loam, Cornell University orchard, Ithaca,N. Y. Maximum distance from trunk to outer branches is14 feet. Soil was sampled at 2, 4, 6, 8, 10, 12, and 16 feetfrom the trunk on the north and southwest sides. At eachdistance from the trunk samples were taken with a soil tubeat depth intervals of 6 inches or less to depths of 4 feet.

there is abundant volunteer cover between the trees.In order to study the effects of lime and fertilizer insuch situations, 32 sets of small plots were set up inApril, 1947, in the zones of greatest shading undereight of the trees that' had no grass growing underthem. All plots received ammonium sulfate at 400pounds per acre. Half of the 32 sets were seeded toa mixture of four grasses and half were seeded to amixture of four legumes.

Each set was composed of four parallel 2 X 3 footplots treated as follows : ( i ) Check, no further treat-ment; (2) dolomitic limestone, 2 tons per acre; (3)20% superphosphate, 400 pounds per acre, and 60%muriate of potash, 133 pounds per acre; and (4) limeplus PK at the above rates. Yield and pH data aresummarized in Table 2. The data indicate that limingresulted in germination and survival of the seedings,and that phosphorus and potash benefited them if theyhad been limed. Low light, however, may be a factorlimiting their survival for another season.

While this extreme acidification may be indirectlyharmful under the conditions of the Cornell Universi-ty orchard whose subsoil has a,tremendous reserve

Treatment*

The pHj at

O-1in. 1-3in.

3-6in.

6-9in.

9-12in.

Dryweight

in gramsper plotj

Legume §PK..O. . .LPK.L. . . .

4.04.07-i7-4

4.03-94.64.6

3-93-94-3

4.04-o4.2

4-24.24-24-3

oo5-5

'2.0

Grass!PK. . . . . . . . . .O . . . . . . . . . . .LPK.. . . . . . . .L . . . . . . . . . . .

•\ Q

3-8•7 &7-°7-8

O 7

5-2

5-2

-2 Q

-2 Q

4-24-1

4.04.04.24.2

4-24-14..^4-3

0u-55-4

*See textNfor rates of lime and fertilizer application.tpH of thin paste with glass electrode. Each figure is the average of 16

borings, i per plot, taken 140 days after liming.JEach figure is the average dry weight for 16 plots, 2 clippings.§Legume seeding: Alfalfa, red clover, ladino clover, birdsfoot trefoil.

Grass seeding: Perennial rye grass, red fescue, orchard grass, rough stalkedmeadow grass.

of bases, we have no evidence of injury to the treesas yet. It is possible however that McIntosh trees onsoils that were initially very acid throughout the pro-

22

DISTANCE FROM TRUNK OF TREE • FEET-

FIG. 2.— Soil pH in relation to distance from the trunks of25-year-old McIntosh apple trees situated on unlimed Hoo-sic gravelly loam, Crowell orchard, Wallkill, N. Y. Maxi-mum distance from trunk to outer branches is 12 feet. Soilwas sampled at 2, 8, and 22 feet from the trunk on four sidesof four trees. Each pH figure represented above is the aver-age of the 16 samples for a given depth.

372 SOIL SCIENCE SOCIETY PROCEEDINGS 1947

file have been brought to magnesium deficiency byfurther acidification of this sort. The situation at the

- Crowell orchard in the southern Hudson Valley isan example. This orchard is situated on Hoosic grav-elly loam, a soil of low exchange capacity derivedfrom acid materials. Not only is there .very sparsegrass cover under the spread of the branches, but.about a quarter of the trees show magnesium deficien-cy symptoms .in some years. Fig. 2 summarizes dataon the relation of soil pH to distance from the trunksof 25-year-old McIntosh trees in the Crowell orchard.It shows the same pattern of acidification as does Fig.I in a situation initially more acid.

In the fall of 1946, high magnesium limestone, at4 tons per acre, was applied to several strips of treesin the Crowell orchard. The application was madewith an endgate truck spreader which distributed uni-formly over the area treated. Fig. 3 shows the relation

.of pH to distance from the tree trunks a year afterliming. There was a measurable increase in pH, dueto liming, but the difference in acidity between thearea under the spread of the branches and the areabeyond persisted. Below the surface inch, soil acidity

• was far greater than that of unlimed grassland out-side of the orchard. The liming benefited volunteercover on the orchard floor, but in September, 1947,II months after treatment, magnesium deficiency wasagain evident on some of the limed trees as well ason some of the unlimed ones.

These preliminary studies of soil acidification in .two apple orchards tend to confirm our fears that theacidity of the soil under McIntosh trees sprayed withelemental sulfur is being increased at a rapid rate.They indicate that inability qf grass to grow underthe trees, and in some instances, magnesium deficien-cy of apple trees may have been brought about by the ,.resultant loss of nutrients. Future work should giveus a better understanding of the prevalence of these

5.3

5.1

4.9

4.7

4.5

4.3

4.1

3.9

3f

as

DEPTH -INCHES-—————— O-1

— —— 64- — — —— 9-12—— ——— 12-18- ———— Id-24

2 8 22

DISTANCE FROM. TRUNK OF TREE • FEET-

FIG. 3.— Soil pH in relation to distance from the trunks ofMcIntosh trees at the Crowell Orchard n months aftersurface application of dolomitic limestone at 4 tons peracre. Maximum distance from trunk to outer branches is12 feet. Soil was sampled at 2, 8, and 22 feet from the trunkon four sides of four trees. Each pH figure representedabove is the average of the 16 samples for a given depth.

conditions and of the maintenance programs neces-sary to prevent their occurrence.