(1959) By L. D. Romberg, C. L. Smith, and H. L. Cranel
The experimental orchard of the U.S. Pecan Field Station at Brownwood, Texas, is located within the flood plain of the Pecan Bayou on Frio clay and clay loam. This is an alluvial soil having a water table at a depth of 17 or 18 feet at this site. About half the pecan acreage was planted to Burkett and Success in early spring 1932 and the rest was planted to Western and Jersey varieties the following year. This orchard was unique because the trees were spaced 35 feet apart on squares, much closer than ordinarily recommended for pecans. An important advantage of close spacing is that it results in greater nut production during the early years of the orchard, the total being in proportion to the number of trees so long as they do not interfere with each other. Disadvantages are that more trees must be planted and brought into bearing and removed later at considerable expense. In addition, close spacing may favor development of fungus diseases. Therefore, it appeared desirable to determine whether close spacing could be justified in the western pecan-growing region, where tree growth is considerably slower than farther east and where fungus diseases are less troublesome. It was intended that the trees would be thinned in stand when they began to "crowd." Reports of experimental obtained in this orchard appeared in the Proceedings of the Texas Pecan Growers' Association in the past (6, 3, 4).
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The chief factors in the performance of pecan trees in the Brownwood area, other than variety, are believed to be the available soil moisture and essential mineral elements. At Brownwood, Texas, the total average annual rainfall for the past 64 years has been 27.44 inches.
Some effects of soil moisture deficiency well known to pecan growers, or to be true, are as follows: (1) A period of extreme deficiency in moisture will cause a pecan tree to die. (2) In a period of less extreme deficiency the outer extremities of the branches may die back but the trunk and large inside branches may alive. A new top is rebuilt with a resumption of normal conditions. (3) A shortage of moisture during the growing season will retard tree growth. (4) Moisture deficiency in July and August, when the nuts are completing their growth, decreases nut size. (5) Moisture deficiency at the time of kernel
Horticulturist, Plant Physiologist, and Principal Horticulturist, respectively, Crops Research Division, Agricultural Research Service, U. S. Department of Agriculture, Brownwood, Texas, Shreveport, La., and Beltsville, Md. respectively.
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Filling reduces the capacity of the leaves to manufacture materials needed to fill the nuts. A common belief that drouth causes immature nuts to drop is refuted by evidence. In an extreme drouth a pecan tree will shed its leaves before it sheds it fruit.
Annual evaporation from a free water surface amounts to some 60 to 70 inches at Brownwood, and during July and August about 10 inches is evaporated per month (1). The pecan tree is deep-rooted on deep, friable soil and in the past years a belief was prevalent in western Texas that the taproot must go to water for the tree to do well. However, the greater portion of the feeding root system lies relatively close to the surface, where the soil is more fertile and better aerated than farther down.
Effects of a deficiency in minerals are generally less apparent than those of a deficiency in moisture because it is usually difficult to know the nutritional status of a tree With respect to most of the essential minerals, such as nitrogen, potassium, phosphorus, and zinc. However, mineral deficiencies are known to result in poor tree vigor and low nut production over a period of years.
Studies on the performance of the trees in this orchard from the time it was established until the winter of 1950-51 indicated that the expected rainfall was not adequate at all times for growth and fruiting of the trees. In addition the trees were small for their age and indicated that they would respond to re-spacing, or thinning of the stand, since they had been much closer than usual. An experiment was then planned to determine their response to supplemental irrigation and re-spacing. Re-spacing will be referred to as thinning.
By the winter of 1950-51 the trees were 11 to 12 inches in diameter and had attained so much branch spread that they appeared to be "crowding" according to generally accepted standards. An experiment was initiated to determine the effects of more space on individual trees. The trees were spaced by removing certain trees in alternate diagonal rows and water was supplied by irrigation. Theoretically, increased space should provide each tree with more light, soil moisture and minerals.
The soil management practiced in this orchard consisted of growing a cover crop each winter, usually a small grain, which was disked under in late April or as soon thereafter as possible. Thereafter the soil was disked as needed to keep the orchard free of weeds until the time for seeding the winter cover crop. Temporary borders were thrown up for each irrigation, which consisted of approximately. 6 acre-inches of water. After each irrigation, the borders were pushed down and the orchard disked.
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The experimental plan provided a comparison of the effects of irrigation and the thinning of the stand of trees as follows:
Treatment 1. Trees thinned to 49.5' x 49.5' spacing on diagonal. Irrigated when needed.
Treatment 2. Trees left at 35' x 35' spacing. Irrigated when needed.
Treatment 3. Trees thinned to 49.5' x 49.5' spacing on diagonal. Not irrigated.
Treatment 4. Trees left at 35' x 35' spacing. Not irrigated.
The orchard was divided first into 8 blocks, half in the Burkett Success orchard and half in the Western-Jersey orchard. Each block was divided into 4 plots. Within each plot, 3 trees each of the two interplanted varieties of approximately average size were selected for records. Thus, the experimental results for each treatment for each variety were based on 12 trees or on 48 trees for all four varieties in each treatment. The plots were randomly distributed so that the trees for each treatment would be comparable.
Rainfall and Irrigation Water Applied
Year 1951: Total rainfall in 1951 was 21.32 inches. Water was applied to irrigated plots on April 2 to 9. On June 11 and 12 heavy rains covered the entire orchard with flood water and the soil was brought to field capacity for a considerable depth. Water was applied to the irrigated plots again during August 2 to 8. A light nut crop was produced.
Year 1952: The winter of 1951-52 was dry and only 19.87 inches fell during the year. Water was applied to the irrigated plots during March 25 to April 4 and during July 23 to 29. A good nut crop was produced,
Year 1953: During 1953, good rains fell in March, April, and May bringing the annual rainfall to 25.15 inches. Water was applied to the irrigated plots during July 14 to 21 and September 3 to 7. The nut was heavy but the nuts were poorly filled. The foliage was by downy spot more than usual and many leaves fell before the nuts matured,
Year 1954: The year 1954 was very dry with only 12.83 inches of rainfall. Water was applied to the irrigated plots during June 14 to 17 August Il to 16. The nut crop was very light and non-irrigated trees shed their leaves early.
Year 1955: Total rainfall for the year 1955 was 22.24 inches. The winter was very dry and water was applied to the irrigated plots from March 8 to 14. Since there was little rainfall in the spring it was feared that non-irrigated trees might become severely injured or die, and a departure was made from the experimental plan by irrigating all plots from May 11 to 24. -Water was applied to the irrigated plots again to 19. A late March freeze almost completely destroyed nut crop.
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Year 1956: Total rainfall during 1956 was 17.42 inches. Because of a dry winter and spring all plots were irrigated between March 19 and 31. On May 2 and 3 the entire orchard was covered by flood water. Water was applied to the irrigated plots from July 9 to 19 and again from August 24 to September 1. A heavy crop of nuts was produced except on the Jersey variety.
Year 1957: Total rainfall during 1957 was 37.07 inches. Because of the dry winter and early spring all plots were irrigated from March 26 to April 3. No further irrigation was applied as rainfall appeared to be adequate. The crop of nuts was light except on the Jersey variety, which produced a heavy crop.
Summary of Rainfall and Irrigation During the Period of the Experiment: The conditions during the 7-year period of the experiment may be summarized as follows: The total annual rainfall of 6 consecutive years of the 7 years of this experiment was below the 64-year average of 27.44 inches. The average annual rainfall for the 7-year period was 22.25 inches, or 5.19 inches below the expected amount. Only in 1957 did the annual rainfall exceed the normal. During the 7-year period much of the rainfall came in floods or in such small amounts that it was lost by surface evaporation.
The irrigated plots were irrigated 3 times in 1955, once in 1957, and twice in each of the other years. In addition, all the plots were covered with flood water once each in 1951 and 1956. No water application was planned for non-irrigated plots; however, by the end of the extremely dry year of 1954 the trees showed the effects of the drought by a thin canopy and early shedding of leaves. Therefore, these plots were given one irrigation early each year in 1955, 1956, and 1957.
In this experiment, there were two factors, irrigation and the thinning of the stand of trees2, both of which could affect tree growth, nut production, and the characteristics of the nuts produced. In reporting the results of this experiment, the (main) effects of these factors on the several criteria of tree performance are stated. The data relating to the effects of irrigation were obtained by totaling the measurements for the trees that received irrigation regardless of thinning and then similarly totaling the same measurements for the non-irrigated trees regardless of thinning. The difference between these totals represents the effects of irrigation. A similar procedure was followed to determine the effect of tree thinning.
Tree Growth: Data giving the growth of all the trees of the four varieties receiving each treatment during the 7-year period of the
2 These will be referred to as irrigated or not irrigated, thinned or not thinned.
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Experiment as measured by the increase in trunk circumference and in cross-sectional area of trunk are presented in Table 1.
Every year the irrigated trees made more growth than the comparable non-irrigated trees. For the 7-year period the irrigated trees made a gain of 27.95 percent in trunk circumference and 33.02 percent in cross-sectional area of the tree trunk over those that were not irrigated. These results clearly indicate that the water supply of the non-irrigated trees was inadequate.
The thinned trees made more growth than the comparable no thinned trees. For example, during the 7 -year period of the experiment the thinned trees made 15.33 percent more growth in trunk circumference and 17.83 percent in cross-sectional area of tree trunk than did the comparable non-thinned trees. The data in Table 1 show that the effects of irrigation and thinning of trees were additive, in that both increased tree growth, but that irrigation was more effective. Maximum growth was attained when the trees were thinned and irrigated and the least when they were not thinned and not irrigated.
Nut Production: The average 7-year yields of air-dry nuts per tree of the four varieties under each treatment are presented in Table 1. The irrigated trees produced 35.66 percent greater weight of nuts per tree than the comparable non-irrigated trees. Similarly, the trees thinned in stand produced a 23.78 percent greater weight of nuts per tree than the comparable non-thinned trees. In addition the combined effects of irrigation and thinning of the stand of trees was more than additive and resulted in the highest yield of nuts per tree of all treatments. The trees that were thinned in stand and irrigated produced 64.83 percent more nuts per tree than those not thinned and not irrigated.
Size (Volume) of Individual Nuts: The average volumes, or sizes, of the nuts produced by the trees in the various treatments during the period of the experiment are given in Table 1. Both irrigation and thinning the stand of trees increased the size of the nuts produced.
Irrigation increased •the size of the nuts by 12.61 percent over those Of non-irrigated trees. Thinning the stand of trees had a similar but smaller effect, since the increase in nut size was only 6.74 percent. The largest nuts were produced by the trees that were thinned and also irrigated, whereas the smallest nuts were produced by the trees that were not thinned or irrigated.
Weight of Individual Nuts: The average weights of the individual nuts produced during the period of the experiment as a result •of the different treatments are given in Table 1. The nut weights were closely proportional to their size, but not altogether so because of differences in their specific gravities. The effect of irrigation was to increase the weight of the individual nuts by 10.69 percent as compared with the weight of those on similar non-irrigated trees. Thinning the sand of
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trees had the effect. of increasing the weight of the individual nuts by 6.05 percent over those produced by similar non-thinned trees. The heaviest nuts were produced by the trees that were thinned and irrigated and the lightest by the trees that were not thinned and not irrigated.
Specific Gravity of Nuts: The specific gravity of pecan nuts grown under similar conditions has been found to be a good index to their degree of kernel development (5). The specific gravity of a representative nut sample drawn from the crop harvested from each tree of each variety in each plot each year was determined. The data are given in Table 1 for all varieties for the 7 year period by treatment given the trees. The data show that the effect of irrigation was to lower the specific gravity of the nuts by 1.66 percent as compared with similar nuts from non-irrigated trees. The effect of thinning the stand of trees was similar in that the specific gravity was reduced by 0.33 percent as compared with the nuts produced by similar non-thinned trees. These effects were essentially additive, since both the irrigated and thinned trees produced nuts that were on the average 2.03 percent lower in specific gravity than those from similar non-irrigated and non-thinned trees.
Filling and Kernel Quality of Nuts as Determined by Shelling of Samples: In 5 of the 7 years of the experiment nut samples representing each treatment of each variety were shelled for nut characteristic determination. The data obtained (not given) show that on the average the amount of edible kernel in the nuts was closely proportional to their specific gravity. Also if the specific gravity was low the quality of the edible kernels was lower than if the specific gravity was high.
Shell Development: The shell weight per unit volume of nut was calculated from the data obtained in the shelling of nut samples. These data (not given) show significant differences in relative weight of the shell in different years and to a less extent between the nuts produced in the different treatments. On the whole, shell weight per unit volume was directly correlated with the specific gravity of the whole nuts and negatively with the size of the individual nuts. In other words, irrigation, thinning, or both reduced the specific gravity of shell •per unit volume of the nuts, but the differences were small.
In the winter of 1950-51 when this experiment was initiated the trees were undersize for those of an 18 and 19-year-old orchard, even though they had received good care from the time they had been planted. Previous orchard management consisted of clean cultivation in the summer and a winter cover crop of small grain. The cover crop was seeded in the fall and turned under in the spring before bloom or as thereafter, as soil conditions permitted. Even so, the trees were small'
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Probably because of the heavy soil on which they were lactated and because of insufficient moisture during critical periods.
Although the soil was deep it consisted of calcareous materials in which there was no development of a soil-profile such as is true of mature soils of the region. However, the surface is darker than subsoil and contains more organic matter. The soil was fertile and under favorable conditions produced good crops of small grain. Small tests showed that nitrogen applications increased growth of winter cars but there was little or no response by any other crop plant to supplemental fertilization regardless of its composition. The trees were unable to obtain sufficient zinc from the heavy alkaline soil; hence, spray applications of zinc sulfate were made as needed annually to prevent the development of deficiency symptoms.
Tree Growth and Nut Productions: Both irrigation and thinking of the stand of trees had pronounced effects on increasing the growth and fruiting performance of the trees. Of the two factors under study, irrigation
As practiced in a limited manner was the more important. However irrigation and thinning produced similar results and their effects were additive. That is, the trees that had been thinned and also irrigated made the most growth and produced the most and largest nuts. On the other hand, the trees that were not thinned or irrigated made the least growth and produced the fewest and smallest nuts. These results clearly indicate that available soil moisture was the most limiting factor in the performance of the trees. It is also apparent that an insufficient number of irrigations were applied to the irrigated trees during the course of this experiment for maximum effects to be attained.
Relation of Nut Production to tree Growth: The amount of food materials produced by a person tree for use in growth and nut production depends on its leaf area and the effectiveness of the area. The effectiveness depends on the amount of water and essential minerals supplied by the root system and freedom of the leaves from fungus disease, scorch, shedding, insert injury, and other troubles. Ordinarily the quantity of food materials produced would be expected to be in proportion to the amount of leaf surface, since the latter is in proportion to the number and lenth of the current-season shoots to which the leaves are attached.
In this experiment, no attempt was unable to made measure: leaf area, but the tree trunk was measured at the same point each winter to determine the increase in circumference and in cross-sectional area during the past year. Trunk growth is collection of the amount of branch growth, and over a period of years a certain fairly constant ratio between branch spread and the circumference or diameter of the tree is ordinarily
maintained. Thus, if nut production is in proportion to the amount of leaf area it is also in proportion to the growth made by the tree. This relation would certainly not be expected to hold true every year because The nut production by pecan trees is highly variable, but it would be expected to apply to a period of years. The data were examined to determine whether nut production had actually been in proportion to trunk growth during the 7-year period. For all varieties combined, the ratios of growth to yield can easily be calculated from the data given in Table 1. Either increase in trunk circumference or in cross-sectional area of trunk can be used as a criterion of the amount of growth if trees are of the same size; otherwise, the cross-sectional area is the more nearly correct criterion. Even though the trees receiving the various treatments grew at considerably different rates, the production per unit of increase in cross-sectional area was very near the same. The ratios of the increase in trunk cross-sectional area to the yield of nuts In pounds were as follows:
Treatment 1, thinned and irrigated…………………………………………………………. .393
Treatment 2, not thinned and irrigated……………………………………………………. .369
Treatment 3, thinned and not irrigated……………………………………………………. .393
Treatment 4, not thinned and not irrigated............................................................................ 393
The data indicate that possibly thinned trees tended to be slightly more productive than similar untinned trees. The significance of the relation between tree growth and yield of nuts is that a pecan grower should endeavor to maintain good tree growth.
Size and Quality of Nuts: Irrigation and thinning of the stand of trees had similar effects on the size (volume) of the individual nuts produced as they had on tree growth and yield of nuts. Both factors, individually and collectively, tended to increase the size of the nuts over those produced by similar non-thinned or non—irrigated trees. The largest nuts were produced by the trees that were thinned and irrigated and the smallest by the non-thinned and non-irrigated.
Apparently the size of the nuts was a function of the factors affecting the growth of the trees as well as the amount of nuts on the tree. In years of heavy crops the individual nuts were smaller than in light crop years. At Brownwood pecan nuts make their growth in size from time of bloom until about the end of August, when the shell becomes hard and further increase in size, if any, is minor. Conditions favorable for tree growth are likewise favorable for the production of large size nuts.
Wide differences were found in the quality of the nuts produced, as measured by their specific gravity. The specific gravity of the nuts was affected by the size of the crop matured by the trees, the condition of the foliage, climatic conditions, variety, and to a small extent by the effects of the treatments given the trees. The greatest effect on the specific gravity of the nuts produced was from the size of the crop matured by the trees. This was found to be especially true of a second large crop following a previous large one. For example, the crop of 1952.
CLARENCE F. HUTCHES
TEXAS, OKLA„ ARK., LA., MISS., AND ALA.
T EL E P H O N ES :
Office: Taylor 6-6334 — Res.: Taylor 2-9532
P. O. Box 1244
SAN ANTONIO 61 TEXAS
November 2, 1959 Dear Fellow Members:
As most of you know, we have been—consistently over 20 years— one of the largest buyers of seedling pecans. In the last ten years, we have become increasingly important factors in improved varieties, hence this message.
It is our suggestion that where growers are reasonably near each other and assuming they are producing the same varieties, it may prove mutually profitable to market similar varieties together. The larger the quantity, the better the price.
We are particularly interested this year in buying the better-known varieties and invite any grower or dealer ready to sell 3,000 pounds or more to telephone me collect.
Again, this year, we will concentrate pecans, both seedlings and improved, in Houston, Dallas, Ft. Worth, San Antonio and we hope Tulsa. Excepting Tulsa, we maintain adequate funds at warehouses in the cities mentioned and when you sell us pecans the warehouses issue their checks as soon as pecans are unloaded and weights determined.
With best wishes, we remain
Clarence F, Hutches
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Averaged 3.4 pounds per tree, had a specific gravity of, 785, whereas the crop of 1953 averaged 40.1 pounds per tree, and had a specific gravity of .68G even though 5.78 inches more rainfall was received in 1958 than in 1952. Furthermore, the 1954 crop was almost a total failure, presumably on account of lack of food reserved in the trees which was exhausted by the two previous crops, as indicated by the low specific gravity of the 1953 crop. In all cases, the quality of the nuts matured was in inverse relation to the size of the nut crop produced.
There were wide differences in the specific gravity of the nuts matured in the different crop years as a result of thinning the stand of trees or water supplied in irrigation. It been pointed out that the effects of thinning and of irrigation were to increase tree growth, nut production, and the size of the individual nuts. The average overall effect of these treatments was to reduce the specific gravity of the nuts matured. This reduction in specific gravity of the nuts was apparently due to a smaller proportional amount of elaborated food materials being available for kernel development than was available to the smaller nuts matured on the trees that were not thinned or irrigated.
Variety Performance: Irrigation or thinking the stand of trees has little or no differential effect on the performance of the trees of the different varieties. The trees of all 4 varieties were affected similarly by the differential treatments as regards tree growth, fruiting, and characteristics of the nuts produced. However, there were large differences in tree growth and especially in nut production. For example, the trees of the Western variety made the most growth in trunk circumference, which averaged for all trees 4.0S centimeters per tree for the 7-year period. Considering the annual increase in trunk circumference of the Western variety as 100 per cent, which of the varieties Burkett, Jersey, and Success were 95.10, 94.36 and 73.04, respectively. It is not known why the trees of the Success variety made so must less growth than those of the Western variety.
During the 7-year period of the experiment, the average yield of nuts per tree per year of the Western variety was 41.77 pounds. Considering this average yield of the Western variety as 100 percent that of the varieties Success, Jersey and Burkett was only 69.77, 40.10 and 26.87 percent, respectively. These wide differences in yield between varieties growth under the same conditions pointedly emphasize the fact that the success of a pecan-production enterprise depends upon the proper selection of the varieties grown.
Effects of Treatments on Average Production of nuts Per Acre per Year: When the quality of the nuts matured is essentially equal it is production per acre rather than per tree that a commercial grower considers in determining the best treatment. In this experiment on half of the plots contained twice as many trees per acre as the other half.
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It was expected that the increase in yield and quality of the nuts produced would largely compensate for the fewer trees per acre in the thinned plots. This was not found to be true, for in 1957 the non-thinned trees were still producing considerably more nuts per acre than similar trees that had been thinned in stand for 7 years. The average annual nut production per acre of trees of each of the 4 treatments of each variety was calculated and the data are given in Table 2. These data show that the highest yield of nuts per acre per year was matured by the trees of all 4 varieties that were irrigated but not thinned. Comparing all the thinned with all the non-thinned trees in all plots, thinning reduced the average yield by 290 pounds per acre per year (Table 3). When the same comparisons are made the data show that irrigation increased the yield of nuts by 171 pounds per acre per year. The lowest yields of nuts per acre per year were produced by the trees that were thinned and not irrigated. The difference in yield between these trees and those not thinned and irrigated was 462 pounds per acre per year.
Indicated Criterion for Thinning: No formula which will indicate exactly when thinning of tree stand is desirable is known because of differences in soil type, soil-moisture supply, climate, orchard management practices, and other factors. This will have to be determined for each orchard by observing the behavior of the trees and perhaps the results of thinning in small test-plots in the orchard. Orchard management would involve the competition between trees and vegetation such as pasture or a cultivated crop. Competition with such vegetation can be as harmful as competition between trees.
Pecan roots are generally considered to extend outward about 21/2 times as far as the branches and by another formula (2) they extend outward from the trunk about as far as the tree is high. On upland soils where the only water available to trees is from scant rainfall there should probably be little or no overlapping of the roots of adjoining trees. Competition from other forms of vegetation should be kept at a minimum. In contrast, in the fertile soil and semi-arid climate where this experiment was conducted the greatest production of nuts was obtained where the trees were closely spaced and irrigated. They did not seem to be too close together as long as the lower branches were not being shaded out. A maximum permissible spacing under these conditions appeared to be approximately 38 square feet of cross-sectional area of tree trunk per acre. Thus, with 35 trees per acre thinning should be done when the trees attain an average trunk diameter of 14 inches one foot above the ground. Hinrichs (2) reported maximum production of nuts when the tree attained 30 square feet of trunk area per acre.
In this experiment, it was found that the soil moisture supply was the major factor in the performance of the individual trees. According to that conclusion, if the trees are not irrigated they should be given
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Distance and orchard management that provides each tree with a reasonable soil moisture supply, but in a humid climate a certain amount of additional space may be desirable as an aid in disease control. Thus, it appears that the spacing of pecan trees for highest production by the environment may range all the way from such wide spacing that the root systems do not overlap to the closest spacing that does not result in shading out and death of the 'lower branches or such spacing that the combined cross-sectional areas of the tree trunks amount to about 38 square feet per acre.
1. Soil moisture was the major factor governing the performance of trees in this experiment. The annual rainfall varied from 12.83 to 25.15 inches during 6 of the 7 year duration of the test.
2. Irrigation increased the growth of the trees, nut production, and the size and weight of the nuts but decreased the specific gravity of the nuts.
3. Thinning of the stand of trees to half the number had the same effect in all respects as irrigation but to a smaller degree.
4. Response of the individual trees to thinning was indicated to be due largely to the increase in the soil moisture supply resulting from the wider spacing.
5. On a percentage basis the most important effects of thinning, irrigation, or both were on Yield, tree growth, nut size (volume) , individual nut weight, and specific gravity of the nuts in the order given.
6. For the total 7-year period of the experiment the yield of nuts of the trees of each variety under each treatment was largely in direct relation to the increase in cross-sectional area of the trunks, thus indicating the importance of maintaining good tree growth.
7. It was indicated that under conditions of low soil moisture that best results were obtained if the roots from adjacent trees did not overlap. However, if good soil moisture conditions prevailed the trees produced best results under the closer spacing of about 38 square feet of cross sectional tree trunk area per acre.
8. The soil moisture supply, relative tree spacing, and the variety were the three major factors determining nut production per acre.
9. Close planting of pecan trees may be desirable.
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1. Bloodgood, Dean W., R. E. Patterson, and R. L. Smith, Jr. 1954. Water evaporation studies in Texas. Texas Agric. Expt. Sta. Bulletin No. 787.
2. Hinrichs, Herman A. 1958. How much room does a pecan tree need? 1958 Proc. Okla. Pecan Growers' Assoc. pp. 35-38.
3. Romberg, L. D. 1951. Growth and yield of Western pecan trees• Proc. Texas Pecan Growers' Assoc. 30:35-38.
4. Romberg, L. D. and C. L. Smith. 1956. Growth and yield of pecan trees at the U. S. Pecan Field Station, Brownwood, Texas. Proc. Texas Pecan Growers' Assoc. 35:40-46.
5. Smith, C. L., Joseph Hamilton, and L. D. Romberg. 1948. Specific gravity and percentage of kernel as criteria of the development of pecan nuts. Proc. Amer. Soc. Hort. Sci. 51:157-170.
6. Smith, C. L. and L. D. Romberg. 1945. Relative behavior of four varieties of pecans during the development of an orchard at the U. S. Pecan Field Station, Brownwood, Texas. Proc. Texas Pecan Growers' Assoc. 24:10-15.
Table 3. Average yield of nuts per acre per year for 1951 to 1957 as affected by thinning the stand of trees and by irrigation, Brownwood, Texas.