2. Test physical and chemical treatments on the Fred Shaeffer ranch to determine their effect on water infiltration, tree growth, and fruit yield.

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1 INFILTRATION IMPROVEMENT FOR PRUNE AND WALNUT ORCHARD SOILS Abstract Michael J. Singer, John R. Munn, Jr., and William E. Wildman Water penetration in many California orchards and vinyards is a serious management problem. Chemical properties of soils related to their aggregate stability and crust strength were measured to determine if one or more properties could be used to predict crusting. None of the measured properties, including aggregate stability, texture, organic matter, and various forms of iron, aluminum, or silica were suitable for this purpose. A field trial in a prune orchard in Yuba county showed that 5 metric tons per hectare phosphogypsum applied to the soil surface greatly improved the final 24 hour infiltration rate and total water entering the soil in 24 hours. Deep disturbance of the soil was also effective, but only until irrigation water was added, after which the soil returned to its original slow water penetration. Two metric tons/hectare phosphogypsum incorporated into the soil did not improve water penetration. The length of time that 5mt/ha phosphogypsum is effective in improving infiltrationrates is unknown, and we do not know if the increased water infiltration increased tree growth or yield. OBJECTIVES 1. Complete the chemical and physical characterizationof 9 soils to determine which properties contribute most to surface sealing and slow water infiltration. 2. Test physical and chemical treatments on the Fred Shaeffer ranch to determine their effect on water infiltration, tree growth, and fruit yield. PROCEDURE 1. Laboratory Iron, aluminum, and silica were extracted with citrate-bicarbonatedithionite and by the acetylacetone in benzene techniques to determine the affects of these components on aggregate stability and crusting. Aggregate stability was determined by standard wet sieving and by the Emerson dispersion test. Modulus of rupture was measured on crusts formed by rainfall simulation. Samples were shaken in distilled water for up to 528 h to determine if the rate of cation release was a measure of crusting susceptability. 2. Field Twelve 4-tree plots were established on a prune orchard in Yuba county in May Three plots were not treated, three were trenched and refilled to simulate deep ripping or slip plowing, three were treatedwith 2 metric tons/hectareof phosphogypsuiii (tilled into the

2 soil) prior to irrigation, followed by 5 metric tons/ha applied to the surface after harvest, and three plots were both trenched, refilled, and treated with phosphogypsum. Double ring infiltrometer measurements of final 24 h infiltration rate and 24 h cumulative water infiltration were used to evaluate the treatments. RESULTS AND CONCLUSIONS 1. Laboratory No clear trends are apparent in the laboratory data (Table 1). Vina has the highest aggregate stability, and the largest amount of dithionite extractable Fe, AI, and Si. This is the expected relationship because the dithionite extractable Fe, AI, and Si are believed to be important in soil structure. However, the Wyman soil has the lowest aggregate stability with moderately high values of Fe, AI, and Si. Other soils have stabilities between the Vina and Wyman. The Emerson dispersion test is used to qualitatively place sa.ples into groups of lik~ dispersion ~nrl~laking (Table 2). Vina is strongly aggregated and Vernalis is weakly aggregated according to this test. These results are similar to and support the data in Table 1. Remolded aggregates of Ryer strongly disperse and those of Wyman moderately disperse. These have the lowest percent stable aggregates. An acetylacetone in benzene extraction of iron was done. There is one report in the literature which suggests that this extraction is related to aggregate stability. Our results do not show a clear relationship between the amount of Fe extracted and the percent water stable aggregates or crust modulus of rupture (Table 3). Modulus of rupture is a measure of crust strength. The higher the modulus of rupture, the stronger is the crust and the slower the water penetration. There was considerable variation in modulus of rupture, but no clear relationship to percent iron. An inverse relationship exists between aggregate stability and crust strength for several of the soils. Wyman has the lowest aggregate stabilty and highest crust strength. This is reasonable since the same rainfall energy was used to create the crusts on all soils. Those which had the lowest aggregate stability should form the strongest crust. However, the relationship did not hold for all the soils. Sodium affects aggregate stability and dispersion. We showed previously (see 1982 report) that the Wyman soil has >4~ exchangeable sodium. Soils which release ions rapidly or in large amounts should have better stability than those which don't. We measured electrical conductivity and salt release from the nine soils by shaking them in water. At the end of 72 h each soil was suction filtered, the filtrate was saved for analysis, the soil was returned to the shaking containers and additional water was added. This was repeated 3 times (Table 4). The soils released cations at different rates and different total amounts were released. We expected those with rapid release rates to be less predisposed to swelling, dispersion, and 105.

3 -- crust formation than those with low rates. The same is true for those which release large amounts of cations compared to small amounts. Sorrento, for example, releases a large amount of cations rapidly, while Wyman releases a small amount slowly. Plots of electrical conductivity over time for the three trials for three soils show the same pattern. Ryer releases salts quickly throughout the three trials (Figure 1), while Vina (Figure 2) and Wyman (Figure 3) release smaller amounts at lower rates. The very low release rate and amount from Wyman is notable. This is the soil which was in our field trial. It has unstable structure and has severe crusting problems. From these data, we cannot determine with certainty that a soil will or will not form a crust or have water penetration problems. Salt release curves may give some useful information, but additional interpretation of the data are necessary. We will continue to evaluate these data to determine how best to predict what soils will have water penetration problems. 2. Field Soil moisture at 15, 30 and 45 cm cumulative 24 hour infiltration, and all three trials are given in Tables separately. for trials 2 and 3, and data on final 24 hr infiltration rate for 5 and 6. Each trial is discussed Trial 1. Surface soil water content was observed to be higher in the trenched plots because of the mixing of moist subsoil materials during the excavation process. This difference in moisture content did not appear to influence the results of Trial 1. Cumulative 24 hour infiltration volume for the trench and trench plus gypsum treatments was significantly greater than for the gypsum or control treatments. There was no significant difference between the control and gypsum treatment or between the trench or trench plus gypsum treatment. The 2 metric ton/ha incorporated gypsum treatment had no beneficial effect on cumulative infiltration, nor did it have an effect on final infiltration rate. When compared to the results of trial 3, it appears that cultivation either dispersed the gypsum throughout the top 10 to 15 cm of soil, thereby diluting its effectiveness, or removed it entirely from the soil surface. The very large increase in cumulative infiltration and final infiltration rate of the two trenching treatments compared to the gypsum and control treatments indicates that one cause of slow water intake at this site is a dense subsoil. This is supported by bulk density measurements on clods from the soil surface and subsoil (.5 to 1 m) which averaged 1.6 and 1.7 g/cm3 respectively prior to trenching. High bulk densities are related to decreased porosity, and trenching helped to reduce the average density and increase infiltration rate. 106.

4 A successful treatment, however, should last for at least an entire season. Subsequent measurements indicated that the beneficial effects of trenching alone did not persist past the first irrigation. Trial 2. A second set of infiltration measurements was made following the first irrigation. Surface soil water content was observed to be higher for Trial 2 than for Trial 1, and there were no significant differences among water contents at 15 cm within Trial 2. Samples from 30 cm in the trenched treatments had higher moisture contents than the control or gypsum treatments. This may be the result of increased infiltration during the first irrigation. There were no significant differences among treatments for either the cumulative 24 hr infiltration or final infiltration rates. However, the cumulative infiltration and final infiltration rates for the control and gypsum treatments were higher (but not significantly different) than for trial one. This increase may result from an interaction of soil condition and measurement method. The rings of the double ring infiltrometer are pounded into the ground before water is added. During the growing season, a compacted subsoil layer became more difficult to penetrate. During the first trial, the rings were ahlf> to penetrate thf' pan; but for the second and third tria1s. tht!v could barely enter it. Thus, the higher infiltration rates for the control and gypsum treatments in Trial 2 compared to Trial 1 reflect increased lateral flow of water between the infiltrometer rings and the pan rather than vertical flow into the soil. It is apparent from these data that neither the 2 metric ton/ha gypsum nor trenching treatments improved water intake after the first irrigation. Either the surface crust reformed, the soil density increased, or water penetration in the trenched plots was inhibited by a residual high water content following irrigation. Data from Trial 3 indicate that both the surface crust and increased subsoil density were reestablished by the time we made the second set of infiltration measurements. Trial 3. After harvest, we ran a final set of infiltration measurements. The original trench plots were reexcavated to 45 cm and an additional 5 metric tons/ha of phosphogypsum was spread on the surface of these and the original gypsum plots. The control plots remained untreated. Cumulative 24 hour infiltration depth on the treated plots was nearly twice as great as on the control plots. This difference was significant at the 0.05 level. Also, when compared to Trial 2, cumulative infiltration depth was more than doubled on the treated plots but only slightly greater on the control plots. As with Trial 2, lateral movement of water between the compacted pan and the infiltrometer rings accounted for a larger proportion of tht! infiltrated water on the control and gypsum plots than on the trenched or trerlch p Ius gypsum plots. 107.

5 The final infiltration rate for Trial 3 followed a pattern similar to cumulative infiltration, except that the final rate on the trenched plots was less than on the gypsum and trench plus gypsum plots. However, this difference was not significant at the 0.05 level because of a high degree of variability in the infiltration rate measurements. The results of Trial 3 clearly demonstrate the benefits of gypsum in promoting cumulative infiltration and in maintaining high infiltration rates. The effect of trenching is less apparent in this trial because of the lateral movement of water on the untrenched plots. Phosphogypsum adds salts to the soil solution as irrigation water enters the soil, thereby promoting aggregate stability. However, it is effective only if it remains on the soil surface prior to irrigation. At this time we do not know how long the 5 metric tonjha treatment will remain effective; but we do know that gypsum is ineffective when it is disked into the soil. Tillage and traffic compact the subsoil and crush the few surface aggregates Deep tillage is required to break up established compaction pans and significantly improves water penetration, but this treatment is short lived if there is continuous traffic over the soil. This study shows that 5 metric tons/ha phosphogypsum is effective in improving water penetration, and it indicates that reduced traffic after deep tillage may be beneficial. We have not determined whether the phosphogypsum application or deep tillage are economical, or if the improvement in water infiltration results in improved tree health, vigor, increased yield, or improved crop quality. 108.

6 Table 1. Percentages of Fe, AI, and Si extracted by citrate-bicarbonatedithionite and aggregate stability for 9 soils. Soil Name Fe Al Si Sand Silt Clay Organic Stable < carbon aggregates (%) > < (%) > (%) (%) Columbia Ryer San Joaquin Sorrento Vernalis Vina Wyman Wyo Yolo

7 Table 2. Results of the Emerson dispersion test. Dry aggregates Soil Name immersed in water Remolded immersed aggregates in water Columbia no dispersion, slight slaking Ryer no dispersion, slight slaking slight strong dispersion dispersion San Joaquin no dispersion, no slaking Sorrento no dispersion, strong slaking moderate moderate dispersion dispersion Vernalis no dispersion, strong slaking strong dispersion Vina no dispersion, slight slaking very slight dispersion Wyman no dispersion, slight slaking moderate dispersion Wyo no dispersion, no slaking very slight dispersion Yolo no dispersion, slight slaking strong dispersion

8 Table 3. Acetylacetone in benzene extractable Fe related to percent water stable aggregates and crust modulus of rupture. Soil Name Percent stable aggregates (%) Fe Crust of modulus rupture (bars) Columbia Ryer San Joaquin Sorrento Vernal is Vina Wyman Wyo Yolo Ill. --

9 Table 4. Sum of cations released during shaking trials. Soil Sum of Cations released Overall Name Trial 1 Trial 2 Trial 3 sum (MEQ/L) (MEQ/L) Columbia Ryer San Joaquin Sorrento Vernalis Vina Wyman Wyo Yolo

10 Table 5. Soil moisture at 15, 30 and 45 cm for the four sets of plots on Wyman soil. 15 cm Soil Moisture (%) 30 cm Soil Moisture (%) 45 cm Soil Moisture (%) TrialII Trial1/ TrialII TREATMENT CONTROL 11.6a 4.5a 10.2a 6.la NM 5.5a GYPSUM 11.8a 5.4a 10.5a 6.0a NM 4.4a TRENCH 11.2a 8.1b 13.2b 9.0ab NM 8.7b GYPSUM a 7.2b 14.8b 9.8b NM 9.5b TRENCH Trial 1/2 was in July 1983 after an irrigation. Trial 1/3 was in September 1983 after harvest. The same letters following numbers indicate no statistical significant difference at 0.06 level using F tests and simple comparison of means. 113.

11 Table 6. Cumulative 24 hr. infiltration and final 24 hr. infiltration rate measured by double ring infiltrometer on the Wyman soil. Cumulative 24 hr infiltration (em) Final 24 hr infiltration rate (em/h) TrialII TrialII TREATMENT CONTROL 5.88a 9.03a 11.86a O.l1a 0.16a 0.2la GYPSUM 6.96a 8.66a 22.15b 0.06a O.17a 0.64b TRENCH 27.40b 9.57a 21.OOb 0.55b 0.14a 0.26ab GYPSUM + TRENCH 30.40b 9.10a 21.60b 0.43b 0.17a 0.55b Trial 111was in May 1983 prior to any irrigation. Trial 112was in July 1983 after an irrigation. Trial 113was in September 1983 after harvest. The same letters following numbers indicate no statistical significant difference at 0.05 level using F tests and simple comparison of means. 114.

12 r\ L RYER 240 o - TRIAL 1 TRIAL 1 Y=A+Be-CX 220 <> - TRIAL 3 [] [] - A=215 B= " C= v R2= >- ('I). - TRIAL I TRIAL t-' U Y=A+Be-CX t-' U'1 ::>. A= Q 120 z B= a u 100 C= J R2= <{ u 80 H" 60 TRIAL 3... u w Y=A+Be-CX -J 40 w A=61.54 B= C= I I I I I I I I I R2= , TIME (h)

13 A L VINA 240 o - TRIAL 1 TRIAL 1 (Y). - TRIAL 2 Y=A+Be-CX TRIAL 3 A=133-8= C= "'0 R2= v 180 >- I:t 160 > TRIAL 2 H t- 140 u Y=A+Be-CX I-' I-' ::> A= '. z a 120 8= C= u 100 -J [] R2a «u 80 H Ck: 60 / TRIAL 3 t- u w Y=A+Be-CX -J 40 A= w 8= C= R2= TIME en)

14 L WYMAN f"'\ (.Y) - "'U v 180 >- t- H 160 > H t- 140 u f-' f-' ::> --.J. Q 120 z a u 100 -I <{ u 80 " t- 60 u -I w 240 o -. TRIAL 1 TRIAL 1 - TRIAL 2 Y=A+Be-CX TRIAL 3 A= = C= H (] R2= TRIAL 2 Y=A+Be-CX A= = C= R2= TRIAL 3 w 40 Y=A+Be-CX A= = C= R2= TIME Ch)