Soil organic material (SOM) is very low (0.7-1.4%) in my Chihuahua Desert farm. For this reason, I sought an effective and efficient carbon source to compost with dairy manure from the nearby Mesilla Valley, New Mexico. The Valley is rich in dairies, pecans, chili and cotton. Pecan shells (C:N 143, measured 2008) appeared to be a reasonable carbon source, which might gradually be broken down, slowly releasing its carbon content and forming a slowly decomposing compost. Following pilot studies in 2008, I composted manure:pecan shells (v:v 9:1) for nine months, creating an approximate C:N 30:1 compost. The compost was applied along with control amendments in four groups: untreated, shells alone, manure alone and compost. Seventy nonbearing pistachio trees (cultivar Kerman grafted onto UCB-1 rootstock) were in each group. Each group received identical irrigation, fertilization and pesticide application.
Nine months after application to the trees, we analyzed soil nutrients. Soil results included: (1) modestly increased SOM and CEC, (2) reduced pH in both the manure and compost groups, (3) reduced sodium content in the compost group compared to the manure group, (4) increased iron and manganese content in the shell group (perhaps because of binding by shell lignin and polyphenols), and (5) shells alone or composted had reduced total nitrates compared to untreated or manure alone. This may have resulted from nitrogen (N) immobilization by the composting pecan shell carbon. After 11 months, we analyzed leaf nutrients and observed differences in tree growth. Leaf N content was highest in the compost group, followed by manure, shell and untreated groups. No changes of tree growth or leaf health were observed at 11 months.
We conclude that, at 9-11 months, mulched or composted pecan shells may lead to N immobilization. Increased SOM and CEC and reductions in soil pH and sodium may result from pecan shell composting. As has been previously reported, we demonstrated that pecan shells may bind iron and manganese. Potential deleterious effects of N immobilization may be corrected by nitrogen application during compost production or application, by reducing the C:N ratio or by using other carbon sources. Alternative carbon sources for composting locally available in the Mesilla Valley (e.g. cotton gin by-product) were discussed and will be proposed in my pending Western SARE grant re-application.
1. Improve low SOM content in the soil of desert farms, such as my own.
2. Gain experience and local producer recognition in large scale agricultural composting.
3. Conduct a systematic process to find an effective and efficient carbon source in the nearby Mesilla Valley, NM for future composting.
4. Measure the effects of manure:pecan shell composting on pistachio tree growth and nutrition.
5. Modify compost composition in future studies and Western SARE grant proposals.
Following pilot studies in 2008, I composted manure:pecan shells (v:v 9:1) for nine months, creating an approximate C:N 30:1 compost. The compost was applied along with control amendments in four groups: untreated, shells alone, manure alone and compost. Seventy nonbearing pistachio trees (cultivar Kerman grafted onto UCB-1 rootstock) were in each group. Each group received identical irrigation, fertilization and pesticide application. Nine months after application to the trees, we analyzed soil nutrients. After 11 months, we analyzed leaf nutrients and observed differences in tree growth.
(Please refer to Table 1 attached for complete data):
Since pistachio trees are highly salt tolerant, manure or compost-related increased salts are of relatively minor concern. This permits SOM enhancement by careful use of these amendments. Cost analysis suggested that we could purchase and transport, via commercial carrier large quantities of dairy manure and pecan shells from nearby (12-30 miles) Mesilla Valley dairy farms and pecan processors. We mixed the by products to create an appropriate C:N ratio 30:1 . The compost was mixed, maintaining proper oxygenation, moisture and temperature for nine months in order to mineralize its components. The resulting compost was applied to 70 pistachio trees in bands four feet wide, dug in 6-12 inches deep, along with three control groups (untreated, shells alone and manure alone).
At nine months, soil samples were obtained by experienced soil samplers who were unaware of study design. Increased SOM and CEC and iron/manganese content and reduced sodium content (29% compared to manure group) were found in the compost group. Soil tests revealed possible N immobilization in groups treated with shells alone or shells composted compared to groups not treated with shells (untreated and manure alone). Nitrate content in the shell group was 53% less than in the control group. Nitrate reduction in the compost group (29%) was greater than could be attributed to shell proportion (approximately 10%). At 11 months the trees were visually inspected for differences in growth and leaf structure and health. No differences were observed among the four groups. Allelopathic effects from the pecan shells’ juglone were not observed. Similarly, leaf analyses at that time revealed the highest N content was in the composted group (2.60%, followed by the manure group (2.49 %), shells group (2.47%) and untreated group (2.45)). Suggested N concentration in August-sampled pistachio trees is 2.2-2.5%.
These data resulted from a relatively small study and were not statistically analyzed. However, the results were both consistent and plausible. The modest results are suggestive of both beneficial effects and potentially one detrimental effect of pecan shell composting with dairy manure. More rigorous study with statistical analyses may be useful.
Educational & Outreach Activities
A fact sheet outlining the problems of desert farming, an introduction to large scale agricultural composting, results of our Western SARE-funded project and recommendations and suggested new hypotheses was produced. This fact sheet will be distributed to producers/processors, as originally proposed in the grant application, at meetings of the Dona Ana Soil and Water Conservation District meeting and at the Western Pecan Growers meeting in March, 2011. The poster produced displaying this project will be displayed at the New Mexico Western SARE meeting in Clovis, NM, December 7, 2010. The fact sheet and a copy of the poster are attached.
We sought a suitable soil amendment to correct the single greatest soil deficiency in my dry land pistachio farm. Composting appeared to be the best solution, not only for my farm, but also for a much wider group of similar desert farms regionally. By using by-products from nearby Mesilla Valley, those producers and processors might benefit by using their by-products to create compost, which could be a value-added, salable product. Composting would also provide a by-product disposal stream for these producers and processors. We chose to use dairy manure and pecan shells to compost. While we demonstrated that this compost may have benefited soil and leaf nutritional status, we also found that N immobilization may have occurred as a result of pecan shell application. This finding may alert others using pecan shells as mulch or compost, preventing unrecognized N deficiency. This finding also lends itself to easy correction by: (1) the addition of the N form of urea, ammonium sulfate, etc., (2) use of other viable carbon source alternatives, e.g. cotton gin by-products, or (3) which includes reducing the carbon source content of the compost (e.g. v:v 9:1 to 6:1). An unexpected finding was increased iron and manganese content in the pecan shell and composted groups. This supports previous studies that pecan shells may be a useful and economic binding agent to reduce heavy metals in contaminated water, providing yet another potential producer/processor benefit.
While I am not aware of other producers adopting the practice of composting manure and pecan shells, mulching with pecan shells is widespread. Our data may have a direct impact on this practice since N immobilization may occur. The concept of producing compost from locally acquired by-products is practical, and ongoing experiments are planned. This will involve a growing number of producers/processors, including dairies and the local cotton gin and its farmers’ cooperative. Large scale agricultural composting in the Mesilla Valley is practical and gaining in producer acceptance.
Reactions from Producers:
Numerous producers inquired about our composting experience and interest level in large scale composting increased. This Western SARE-funded project was discussed by both producers and processors in the Mesilla Valley, Tularosa Basin, NM, and at the Texas A&M AgriLife Research Center in El Paso, TX. In these informal settings, I shared my experience in agricultural composting and encouraged others to pursue similar efforts.
Our successful production of large quantities of compost made from two readily available agricultural by-products demonstrated that the addition of pecan shells as a carbon source might lead to N immobilization in the soil at nine months. By 11 months modest but plausible and consistent beneficial N % increases were seen by leaf analysis. Longer term observations and analyses might reveal diminishing N immobilization and increased mineralization of compost components.
Because of the confidence and skills we gained in this useful experience, I sought other available carbon sources for composting from the Mesilla Valley. As a result, I proposed another Western SARE project in 2009 in which cotton gin by products would be used with dairy manure. Although this proposal was not accepted in 2009, I focused on the comments and recommendations made by reviewers and will re-submit the proposal in December 2010.
An intriguing hypothesis revolves around heavy metal binding by pecan shells observed in this study. Other prior studies have reported iron and manganese binding by pecan shells and other nut shell and tree barks lignin cellulose compounds and polyphenols. The exciting possibility that pecan by-products might serve as a heavy metal decontaminant in polluted water was discussed.
Finally, in addition to clearly observed soil quality improvements on my farm, I gained valuable composting experience and confidence, made many useful contacts with soil scientists and other Mesilla Valley producers/processors interested in sustainable farming.