Chaney, Ph.D., William E

Jackson Dr, Louise E

Klein, Jill A. : J. A. Klein was Postgraduate Researcher, Department of Vegetable Crops, UC Davis

Koike, Steven T.

Smith, Richard

Wyland, Lisa J. : L. J. Wyland is StaflResearch Associate, Department of Vegetable Crops, UC Davis

In lettuce production . . . Winter cover crops can decrease soil nitrate , leaching potential Louise E . Jackson CI Lisa J . Wyland u Jill A . Klein o Richard F . Smith CI William E . Chaney Steven T . Koike tices can also increase some soil insect fallow ; net N mineralization reaches its The large amounts of soil nitrate annual maximum at this time . One ob - populations , although this can be benefi - that can accumulate in annual row cial in the case of natural predators . jective in trying to reduce nitrate leach - crop production during the winter Cover crops and crop rotations have also ing therefore involves ways to recycle fallow period can leach during the excess residual soil N after the au - been shown to suppress some soilborne winter storms and spring irriga - tumn harvest and to synchronize its re - diseases . For example , a study con - ducted in Salinas in 1986 - 88found that lease with uptake by the subsequent tion . In Monterey County , 48 % of vegetable crop in early spring . corky root of lettuce can be partially the wells in the upper unconfined suppressed by a winter cover crop of In other cropping systems , nonlegu - aquifer exceed the public health cereal rye . minous winter cover crops have been drinking water standard of 10 ppm successfully employed to take up excess Field station trials of nitrate - N . Nonleguminouscover water and nitrate during the rainy fal - low season , as well as to contribute to A preliminary trial , conducted on crops , planted during the winter soil organic matter content after incorpo - field station plots , evaluated several fallow and incorporatedin early ration . species for use as winter cover crops in A large volume of research has spring using reduced tillage shown that increased organic matter rotation with annual row crops in the equipment to maintain intact leads to increased microbial activity in Salinas Valley . Desired characteristics beds , have been found to reduce the soil , greater soil included rapid growth and extensive N turnover , greater aggregate stability , decreased soil root development in the upper soil pro - nitrate leaching without disrupting file during winter , to maximize nitrate crusting , increased water infiltration cropping schedules . and water uptake . In addition , it was and ultimately enhanced fertility for assumed that the cover crop should be the subsequent cash crop . With the de - The most efficient use of fertilizer and easy to incorporate on the beds , using velopment of techniques to grow and soil - derived nitrogen ( N ) occurs when minimum tillage techniques , and should incorporate cover crops directly on availability coincides with plant de - not harbor diseases threatening to the semi - permanent beds , the constraints mand . In cool - season vegetable pro - subsequent cash crop . of time and expense typically involved duction systems , most nitrate leaching in disking and reshaping beds will be Methods . A cover crop trial was es - eliminated . occurs : ( 1 ) in the fallow period during tablished on field station research plots winter rains when excess soil nitrate ac - in Salinas , California , on November 15 , In other studies , cover cropping has cumulates from residual fertilizer and been shown to affect crop disease and 1989 . Six species were planted in a ran - from N mineralization and nitrification domized complete block design : oilseed insect pest management both positively of crop residues and soil organic matter , radish ( Raphanus sativus cv . Renova ) , and negatively . Cover crop cultivation and ( 2 ) during frequent irrigations in the may promote some soil fungal patho - white senf mustard ( Brassica kirta cv . final vegetable production growth Martigena ) , white mustard ( Brassica gens . Previous research has found that stages . alba ) , phacelia ( Phacelia tanacetijolia cv . some cover crops can increase Sclerotinia Test results in the Salinas Valley Phaci ) , rye minor inoculum which , combined with ( Secale cereale cv . Merced ) and show that high soil nitrate levels remain reduced tillage techniques , might annual ryegrass ( Lolium rnultiflorurn ) , after vegetable harvest , and that concen - along with a bare fallow plot in each threaten subsequent lettuce crops . Cover trations often double during the winter crop residue and reduced tillage prac - block as a control . Soil samples to 60 cm 12 CALIFORNIA AGRICULTURE , VOLUME 47 , NUMBER 5 Left , field station cover crop trial at Sali - nas in December , 1989 , one month after planting . Cover crops were seeded in two rows on 40 - inch beds . Species ( left to right ) in the front row are : white senf mus - tard , annual rye grass , barley , oilseed rad - ish , white mustard , annual ryegrassand Merced rye . Phacelia is in the second row in lines 4 and 5 from the right side of the plot . Right , on - farm cover crop trial at Salinas in February , 2.5 months after planting . Treatments ( left to right ) are : bare soil , phacelia and Merced rye . ( 24 inches ) deep were taken at planting , in midseason ( January 7,1990 ) and be - fore incorporation ( March 8,1990 ) . The stands were incorporated on March 20 to a depth of 20 cm ( 8inches ) with a rotary tiller ( Marvin Rowmaster bedshaper / in - corporator ) . Soil samples were taken weekly from the top 15 cm ( 6 inches ) soil layer for 7 weeks following incorpora - cided with the zone of greatest nitrate Results . Cover crops significantly depletion . tion . reduced soil moisture and nitrate con - Phacelia and Merced rye were se - centrations relative to a fallow control On - farm trials lected from the 1989 - 90experiments as during the winter of 1989 - 90 ( fig . 1 ) . By Although cover ps can reduce ni - the most promising of the cover crops the time of incorporation in March 1990 , ease soil N turn - trate leaching and for the second year of the study because the average nitrate concentration in the over , it is difficult in commercial practice of their fibrous root systems and diffuse cover - cropped plots ( 0 to 30 cm , or 0 to to include them in crop rotations be - growth forms , which facilitate direct in - 12inches , deep ) was 15 pg nitrate - N per corporation on pre - formed beds . The cause of the time required for disking , gram ( NO3 - - N / g ) soil lower than the cruciferous cover crops were not chosen listing and shaping beds before planting bare soil plots , which is equal to a differ - the cash crop . In 1991 , we initiated a se - because they are potential alternate ence of 63 kg NO3 - - N / ha ( 71lb NO3 - - ries of on - farm studies that included de - hosts for Brassica cash crop diseases such N / acre ) . In addition , the cover - cropped veloping reduced tillage techniques to as turnip mosaic virus . Phacelia has plots sustained a higher level of micro - incorporate cover crops directly on the been used in sugar beet rotations as a bial activity , as indicated by higher net beds into which the subsequent crop cyst nematode trap crop . It is mildly mineralizable N concentrations : 36.2f sus - ceptible to beet western yellows virus , 5.19 pg ammonium - N per gram ( NH4f - / g ) soil in cover - cropped soil versus and mature flowering stands have been N 10.4f found to harbor Lygus spp . 0.67 pg NH4 + - N / g soil in the bare soil ( x f S.E . ) , averaged over 7 sampling In the second year of this study , phacelia and Merced rye were seeded on dates in the 7 weeks following incorpo - November 13 , 1990 , after a preplant soil ration . sample was taken . The design was a ran - In the study's second year , the winter domized complete block , with a bare fal - of 1990 - 91 , rainfall was 15.1cm ( 5.94 low control plot in each of six blocks . inches ) below average . Soil nitrate con - Subsequent soil samples were taken on centrations increased 500 to 540 kg NO3 - January 2 and February 12,1991 . Plant - N / ha ( 446to 482 Ib / acre ) in the cover - shoot and root samples were also col - cropped plots and 650 kg NO3 - - N / ha lected on these dates . The cover crops ( 580 lb / acre ) in the bare plots ( 0 to 60 were incorporated on the beds with a ro - cm , or 0 to 24 inches , deep ) . This large tary tiller on February 12 . increase illustrates the magnitude of net All soil samples were extracted in 2 mineralization and nitrification that moles / liter potassium chloride ( KCl ) , leads to soil nitrate accumulation . and the supernatant was analyzed for Aboveground biomass production in nitrate and ammonium content with a the second year by phacelia and Merced Wescan Ammonia Analyzer . Gravimet - rye was similar , averaging 1,600 kg / ha ric soil moisture content was determined ( 1,427lb / acre ) after a 13 - weekgrowing for each sample . Net mineralizable N period that included both severe cold Fig . 1 Mean nitratecontent in the top 30 cm was determined using a 7 - day anaerobic and drought . Root biomass averaged 490 of soil , and N in the aboveground biomass incubation procedure . Aboveground kg / ha ( 437Ib / acre ) at a depth of 0 to 60 of six cover crops compared to a baresoil plant and root samples were oven - dried cm ( 0 to 24 inches ) for both species . The controlat mid - season in January ( gray ) and at 65 ° C ( 150 OF ) , weighed and analyzed at incorporation in March ( black ) , in an ex - majority of roots were located in the top perimentaltrial in Salinas in 1990 . for total N by the Kjeldahl method . 30 cm ( 12 inches ) of soil , which coin - CALIFORNIA AGRICULTURE , SEPTEMBER - OCTOBER 1993 13 corporation , at lettuce planting ( April 22,1992 ) , at thinning ( May 20 ) and at harvest ( June 29 ) . Lettuce plant samples were collected at thinning and at har - vest . Soil samples from all sites were pro - cessed by extraction in 2 moles / liter KCl , and analyzed for nitrate and am - monium content as described above . A separate sample from each depth was taken for gravimetric measurement of soil moisture . Samples at Farms 1and 2 were taken to 45 cm ( 18inches ) deep and divided into two depth increments ( 0 to 15and 15to 45 cm or 0 to 6 and 6 to 18inches ) . Samples at Farm 3 were taken to 105cm ( 3.5ft ) deep and divided into four depth increments . In addition , samples from the top ( 0 to 15and 15to 30 cm , or 0 to 6 and 6 to 12inches ) soil layers at Farm 3 were analyzed for net mineralizable N using an anaerobic in - cubation assay . Cover crop and lettuce plant samples were oven - dried , weighed was to be direct - seeded . Growth and to determine dry matter accumulation , took place on October 23 , using a flail ground and analyzed for total shredder and then a Sundance System disease incidence were monitored in the N content to incorporate the cover crops without lettuce crops at these sites to determine by Kjeldahl digestion . Data analysis was whether they were affected by the pres - disturbing the drip tubing . Lettuce was conducted using t - tests or analysis of ence of cover crop residues . direct - seeded approximately variance . Statistical significancewas set 2 months Methods . In the autumn of 1991 , at P < 0.05for all tests unless otherwise later . Lettuce samples were collected at indicated . cover crop trials were established at thinning , March 2,1992 , and at harvest , three farm sites in Salinas , California , April 27 . Results.At Farm 1 , phacelia plots that were subsequently planted with Farm 3 delayed planting cover crops contained significantly lower nitrate and crisphead lettuce following cover crop until December , after harvesting Cole moisture levels than bare soil plots after incorporation . The three farms encom - crops in late August . The site was 5.7 ha 8 weeks of growth . The nitrate content passed several management regimes for of bare soil exceeded cover - cropped soil ( 14 acres ) on a Salinas clay loam soil . large - scale production of vegetables . by 11.0kg N03 - - N / ha ( 9.8lb / acre ) in Two soil samples were taken before Farm 1utilized a fall - planted phacelia the cover crop planting : one on September 0 - to 15 - cmsoil layer , and by 61.7 kg cover crop . The site was a 9.7 - ha ( 24 - 17and one on November 14,1991 . N03 - - N / ha ( 55.0 lb / acre ) in the 15 - to acre ) field on a Chualar loam soil type . 45 - cm soil layer . There were no signifi - Phacelia and Merced rye were planted Phacelia was planted on September 7 at cant differences in the dry weight or N on December 6 in a randomized com - content of the lettuce collected at harvest / ha ( 3.1 lb / acre ) and sprinkler - 3.5 kg plete block design that included a bare irrigated to a stand . Two areas of the fallow control . The field was sprinkler - from the cover - cropped and bare plots . field were left unplanted ( bare ) as a con - irrigated The lettuce crop was evaluated for corky 1and 2 weeks after planting . trol . The crop was incorporated on No - root and Aboveground plant samples were col - Sclerotinia diseases , and there vember 6 using a rotary chopper fol - lected from the cover crops throughout was no difference in disease incidence lowed by two passes with a Sundance between treatments . the season to measure dry matter and System to maintain intact beds . Soil Farm 2 , utilizing subsurface irriga - total N accumulation . On February 28 , samples were taken from the cover - 1992 , the cover crops were flail mowed tion , displayed similar trends for several cropped and corresponding bare plots 2 and then incorporated with a Sundance cover crops . Phacelia significantly re - days before cover crop planting and 2 System . Soil samples were taken before duced soil nitrate at both sampling days after incorporation . Lettuce was planting , at midseason , at cover crop in - depths compared with adjacent bare planted on December 28 . Lettuce samples were collected just before har - vest to measure biomass and N content . Farm 2 was a 3.5 - ha ( 8.6 - acre ) subsur - face drip - irrigated site on a Salinas clay loam . Merced rye ( 40kg / ha , or 35.7 lb / acre ) , phacelia ( 10kg / ha , 8.9 lb / acre ) and oilseed radish ( 17.5kg / ha , 15.6lb / acre ) were planted on September 5 , each with corresponding bare control plots . The field was drip - irrigated until Octo - ber 11 . Soil samples were taken at cover crop planting and a week before incor - poration , when cover crop biomass samples were collected . Incorporation 14 CALIFORNIA AGRICULTURE , VOLUME 47 , NUMBER 5 fully decrease soil nitrate and reduce the plots , rye reduced soil nitrate levels at 15 potential for leaching . Of all the cover to 45 cm deep , and radish reduced ni - crops tested , phacelia was selected as the trate levels at 0 to 15cm deep . In the en - most advantageous for this cropping tire 0 - to 45 - cm profile , the bare plots system because contained 34.1,59.4 and 201.7 kg NOS - - ( 1 ) it has fibrous roots and succulent tops that facilitate incor - N / ha ( 30.4,53.0and 180.0Ibtacre ) more poration , ( 2 ) it is late - floweringand can than their corresponding phacelia , ryeois - and radish plots , respectively.Soil m withstand cool coastal winters , ( 3 ) it ture was reduced by the presence of does not harbor the diseases of the cru - cover crops in all plots . Oilseed radish ciferous cover crops and ( 4 ) it does not produced approximately twice the biom - resprout after incorporation using re - ass , with twice the N content , as the duced tillage techniques , which is typi - cal of the grasses.Although phacelia is phacelia and rye in this short ( 7 - week ) growing period . Total aboveground bio - native to California , it is presently being mass production was 1,250,1,355 and bred for use as a cover crop only in Eu - Phacelia tanacetifoliaafter 2 months of rope , and seed availabilityin this coun - 2,761 kg / ha ( 1,115,1,208 and 2,463 Ib / winter growth . acre ) from phacelia , rye and radish try is limited . As phacelia becomes more crops , respectively.The corresponding widely recognized as a valuable winter cover crop in this region , its availability N content of these cover crops was 54.9 , 57.0 and 113.5 kg N / ha ( 49.0,50.8and and affordabilityare expected to im - 105cm relative to the bare plots by the 101.2 lb / acre ) . There were no significant prove . We found it possible to grow 12weeks after time of incorporation , differences in the dry weight or N con - and incorporate cover crops on semi - planting ( fig . 2 ) . Soil nitrate at the lower tent of the lettuceharvested from all permanent beds using reduced tillage depths in the bare plots increased dur - cover - croppedand adjacentbare plots . equipment ing this time . Nitrate content was signifi - such as the Sundance System cantly lower in the cover - croppedplots Sclerotinia infection was minimal in this / incorpo - or a power - driven bed shaper compared with the bare fallow : 8.6 kg field , and there was no difference ob - rator after flail - mowing the stand.gy By reducing the time and ener re - N03 - - N served between treatments . / ha ( 7.7lb / acre ) lower in the 15 - At Farm quired for management , integrating a 3 , the soil nitrate level fol - to 45 - cm soil depth , and 32.0 and 31.2 kg winter cover crop rotation into econven - lowing the autumn vegetableharvest N03 - - N / ha ( 28.5 and 27.8 Ib / acre ) tional farming practicesbecomes mor was 56.4 kg N03 - - N lower at the 45 - to 75 - cm and 75 - to 105 - / ha ( 50.3 lb / acre ) at feasible.No detrimental effecton slettuce 0 to 105cm ( 0 to 3.5 ft ) , and it cm ( 18 - to 30 - inch and 30 - to & inch ) a depth of crop yield was found at any of the site more than doubled to 128.4kg N03 - - N / soil depths , respectively ( table 1 ) . Soilhe as a result of having cover crops in the ha ( 114.5Ib / acre ) in the next 2 months moisture was significantly lower in t rotation . The fertilizer applied as a pre - of fallow before the cover crops were cover - croppedplots in the top two soil plant and / or side - dress during lettuce planted ( table depths . Aboveground biomass produc - s 1 ) . Cover crops signifi - h tion and N content of the phacelia wa crop production at all three field sites cantly depleted soilnitrate to a dept of significantly greater than the rye ( 23.3.1 ( between 100and 150kg N / ha , or 89.2the versus 8.0 kg N / ha , or 20.7 versus 7 and 133.8Ib / acre ) probably masked lb / acre ) after the first 6 weeks of N contribution of the cover crops , and explains why there was no significant growth . At incorporation , phacelia shad produced significantly more biomas increasein the yield or N content of the lettuce in the cover - croppedplots . than the rye ( 1,090 and 810 Kg / ha , or 972 and 722 lb acre , from phacelia and Future studies should test yields from cover - croppedand fallow soilswith rye , respectively ) , but there was no dif - ference in total N / ha between the two lower amounts of added fertilizerand in cover crops ( 87.0 and 71.6 kg N / ha , or fields cover cropped for successive 77.6 and 64.0 lb / acre , from phacelia and years , to determine the cumulative influ - rye , respectively ) . There were no signifi - ence of cover cropping on soil fertility . cant differences in dry weight or Further research is also needed to pro - N con - vide a more in - depth evaluation of the tent of lettuce at thinning or at harvestw between the cover - croppedand fallo effect of cover cropping on plant diseaseer and insect pest incidenceas well as oth plots . Sclerotinia infection was very low long - termmanagement effectsand eco - in this field , and no differenceswere iob - n nomic profitability . served between cover crop treatments the incidence of either Sclerotinia or corky root of lettuce . No damping off L . E . Jackson is Assistant Professor , Coop - - ( due to Pythium spp . or Rhizoctonia spp . erative Extension Specialist and Physiolo gist , Department of Vegetable Crops , UC root infections that damage and kill young seedlings ) was observed in our Davis ; L.J . Wyland is StaflResearch Associ - experimentalor on - farm field trials . ate , Department of Vegetable Crops , UC kg Nha in cover crop biomass Davis ; J . A . Klein was Postgraduate Re - Conclusions searcher , Department of Vegetable Crops , Fig . 2 . Mean nitrate contentto 105 cm soil Data from all three on - farm trials con - UC Davis . R . F . Smith is Small Farm Advi - depth , and N in aboveground biomass of firm the conclusions of the earlier field sor , San Benito - Monterey - Santa Cruz coun - two cover crops compared to a bare soil station experimental trials . Nonlegumi - ties ; and W.E . Chaney and S . T . Koike are control at mid - season in January ( gray ) nous cover crops grown during the Farm Advisors , Monterey - Santa Cruz and at incorporationin March ( black ) , in a growerâ??s field in Salinas in 1992 . rainy winter fallow season can success - counties . CALIFORNIA AGRICULTURE , SEPTEMBER - OCTOBER 1993 15