Nitrogen Continued from Page 6
the weeks after planting but before fruit- ing, andthenincreasedto3poundsaweek while the plantswere producing berries. “You have to match nitrogen supply with the nitrogen demand of the plants,” said Joji Muramoto, UCCE specialist in organic production. “These figures were takenfromfour yearsof studyat anorganic strawberry fieldinMossLandingaveraging 37,800 pounds of berries.” Therecanalsobe significant differences in howmuch nitrogen the plants take up from the soil, as a deep-rooted crop such as broccolimay scavenge the soil formore thanmore than 150 pounds. From2016 to2019,UCresearchers eval- uated fertilizer use on 28 blocks at large- scale Salinas Valley organic vegetable op- erations and found that growers applied on average 205%asmuch nitrogen as was taken up by the crop, but the fertilizer still supplied only 85% of the nitrogen taken up by the crop. Much of the nitrogen in the fertilizer onlybecameavailableovera longperiodof time, but soilmicrobesweremakingother sources of nitrogenavailable to the crop to take up the slack. Soil quick tests can take some of the guesswork out of fertilizer application decisions in both conventional and or- ganic systems. When soil tests show 20 parts per mil- lion nitrates, that translates into 70 to 75 pounds of cropavailablenitrogenanacre, which should be enough to feed a vegeta- ble crop for aweek or two. Useof the soil quick test todecidewhen an application is needed is trickier in or- ganic systems, however, because unlike conventional fertilizers, organic fertilizers applied after the soil nitrate supply is ex- hausted release their nitrogen gradually over time. ( Bo b J o hn s o n i s a r e p o r t e r i n Sacramento. He may be contacted at bjohn11135@gmail.com.)
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in order tominimize the loss of nitrogen.” Another important nutrient and wa- ter management tool is to encourage the growth inthesoil of beneficialmycorrhizal fungi, which attach to the roots andmine the soil for nutrients andwater. “Lots of information shows that con- tinuous cropping with less tillage and less fallow time is conducive to the symbiosis between mycorrhizae and crop roots,” Jackson said. Matching the nitrogen available in the soil with the amount needed by the crop begins with an understanding of the amount taken up by various crops over the course of the season. Spinach takes up 90 to 130 pounds an acre of nitrogen, full term lettuces from 120 to 160 pounds, and broccoli between 250 and 350 pounds, according to studies conducted byUCCE specialist TimHartz. Irrigationmanagement is another cru- cial pieceof thepuzzleofmakingnitrogen available in the root zone when the crop needs it. “You can do a lot of nitrogen budgeting, but if you’re not paying attention to water management, thatbudgetingfor theseason isnotgoingtobethataccurate,”saidMichael Cahn,UCCE irrigation farmadvisor. Spinach, for example, takesup6pounds a day of nitrogen in the final two weeks before harvest. That nitrogen must be available in the narrowband of soil that is reached by the shallow roots. “In spinach, we found that most of the nitrate is coming froma narrow band be- tween 4 and 12 inches deep, whichmakes irrigationmanagement critical tokeep the nitrate in this zone,” Smith said. Strawberriesalsohaveauniquecycle, as astudyshowedthecropat theorganic field inMoss Landing took up a littlemore than 90poundsofnitrogenover thecourseof the season. Uptakewas just apoundaweek in
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