Waar peulplante wél stikstofbemesting benodig

Dr. Dries Bloem (GeoLab) werk in noue verbintenis saam met die GWK Landboudienste-span en sy kennis en bydrae is van groot waarde vir hierdie besigheid. Hy het hierdie artikel oor stikstofbemesting op peulplante oorspronklik in Engels geskryf, en dit word dus so in Kommuniek geplaas.
Peulplante soos lusern, sojabone en grondbone word normaalweg nie met stikstof (N)-kunsmis bemes nie, aangesien hierdie gewasse die vermoë het om hulle eie stikstof te bind deur middel van simbiose met Rhizobium-bakterieë.
Verskeie navorsingsresultate dui daarop dat daar wel sekere toestande kan wees waar dit winsgewend kan wees om 10 – 40 kg/ha N as voorplantbemesting of bandplasing (5cm weg van ingeënte saad) toe te dien, onder meer:
· Waar stikstofvlakke in grond laag is, soos by sandgronde, waar die vorige jaar hoë opbrengs behaal is, waar hoë reënval was voor planttyd of waar peulplante direk tussen vorige jaar se plantreste geplant word.
· Waar grondtemperature laag is wat veroorsaak dat Rhizobium minder doeltreffend stikstof bind.
Dit kan ook die moeite werd wees om N-toedienings laat in die seisoen te oorweeg omdat Rhizobium minder doeltreffend kan word met ouderdom, temperature laer is en groot hoeveelhede energie deur die plant na peule getranslokeer word in daardie stadium. Toediening van stadig-vrystellende stikstofkunsmis is ʼn belowende praktyk vir peulplante, aangesien daar navorsing is wat toon dat dit opbrengs kan verhoog sonder om Rhizobium-werking te beperk.
Introduction
Legumes like groundnut, soybean and lucerne have the ability to satisfy their nitrogen (N) requirements through biological nitrogen fixing (BNF) by means of symbiosis with Rhizobium bacteria. Seeds are normally inoculated with Rhizobium bacteria to ensure the process works.

Visible nodules are formed on the roots of legumes.
Focus is mostly on the nitrogen contribution through BNF and the other side of the symbiosis is often forgotten. The other side of the symbiosis is that the plant supplies some of its energy to the Rhizobium.
In some trials it has been found that nitrogen fertilisation in the grain-fill period of soybean caused a significant yield increase. This has been explained that the legumes translocate large amounts of energy to the pods in this period and some energy still needs to be allocated to the Rhizobium. When nitrogen fertiliser is applied at this stage, the legumes utilise applied N rather than BNF and translocate more energy to the pods. Another explanation might be that nodules lose some efficiency with age.
Trial results
In a soybean trial in the Netherlands under controlled conditions and in an artificial growth medium (very little nitrogen mineralisation), the results in Table 1 were obtained.

Table 1
Plant yield with sufficient N fertiliser was much better than where only Rhizobium inoculation was done (compare Treatments 2 and 3). Where sufficient N fertiliser was applied and Rhizobium inoculation was done in Treatment 4, the Rhizobium provided more N to the plant than in Treatment 3, where no N fertiliser was applied.
In a next phase of the same trial, where all treatments were inoculated with Rhizobium, the results in Table 2 were obtained.

Table 2 (*Approximately 10% of sufficient N fertiliser)
Nodulation was considerably better with a little N fertiliser application (Treatment 2) in comparison to the control (Treatment 1) where no N fertiliser was applied. Sufficient N fertiliser (Treatment 3) caused a lower nodulation.
Results from both trials indicate that both nodulation and yield of soybean were improved with a small amount of N fertiliser. The reason might be that N fertilisation stimulates initial growth and root development, which means that the plant (host) can translocate more energy earlier to Rhizobium nodules, and thereby increase nodulation. Reports indicate that it can take 21 to 30 days after planting for efficient nodules to develop.
In field trials with lucerne in Oregon, USA, the results in Table 3 were obtained with application of different levels of pre-plant N fertiliser.

Table 3
Average 10cm deep soil temperate two weeks after planting was 18°C in 1987 and 10°C in 1988. Yield response to N fertiliser application in 1988 was due to inefficiency of Rhizobium nodules to fix sufficient N at low temperature. After field and growth medium trials these researchers made the following conclusions: pre-plant application of 20-40 kg/ha N on lucerne may increase yield without decreasing BNF in conditions where 1) soil temperatures are below 15°C two weeks after planting and 2) soil nitrate levels are below 16 mg/kg.
Slow-release nitrogen fertiliser is a promising technology to enhance production of legumes. Researchers of the University of Nebraska-Lincoln found that pre-plant application of 180kg nitrogen per hectare, 20cm deep between rows in the form of slow-release fertiliser increased the yield of soybean without affecting the BNF rate.
*References:
· Gan, Y, Stulen, I, Van Keulen, H & Kuiper, P.J.C., 2004. Low concentrations of nitrate and ammonium stimulate nodulation and N2 fixation while inhibiting specific nodulation and specific N2 fixation in soyabean. Plant and Soil, 258: 281-292.
· Salvagiotti, F., Specht, J.E., Cassman, K.G., Walters, D.T., Weiss, A. & Dobermann, A., 2009. Growth and nitrogen fixation in high-yielding soybean: Impact of nitrogen fixation. Agronomy J., 101: 958-970.
· Shuler, P.E., 1991. The effect of preplant nitrogen fertilization and soil temperature on biological nitrogen fixation and yield of alfalfa (Medicago sativa L.). Ph.D. Thesis, Oregon State University.