Return to search

The influence of fertiliser nitrogen on soil nitrogen and on the herbage of a grazed kikuyu pasture in Natal.

The work reported in this thesis was designed to develop a better



understanding of the fate of fertiliser nitrogen applied to a



tropical pasture under field conditions, with the eventual



objective of improving the economy of livestock production off



such pastures. This involved an examination of the



concentrations of soil total nitrogen, ammonium nitrogen and



nitrate nitrogen at different depths within the soil profile



following the application of different levels of fertiliser



nitrogen to a grazed kikuyu (Pennisetum clandestinum) pasture,



as well as the influence of such applications on pasture yield



and some elements of pasture quality. The trial was conducted



over a two year period at Broadacres in the Natal Mistbelt.



A labelled [15]NH[4]N0[3] fertiliser experiment was also conducted



to ascertain how the labelled ammonium ion moved through the



soil, roots and herbage after being applied in spring onto a



kikuyu pasture.



In the absence of fertiliser N, a total of 15.45 t/ha of



soil N was recorded at an average concentration of 0.15%. More



than 30% of the soil total N was, however, situated within the



top 10cm of soil. organic matter (OM) content in the top 0-10cm



of the profile was high (4.75%), reflecting an accumulation of



organic matter in this zone. However, as organic C (and thus c: N



ratios) declined with depth, so too did soil total N



concentration.



Not surprisingly, fertiliser



measurably increase soil total N,



N applications did not



but indirectly may have affected soil N dynamics by increasing net mineralisation (due



to its "priming" effect) thereby stimulating plant growth and



thus increasing the size of the organic N pool through greater



plant decay.



Total soil N concentration did not change significantly from



the first to the second season. This could be attributed to the



fact that N gains and losses on the pastures, being over 15 years



old, were probably in equilibrium. Generally similar trends in



soil total N down the profile over both seasons was further



confirmation of this.



Before the application of any fertiliser, 331.9 kg NH[4]-N was



measured in the soil to a depth of 1m, on average, over both



seasons. This amount represented only 2.1% of the soil total N



in the profile. The concentration of NH[4]-N followed a quadratic



trend down the soil profile, irrespective of the amount of



fertiliser N applied, with the largest concentrations



accumulating, on average, in the 0-10cm and 75-100cm depth



classes and lowest concentrations in the 50-75cm depth class.



Laboratory wetting/drying experiments on soil samples collected



from a depth of 75-100cm showed that NH[4]-N concentrations



declined only marginally from their original concentrations. A



high organic C content of 1.44% at this depth was also probable



evidence of nitrification inhibition. Analysis of a similar



Inanda soil form under a maize crop did not exhibit the



properties eluded to above, suggesting that annual turn-over of



the soil was causing mineralisation-immobilisation reactions to



proceed normally.



Addition of fertiliser N to the pasture significantly increased the amount of NH[4]-N over that of the control camps.



Furthermore, the higher the application rate, the greater the



increase in NH[4]-N accumulation within the soil profile. As N



application rates increased, so the NH[4]-N:N0[3]-N ratio narrowed in



the soil complex. This was probably due to NH[4]-N being applied



ln excess of plant requirements at the high N application rates.



On average, 66.7 kg more NH[4]-N was present in the soil in



the first season than in the second after fertilisation.



Although this amount did not differ significantly from spring



through to autumn, during early spring and late summer/autumn



concentrations were higher than in mid-summer. Observed soil



NH4-N trends were also very similar to the soil total N trends



within both seasons, suggesting that soil total N concentrations



might well play an important role in determining soil NH4-N



concentrations.



Before fertilisation, only 45.6 kg N0[3]-N, representing 0.29%



of the soil total N, was on average, found in the profile to a



depth of 1m. The highest concentration of N0[3]-N was lodged in



the top 10cm of the soil. Nitrate-N declined, on average, with



depth down the profile. However, during the second season, even



though the concentration of N03-N declined down the profile, it



increased with depth during relative to that of the first season,



suggesting the movement of N0[3]-N down the profile during this



period.



Fertilisation significantly increased the concentration of



N0[3]-N above that of the control camps. Concentrations increased



as fertiliser application rates increased, as did N0[3]-N



concentrations with depth. This has important implications regarding potential leaching of N03-N into the groundwater,



suggesting that once applications reach levels of 300 kg



N/ha/season or more, applications should become smaller and more



frequent over the season in order to remove this pollution



potential.



On average, 94.3 kg N0[3]-N/ha was present down to a depth of



1m over both seasons. However, significantly more N0[3]-N was



present in the second season than in the first. This result is



in contrast to that of the NH[4]-N, wherein lower concentrations



were found in the second season than in the first.



No specific trends in N0[3]-N concentration were observed



within each season. Rather, N0[3]-N concentrations tended to vary



inconsistently at each sampling period. Nitrate N and ammonium



N concentrations within each month followed a near mirror image.



A DM yield of 12.7 t/ha, averaged over all treatments, was



measured over the two seasons. A progressive increase in DM



yield was obtained with successive increments of N fertiliser.



The response of the kikuyu to the N applied did, however, decline



as N applications increased.



A higher yield of 1.8 t DM/ha in the first season over that



of the second was difficult to explain since rainfall amount and



distribution was similar over both seasons.



On average, 2.84% protein N was measured in the herbage over



both seasons. In general, protein N concentrations increased as



N application rates increased.



On average, higher concentrations of protein-N were measured



within the upper (>5cm) than in the lower <5cm) herbage stratum,



irrespective of the amount of N applied. Similar bi-modal trends over time in protein-N concentration



were measured for all N treatments and within both herbage strata



over both seasons, with concentrations tending to be highest



during early summer (Dec), and in early autumn (Feb), and lowest



during spring (Oct), mid-summer (Jan) and autumn (March). spring



and autumn peaks seemed to correspond with periods of slower



growth, whilst low mid-summer concentrations coincided with



periods of high DM yields and TNC concentrations.



The range of N0[3]-N observed in the DM on the Broadacres



trial was 0.12% to 0.43%. As applications of fertiliser N to the



pasture increased, N0[3]-N concentrations within the herbage



increased in a near-linear fashion.



On average, higher concentrations of N0[3]-N, irrespective of



the amount of fertiliser N applied, were measured wi thin the



upper (>5cm) than the lower <5cm) herbage stratum.



A similar bi-modal trend to that measured with protein-N



concentrations was observed in both seasons for N0[3]-N in the



herbage. High concentrations of N0[3]-N were measured during



spring (Nov) and autumn (Feb), and lower concentrations in midsummer



(Dec & Jan), very early spring (Oct) and early autumn



(March). During summer, declining N0[3]-N concentrations were



associated with a corresponding increase in herbage DM yields.



A lack of any distinctive trend emerged on these trials in



the response of TNC to increased fertilisation with N suggests



that, in kikuyu, applied N alone would not materially alter TNC



concentrations.



Higher concentrations of TNC were determined in the lower



<5cm) height stratum, on average, than in the corresponding upper (>5cm) stratum. This may be ascribed to the fact that TNCs



tend to be found in higher concentrations where plant protein-N



and N0[3]-N concentrations are low.



A P concentration of 0.248% before N fertilisation, is such



that it should preclude any necessity for P supplementation, at



least to beef animals. Herbage P concentrations did, however,



drop as N fertiliser application rates were increased on the



pasture, but were still high enough to preclude supplementation.



Even though no significant difference in P concentration was



measured between the two herbage strata, a higher P content



prevailed within the lower <5cm) herbage stratum.



On average, 2.96% K was present within the herbage material



in this trial. The norm for pastures ranges between 0.7 and



4.0%.



On these trials, applications of fertiliser N to the camps



did not significantly affect K concentrations within the herbage.



The lower <5cm) herbage stratum, comprising most of the



older herbage fraction, was found to contain the highest K



concentration in the pasture.



The presence of significantly (although probably



biologically non-significantly) less K within the herbage in the



second season than in the first may be linked to depletion of



reserves of ยท this element in the soil by the plant and/ or



elemental interactions between K and other macro-nutrients.



An average Ca content of 0.35% within the herbage falls



within the range of 0.14 to 1.5% specified by the NRC (1976) as



being adequate for all except high-producing dairy animals.



Increasing N application rates to the pasture increased the Ca content within the herbage .



No significant differences in Ca concentration were found



between the upper (>5cm) and lower <5cm) herbage strata over



both seasons, even though the lower stratum had a slightly higher



Ca concentration, on average, than the upper stratum.



Calcium concentrations did not vary between seasons,



probably because concentrations tend rather to vary according to



stage of plant maturity, season or soil condition. However,



higher concentrations of the element were measured in the second



season than in the first. The reason for this is unknown.



On average, 0.377% Mg was present within the herbage over



both seasons. This compares favourably with published data



wherein Mg concentrations varied from 0 . 04 to 0.9% in the DM,



with a mean of 0.36%.



All camps with N applied to them contained significantly



more Mg in their herbage than did the material of the control



camps.



On these trials, the Ca :Mg ratio is 0.92: 1, which 1S



considered to be near the optimum for livestock and thus the



potential for tetany to arise is minimal.



Magnesium concentrations remained essentially similar within



both herbage strata, regardless of the rate of fertiliser N



applied.



As in the case of Ca, Mg concentrations within the herbage



were significantly higher in the second season than in the first.



Calcium:phosphate ratios increased, on average in the



herbage, as N application rates increased. This ratio was high



in spring, dropped off in summer and increased again into autumn, suggesting that the two ions were following the growth pattern



of the kikuyu over the season.



The K/Mg+Ca ratios were nearly double that of the norm,



suggesting that the pasture was experiencing luxury K uptake



which may be conducive to tetany in animals grazing the pasture.



This ratio narrowed as N application rates were increased,



probably as a result of ion dilution as the herbage yields



increased in response to these N applications. The ratio was low



in spring (October), but increased to a peak in December, before



declining again to a low in March. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1994.

Identiferoai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10903
Date January 1994
CreatorsHefer, Graham Daniel.
ContributorsTainton, Neil M., Miles, Neil.
Source SetsSouth African National ETD Portal
Languageen_ZA
Detected LanguageEnglish
TypeThesis

Page generated in 0.0035 seconds