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Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures
Thesis (PhDAgric)--Stellenbosch University, 2025.
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Stellenbosch : Stellenbosch University
2026
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| _version_ | 1867614138303250432 |
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| access_status_str | Open Access |
| author | Phohlo, Motsedisi Portia |
| author2 | Swanepoel, Pieter |
| author_browse | Phohlo, Motsedisi Portia Swanepoel, Pieter |
| author_facet | Swanepoel, Pieter Phohlo, Motsedisi Portia |
| author_sort | Phohlo, Motsedisi Portia |
| collection | Thesis |
| dc_rights_str_mv | Stellenbosch University |
| description | Thesis (PhDAgric)--Stellenbosch University, 2025. |
| format | Thesis |
| id | oai:scholar.sun.ac.za:10019.1/134774 |
| institution | Stellenbosch University (South Africa) |
| last_indexed | 2026-06-10T12:47:16.314Z |
| license_str | Other — see source repository |
| provenance_str_mv | Harvested via OAI-PMH from SUNScholar — Stellenbosch University Repository |
| publishDate | 2026 |
| publishDateRange | 2026 |
| publishDateSort | 2026 |
| publisher | Stellenbosch : Stellenbosch University |
| publisherStr | Stellenbosch : Stellenbosch University |
| record_format | dspace |
| source_str | SUNScholar — Stellenbosch University Repository |
| spelling | oai:scholar.sun.ac.za:10019.1/134774 Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures Phohlo, Motsedisi Portia Swanepoel, Pieter Hinck, Stefan Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy. Pastures -- Fertilization Pastures -- Fertilizers -- South Africa -- Eastern Cape Soils -- Health Dairy farming -- South Africa -- Eastern Cape Nitrogen fertilizers -- Environmental aspects -- South Africa Biogeochemical cycles -- Seasonal variations UCTD Thesis (PhDAgric)--Stellenbosch University, 2025. Phohlo, M. P. 2025. Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures. Unpublished doctoral dissertation. Stellenbosch: Stellenbosch University [online]. Available: https://scholar.sun.ac.za/items/fcf3629a-6793-4e61-b03a-975c40f4be66 ENGLISH ABSTRACT: Nitrogen (N) fertilisation is a cornerstone of pasture-based dairy production, yet its inefficient use contributes to environmental degradation, economic losses and diminished long term soil productivity. In South Africa’s Eastern Cape the nation’s largest dairy producing province blanket fertilisation practices often overlook site specific soil health constraints, resulting in suboptimal nitrogen use efficiency (NUE) and potential nutrient imbalances. This study investigates the relationships between N fertilisation rates, pasture herbage yield, soil N dynamics and biological indicators of soil health, with the aim of identifying fertiliser strategies that maximise productivity while safeguarding environmental sustainability. Chapter 3 evaluates the impact of three N fertilisation categories, low (<200 kg N ha⁻¹), moderate (200–350 kg N ha⁻¹), and high (>350 kg N ha⁻¹) on pasture yield and NUE using a five-year dataset (2015–2019) from 153 commercial dairy pasture fields. Contrary to conventional expectations, high N rates did not increase annual herbage yields compared to moderate or low rates. Instead, a weak but significant negative correlation between yield and N rate was observed, indicating that excessive applications may suppress productivity. NUE was highest at lower N inputs, requiring only 11.3 kg N to produce 1 t of herbage, compared to 25 kg N t⁻¹ at the highest input levels. Seasonal analysis revealed that yields peaked in spring and summer when temperature and moisture conditions favoured natural N mineralisation, highlighting the benefits of aligning fertiliser applications with these periods. Chapter 4 explores how N fertilisation influences soil N cycling, particularly the mineralisation of organic N, and its relationship to yield. Soil analyses from 2016 to 2019 measured total N, nitrate, ammonium, potentially mineralisable N (PMN) correlated to herbage yield. Results showed that excessive N fertilisation (>350 kg N ha⁻¹) reduced the soil’s ability to mineralise N, leading to nitrate accumulation that negatively correlated with yield and increased risks of leaching and groundwater contamination. In contrast, low N input fields maintained higher ammonium levels, healthier microbial activity, and more efficient N cycling, sustaining or improving yields with little environmental compromise. Chapter 5 examines the influence of soil properties on pasture productivity in two dairy regions: Tsitsikamma (high rainfall, acidic soils) and Cookhouse/Cradock (semi-arid, irrigated, neutral to alkaline soils). Using 263 field year observations, region-specific analyses identified distinct yield drivers. In Tsitsikamma, excessive nutrient saturation particularly N, phosphorus (P), potassium (K), and magnesium (Mg) was detrimental, while balanced levels of calcium (Ca) and Mg improved soil structure and enhanced yields. In Cookhouse/Cradock, K% base saturation and active carbon (C), indicating strong biological function, were the most important yield determinants. Across both regions, excessive N fertilisation consistently reduced productivity. Collectively, these findings challenge the long-held belief that higher N inputs guarantee higher yields. Instead, they demonstrate that low to moderate N fertilisation, timed with seasonal mineralisation peaks and supported by balanced soil fertility management, can maintain competitive yields, improve NUE, lower production costs, and reduce environmental impacts. Furthermore, region-specific strategies, structural and nutrient balance optimisation in Tsitsikamma and potassium management with organic matter enhancement in Cookhouse/Cradock offer targeted pathways to more sustainable and profitable dairy pasture systems. AFRIKAANSE OPSOMMING: Geen opsomming beskikbaar. Doctoral 2026-01-07T10:21:08Z 2026-01-07T10:21:08Z 2025-12 Thesis https://scholar.sun.ac.za/handle/10019.1/134774 Stellenbosch University vi, 6 unnumbered, 72 pages : illustrations, maps application/pdf Stellenbosch : Stellenbosch University |
| spellingShingle | Pastures -- Fertilization Pastures -- Fertilizers -- South Africa -- Eastern Cape Soils -- Health Dairy farming -- South Africa -- Eastern Cape Nitrogen fertilizers -- Environmental aspects -- South Africa Biogeochemical cycles -- Seasonal variations UCTD Phohlo, Motsedisi Portia Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title | Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title_full | Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title_fullStr | Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title_full_unstemmed | Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title_short | Optimising nitrogen use efficiency through soil health in Eastern Cape dairy pastures |
| title_sort | optimising nitrogen use efficiency through soil health in eastern cape dairy pastures |
| topic | Pastures -- Fertilization Pastures -- Fertilizers -- South Africa -- Eastern Cape Soils -- Health Dairy farming -- South Africa -- Eastern Cape Nitrogen fertilizers -- Environmental aspects -- South Africa Biogeochemical cycles -- Seasonal variations UCTD |
| url | https://scholar.sun.ac.za/handle/10019.1/134774 |
| work_keys_str_mv | AT phohlomotsedisiportia optimisingnitrogenuseefficiencythroughsoilhealthineasterncapedairypastures |