{"id":17045,"date":"2026-08-23T12:03:11","date_gmt":"2026-08-23T10:03:11","guid":{"rendered":"https:\/\/www.proefschriftmaken.nl\/portfolio\/yussif-baba-kassim\/"},"modified":"2026-08-23T12:03:19","modified_gmt":"2026-08-23T10:03:19","slug":"yussif-baba-kassim","status":"publish","type":"us_portfolio","link":"https:\/\/www.proefschriftmaken.nl\/en\/portfolio\/yussif-baba-kassim\/","title":{"rendered":"Yussif Baba Kassim"},"content":{"rendered":"","protected":true},"excerpt":{"rendered":"","protected":true},"author":8,"featured_media":17046,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"footnotes":""},"us_portfolio_category":[45],"class_list":["post-17045","us_portfolio","type-us_portfolio","status-publish","post-password-required","hentry","us_portfolio_category-new-template"],"acf":{"main_text":"","naam_van_het_proefschift":"Enhancing crop response to fertilization in northern Ghana","samenvatting":"Afrikaanse regeringen en hun internationale organisaties hebben minerale meststoffen beschikbaar gesteld aan boeren als middel om de gewasopbrengsten te verbeteren. Onderzoek heeft echter overtuigend aangetoond dat de agronomische effici\u00ebntie van toegepaste minerale meststoffen, die vaak alleen stikstof (N), fosfor (P) en\/of kalium (K) bevatten, laag is in Ghana. Dit suggereert dat er andere opbrengstbeperkende of opbrengstverlagende factoren zijn die de gewasrespons op toegepaste minerale meststoffen belemmeren. Dit proefschrift analyseert hoe verschillende gewasintensiveringsmaatregelen, bedoeld om beperkingen in de sluiting van het gewasdek, de beschikbaarheid en het gebruik van zwavel, de worteldiepte van het gewas en de beschikbaarheid en het gebruik van bodemvocht aan te pakken, de opname en het gebruik van N, P en K uit minerale meststoffen verbeteren voor soja en ma\u00efs in Noord-Ghana. De geteste intensiveringsmaatregelen omvatten plantdichtheid, zwavelbemesting, het aanleggen van ruggen en mulchen, evenals een selectie van hun combinaties.\n\nHoofdstuk 1 geeft de algemene achtergrond en een grondige pre-analytische diagnose van opbrengstbeperkende factoren in Noord-Ghana. In het bijzonder geeft het een vroege indicatie van hoe de opbrengstrespons van soja en ma\u00efs op N, P en K (NPK) kan worden verbeteren. Het concept van nutri\u00ebntenopname en -gebruik als analytisch instrument voor het begrijpen van mechanismen waarmee intensiveringsmaatregelen de gewasopbrengst verhogen, wordt gepresenteerd. Verder worden in dit hoofdstuk de belangrijkste onderzoeksvragen en de stappen om deze te beantwoorden uiteengezet.\n\nHoofdstukken 2 en 3 analyseren de impact van plantdichtheid, zwavelbemesting, ruggen en mulchen op soja- en ma\u00efsopbrengsten onder NPK-bemesting. Het onderzoek werd uitgevoerd in twee dorpen, Bognaayili en Gauwogo. Zwavel en ruggen waren niet effectief voor soja in Bognaayili. In Gauwogo verhoogde het aanvullen van NPK met een hogere zaaidichtheid, zwavel, ruggen of mulch de soja-opbrengst aanzienlijk. Ma\u00efsopbrengsten namen toe met 480 tot 2210 kg ha\u22121 wanneer NPK werd gecombineerd met deze maatregelen. De intensiveringsmaatregelen werden als even effectief beschouwd en vertegenwoordigen een reeks opties voor systemen waar NPK wordt toegepast. Tegen de verwachting in werden geen synergie\u00ebn waargenomen tussen de verschillende maatregelen. De agronomische effici\u00ebntie van toegepaste fosfor (AEP) was hoog in vergelijking met vergelijkbare gebieden, wat duidt op een hoge herwinning of verdunning van P in de planten.\n\nLage bodemvochtigheid lijkt een belangrijke factor die de opbrengsten beperkte in percelen met alleen NPK. De gewasbedekking nam toe bij intensiveringsmaatregelen, wat de bodem beter beschermt tegen direct zonlicht, waardoor de bodemtemperatuur en verdamping dalen. Dit houdt meer water beschikbaar voor transpiratie, wat essentieel is voor fotosynthese.\n\nDe soja-opbrengsten waren lager in Bognaayili dan in Gauwogo, waarschijnlijk door een lagere bodempotentie. Sojagroei was matig op percelen met minder dan 0,5% organische stof en goed op percelen boven deze drempel. Percelen met minder dan 0,5% organische stof lijken daarom van onvoldoende kwaliteit voor soja-intensivering.\n\nHoofdstuk 4 onderzoekt mulchen onder boerenomstandigheden. In tegenstelling tot onderzoeksbeheer had mulchen onder boerenbeheer geen invloed op de ma\u00efsopbrengst. Boeren begonnen pas 19-28 dagen na het zaaien met mulchen, wanneer het kritieke stadium voor bescherming tegen verdamping mogelijk al voorbij was. Boeren zagen mulchen als nuttig voor groenten op kleine schaal, maar niet voor ma\u00efs op grote schaal vanwege de benodigde arbeid en materialen.\n\nHoofdstuk 5 verkent het gebruik van drone-beelden om variatie in bodemvruchtbaarheid (organische stof) binnen velden te beoordelen. Spectrale vegetatie-indices van vroege onkruidbedekking toonden correlaties met organische stof in Bognaayili (grote variatie), maar niet in Gauwogo (lage variatie). Dit is een veelbelovende optie voor het in kaart brengen van variatie, maar vereist nog gelijktijdige bodembemonstering voor modelverbetering.\n\nHoofdstuk 6 synthetiseert de resultaten en stelt vervolgstappen voor. De geteste maatregelen werkten via verbeterde nutri\u00ebntenopname. Hoewel intensivering leidde tot grotere vegetatieve groei, waren er geen tekenen van verhoogde gewasstress, aangezien SPAD-waarden en de oogstindex ongewijzigd bleven.","summary":"African governments and their international organizations have been making mineral fertilizers available to farmers as a means to improve crop yields. However, research has convincingly shown that the agronomic efficiency of applied mineral fertilizers which often contain nitrogen (N), phosphorus (P), and\/or potassium (K) only, is low in Ghana. This suggests there are other yield-limiting or yield-reducing factors that impede crop responses to applied mineral fertilizers. This thesis analyzes how various crop intensification measures that are meant to address constraints in crop canopy closure, sulfur availability and use, crop rooting depth, and soil moisture availability and use, enhance soybean and maize uptake and utilization of N, P, and K supplied through a mineral fertilizer, in northern Ghana. The crop intensification measures tested include planting density, sulfur fertilization, ridging, and mulching alone, and a selection of their various combinations.\n\nChapter 1 gives the general background and a thorough pre-analytical diagnosis of yield-limiting factors in northern Ghana. Specifically, it provides an early indication of how soybean and maize yield responses to N, P and K (NPK) could be enhanced in northern Ghana. The concept of nutrient uptake and utilization as an analytical tool for understanding mechanisms through which intensification measures enhance crop yields, is presented. Further, in this chapter, the key research questions addressed by the various chapters and the steps taken to answer these questions are outlined.\n\nChapters 2 and 3 analyze the impact of planting density, sulfur fertilization, ridging, and mulching alone, and a selection of their various combinations on soybean and maize yield responses to NPK fertilization. The study was conducted in two villages, Bognaayili and Gauwogo, which were relatively close to each other, in northern Ghana. Sulfur and ridging were not effective in enhancing soybean yield response to NPK in Bognaayili. Complementing N, P, and K compound fertilizer (NPK) with increased sowing density, sulfur fertilization, ridging, or mulching increased soybean grain yield by 520 \u2013 590, 390 \u2013 760, 390 \u2013 580, and 550 \u2013 770 kg ha\u22121, respectively, compared to NPK alone, in Gauwogo. Maize grain yield increased by 680 \u2013 1580, 910 \u2013 2210, 480 \u2013 1370, and 590 \u2013 1600 kg ha\u22121, when NPK was complemented with increased sowing density, sulfur fertilization, ridging, or mulching, compared to NPK alone. The patterns in stover yields were similar to the patterns observed in grain yields for both crops. The intensification measures were considered equally effective and represent a set of options for intensifying cropping systems when NPK is applied. Against expectations no synergies were observed between the different measures. Agronomic efficiency of applied phosphorus (AEP) observed in the study reported here was high compared to AEP reported in areas with conditions similar to northern Ghana. The high AEP values observed suggest that either recovery of applied P was high, or P dilution occurred in the plants, in this study.\n\nLow soil moisture seems to be an important factor that limited the crop yields of grain and stover in plots fertilized with NPK only, thus limiting the efficiency of the fertilizer application. This assertion was made because crop cover increased when NPK was implemented along with the crop intensification measures, compared to NPK alone. Larger crop canopy cover shades soil in crop fields better, protecting the soil from direct sunlight better, thus reducing soil temperature and water evaporation. Reduced soil water evaporation keeps more water available for transpiration which in turn allows to keep the stomata open and thus to maintain the influx of carbon dioxide for subsequent fixation through photosynthesis.\n\nFor individual intensification measures implemented in soybean, grain and stover yields were lower in Bognaayili than in Gauwogo, whereas in maize, grain and stover yields were comparable across the two sites. The lower soybean (but not maize) grain and stover yields in Bognaayili were likely due to lower potential soil productivity in that site for soybean, compared to Gauwogo. Measured soil organic matter of individual plots ranged between 0.19 and 0.69% in Bognaayili, and between 0.53 and 0.77% in Gauwogo. Soybean growth was poor in plots with soil organic matter content of less than 0.5%, and good in plots with soil organic matter content of 0.5% or higher. Hence, crop fields with soil organic matter content less than 0.5% seem to be of poor quality for soybean intensification.\n\nChapter 4 explores, collaboratively, organic mulching under farmer conditions, which allowed comparison to the mulching effect on maize under management reported in chapter 3. Unlike under researcher management, implementing mulching under farmer conditions did not have impact on maize grain yield. Maize growers in northern Ghana used grass, groundnut vines, groundnut husks, tree leaves, false bean and groundnut vines, and tree leaves and groundnut husks as mulching materials. They began mulching their fields 19 \u2013 28 days after sowing the maize, when the crop might have passed the critical stage when it needed protection against excessive soil water evaporation. Farmers perceived mulching as a useful intensification measure for crops such as pepper and tomato which are cultivated on a smaller scale, but not for maize which is cultivated on a large scale. These findings suggest that mulching larger crop fields such as maize would not fit the farming context of smallholder maize growers in northern Ghana.\n\nChapter 5 shows an attempt to develop a method for assessing within-field variation in soil fertility in crop fields. Specifically, the chapter explores the use of drone imagery as an approach to assess within-field variation in soil organic matter, which is a good indicator of inherent soil fertility. Spectral vegetation indices from early season weed cover images showed correlations with measured soil organic matter in Bognaayili which is the site with the larger within-field variation (range 0.19 \u2013 0.69%), but not in Gauwogo (range 0.53 \u20130.77%). Model-predicted soil organic matter from spectral vegetation indices significantly correlated with measured soil organic matter in Bognaayili, but not in Gauwogo. These findings suggest that the use of spectral vegetation indices from early season weed cover images as proxy for large within-field variation in soil organic matter is a promising option, but for now, it still requires simultaneous vegetation observations and soil sampling for organic matter analysis to improve the model.\n\nChapter 6 synthesizes the outcomes of the research chapters in this thesis and proposes further steps that need to be taken to realize improved fertilizer use efficiency under farmer conditions. It can be asserted that nutrient uptake (kg nutrient taken up ha\u22121) of N (from biological nitrogen fixation in soybean, from fertilizer in maize), and P and K (from fertilizer), was the mechanism through which the tested intensification measures enhanced crop yield when NPK was applied. After all, SPAD values were comparable among all NPK amended plots, despite the larger canopy in plots in which the intensification measures were implemented compared to in plots in which NPK alone was implementing. Similar SPAD values of top leaves combined with larger crop cover indicates more nitrogen was taken up and partitioned into canopy. Although intensifying the cropping systems led to enhanced vegetative growth, there were no indications of enhanced crop stress as crop developmental stages, SPAD values, canopy senescence, and harvest index remained unchanged.","auteur":"Yussif Baba Kassim","auteur_slug":"yussif-baba-kassim","publicatiedatum":"4 september 2026","taal":"EN","url_flipbook":"https:\/\/ebook.proefschriftmaken.nl\/ebook\/yussifbabakassim?iframe=true","url_download_pdf":"https:\/\/ebook.proefschriftmaken.nl\/download\/c3e3f2b1-213a-43a6-a928-81b7d848a55b\/highres","url_epub":"","ordernummer":"18328","isbn":"","doi_nummer":"","naam_universiteit":"Wageningen University","afbeeldingen":17047,"video_url":"","podcast_url":"","naam_student:":"","binnenwerk":"","universiteit":"Wageningen University","cover":"","afwerking":"","cover_afwerking":"","design":""},"_links":{"self":[{"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/us_portfolio\/17045","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/us_portfolio"}],"about":[{"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/types\/us_portfolio"}],"author":[{"embeddable":true,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/users\/8"}],"replies":[{"embeddable":true,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/comments?post=17045"}],"version-history":[{"count":1,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/us_portfolio\/17045\/revisions"}],"predecessor-version":[{"id":17048,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/us_portfolio\/17045\/revisions\/17048"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/media\/17046"}],"wp:attachment":[{"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/media?parent=17045"}],"wp:term":[{"taxonomy":"us_portfolio_category","embeddable":true,"href":"https:\/\/www.proefschriftmaken.nl\/en\/wp-json\/wp\/v2\/us_portfolio_category?post=17045"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}