Evaluation of a peri-urban smallholder farmers’ soil amendment practices on soil quality and crop growth, yield and quality

Gobozi, Thamsanqa Khanya Sikho (2018-03)

Thesis (MScAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Peri-urban, smallholder farmers surrounding Cape Town, which are the main producers of fresh vegetables in the region, are generally not producing at their optimum level due to lack of agronomic support, marginal sandy soils and socio-economic constraints. The aim of this study was to evaluate the soil amendment practices of an organic smallholder farmer from Raithby, near Stellenbosch, in comparison with potential alternative organic and chemical amendments on soil fertility and vegetable crop growth, yield and quality and economic profitability. During the first winter field trial, the farmer’s routine soil amendment practice of adding 10 t/ha of commercially bought compost was compared with three alternative organic amendment practices and a commercial chemical fertilizer programme on broccoli production. Two on-farm produced composts, composted plant and animal waste (CW) and composted waste containing 20% biochar (CB), and the commercial compost (CC) were applied at typical smallholder application rate of 10 t/ha. The CW was also applied at broccoli N requirement equivalent to 22 t/ha (CWCR). These organic treatments were compared with a control soil (C) and a chemical fertilizer (CF) programme designed specifically for broccoli. There were no significant differences in soil quality at planting or at harvest (pH, EC, ECEC, plant available macro or micro-nutrients) or broccoli head nutrient content between treatments. However, the CF significantly (p<0.05) increased soil mineral N compared to all other treatments, whereas, CB significantly (p<0.05) enhanced soil C. Application of CF significantly (p< 0.05) increased broccoli yields (88% increase compared to CC) which was correlated with the higher soil mineral N, followed by CW (28% increase compared to CC). Application of CC, CB and CWCR resulted in non-significant changes in yield compared to the control, which was attributed to too much C being added to soil compared to N. Compared to the farmer’s routine amendment practice (CC), the CF resulted in the greatest income increase (455%) followed by CW at 10 t/ha (151%). During the second summer field trial, the effect of two composts, i.e., university compost (UC) and farmer’s compost (FC), two commercial organic fertilizers, i.e., OF1 (blood and bone meal based) and OF2 (chicken manure based), and commercial chemical fertilizer (CF) programme was evaluated on green bean production. The commercial organic and mineral fertilizers were applied at green bean N requirement rate of 158 kg N/ha. Whereas, the two composts were inadvertently applied at different N application rates relative to the commercial fertilizers (UC was added at 8.9 ton/ha (~49 kg N/ha), while FC was added at 17.8 ton/ha (~181 kg N/ha) due to a commercial laboratory providing incorrect elemental analysis of the composts prior to the field trial. All compost and fertilizer treatments significantly (p< 0.05) increased soil Bray II P contents above the critical value 25 mg/kg at planting except FC. The commercial organic fertilizers increased soil EC by a factor of 2-3, which resulted in lower bean plant survival. There were no significant differences in bean nutrient content between treatments, except for OF1 which contained significantly lower Mg content. Application of CF significantly (p< 0.05) increased (56% compared to control) green bean yields which was associated with a significantly (p< 0.05) higher (168% compared to control) cumulative soil mineral N, while the FC applied at 17.8 t/ha produced the second highest increase (37% compared to control) which was associated with higher (5%) number of plants that survived to harvest and the order was consistent in terms of economic feasibility. The availability of mineral N was the main driver of crop yields and size of economic yield per plant in this study. Composts, especially commercial composts with low inherent N content, are not reliable sources of mineral N for intensive crop production. The commercial organic fertilizers, although better sources of mineral N, were prohibitively expensive and decreased plant survival. The organic smallholder farmer is likely to generate more income when he produces his own compost using animal and plant waste and applily the on-farm produced compost at N requirement of the crop in production rather than buying composts or organic fertilizers. The study also indicated that the farmer would generate much higher income, especially in winter when organic N mineralisation is slowest, if he would use a chemical fertilizer programme for both model crops.

AFRIKAANSE OPSOMMING: Kleinhoewe kleinboere rondom Kaapstad is hoofsaaklik verantwoordelik vir die produksie van vars groente in hierdie streek. Weens die gebrek aan agronomiese ondersteuning, marginale sanderige gronde en sosiale- ekonomiese beperkings, produseer hulle nie op hul optimale vlakke nie. Hierdie studie vergelyk die invloed van grondverbeteringspraktyke van ‘n organiese kleinboerdery, van Raithby naby Stellenbosch, met potensiële alternatiewe organiese en chemiese toevoegings, op grondvrugbaarheid en oesopbrengs, asook ekonomiese winsgewendheid. Gedurende die eerste winter se veldproef, was die boer se gewone grondverbeteringspraktykte om 10t/ha van ʼn kommersiële gekoopte kompos toe te dien. Tesame met die is drie alternatiewe organiese grondverbeteringspraktyke met ʼn kommersiële chemiese bemestingsprogram op broccoli produksie vergelyk. Twee plaaslik (op die kleinhoewe) geproduseerde komposte, wat plant en diere-afval (CW) bevat en gekomposteerde plant- en diere-afval wat 20% Biochar (CB) bevat, en kommersiële kompos (CC) was teen 10t/ha toegedien. Die CW het ook aan broccoli se stikstofbehoefte vereiste van 22t/ha (CWCR) voldoen. Hierdie organiese behandeling was met ‘n kontrole grond (C) en ‘n chemiese kunsmis (CF) program wat spesiaal vir broccoli ontwerp is,vergelyk. Daar was geen beduidende verskil op die grondgehalte tydens oes (pH, EC, ECEC, plantbeskikbare voedingstowwe, (makro- of mikro-voedingstowwe) of voedingstowwe in die broccolikop se inhoud tussen behandelinge gekry nie. Hoewel, die CF ‘n beduidende verhoogte grondmineraal N-inhoud in vergelyking met all die ander behandelings, gehad het, terwyl CB tot ʼn aansienlike (p< 0.05) verbeterde grond C-inhoud aanleiding gegee het. Toepassing van CF het aansienlik (p< 0.05) die opbrengs van broccoli verhoog (88% verhoging in vergelyking met CC) wat met hoër grond minerale N gekorreleer was, gevolg deur CW (28% verhoging in vergelyking met CC). Toepasing van CC, CB en CWCR het egter geen beduidende bydra tot opbrengs gehad nie, wat toegeskryf is aan te veel C en te min N wat aan grond in hierdie behandelings toegevoeg is. In vergelyking met die boer se gewone grondvebeteringspraktyk (CC), het CF ‘n baie hoër inkomste getoon (455%) gevolg deur CW teen 10 t/ha (151%). Tydens die tweede (somer) veldproef, was die effek van die twee komposte, naamlik universiteitskompos (UC) en boerekompos (FC), twee kommersiële organiese bemestingstowwe, OF1 (bloed- en beenmeel gebaseerd) en OF2 (hoendermis gebaseerd), en kommersiële chemiese kunsmis (CF) -program op die produksie van groenboontjies vergelyk. Die kommersiële organiese en minerale kunsmis was op ‘n groenbone se N-behoefte van 158 kg N/ha toegepas. Die twee komposte was per ongeluk op verskillende N vlakke in vergelyking met kommersiële kunsmis (UC) toegedien. UC is teen 8.9 ton/ha (~49kg N/ha) toegedien, terwyl FC teen 17.8 ton/ha toegedien is (~181 kg N/ha). Die fout was weens ʼn kommersiële laboratorium wat ʼn verkeerde elemente ontleding van die kompos vir die veldproef verskaf het. Alle kompos en kunsmisbehandelings het aansienlik (p<0.05) verhoogde grond Bray II P-inhoud bo die kritiese waarde tydens plant van 25 mg/kg behaal, behalwe FC. Die kommersiële organiese kunsmis verhoog die grond EC met ‘n faktor van 2-3, wat tot gevolg gehad het dat minder groenboontjie plante oorleef het. Daar was geen betekenisvolle verskille in die voedingstofinhoud van die groenbone nie, behalwe vir OF1 wat ʼn betekenvolle laer Mg-inhoud gehad het. Toepassing van CF het die opbrengs van groenbone aansienlik (56% in vergelyking met die kontrole) verhoog wat geassosieer was met ʼn aansienlike (p<0.05) hoër (168% met vergelyking met kontrole) kumulatiewe grondminerale N. Behandeling FC het teen 17.8 t/ha die tweede hoogste (37% vergelyking met kontrole) produksie behaal wat geassosieer was met (5%) meer plante wat oorleef het. Die volgorde was konsekwent in terme van ekonomiese lewensvatbaarheid. Die beskikbaarheid van grondminerale N was die belangrikste vir oesopbrengs en die groote van die ekonomiese oprengs per plant in hierdie studie. Kompos, veral kommersiële kompos wat lae inherente N bevat, is nie voldoende vir intensiewe produksie nie. Kommersiële organiese kunsmis is ‘n goeie bron van minerale N, maar dit is baie duur en verlaag die saailinge se oorlewing weens ʼn hoë soutinhoud. Die organiese kleinboer, sal waarskynlik meer inkomste kan maak wanneer hy sy eie kompos met die gebruik van plant en diere-afvalmateriaal kan maak, en verseker dat dit die nodige N bevat, inplaas daarvan om kompos of organiese kompos te koop. Die studie het ook aangetoon dat die boer ʼn baie hoër inkomste kan genereer, veral in die winter wanneer organiese N mineralisasie stadig is, as hy van ʼn anorganiese bemestingsprogram vir albei die gewasse gebruik sal maak.

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