Although corn plants are tolerant of low levels of available soil Mn, Mn deficiencies in corn were suspected on soils where Mn applications had previously increased soybean seed yields. Five experiments were conducted in farmer's fields to evaluate the response of corn to Mn applications. Three band Mn and two broadcast Mn studies were conducted in five field experiments on Atlantic Coastal Plain soils. The mean increase in corn grain yield in the band studies on three soils was 960 kg ha⁻¹. Corn grain yields were not increased in the broadcast Mn studies on Slagle and Dragston fine sandy loams. Manganese uptake from the Slagle soil was so high that Mn deficiency did not occur; whereas, Mn uptake from the Dragston soil was so low that the deficiency was not completely corrected by up to 24 kg Mn hha⁻¹ as broadcast MnSO₄. / Master of Science

Thesis (M.Sc. (Soil Science)) --University of Limpopo, 2013 / Large amount of pre-plant nitrogen (N) fertilizer results in low nutrient-use-efficiency due to poor synchrony between soil N supply and maize demand, especially during N sensitive growth stages. Optimum maize production is dependent on adequate N availability to the crop during the critical vegetative and reproductive growth stages. High N fertilizer prices and maize yield decline are the main challenges faced by the Limpopo Province farmers. The objectives of this study were to compare growth and yield of maize under conventional and site-specific N management in a dryland farming system. The study was conducted in Leeukraal, Towoomba, Ga-Marishane and Radium in the Limpopo Province, South Africa. Experimental plots were laid out in a randomized complete block design, with four replications. Phosphorus was applied through band placement using a planter in all plots at a rate of 42 kg P/ha. Hybrid maize SNK 2147 was planted on a 20 by 20 m plot with Inter-row and Intra-row spacing of 0.9 and 0.35 m respectively. Treatments consisted of 3 N management strategies as follows, (i) No N application (N0), (ii) Site-specific N at a rate ranging between 18 and 33 kg N/ha (N1) and (iii) Conventional N application at 58 kg N/ha (N2). Treatment N2 was applied at a uniform rate during maize planting. Sufficiency index as an indication for N deficiency was determined using CCM-200 for treatment N1. The sufficiency index was determined during leaf stage V6, V10 and V14, and thereafter N was applied only when needed. Data were subjected to analysis of variance through Statistical Analysis System package. Mean separation tests were computed using Duncan’s Multiple Range Test. Maize grain yield at Leeukraal of 5.2 t/ha for N1 was higher than 3.2 and 4.0 t/ha of N0 and N2, respectively. There was no difference amongst the three N management approaches on the grain yield at Towoomba. The grain yield at Ga-Marishane for N1 of 2.2 t/ha was significantly higher than 1.7 t/ha of the N0. Conventional management approach, which is a traditional approach used by farmers in the Limpopo Province, had 2.6 t/ha grain yield that was significantly higher than the N0 and N1. The maize growth and yield under N2 and N1 was compared, N1 required between 43 and 69% lesser N fertilizer as compared to N2. Therefore site-specific nutrient management approach sustains and improves growth and yield of maize using minimal inputs of N compared to conventional approach. This therefore saves input costs and avoids unnecessary environmental consequences. Key words: maize yield, nitrogen management, site-specific approach / Vlaamse Interuniveritatire Raad and Limpopo Department of Agriculture

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Globally, an estimated 25% of the CO2 and 90% of the N2O is believed to come from agroecosystems. The objective of this study was to investigate the dynamics of the below-ground CO 2 and N2O concentrations and efflux in corn and soybean systems. In our field study, changes in the below-ground concentrations of CO 2 and N2O were closely related to seasonal changes in soil moisture, with the first two months of the growing season being particularly critical to the production of these gases. Tillage significantly increased CO2 content in the soil profile, however, this effect was greater in the soybean plots than in the corn plots. In our greenhouse studies, an average of about 79% of the soil respiration in corn came from rhizosphere respiration, compared to an estimated 58% in the case of soybean. Specific rhizosphere respiration was significantly higher in soybean (0.29 mg C g -1 root h-1) than corn (0.09 mg C g-1 root h-1), which supports previous observations made with regards to slower-growing plants (e.g. soybean) having relatively higher root respiration than faster growing plants. We observed a nonsignificant difference between N2O efflux in the soybean-planted soil and unplanted bulk soil, which is in contrast to the perception that legumes could stimulate more N 2O production from the soil by increasing the N pool through N 2 fixation. While corn had the greatest uptake of fertilizer N, N 2O efflux in corn pots was higher (2.84 mug N pot-1 h-1) than the soybean pots (0.06 mug N pot-1 h-1). In the laboratory setting, denitrification in the microaggregates proceeded at about 4.4 to 39.6 times higher rate than in large macroaggregates, small macroaggregates or the bulk soil, and showed the greatest response to high moisture levels (80% WFPS).

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An incubation study was conducted to evaluate the effects of added urea, NH$ sb4$Cl, and KCl on fixation of NH$ sb4 sp{+}$ and K$ sp{+}$. A field study was carried out to observe the effects of rates and methods of added urea and KCl on the forms and distribution of N and K in soils, and on yields and nutrient uptake of silage corn. / Generally, the fixation of NH$ sb4 sp{+}$ was increased by added N but reduced by added K and vice versa. One exception was that added K increased NH$ sb4 sp{+}$ fixation when NH$ sb4$Cl was the N source. In comparison with NH$ sb4$Cl, urea resulted in greater K$ sp{+}$ fixation but less NH$ sb4 sp{+}$ fixation. / Band placement of urea and KCl showed apparent accumulation of NH$ sb4$-N and NO$ sb3$-N, and exchangeable K$ sp{+}$ in the fertilizer bands, particularly early in the growing season. On the other hand, band placement resulted in lower levels of available N and K in soil between the bands in comparison with broadcast. / Yields of silage corn and nutrient uptake values were increased by N and K fertilizers, except for one soil in the first year. Band placement of urea was superior in the case of frequent and light rainfall but inferior with dry surface soil conditions followed by a relatively large rainfall 3 or 4 days after application. In the latter condition, banding K resulted in less yields on one soil, and banding the two fertilizers together further reduced N utilization on another soil. / Additions of K suppressed Ca and Mg uptake but did not influence N uptake, whereas K uptake was usually enhanced by added N.

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Concern about NO$ sb3 sp-$-N leaching and groundwater pollution from monoculture corn (Zea mays L.) has prompted investigation of alternative production systems which reduce N leaching. Both intercrop systems and water table controls alone have been shown to increase nitrogen (N) uptake and reduce soil NO$ sb3 sp-$-N accumulation in cropping systems. There is a need to maintain crop productivity while reducing the potential for soil NO$ sb3 sp-$-N leaching into groundwater. However, there has been no information available regarding agronomic and physiological aspects of N and water management for monocrop corn and corn competing with annual Italian ryegrass (Lolium multiflorum Lam) in an intercrop system. A study was conducted in southwestern Quebec during 1993 and 1994. Nitrogen and dry matter components in the plant-soil system were determined. Intercropped corn grain yield did not differ from monocropped corn under high N fertility. At harvest, the corn-annual ryegrass intercrop system increased total aboveground N uptake by 77.2 and 50.7 kg ha$ sp{-1}$ when compared with the corn monocrop system in 1993 and 1994, respectively. The intercrop system reduced the amount of NO$ sb3 sp-$-N in the top 1 m of soil by 47% (92.3 kg N ha$ sp{-1}$) at harvest in 1993. Water table controls had little effect on corn yield, N use efficiency and soil NO$ sb3 sp-$-N accumulation over the two years of this study. Both plant establishment and weather conditions affected the ability of annual ryegrass to aid in the uptake of soil NO$ sb3 sp-$-N. The reproductive development of water stressed plants after silking was limited more by overall plant changes due to water stress than assimilate supply. The delivery of C (sucrose) and N ($ sp{15}$N urea) into corn plants via stem-injection showed that externally supplied C changed both the source strength (photosynthetic inhibition) and sink strength (decreased total grain production), while distribution of $ sp{15}$N was affected by p