Aims

Perennial forage grass production has the potential to improve smallholder livelihoods in the tropics. However, nutrient management is often challenging, especially on infertile sandy soils. This study tested whether typical nutrient management limits the productivity and sustainability of perennial forage grass systems on sandy soils.

Methods

Nutrient balances were estimated for four fields of either Megathyrsus maximus cv. Tanzania or Urochloa hybrid Mulato II. Nutrient limitations were then evaluated in a nutrient omission experiment.

Results

All soils were sandy (< 10% clay), neutral to acidic (pH (CaCl₂) 4.6 to 6.7) and had plant-limiting concentrations of total nitrogen, extractable potassium and extractable sulfur. Nitrogen inputs were typically higher than outputs, with balances ranging from −16 kg ha⁻¹ yr⁻¹ to 293 kg ha⁻¹ yr⁻¹, yet concentrations in forage shoots were low at all sites. Phosphorus balances ranged from −5 kg ha⁻¹ yr⁻¹ to 77 kg ha⁻¹ yr⁻¹ and concentrations in forage shoots were adequate. Potassium inputs were low, resulting in balances from −79 kg ha⁻¹ yr⁻¹ to −138 kg ha⁻¹ yr⁻¹ at locations that did not apply inorganic potassium fertilizer. Potassium concentrations in forage shoots were low and omission of potassium resulted in severely depressed biomass production. Inorganic sulfur fertilizers were not applied to forages and the sulfur balance varied from −1 kg ha⁻¹ yr⁻¹ to −24 kg ha⁻¹ yr⁻¹. Sulfur concentrations in shoots were correspondingly low and production was depressed when sulfur was omitted in the experiment.

Conclusion

Balanced nutrition was not achieved, despite substantial fertilizer inputs, resulting in deficiencies of potassium and sulfur, inefficient use of nitrogen and excesses of phosphorus. If current practices continue, potassium and sulfur depletion, phosphorus accumulation and soil acidification can be expected. Recommendations for balanced nutrient management that accounts for high rates of removal in biomass, variable concentrations in organic fertilizers, and leaching potential, are needed to sustain the productivity of perennial forages on tropical sandy soils.

中文翻译：

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钾和硫供应不足威胁热带沙质土壤小农户多年生牧草的生产力

目的

多年生牧草的生产具有改善热带小农生计的潜力。但是，养分管理通常面临挑战，特别是在不肥的沙质土壤上。这项研究测试了典型的养分管理是否限制了沙质土壤上多年生牧草系统的生产力和可持续性。

方法

估计了两个最大背甲（Megathyrsus maximus cv）的四个领域的营养平衡。坦桑尼亚或Urochloa杂交Mulato II。然后在营养物遗漏实验中评估营养限制。

结果

所有土壤均为沙质（<10％黏土），中性至酸性（pH（CaCl ₂）4.6至6.7），并且具有限制植物的总氮，可提取钾和可提取硫的浓度。氮输入通常高于输出，其平衡范围为-16 kg ha ^-1  yr ^-1至293 kg ha ^-1  yr ^-1，但是在所有地点的牧草芽中浓度都较低。磷的平衡范围为-5 kg ha ^-1  yr ^-1至77 kg ha ^-1  yr ^-1，草料芽中的浓度足够。钾输入量低，导致-79 kg ha ^-1  yr ^-1到-138 kg ha的平衡^-1  yr ^-1在未施用无机钾肥的地点。牧草芽中钾的含量低，钾的缺乏导致生物量的生产严重下降。无机硫肥料未施用于草料，硫平衡从-1 kg ha ^-1  yr ^-1到-24 kg ha ^-1  yr ^-1不等。当在实验中省略硫时，枝条中的硫浓度相应较低，并且产量降低。

结论

尽管投入了大量肥料，但营养仍未达到平衡，导致钾和硫缺乏，氮利用效率低下和磷过多。如果目前的做法继续下去，可以预期钾和硫的消耗，磷的积累和土壤酸化。为了维持热带沙质土壤上多年生牧草的生产力，需要平衡养分管理的建议，以解决生物质的高去除率，有机肥料中的浓度可变和淋溶潜力。