Sweet potato yield and quality as a function of phosphorus fertilization in different soils

ABSTRACT Phosphorus (P) is an essential nutrient for growth and yield of sweet potatoes; in sandy soils, sweet potato yield and quality may be limited by application of low P doses, mainly in degraded areas. The objective of this work was to evaluate the effect of P doses on yield and quality of sweet potato root tubers grown in tropical sandy soils with different initial P availabilities: 23.2 mg dm-3 (high P - crop rotation) and 3.7 mg dm-3 (low P - post degraded pasture). Two experiments were carried out in a randomized block design, with four replicates. The treatments consisted of P doses (0, 22, 44, 88, and 176 kg ha-1) applied to the planting bed. Tuber yield, number and mean weight of tubers, P concentration in leaves and tubers, exported P, as well as the percentage of starch, reducing sugars, total sugars, and sucrose in the tubers were evaluated. P concentration in the soil was evaluated at the end of the experiment. In the area with high P availability (> 20 mg dm-3 - resin), increasing P doses did not increase root tuber yield and decreased root starch concentrations. In the area with low P availability (< 3.7 mg dm-3 - resin), root tuber yield and starch concentration peaked when applying 68 and 33 kg ha-1 of P, respectively. In areas after degraded pasture, with low initial soil P concentration, P fertilization for sweet potato crops should be carried out with a P dose of 68 kg ha-1. In areas with crop rotation and adequate soil P concentration, P fertilization should be carried out to replace the amount of P exported by the harvested root tubers to avoid decreases in soil fertility.

摘要 磷（Phosphorus，P）是维持甘薯（sweet potato）生长与产量的必需营养元素；在砂质土壤中，尤其是退化区域，低磷施用量可能限制甘薯的产量与品质。本研究旨在探究不同初始有效磷水平的热带砂质土壤中，施磷量对甘薯块根产量与品质的影响，供试土壤初始有效磷水平分别为23.2 mg dm⁻³（高磷区——轮作农田）与3.7 mg dm⁻³（低磷区——退化牧场撂荒地）。

本研究采用随机区组设计（randomized block design）开展两组试验，每组设置4次重复。试验处理设置施磷量梯度为0、22、44、88及176 kg ha⁻¹，均施加于种植畦中。测定指标包括块根产量、块根数量与平均单重、叶片及块根中的磷浓度、土壤磷携出量，以及块根中的淀粉、还原糖、总糖与蔗糖的百分含量；试验结束后，同步测定土壤有效磷浓度。

在初始有效磷水平较高（>20 mg dm⁻³，树脂法（resin））的区域，提升施磷量未显著提升块根产量，反而降低了块根的淀粉含量。在初始有效磷水平较低（<3.7 mg dm⁻³，树脂法）的退化牧场撂荒区域，块根产量与淀粉含量分别在施磷量为68 kg ha⁻¹与33 kg ha⁻¹时达到峰值。

对于退化牧场撂荒且初始土壤磷浓度较低的区域，甘薯种植的适宜施磷量为68 kg ha⁻¹；对于轮作农田且土壤磷浓度充足的区域，应通过施磷补充收获块根所携出的磷素，以维持土壤肥力。