ABSTRACT

Vertisols of India are calcareous and have a heavy texture with high clay content but low to medium content of Olsen estimated phosphorus (P). The on-farm and on-station trials revealed that Vertisols with various levels of P₂O₅ responded negligibly to fertilizer application on soybean crops. In Vertisols, the inorganic P is primarily associated with Ca (Ca-P), which gradually solubilizes under acidic conditions surrounding rhizosphere. Hence, in order to correlate both the findings, the current study investigated the rhizosphere/rhizoplane pH. The study used agar plate technique to observe that the pH of the rhizosphere and rhizoplane soils, which was observed to be acidic although the soil pH was 7.6. The correlation between P uptake and soil-available P measured using the four test methods (Bray 2, Olsen, Truog, and Mehlich 3) revealed that Bray 2 had high correlation with the amount of P absorbed by soybean. Bray 2 method extracts approximately 10 times more P from Vertisols than does the Olsen extractant. This could be a plausible reason for insignificant difference in crop post fertilizer application due to sufficient P fertility of the Vertisols. Hence, Bray 2 method was established as a suitable method for the extraction of P from Vertisols, in which the pH of the extraction solution was the most acidic, among the selected four extraction methods and had high correlation with the amount of P absorbed by soybeans. Hence, this method is recommended for P estimation for Vertisols present in Central India. Essentially, this study provided information that an accurate measurement of P availability is required to explain the non-responsive nature of soybean to P fertilizers in Vertisols. Additionally, the current study emphasized the need for optimum recommendations of P from the view point of rhizosphere; it may facilitate the reduction of the application rates of P to the crops, without reducing the crop yields.

摘要

印度的 Vertisol 是钙质的，质地厚实，粘土含量高，但奥尔森估计的磷 (P) 含量低至中等。农场和站内试验表明，具有不同水平 P ₂ O _{5 的}Vertisols对大豆作物施肥的反应微乎其微。在 Vertisols 中，无机 P 主要与 Ca (Ca-P) 结合，后者在根际周围的酸性条件下逐渐溶解。因此，为了将这两个发现相关联，目前的研究调查了根际/根际 pH 值。该研究使用琼脂平板技术观察根际和根面土壤的 pH 值，虽然土壤 pH 值为 7.6，但观察到呈酸性。使用四种测试方法（Bray 2、Olsen、Truog 和 Mehlich 3）测量的磷吸收和土壤有效磷之间的相关性表明，Bray 2 与大豆吸收的磷量具有高度相关性。Bray 2 方法从 Vertisols 中提取的磷大约是 Olsen 提取剂的 10 倍。由于 Vertisols 的磷肥力充足，这可能是作物施肥后差异不显着的合理原因。因此，Bray 2 法被认为是从 Vertisols 中提取 P 的合适方法，其中提取液的 pH 值是最酸性的，在所选的四种提取方法中，与大豆吸收的 P 量具有高度相关性. 因此，建议将此方法用于印度中部存在的 Vertisols 的 P 估计。从本质上讲，这项研究提供的信息表明，需要准确测量磷的可用性来解释大豆对 Vertisols 中磷肥的无反应性质。此外，目前的研究强调需要从根际的角度对磷进行最佳推荐；