Determination of Available Phosphate and Exchangeable Potassium in the Soil with a Hand-made Colorimeter Equipped with Different Wavelength LED, Together with a Commercial Water Analysis Kits.

Abstract

In recent years, the content of available phosphate and exchangeable potassium in the soil of Japanese tea gardens has been increasing. Especially the amount of available phosphate in the soil has far exceeded the permissible level. These excessive accumulation of minerals in the soil may have been caused by the following factors: 1) phosphate fertilizer applied to the soil surface would not easily leach to the subsoil, 2) potassium is supplied by the decomposition of leaves and stems add to the soil by periodical pruning, and 3) specified amount of fertilizer is applied only the between-row spacing (about 20% of tea garden area).
When considering the cost of fertilizer application, the amount of fertilizer should be determined based on the mineral concentrations in the soil. In order to easily determine the mineral concentration in the soil without cost increase, we used a hand-made colorimeter equipped with different wavelength LED, together with a commercial water analysis kits. In this analysis, 0.002 N H₂SO₄ was used as an agent extracting available phosphate and exchangeable potassium.
This analysis procedure can easily detect the excessive levels of available phosphate and exchangeable potassium in the soil with sufficient accuracy.