The accuracy of soil test analysis data is restricted by many factors, and many seemingly simple details can actually affect the measurement results. This blog focuses on some issues that should be paid attention to in the process of soil sample collection, processing and analysis from several aspects.
1 Collection, preparation and storage of soil samples
Soil is not homogeneous. If we do not pay attention to the characteristic of soil heterogeneity, and do not effectively control the sampling error from theory and practice, so it is impossible for any sophisticated analytical instrument and skilled analytical technique to make the measurement results truly reflect the objective conditions of the soil. The objective factors affecting soil heterogeneity are complex, including natural factors, including topography and erosion, and human factors, such as tillage and fertilization. Therefore, the collection of soil samples is the most important and the most critical link in the soil laboratory analysis, which is related to the correctness of the analysis data and the conclusions drawn therefrom.
There are different sampling standards for different experimental purposes. Generally speaking, the soil to be tested must be sufficiently representative. One soil sample can only represent one soil condition. When sampling, the sampling standards and sampling rules must be determined according to the purpose of the experiment (detailed rules will not be introduced in this article, Individual sampling has different criteria). The sampling weight of a mixed soil sample is preferably 1 kg. At the same time, the sampling location, time, basic information of the sampling location, sampling method, sampling person, etc. shall be marked, and the number and sampling records shall be made. For general laboratory analysis, air-dried soil samples can be used. If there are special requirements, they must be explained in advance so that they can be treated separately during soil preparation.
1.1 Collection of soil samples
When taking soil samples, it is necessary to clarify the purpose of sampling, what indicators to measure, and what can be explained by the measured indicators. In addition, survey the sampling sites and set up a certain number of sampling points based on various factors. The sampling points must be representative. The number of sampling points depends on the requirements of the test. Generally, 10 to 20 points are suitable for a small sample plot. Sampling must be carried out in accordance with the principles of random, equal, and multi-point mixing. Soil profile for soil sampling. If there is no special requirement for sampling time, autumn sampling is the best, because autumn can better reflect the soil’s demand for nutrients.
1.2 Preparation of soil samples
1) Samples for general chemical analysis: After the soil samples are taken back, if there is no special requirement, they should be placed in a clean and tidy indoor ventilated place to dry naturally. Explosion is strictly prohibited, and attention should be paid to prevent pollution by acids, alkalis and other gases and dust. At the same time, remove intrusions other than soil. After the air-dried soil samples were sorted out plant residues, stones and other intrusive bodies and new bodies, they were all crushed and passed through a 1 mm aperture sieve. The soil particles that had not been sieved had to be re-rolled and sieved until all passed through. The soil samples passed through the 1 mm aperture sieve can be used for the determination of pH value, salinity, exchange performance, and effective nutrients.
2) Take out a part of the soil sample that has passed through the 1 mm aperture sieve and continue to grind it, so that all of it passes through the 0.25 mm aperture sieve for the determination of organic matter, humus, total nitrogen, calcium carbonate and other items.
3) Samples for trace element analysis: The treatment method is the same as that of general chemical analysis samples, but do not touch metal utensils during the treatment process to prevent pollution. Wooden, bamboo or plastic tools should be used for the above steps. The sieve should use a nylon sieve.
