How to collect soil testing samples? A detailed guide to the self-sampling method for farmers
Since the soil testing and formula fertilization subsidy project was launched in 2005, it has now entered the stage of promoting formula fertilizers based on regional grand formulas. To put soil testing and formula fertilization technology into practice in farmland and orchards, make good "small adjustments" according to local conditions, accurately collect soil samples, and accumulate big data for soil "health checkups" is the prerequisite. In recent years, as the pace of appropriately scaled land management has accelerated, new types of business entities such as large growers, family farms, farmers' professional cooperatives, and agricultural social service organizations have continued to develop. Compared with traditional farmers, they have new changes in farming methods and new requirements for scientific fertilization, quality improvement, and efficiency enhancement. It can be said that new agricultural business entities will be the main body implementing soil testing and formula fertilization technology. Today, this edition specially invites experts from the Meixian Agricultural Technology Extension Service Center in Shaanxi to popularize relevant professional knowledge, laying a good foundation for farmers to use soil testing and formula fertilization technology to increase production and income.
To fertilize scientifically and improve fertilizer use efficiency, the first step is to understand soil nutrient status, so soil samples must be collected. The collection of soil samples is an important link in determining whether soil testing results are reliable. Because soil, especially cultivated soil, varies greatly in nutrients, sampling error can be several times greater than analytical testing error. The sampling standard is that the collected soil sample can represent the average nutrient status of the sampled field. Therefore, collecting representative soil samples is the foundation and prerequisite of soil testing and formula fertilization. When soil and fertilizer professionals and technicians are relatively scarce, having agricultural producers collect soil samples themselves is an effective measure to accelerate the application of soil testing and formula fertilization technology. Therefore, teaching them how to collect soil samples is especially important for promoting soil testing and formula fertilization technology and raising farmers' awareness of scientific fertilization.
1 Determination of sampling units
Because people's production practices, such as uneven fertilization, banded or hole application of fertilizer, ridging for planting, deep plowing of soil, and different crops planted, cause great differences in local soil nutrient changes. Therefore, according to differences in soil nutrients, the soil is divided into several sampling areas, which we call sampling units. The soil nutrients in each sampling unit should be as similar as possible. Each sampling unit can also be divided into several small units according to area size. The smaller the area represented by each small unit, the more reliable the representativeness of the soil sample. However, the smaller the area, the greater the sampling workload and the greater the analytical testing workload. So how large an area can one composite sample represent in a relatively reliable and economical way? Apart from the fact that different soil types must be separated for sampling, generally one sampling unit can range from a few mu to several dozen mu.
2 Determination of sampling time
The content of available nutrients in soil varies greatly with seasonal changes and changes in soil temperature and moisture. For example, in winter, available phosphorus and potassium in soil both increase. Therefore, only the analytical testing results of soil samples collected at the same time can be compared with each other. The determination of sampling time should differ according to crop type and fertilization purpose. (1) For grain crops such as wheat, rice, and corn, soil samples are generally collected before or after the harvest of the previous crop, or before sowing of that crop. (2) For vegetable economic crops such as tomatoes, cauliflower, and baby bok choy, soil samples are generally collected before sowing. (3) For orchard soils of fruit tree economic crops such as apples, pears, and kiwifruit, the soil sampling time differs according to fertilization purpose. If topdressing during the fruit expansion period, soil samples should be collected in mid-to-late May. If determining the annual fertilization plan for fruit trees, soil samples should be collected from late September to early-to-mid October, that is, before or after fruit harvest, for analysis and testing.
3 Selection of sampling tools
The commonly used professional sampling tools mainly include three types: small soil shovel, tubular soil auger, and ordinary soil auger (see Figure 1). Farmers collecting soil samples themselves generally choose a spade or small soil shovel. Whether using professional sampling tools or a farmer's own spade for sampling, special attention should be paid to keeping the upper and lower soil body consistent when sampling, especially preventing surface soil from falling off; otherwise, it will have a relatively large impact on the analysis results.
4 Collection of composite samples
Due to soil heterogeneity, within one sampling unit, several points are arbitrarily selected along a certain route, and the soil samples collected from each point are mixed together to form a composite sample. A composite sample is actually equivalent to an average value, used to reduce soil differences and improve the representativeness of the sample. Generally, according to the area size of the sampling unit, one composite sample consists of 5-15 sampling points. Composite sample collection methods:
1. Requirements for collecting composite samples The soil sampling depth at each point must be consistent, and the upper and lower soil body must be consistent. Each point is arbitrarily selected, that is, all points within a sampling unit have an equal chance of being sampled. In order to avoid systematic error, the general sampling routes include the diagonal sampling method, the checkerboard sampling method, and the snake-shaped or "S"-shaped sampling method, etc. (see Figure 2).
2. Depth and position of sampling points (1) For grain crop and vegetable economic crop fields, generally 5-100 mu is one sampling unit. Within one sampling unit, 5-15 points are evenly selected for soil sampling, with a sampling depth of 0-20 cm, that is, a 0-20 cm soil sample is taken vertically from the surface. The sampling position should preferably be 3-5 m from the outer edge of the field. (2) For fruit tree economic crop fields, generally 2-20 mu is one sampling unit. Within each sampling unit, 5-15 points are evenly selected for soil sampling. The soil sampling points should be selected at 2/3 of the distance from the fruit tree canopy projection, with a sampling depth of 0-30 cm, that is, a 0-30 cm soil sample is taken vertically from the surface (see Figure 3). Similarly, the sampling position should preferably be 3-5 m from the outer edge of the field.
3. Weight of the composite sample Generally, about 1 kg of soil sample is sufficient for sending to the laboratory. If the collected sample weighs much more than that, the collected soil sample can be placed on a plastic sheet or wooden tray, and the soil clods can be crushed by hand. Remove stones, grass roots, and dead branches and fallen leaves, mix the soil samples from each point evenly, then use the "quartering method" (see Figure 4) to divide out about 1 kg of soil sample, put it into a sample bag (such as a plastic bag or paper bag), and place a soil sample label inside the bag. On the soil sample label, clearly write the farmer's name, sampling location, field position, sampling depth, sampling date, sampler, crop planted, and variety. If it is a fruit tree, note the tree age, average annual yield, average annual fertilization and management conditions, and other information on the farmer's cultivation and management measures. The soil sample label should be written clearly in pencil.
Send the collected soil sample to the soil and fertilizer laboratory, where laboratory technicians will conduct testing and analysis. Based on the soil sample test results and in combination with the long-term fertilizer efficiency trial results of the soil and fertilizer station, a fertilization plan for that crop can be formulated to guide farmers in formula fertilization.