Soil testing and fertilization includes four phases, namely, taking soil samples, soil testing, and the results of laboratory tests and suggesting fertilization. Taking soil samples is the primary link in the success or failure of soil testing. If you sample improperly, you will not only waste the cost of the test, but will also draw erroneous conclusions. 1. Reasonable sampling points Because the soil is an inhomogeneous body, the peasants said that "one step three soil replacement" makes sense. This is because the soil is formed under a variety of environmental factors, including climate, parent material, vegetation, topography, time, and human factors. These environmental factors are all changing. A slight change in each factor will make the soil Corresponding changes have taken place. We took a soil sample every 10m on the soil of 4 hectares (50100m) in 4 provinces of the province. The test results showed that the change of soil pH was small and the effective nutrient changed greatly, the difference between high and low was 1-5 times (see table 1). Another example is the distribution of available phosphorus content based on the results of the tests on our 3 hectare experimental site, after one year of uniform sampling, and then every 10 meters in the autumn before sampling. From Figure 1, it can be seen that the distribution of available phosphorus at different levels of content is very different at the test site. For this reason, in soil testing and fertilization, it is necessary to take multiple mixed samples. 2. The correct sampling time The sampling time should be after October 1st and before freezing. The effective nutrients in the soil change with the hydrothermal conditions and crop absorption. After October 1st the temperature is reduced, the crop is mature and no longer absorbs nutrients from the soil. At this time, the effective nutrients in the soil tend to be stable. It can represent the level of Fertilizer supplied to the soil. Therefore, in the area of ​​soil testing at home and abroad at mid-latitude, it is advocated that samples should be taken after autumn harvest. For example, the Ministry of Agriculture of Alberta, Canada, in the Fertilization Guidelines, stipulates that “Spring sowing of cultivated land samples after October 1st, and autumn sowing of farmland samples during the month before sowing”. The measurement and fertilization work in Jilin Province, which is adjacent to Heilongjiang Province, was conducted earlier and was also sampled after autumn harvest. According to reports, the available phosphorus and potassium in the soil were measured continuously from April to September. The content ranges were 0-2.1 mg/kg and 1-26 mg/kg, respectively. The highest nutrient in spring and the lowest in September. We do not advocate spring sampling because the soil in the spring tillage layer has not been completely thawed and cannot meet the requirements for the use of mixed soil samples. It is also due to the fact that with the increase of temperature in the spring and the activity of soil microorganisms, the effective nutrients in the soil have increased dramatically. The highest value was reached at the time of sowing. During this period of sampling, due to the instability of the available nutrients in the soil, it was difficult to make a correct evaluation of the soil fertility. The study of the relationship between soil measurement and fertilization is called calibration study. What time soil samples are taken is also related to calibration studies. If the calibration study is spring soil sampling, find out the relationship between soil available nutrients and fertilizer amount in spring. Afterwards, sampling in the spring is not always possible, but it is difficult to do it due to time constraints. Many of the domestic soil testing and fertilization tests fail, that is, they cannot find out the relationship between soil measurement and fertilization, and even come to the opposite conclusion. Most of them are caused by irregular sampling of soil samples. 3. It is good to have a mixed sample with respect to the area of ​​a representative sample of mixed samples. Reports vary greatly from place to place, and the United Nations Food and Agriculture Organization of the United Nations has done the best in 15 acres of tests in India. The former Soviet Union stipulates that each mixed sample represents an area with no more than 10 hectares of old cultivated land, 5-7 hectares of newly reclaimed land, and 1-2 hectares of undisturbed land. In the United States in 1951, a mixture of 2-4 hectares was recommended. It was later thought that 2-4 hectares would be unrealistic due to the large-scale mechanical cultivation plots. When the land was contiguous, the difference was not significant. A mixed sample of 8 hectares was used. Many foreign countries think that each mixed sample represents an area of ​​2.0-8.1 hectares. The higher the soil uniformity, the larger the representative area. In fact, we are now using plots as the operating unit, that is, the amount of fertilizer used in a plot is the same, and it is difficult to consider the differences within plots. Therefore, the amount of fertilizer applied can only be determined by the majority of soil conditions in the plot. To this end, small plots within 10 hectares are recommended, and plots should be taken as sample plots, taking multiple samples. If it is a large plot, a representative lot can be selected within the plot, and about 10 hectares is a sampling area. The measured value represents the fertility of this plot. At present, “precision agriculture” emerging from abroad does not use plots as operating units, but instead plots plots of 0.5-1 hectares of small squares, fertilizing according to the soil differences in each small square. For us, the first step is to get to the plot first, and the difference within the plot cannot be considered at present. 4. Depth of sampling and sampling method Traditionally, the depth of cultivation of cultivated land is usually 15-20cm, wasteland is 15cm, and no-tillage agriculture is 7.5cm. If soluble salt is measured, it should be 30-120cm deep. The most basic requirement for sampling is to follow the random principle, so the sampled route should be zigzag-shaped and take a soil sample every 10-20 steps. The tools used for sampling shall meet the following conditions: (1) The volume of the soil core is the same for each sample point; (2) It is convenient to use and the sampling is rapid; (3) The required depth and the cross-sectional area are the same. Therefore, it is best to use an earth drill for sampling. In the absence of soil drills, when sampling with earthworms, first dig a pit equal to or slightly deeper than the plough layer, and cut a 2 cm thick piece of soil along the edge of the earth pit. A 2 cm wide strip of soil was left in the middle of the sheet, ie a 22 cm soil core was formed as a soil sample. If it is a ridge plot, it is necessary to take samples of ridges, ridges, and furrows into consideration to form a single sample. For simplicity, it is also possible to sample only from the ridge. Sampling considerations: (1) Different plots should be sampled separately for soil structure, color, slope, soil erosion, drainage, and improvement measures taken in the past; (2) Roadsides, trenches, dung, channels, alkalis Can not be sampled in abnormal places; (3) Frozen soil and silt should not be sampled. About 500g for each sample. Take it well and put it in a ventilated place, dry it in a timely manner, put it into a cloth bag or plastic bag, fasten the label, prevent it from mixing and send it to the laboratory for testing. 5. The frequency of sampling As the amount of fertilizer applied is offset by the amount of crop absorption, soil nutrients will not change drastically. It is therefore recommended that soil extraction be analyzed once every 3-5 years. In the U.S., it is recommended that the earth-fetching analysis be conducted once in 2-4 years, and in the irrigated land, the annual earth-fetching analysis should be performed once. This may be related to the large changes in effective nutrients under irrigation conditions.

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