Contents of nitrogen in the soil: N-min method

In order to perform proper and quality nutrition of wheat and other small grains, it is necessary to implement sampling and analysis of the soil in order to determine the contents of the mineral nitrogen (N-min method). Given the fact that the nitrogen is a nutrient that dominantly influences the yields, it is very important that quantity of the nitrogen fertiliser that is to be applied is correctly determined. Bad influence of the weak provision of plants with nitrogen, or its lack, on the yield is well known. During the determining of the quantity of nitrogen fertiliser that should be applied, a dominant role is given to the results of the soil analysis according to the N-min method. Only after we get acquainted with the contents of the readily available nitrogen in the soil, we can determine the quantity which the plants lack for their proper growth and development.

Unlike sampling of the soil for analysis of fertility, which is made every four or five years, sampling for the N-min method is made every year. As a rule, time for sampling is immediately before the time when we want to know the contents of the readily available nitrogen. In our conditions, it is mainly made in the following periods:

  • For the wheat: February-March;
  • For the sugar beat: in the 3rd week of February and the 1st week of March;
  • For the maize: in the 2nd and the 3rd weeks of March.

It is very important to perform the sampling procedure properly, since based on a small quantity of the soil from the sample a conclusion is made about the average contents of the nitrogen on the whole plot. Reliability of obtained results depends largely on the quality of taking the samples. The average sample that contains 20-25 individual samples should be taken from an area not larger than 10-20ha from the same preceding crop, the same type of soil and the uniform configuration of the terrain. Distribution of individual samples may be in a diagonal, double diagonal or in a chess pattern, which is the best. Samples for the N-min method are mainly taken from the depth of 0-30cm, 30-60cm, 60-90cm with small grains, while for the root crops a layer at the depth of 90-120cm is also taken.

Every average sample should be accompanies with an adequate label attached, which should contain the following data:

  • Owner of the plot;
  • Place where the sample is taken from;
  • Cadastre number of the plot and its surface;
  • Number of the samples;
  • Depth of the sampling;
  • Cultivated plant species;
  • Date of the sampling;
  • Name of the sampler.

Samples may be taken using the automatic equipment for soil sampling or with a manual probe or a shovel. Individual samples for every soil layer should be placed in one container and, in the end, the soil should be chopped, well stirred, excess of the soil removed, and the obtained average sample should be placed in a bag with a corresponding label, which should weigh 0.5-1 kg. Labels should be written with a lead pencil, since if data is written with ink it will become illegible in the laboratory due to the smearing under the influence of moisture. It is very important that the samples prepared in this way are put immediately in portable refrigerators and kept in a cold place until the time of analysis, in order that most relevant results are obtained.