Preparation of Soil Test Summaries and Soil Fertility Maps
Besides their use in making individual fertilizer recommendation to the farmers, soil test data usually are summarized for a respective block and district and on an all India level. Such soil fertility summaries are useful to administrators and planner in deciding the kind and amount of fertilizer most suitable in each area or district and determining the policy of fertilizer, distribution and consumption in different region. The data also are of use to fertilizer association, fertilizer industries and extension workers in promoting their respective programme and to research workers, particularly from the point of view of changes in fertility levels, conditioned by different fertilizer use or by different soil and crop management practices. Its satisfactory soil test summaries of any unit have been sufficiently large and the samples are representative of the whole area. For district summaries there should be at least one sample for one every 500 acres for block summaries one sample for every 50 acres and for village summaries one sample for every 5 acres.
Many soil test summaries include tables giving percentage of the total number of samples falling in different categories of classification. E.g . Low, medium and high.
Parker’s Nutrient Index:
In order to compare the levels of soil fertility of one area with those of another it is necessary to obtain a single value for each nutrient. Here the nutrient index introduced by Parker et. Al. (1951) is useful. The percentage of samples in each of the three classes, low, medium and high is multiplied by 1,2 and 3 respectively. The sum of the figures thus obtained is divided by 100 to give the index or weighted average.
Three Tier System:
PI = No .of samples (low) X 1 + No of samples (medium) X 2 +
No. of samples ( high) X 3
Total number of samples
Six Tier System :
No. of samples (V.L) X 0.5 + No. of samples (L) X 1.0 + No. of samples
(Medium) X 1.5 + No of sample (M high) X 2.0 + No. of sample ( High) X
2.5 + No of samples (V. high) X 3.0
Total number of sample
Rammoorthy and Bajaj ( 1969) modified the index classification as low 1.67 , medium 1.67 – 2.33 and high above 2.33 to avoid under weight age to the medium categories.
The use of nutrient indices in preparation of soil summarises and for fertilizer recommendations.
The nutrient indices permit a comparison of soil conditions within a given region, clearly reflects management effects and tell at a glance the nutrient sufficient and deficient area. The indices are used in preparing the nutrient index map which serve many useful purpose.
Following samples should be avoided for to purposes:
1. Unusual area, saline, sodic etc.
2. Only samples from one common crop should be grouped together. E.g. for making recommendation to rice soil test values for rice soils should be taken in to consideration. Preparation of soil fertility maps and fertilizer based on indices: Village wise, Taluka wise, District wise and State wise.
1. Use of concentric circles to depict indices.
2. Use of different colour combination.
3. Use of other symbols used for the purpose by the local soil testing laboratories of government of Maharashtra.