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The study of crop combinations constitutes an important aspect of agricultural geography. In fact, it provides a good basis for agricultural regionalisation and helps in the formulation of strategy for agricultural development.
Crops are generally grown in combinations and it is rarely that a particular crop occupies a position of total isolation. The distribution maps of and their concentration are interesting and help in knowing the density and concentration of individual crops, but it is even more important to view the integrated assemblage of the various crops in a region. For example, the demarcation of India into rice or wheat region does not explain the agriculturally significant fact that very often the wheat/rice region also has mustard, gram, pulses, and maize. For a comprehensive and clear understanding of the agricultural mosaic of a region, a systematic study of the crop combinations has great planning significance. The methods applied for the demarcation of crop combination regions may be summed up under two categories:
1. The arbitrary choice method, and
2. The statistical method
an objective, constant and precisely repeatable procedure and would yield comparable results for different years and localities. In his work, Weaver calculated deviation of the real percentages of crops (occupying one per cent of the cropped area) for all the possible combinations in the component areal units against a theoretical standard. The theoretical curve for the standard measurement was
employed as follows:
Monoculture
2-crop combination 3 crop combination 4-crop combination 5-crop combination
= 100 per cent of the total harvested crop land in one crop
= 50 per cent in each of the two crops
= 33.3 per cent in each of the three crops
= 25 per cent in each of the four crops
= 20 per cent in each of five crops
10-crop combination = 10 per cent in each of 10 crops
For the determination of the minimum deviation the standard deviation method was used:
Where is the difference between the actual crop percentage in a given county (areal unit) and the appropriate percentage in the theoretical curve and ‘n’ is the number of crops in a given combination.
As Weaver pointed out, the relative, not absolute, value being significant, square roots were not extracted; so the actual formula used was as follows:
, Xd2 d = ---
n
To illustrate Weaver’s technique an illustration can be given from the Gorakhpur district where the percentage share of crops in the total harvested area in a year was as follows: rice—48 per cent, wheat—23 per cent, barley—15 per cent, sugarcane—6 per cent, and pulses—5 per cent.
(100 — 48)2
Monoculture = =2704
1 crop
(50- 48 )2 + (50-23)2
2-crop combination = =366.5
2 crops
„ , (33.3-48)2+(33.3-23)2 +(33.3-15)2
3 crops
, (25 — 48)2 + (25 23)2 + (25 —15)2 + (25 — 6)2
4 crops
, (20 - 48)2 + (20 - 23)2 + (20 - 15)2 + (20 - 6)2 + (20 - 5)2
5-crop combination =---------------------
5 crops
The deviation of the actual percentage from the theoretical curve is seen to be the lowest for a 3-crop combination. This result established the identity and the number of crops in the basic combination for the district of Gorakhpur as rice-wheat and barley.
Weaver’s technique when applied at the district level for the data of 2003-06 gives the following eight-crop combinations to India which have been plotted in