## Soiltestbased Fertilizer Recommendations in Soybean

Liebig's law of minimum states that the growth of plants is limited by the plant nutrient element present in the smallest amount, all others being in adequate quantities. From this, it follows that a given amount of a soil nutrient is sufficient for any one yield of a given percentage nutrient composition. Ramamoorthy et al. (1967) established the theoretical basis and experimental proof for the fact that Liebig's law of minimum operates equally well for nitrogen, phosphorus and potassium. This forms the basis for fertilizer application for targeted yields, first advocated by Truog (1960). Among the various methods of fertilizer recommendation, the one based on yield-targeting is unique in the sense that this method not only indicates a soil-test-based fertilizer dose, but also gives the level of yield the farmer can hope to achieve if good agronomic practices are followed in raising the crop. The essential basic data required for formulating fertilizer recommendations for a targeted yield are: (i) the nutrient requirement in kg q-1 of produce, grain or other economic produce; (ii) the percentage contribution from the soil-available nutrients; and (iii) the percentage contribution from the applied fertilizer nutrients (Ramamoorthy et al., 1967).

The above mentioned three parameters are calculated as follows (Subba Rao and Srivastava, 2001):

• Nutrient requirement of nitrogen, phosphorus and potassium for grain production (NR):

Grain yield (q)

• Percentage contribution of nutrient from soil (%CS):

% CS = Total uptake in control plot (kg ha-1 ) x o Soil test value of nutrient in control plot (kg ha

• Percentage contribution of nutrient from fertilizer (%CF):

Contribution from fertilizer in treated plots (CF) = Total uptake of nutrients -(Soil test values of nutrients in fertilizer treated plots xCS)

Fertilizer dose (kg ha )

• Calculation of fertilizer dose (FD):

The above basic data are transformed into workable adjustment equation as follows:

FD = (a consant x T)-(b consant x soil test value in kg ha- ) Where T is the yield target (q ha-1).

Ramamoorthy et al. (1967) refined the procedure of fertilizer prescription initially given by Truog (1960) and it was later extended to different crops in different soils (Randhawa and Velayutham, 1982). The targeted yield concept strikes a balance between 'fertilizing the crop' and 'fertilizing the soil'. The procedure provides a scientific basis for BF and balance between applied nutrients and soil-available nutrients. In the targeted yield approach, it is assumed that there is a linear relationship between grain yield and nutrient uptake by the crop; as for obtaining a particular yield, a definite amount of nutrients is taken up by the plant. Once this requirement is known for a given yield level, the fertilizer needed can be estimated taking into consideration the contribution from soil-available nutrients.

Conditions for the successful use of the targeted yield approach are as follows:

• The targeted yield approach should be used for similar soils occurring in a particular agro-ecoregion.

• Targets chosen should not be unduly high or low and should be within the range of experimental yield obtained.

 Basic data Fertilizer adjustment equation Relative yield kg-1 nutrient Nutrient NR (kg q- 1) CS (%) P2O5 k2o 1.33 4.54 45.4 21.3 25.6 116 F N = 5.91 T - 0.48 SN F P2O5 = 5.2 T - 4.1 SP F KP = 3.9 T - 0.22 SK 7.00

CF, contribution of nutrient from fertilizer; CS, contribution of nutrient from soil; F, fertilizer; NR, nutrient requirement; T, target yield; SK, soil test potassium (ammonium-acetate-extractable potassium); SN, available nitrogen in soil (kg ha-1); SP, soil test (Olsen's) phosphorus.

CF, contribution of nutrient from fertilizer; CS, contribution of nutrient from soil; F, fertilizer; NR, nutrient requirement; T, target yield; SK, soil test potassium (ammonium-acetate-extractable potassium); SN, available nitrogen in soil (kg ha-1); SP, soil test (Olsen's) phosphorus.

Table 8.7. Fertilizer recommendations for soybean at varying soil test values at specific yield targets (reprinted with permission from Puri and Gorantiwar, 2001).

Fertilizer doses (kg ha-1) for yield Available nutrient status in soil (kg ha-1) target of 2.5 t ha-1

 Olsen's phosphorus extractable potassium P2O5 K2O 5 200 109 53 10 250 89 42 15 300 68 31 20 350 48 20 25 400 27 9 30 450 - - 35 500 - - 40 550 - -

• Adjustment equations must be used within the experimental range of soil test values and cannot be extrapolated.

• Good and recommended agronomic practices need to be followed while raising crops.

• Other micro- and secondary nutrients should not be yield limiting.

• For leguminous crops such as soybean, a minimum dose of nitrogen (20-30 kg ha-1) may be applied.

Puri and Gorantiwar (2001) developed fertilizer adjustment equations for soybean grown in the monsoon season on medium black soils of central India. The basic data and fertilizer adjustment equations for a targeted yield are given in Table 8.6. Fertilizer nitrogen, phosphorus and potassium doses computed from the fertilizer adjustment equations are shown in Table 8.7.

Table 8.8. Seed yield of soybean under farmers' practice and soil test crop response (STCR)-based fertilizer dose (reprinted with permission from Srivastava etal., 2001).

Fertilizer dose (kg ha-1)

- Grain yield

Site Treatment N P2O5 K2O (kg ha-1)

Table 8.8. Seed yield of soybean under farmers' practice and soil test crop response (STCR)-based fertilizer dose (reprinted with permission from Srivastava etal., 2001).

Fertilizer dose (kg ha-1)

- Grain yield

Site Treatment N P2O5 K2O (kg ha-1)

 Site 1 Farmers' practice 9 23 0 1080 STCR (1680 kg ha-1) 0 57 9 1650 Site 2 Farmers' practice 32 23 0 2030 STCR (1680 kg ha-1) 44 0 19 2260 Site 3 Farmers' practice 34 12 0 1760 STCR (2000 kg ha-1) 42 65 0 2270 Site 4 Farmers' practice 45 82 12 1770 STCR (2000 kg ha-1) 17 37 38 2170 Site 5 Farmers' practice 80 100 36 1550 STCR (2400 kg ha-1) 1 0 0 2390 Site 6 Farmers' practice 23 80 29 2100 STCR (2400 kg ha-1) 34 13 11 2310

aFigures in parentheses are the target yield of soybean.

aFigures in parentheses are the target yield of soybean.

The advantage of application of fertilizer nutrients based on the target yield approach has been demonstrated to the farmers of different villages (Srivastava et al., 2001). The soil-test-based fertilizer dose computed from the above equations produced significantly higher yields over the farmers' practice at all sites (Table 8.8).

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