The cost of harvesting a square field at ₹900 per hectare is ₹8100.

As, the rate of the harvesting $=$ ₹ 900 per hectare

And, the cost of harvesting $=₹ 8100$

So, the area of the square field $=\frac{8100}{900}=9$ hectare

$\Rightarrow$ the area $=90000 \mathrm{~m}^{2} \quad\left(\right.$ As, 1 hectare $\left.=10000 \mathrm{~m}^{2}\right)$

$\Rightarrow(\text { side })^{2}=90000$

$\Rightarrow$ side $=\sqrt{90000}$

So, side $=300 \mathrm{~m}$

Now, perimeter of the field $=4 \times$ side

$=4 \times 300$

$=1200 \mathrm{~m}$

Since, the rate of putting the fence $=₹ 18$ per $\mathrm{m}$

So, the cost of putting the fence $=1200 \times 18=₹ 21,600$