Plot Fertiliser applied Yield of cabbage (kg) 1 No fertiliser (control) 120 2 Farmyard manure 185 3 NPK compound fertiliser 210 4 Nitrogen fertiliser only 1...

(a) The plot that produced the highest yield was Plot 3, which received NPK compound fertiliser and yielded 210 kg. [1]

(b) The difference in yield between Plot 3 and Plot 1:

\[ 210 - 120 = 90 \text{ kg} \]

Working: Plot 3 yield = 210 kg, Plot 1 yield = 120 kg. [1]
Difference = 210 - 120 = 90 kg. [1]

This 90 kg increase represents a 75% improvement over the unfertilised control, demonstrating the substantial benefit of balanced NPK fertilisation.

(c) Plot 4 produced a lower yield than Plot 3 because it only received nitrogen and therefore lacked phosphorus and potassium. [1] Phosphorus is essential for root development and energy transfer, while potassium regulates water uptake and enzyme activity. Without all three major nutrients, the crop cannot achieve its full growth potential. This illustrates the principle that plant nutrition requires a balance of elements, not just one.

(d) What each letter in NPK represents:

1. N = Nitrogen [1] - promotes leafy vegetative growth and is a component of chlorophyll and proteins.
2. P = Phosphorus [1] - essential for root development, flowering, and energy transfer within the plant.
3. K = Potassium [1] - important for water regulation, disease resistance, and overall plant vigour.

(e) One advantage of using farmyard manure instead of chemical fertiliser: farmyard manure improves soil structure by adding organic matter, which increases the soil's ability to retain water and nutrients over the long term. [1] Other acceptable advantages: releases nutrients slowly; is cheaper if livestock are kept on the farm; less risk of polluting waterways.

(f) The farmer included a plot with no fertiliser because:

1. It acts as a control. [1]
2. It allows the farmer to compare the yields and measure the actual effect of each fertiliser treatment against a baseline. [1] Without a control, the farmer could not determine how much of the yield was due to the fertiliser and how much was due to the natural fertility of the soil.