Please help with this question guys!!!!,

1. Fertilizer: usually potash, phosphorus and nitrogen based - plants will use these to:-

a) make compounds that re used as co-enzymes/intermediaries for energy generation (phosphorus is needed for phosphorylation of NAD [-----> NADP] and of ADP [-----> ATP]
b) potassium is the main intracellular cation
c) nitrogen is needed to make protein [you know proteins have an amino group - NH2] and DNA/RNA

At A level, you must then connect this with "high yield" by saying that plants supplied with better nutrients are likely to produce and utilize energy better, and to improve the efficiency of photosynthesis, both of which will help increase growth of the parts of the plant that are harvested e.g. the corn grains that we eat are fruits [we know this because they are enveloped by a pericarp, and because they contain thin elongated seeds][this "fruit" has an outer layer called the aleurone layer which is rich in protein; the edible part of an onion is stem [as a corm] and is rich in glucose which requires photosynthesis to make it.

2. Crop rotation to prevent depletion of specific minerals in the soil - different plants use different ions to varying extents, so rotation allows soil to recover.