As soon as your seeds are planted in your nursery, plough your land. Break up lumps of soil. Mix in well-rotted manure. Smooth the soil surface.
Your soil test will tell you which fertilisers to add to your soil and how much. Usually you will be told to add an NPK fertiliser at planting. This contains Nitrogen, Phosphorous and Potassium, and sometimes other nutrients too.
Mavuno Fertilizers (ARM Cement) make fertilisers specific to your crop. Buy Mavuno fertilisers from agrovets.
Weeds take water and nutrients from your crop. They also hide pests and diseases. Take out weeds when you see them.
You can also use herbicides. Always follow instructions and wear protective gear when using herbicides.