Explain clearly why it is desirable to collect quantitative data where possible in biological studies

Quantitative = quantity 
it's good to have a lot of data because then you know that you don't have a skewed sample, like if you were doing a survey between like coke vs pepsi and asked ten people, you'd probably get a different result (percentage wise) than if you asked 100 people

I believe it is Organic matter is decayed in the soil because plants get their nutrients from animal waste and dead organisms.