Land growing continuous grain corn as a cash crop loses soil at a rate of

14.0 metric tonnes ha-1 yr-1. This is a fertile soil, which has an organic matter content of 3.8% and nitrogen content of 0.35% (both by mass).

a)
we did this question before,
lost soil=soil mass
14000kg/10000m^2 yr * time=7in*1m/39.37in*1.25E3kg/m^3

time=7/39.37 * 1.25E3 /(1.4) years
and the answer was: 158.7 years..

I don't understand part b!

b) If the entire Ah horizon is lost, calculate how many kg of manure you would need to add per hectare in order to replace the lost organic matter if manure is assumed to be 100% organic matter .

Im not sure but since 14000 kg/ is the soil that is lost per 10000m^2. Do i multipy this by 3.8% to get how much organic material was there?

Yes, to calculate the amount of organic matter lost in the Ah horizon, you would multiply the lost soil (14,000 kg per 10,000 m^2) by the organic matter content of the soil (which is given as 3.8% or 0.038). This will give you the amount of organic matter that was present in the lost soil.

So, the calculation would be:

Amount of organic matter lost = Lost soil * Organic matter content
= 14,000 kg/10,000 m^2 * 0.038

This will give you the amount of organic matter lost (in kg) in the Ah horizon.