Now, I succession garden 9 months a year.  In this video I am putting in carrots after I harvest lettuce.  If I kept planting lettuce in the same spot the nutrients needed for the lettuce would quickly be used up.  So each time I change crops, I try to plant ones that have different nutrient needs.   After the crop is done, I pull out the spent plants and then amend the soil with some manure, peat and new soil.  This helps to get soil ready for the new plants.