"3. The proceeds of the loan must be used for the extinguishment of preexisting indebtedness or for productive purposes, which include the purchase of live stock, fertilizers, equipment, and improvements.
"5. The amount of each installment may be fixed by the borrower, but can not be less than sufficient to pay off the debt in 40 years, nor greater than to pay it off in 5 years.
"6. The rate of interest charged any borrower can not exceed 6 per cent per annum.
"7. The borrower can not be called upon to pay the debt except by the installments he originally fixes unless he defaults, but after five years he may pay off the whole or any portion at his option at any installment period. Under the amortization plan the term of the loan and the amount of each installment are relative; determining one fixes the other."