One of the few positives for banks over the past year has been the relatively high rate of deposit growth. Business and household deposit rates have grown ahead of lending rates, helping banks move towards their Basel III stable funding ratio and liquidity targets.However, a joint study by Macquarie Equities and East & Partners suggests the weak corporate earnings performance in the December half-year will flow into softer business deposit growth.If things play out as the Macquarie and East report suggests, deposit-takers will be forced to pay more for deposits or will have to budget for lower rates of loan growth.According to Australian Bureau of Statistics figures, company profits rose 2.7 per cent in the six months to December 2011 compared with the six months to the end of 2010. CommSec's review of the 131 companies in the S&P/ASX 200 that reported half-year results to December showed average profit growth of 5.5 per cent. Only 47 per cent of those companies reported growth over the previous corresponding period.One impact of softer earnings is that companies are eating into their savings to maintain investment programs and dividend payout ratios. CommSec found the cash balances in the companies it surveyed had been trimmed by an average of 14 per cent.Macquarie and East also observed that companies were eating into their cash reserves to fund working capital.  Australian Prudential Regulation Authority figures show that business deposits grew by seven per cent in the 12 months to January compared with growth in business lending of 5.2 per cent over the same period. Household deposits grew by 8.3 per cent over the 12 months to January compared with growth in mortgage lending of 5.9 per cent over the same period.Macquarie and East forecast that business deposit growth will fall back to four or five per cent this year. They estimate that business deposit growth has been responsible for as much as 70 per cent of total system deposit growth over the past four years (after re-classifying micro-business deposits from the household to the business category).Macquarie and East said: "Deposit growth should be driven by profit growth. Business deposit growth may start to slow over the next few years."This could be an issue for the majors, given the strong support business deposit growth has provided in reducing the funding task. "One outcome could be a further slowing of credit growth."Among the Big Four banks, Westpac is the most exposed to a slowdown in deposit growth. Only 53 per cent of Westpac's funding comes from customer deposits compared with 54 per cent for National Australia Bank, 60 per cent for Commonwealth Bank and 61 per cent for ANZ.