Consider the following statements regarding alluvial soils in India:

1. They cover approximately 40 per cent of India’s total area.
2. They are generally rich in potash but poor in phosphorus.
3. Their sand content increases from west to east across the northern plains.
4. They become more loamy and clayey in the lower and middle Ganga Plain and the Brahmaputra Valley.

Which of the statements given above are correct?