It's because each kernel has to be fertilised in order to develop. On the top of the plant is the male part that releases about 16 - 20 million specks of 'sperm'. This falls in the air/gets blown by the wind and attaches itself to the little silks that come out of the female cobs - each silk is a potential kernel. If the silk doesn't catch a sperm it doesn't grow. It gets slightly more complicated as the sperm is actually in two parts and the two parts have different jobs to do for the pollination to be complete. Needless to say that if either of those two parts are malfunctioning then there's no growth of a kernel either.
I've heard that if it's exceptionally dry when all this happens - only takes about two days - then there can be lots of bald patches. Don't know about the biology behind that though.
Both of the above answers are correct. Since corn is wind pollinated, small patches should be planted in blocks, rather than rows. A single row of 100 plants will only produce 10-20% as much corn as will 10 rows of 10 plants each.
Heavy rains at pollination time may also limit proper pollination, as can excessive humidity.
All correct answers and suggestions. My experience in small plots ... hand pollinate to get the best results. I live in a dry hot climate when the corn is pollinating. My best pollination time is 6 to 9 am. I have no shortage of wind or insect pollinators. I planted 4 row blocks and still only moderate filled ears (some very good but most imperfect). It all makes good corn for stews but could have been better yield. I went for the wind pollination this year instead of hand pollinating. Given the small effort to hand pollinate and the big result I will always hand pollinate in the future. Good luck all.
PS. Hand pollinating can be as simple as shaking the pollen from one plant onto the silk of itself or other nearby.
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