Planting corn, beans and squash together is credited to the Native American practice of growing "The Three Sisters" together in one plot of land for the mutual benefit of all plants. According to the New Mexico State University, this early practice of companion planting is thought to originate with the Haudenosaunee tribe. When planting in a group, corn provides support for beans while the beans "fix" nitrogen from the air, improving the soil. Squash creates a living mulch for both to keep soil cool and moist.
Prepare the soil in s sunny location that receives at least 6 to 8 hours of direct sunlight a day. Till to a depth of 8 to 12 inches. Remove rocks and roots form the soil. Add a 2- to 3-inch layer of well-rotted manure or compost and work it in well with the soil.
Hoe the soil into round mounds approximately 1 1/2 to 2 feet in diameter and 12 inches high. Space the mounds 4 feet apart, measured from the center of each mound.
Apply 5-10-10 fertilizer, following the recommended application rate on the container, and mix in well with the existing soil. Rake the top smooth.
Plant five to six corn seeds to a depth of 1/2 inch in the center of each mound. Water to moisten the soil, and keep the soil moist until seedlings emerge.
Plant seven to eight bean seeds to a depth of 1/2 inch, spaced 6 inches from the corn stalks, when the corn is 5 to 6 inches high. Water to moisten the soil.
Sow seven to eight squash seeds evenly spaced around the perimeter of the hill one week later. Water to moisten the soil.
Thin plants to three to four corn stalks and four to six beans and squash to allow the biggest and healthiest plants to grow.
Water thoroughly once a week to saturate the soil to the root level.