Reducing Tillage

Reducing Tillage

Reducing Tillage

. . . the crying need is for a soil surface similar to that which we find in nature . . . [and] the way to attain it is to use an implement that is incapable of burying the trash it encounters;in other words, any implement except the plow.

—E.H. FAULKNER, 1943

Figure 16.1. Rolled-rye cover crop being prepared for row-crop planting. Photo by Anu Rangarajan.

Figure 16.1. Rolled-rye cover crop being prepared for row-crop planting. Photo by Anu Rangarajan.

Although tillage is an ancient practice, the question of which tillage system is most appropriate for any particular field or farm is still difficult to answer. Before we discuss different tillage systems, let’s consider why people started tilling ground. Tillage was first practiced by farmers who grew small-grain crops, such as wheat, rye, and barley, primarily in western Asia (the Fertile Crescent), Europe, and northern Africa. Tillage was primarily practiced because it created a fine seedbed, thereby greatly improving germination. It also gave the crop a head-start before a new flush of weeds, and stimulated mineralization of organic nitrogen to forms that plants could use. The soil was presumably loosened by a simple ard (scratch plow) in several directions to create fine aggregates and a smooth seedbed. The loosened soil also tended to provide a more favorable rooting environment, facilitating seedling survival and plant growth. Animal traction was employed to accomplish this arduous task. At the end of the growing season, the entire crop was harvested, because the straw also had considerable economic value for animal bedding, roofing thatch, brick making, and fuel. Sometimes, fields were burned after crop harvest to remove remaining crop residues and to control pests. Although this cropping system lasted for centuries, it resulted in excessive erosion, especially in the Mediterranean region, where it caused extensive soil degradation. Eventually deserts spread as the climate became drier.

JETHRO TULL AND TILLAGE: A MIXED LEGACY AND AN IMPORTANT LESSON

Jethro Tull (1674–1741) was an early English agricultural experimentalist whose book The New Horse Hoeing Husbandry: An Essay on the Principles of Tillage and Vegetation was published in 1731. It was the first textbook on the subject and set the standard for soil and crop management for the next century (it is now available online as part of core historical digital archives; see “Sources” at the end of the chapter). In a way, Tull’s publication was a predecessor to this book, as it discussed manure, rotations, roots, weed control, legumes, tillage, ridges, and seeding.

Tull noticed that traditional broadcast sowing methods for cereal crops provided low germination rates and made weed control difficult. He designed a drill with a rotating grooved cylinder (now referred to as a coulter) that directed seeds to a furrow and subsequently covered them to provide good seed-soil contact. Such row seeding also allowed for mechanical cultivation of weeds, hence the title of the book. This was a historically significant invention, as seed drills and planters are now key components of conservation agriculture and building soils. But the concept of growing crops in rows is attributed to the Chinese, who used it as early as the 6th century B.C.E.

Tull believed that intensive tillage was needed not only for good seed-soil contact but also for plant nutrition, which he believed was provided by small soil particles. He grew wheat for thirteen consecutive years without adding manure; he basically accomplished this by mining the soil of nutrients that were released from repeated soil pulverization. He therefore promoted intensive tillage, which we now know has long-term negative consequences. Perhaps this was an important lesson for farmers and agronomists: Practices that may appear beneficial in the short term may turn out detrimental over long time periods.

Ancient agricultural systems in the Americas did not use intensive full-field tillage for grain production, as they did not have oxen or horses to perform the arduous tillage work. Instead, the early Americans used mostly direct seeding with planting sticks, or manual hoes that created small mounds (hilling). These practices were well adapted to the staple crops of corn and beans, which have large seeds and require lower plant densities than the cereal crops of the Old World. Several seeds were placed in each small hill, which was spaced several feet apart from the next one. In temperate or wet regions the hills were elevated to provide a temperature and moisture advantage to the crop. In contrast with the cereal-based systems (wheat, rye, barley, rice) of growing only one crop in a monoculture, these fields often included the intercropping of two or three plant species growing at the same time, like the corn, bean, and squash of the Three Sisters system in North America. This hilling system was generally less prone to erosion than whole-field tillage, but climate and soil conditions on steep slopes still frequently caused considerable soil degradation.

TECHNOLOGIES THAT HAVE LESSENED THE NEED FOR TILLAGE…

  • herbicides
  • new zone and strip tillage tools that provide targeted decompaction
  • new planters and transplanters
  • new methods for cover crop management

A third ancient tillage system was practiced as part of the rice-growing cultures in southern and eastern Asia. There, paddies are tilled to control weeds and puddle the soil to create a dense layer that limits downward losses of water through the soil. The puddling process occurs when the soil is worked while wet—in the plastic or liquid consistency state; see chapter 6—and is specifically aimed at destroying soil aggregates. This system was designed to benefit rice plants, which thrive under flooded conditions, especially relative to competing weeds. There is little soil erosion, because paddy rice must be grown either on flat or terraced lands, and runoff is controlled as part of the process of growing the crop. Recent research efforts have focused on less puddling and ponding to conserve soil health and water.

Graph illustrating soil erosion

Full-field tillage systems became more widespread because they are better adapted to mechanized agriculture, and in time some of the traditional hill crops like corn became row crops. The moldboard plow was invented by the Chinese 2,500 years ago but was redesigned into a more effective tool in England in the 1700s. It provided weed control by fully turning under crop residues, growing weeds, and weed seeds. Its benefits were compelling at first; it allowed for a more stable food supply and also facilitated the breaking of new lands in the Americas. The development of increasingly powerful tractors made tillage an easier task (some say a recreational activity) and resulted in more intensive soil disturbance, ultimately contributing to the degradation of soils.

New technologies have lessened the need for tillage. The development of herbicides reduced the need for soil plowing as a weed control method. New planters achieve better seed placement, even without preparing a seedbed beforehand. Amendments, such as fertilizers and liquid manures, can be directly injected or band-applied. Now there are even vegetable transplanters that provide good soil-root contact in no-till systems. Although herbicides often are used to kill cover crops before planting the main crop, farmers and researchers have found that they can obtain good cover crop control through well-timed mowing or rolling (figures 16.1, 16.7)—greatly reducing the amount of herbicide needed. If there is sufficient cover crop biomass, the mat acts as an effective barrier to weeds and provides nearly complete control.

Increased mechanization, intensive tillage, and erosion have degraded many agricultural soils to such an extent that people think tillage is required to provide temporary relief from compaction. As aggregates are destroyed, crusting and compaction create a soil “addicted” to tillage. Except perhaps for organic production systems, in which tillage is often needed because herbicides aren’t used, a crop produced with limited or no tillage can generate better economic returns than one produced with conventional tillage systems. Managing soil in the right way to make reduced tillage systems successful, however, remains a challenge.

Chapter Fifteen: Summary and Sources | Top | Tillage Systems