Recall in module 5, we examined how soils, climate, and markets play major roles in determining which crops farmers cultivate. In many cases, farmers cultivate multiple crops of more than one life-cycle because the diversity provides multiple benefits, such as soil conservation, interruption of pest lifecycles, diverse nutritional household requirements, and reduced market risk. In this module, we examine some ways that farmers cultivate crops in sequence and define some of the terms for this crop sequencing.

A sole crop refers to planting one crop in a field at a time. Recall from Module 5, the seasonal crop types (Figure 7.1.1) and note that different seasonal crops could be planted in succession. A monoculture refers to planting the same crop year after year in sequence (See Figure 7.1.2). By contrast in a crop rotation, different crops are planted in sequence within a year or over a number of years, such as shown in Figures 7.1.3a and 7.1.3b. When two crops are planted and harvested in one season or slightly more than one season, the system is referred to as double cropping, as illustrated in Figure 7.1.4. Where growing seasons are long and/or crop life cycles are short (ex. leafy greens), three crops may be planted in sequence within a season, as a triple-crop.

Figure 7.1.1: Crop Term: Seasonal Types and Example Crops

Credit: Heather Karsten

Figure 7.1.2: Monoculture

Credit: Heather Karsten

Figure 7.1.3a: Simple Summer Annual Crop Rotation

Credit: Heather Karsten

Figure 7.1.3b: Dairy Perennial - Annual Crop Rotation

Credit: Heather Karsten

Figure 7.1.4: Double-cropped annual crops

Credit: Heather Karsten

Crop rotations and double cropping can provide many soil conservation and soil health benefits that are discussed in the reading assignment at the end of this page, and in Module 7.2. Crop rotations can provide additional pest control benefits particularly when crops from different plant families are rotated, as different families typically are not hosts of the same insect pest species and crop pathogens. Integrating crops of different seasonal types and life cycles in a crop rotation also interrupts weed life cycles by alternating the time when crops are germinating and vulnerable to weed competition. Rotating annual crops with perennial forage crops that are harvested a couple of times in a growing season also interrupts annual weed life cycles, because most annual weeds don't survive the frequent forage crop harvests.

When all or most of a crop is grazed or harvested for feed for ruminant livestock, such as dairy and beef cattle or sheep, the crop is referred to as a forage crop. Examples of forage crops include hay and pasture crops, as well as silage that can be produced from perennial crops and most grain crops. For instance, silage from alfalfa, perennial grass species, corn, oat, and rye is made when most of the aboveground plant material (leaves, stems and grain in the case of grain crops) is harvested and fermented in a storage structure called a silo or airtight structure. To preserve the silage, air is precluded from the storage structure and microbes on the plant material initially feed on the crop tissues, deplete oxygen in the storage structure, and produce acidic byproducts that decrease the pH of the forage. This acidic environment without oxygen prevents additional micro-organisms from growing, effectively "pickling", and preserving the forage.

Figure 7.1.5: Airtight upright silo

Credit: Heather Karsten

Figure 7.1.6: Bunker silos are packed tightly with heavy equipment and covered with plastic to keep out air and moisture.

The bunker silo on the right is uncovered because the silage is being removed to feed to dairy cattle.

Credit: Heather Karsten

Intercrops are two or more crops that are planted together in a field at the same time or to be planted close in time and overlap for some or all of their life cycle. Intercrops may provide a range of benefits including: i. improving soil fertility, ii. increasing crop diversity and iii. reducing pest pressure. The mixtures also often produce higher yield and crop quality. There are multiple types of intercrops that vary in their spatial arrangement.

Strip intercrops are wide strips with multiple rows of one crop, that are alternated on the field with strips of one or more different crop(s). Strip intercrops are typically planted on the field contour with crops of different life cycles that protect soil from erosion throughout the year.  For instance, strips of corn may be alternated with strips of perennial forage grasses that can reduce soil erosion across the field when the corn isn't growing. Or, as in the photo below, winter wheat provided live plant coverage on portions of the field in spring, prior to corn and soybean were planted. In mid-summer, corn and soybean provide live coverage after wheat is harvested; and in fall, winter wheat will be growing on some strips after corn and soybean are harvested. Having strips of different crop species can also reduce the spread of insect pests and crop pathogens compared to cultivating one crop on the entire field. 

Figure 7.1.7. Strip intercrop: Alternating strips of corn, soybean and winter wheat planted on the contour.

Credit: Heather Karsten

Row intercrops alternate rows of different crop species, usually every other row or every two rows.

Figure 7.1.8. Row intercrop: Alternating rows of onion and hairy vetch. Hairy vetch is a winter annual legume that is mowed frequently to reduce competition with the onions. In this system, hairy vetch is planted to provide soil protection, suppress weeds, and add nitrogen to the soil.

Credit: Heather Karsten

Mixture intercrops tend to be combined randomly when planted; such as grass and legume forage mixtures. Intercrops of different crop species (ex. native tuber mixtures) or different varieties of a crop species (ex. rice) are sometimes planted to reduce pathogen and insect pest infestations. Crop rotation and intercropping increase agrobiodiversity across an agricultural landscape, providing multiple potential agroecosystem benefits, such as i. reducing the risk of crop loss to pests and climatic stresses (ex. frosts, floods, and drought), ii. providing habitat for beneficial organisms such as pollinators and pest predators, and iii. enhancing the diversity of nutritional crops for farmers and markets. Further, integrating crops from the grass family tends to promote soil structure, while legumes enhance soil nitrogen, and integrating perennial crops protects the soil from erosion and builds soil organic matter and soil biological activity because perennials allocate a high proportion of their growth to storage organs. For instance, the photos below illustrate how both intercropping and crop rotation enhance agrobiodiversity in the high Andes of Peru.

Figure 7.1.9. Pasture intercrop of four perennial forage crops: tall fescue, orchardgrass, Kentucky bluegrass, and white clover.

Credit: Heather Karsten

Figure 7.1.10. Four major native tuber crops: Maca, Oca, Ulluco, and Mashua at the CIP International Potato Center in Lima, Peru.

Credit: Heather Karsten

Figure 7.1.11. Example High Altitude Andean Crop Rotation from Peru

Credit: Heather Karsten

Figure 7.1.12: Sheep grazing perennial pastures that are typically rotated to annual crops: potato, native tuber crops, legumes, and small grains before rotating back to perennial pasture.

Credit: Heather Karsten

Figure 7.1.13. Agrobiodiversity is high across the high altitude Andean landscape due to crop rotation and genotypic diversity within fields.

Credit: Heather Karsten

Figure 7.1.14. Diversity of potato and native tuber crops in a grocery store in Lima, Peru

Credit: Heather Karsten

Cover Crop: A cover crop is planted after a crop that is harvested and is terminated before the subsequent crop is planted. Cover crops tend to be annual crops that they can quickly establish after a harvested crop to protect the soil from erosion and provide other benefits including i. to add organic matter to the soil; ii. to scavenge nutrients and prevent nutrients from leaching out of the topsoil (also called a catch crop); iii. to support soil organisms in the root zone, iv. to suppress weeds, and v. to provide habitat for aboveground beneficial organisms, such as insects that predate on crop pests or weed seeds. Leguminous cover crops also add nitrogen to the soil when they are terminated and returned to the soil and are therefore often referred to as green manure crops. Cover crops are also sometimes referred to as "catch crops" because they can take up and retain nitrogen and other nutrients that might otherwise leach out of the rooting zone and be lost to deeper soil profiles, and potentially to groundwater.

Cover Crop Intercrops

Because cover crop species have different plant traits that provide different cropping system benefits, often two or more species of cover crops are planted together as a cover crop intercrop or cover crop mixture. For instance, small grains that scavenge nitrogen well and have fibrous roots that bind soil particles and promote soil structure are often mixed with tap-rooted legumes that fix nitrogen. Some cover crop mixtures combine plant species that establish quickly in the late summer or early fall but don't typically survive the winter, such as oats or deep-rooted radish species. Non-winter hardy species are sometimes combined with winter-hardy species such as hairy vetch, cereal rye or annual ryegrass that survive the winter and provide cover in early spring.

Figure 7.1.15. Annual crimson clover and winter wheat cover crop intercrop photographed in spring.

Credit: Heather Karsten

Figure 7.1.16. Cover crop intercrop of annual cereal rye and hairy vetch (a legume) photographed in spring.

Credit: Heather Karsten

As discussed in Module 5, soil is a complex matrix of minerals, air, water, organic matter, and living organisms. Historically, the emphasis in agriculture has been on reducing soil erosion. But since the 1990s, soil scientists and conservationists have recognized and described multiple valuable properties and ecosystem functions of soil that are referred to as indicators of soil quality or soil health. In 1997, the Soil Science Society of America's Ad Hoc Committee on Soil Quality (S-581) defined Soil Quality as:

"the capacity of a specific kind of soil to function, within natural or managed ecosystem boundaries, to sustain plant and animal productivity, maintain or enhance water and air quality, and support human health and habitation" (Karlen et al., 1997).

Indicators or measures of soil quality describe a soil's biological, chemical and physical properties. In addition to the soil chemical properties such as nutrient content and pH, additional indicators of soil quality include a soil’s:

• Organic matter content. Organic matter stores carbon can release nutrients, support soil biological activity, buffer soil pH, hold plant nutrients, and increase a soil's water-holding capacity
• Biological activity in the soil. Soil organisms can provide multiple benefits such as nutrient cycling, secreting sticky polysaccharides that help bind together soil particles and increase soil porosity and predation, and suppression of plant pests such as plant pathogens and weed seeds.
• Soil structure and porosity. Soils with good structure and porosity can support water and air infiltration and resist compaction. Water stable aggregates are an important physical indicator of soil health. Water stable aggregates contain soil mineral particles such as sand, silt, and clay that are typically held together by a combination of binding materials including fine root hairs, soil fungal hyphae (fungal filaments), and sticky polysaccharides that are exuded from soil microorganisms. Because they are stable when wet (during or after a precipitation event) they maintain soil pores that can contain and allow air and water to infiltrate the soil, reducing water and soil run-off. Water stable aggregates can also protect organic matter from degradation and soil microorganisms from predatory micro-organisms.

Figure 7.1.17: Plant roots, fungal hyphae, and microbial polysaccharide exudates contribute to binding soil mineral particles together into soil macroaggregates, creating macropores between them.

Credit: Illustration by Heather Karsten

Read

Chapter 1 (Healthy Soil) and Chapter 2 (Organic Matter: What it is and Why it’s so important?) from the book that you downloaded: Building Soils for Better Crops. Edition 3. Sustainable Agriculture Network, USDA. Beltsville, MD.

Then watch the following video about soil biology and list four kinds of soil organisms and how they influence soil.: The Living Kingdoms Beneath our Feet. (USDA NRCS).

Video: International Year of Soils July: The Living Kingdoms Beneath Our Feet (2:08)