Photo by Lan Ngo

Soil
Management

The Mediterranean climate and productive soils of California make the state one of the most productive agricultural regions in the world. California produces over 250 different crops and leads the nation in production of 75 commodities. Californian agriculture is characterized by its high production capacity through very intensive management of irrigated cropping systems. Intensive management is characterized by a combination of chemical inputs and a combined use of extensive tillage and surface irrigation practices. Increasing water use efficiency and decreasing tillage, as well as building healthier soils more broadly, are necessary to enhance the competitiveness of the region’s agriculture.

Reduced tillage practices have been shown to significantly increase water availability and reduce soil erosion in arid environments. The adoption of reduced tillage, or conservation tillage, practices has been problematic in California due to the accumulation of crop residues on the soil, which impedes water movement in the most common irrigation practice of furrow run water. The residues act to dam the water in the furrow resulting in increased infiltration and requires additional water to be added to the field to compensate. However, the mulch effect of crop residues decreases evapotranspiration from soil, resulting in a small potential water savings. The significance in the reduction of evapotranspiration losses from the residue mulch effect compared to increased water inputs from impeded water flow has not been adequately studied to determine the potential benefits of reduced tillage in furrow irrigated systems in California. In overhead or drip irrigated systems, residues do not necessitate the application of additional water and in fact may require less applied water to meet crop needs.

Research on the effects of reduced tillage on winter runoff has produced mixed results in California. In other regions of the US, long-term reduced or no-till systems generally reduce runoff by infiltrating a greater degree of precipitation. In California, our heavier soils can lead to increased soil surface crust formation and compaction, often leading to greater runoff of winter precipitation. However, the long-term effects of reduced tillage may eliminate these issues through accumulation of surface soil organic matter that will minimize crust formation and the effects of compaction.

Overall, the practices of reduced tillage and winter cover cropping have additional soil benefits, namely increasing soil organic matter content. Other practices such as the application of manure and compost can significantly increase soil organic matter. Combining these practices has an additive effect on building soil organic matter. Increasing soil organic matter improves fertility by adding nutrients and cation exchange capacity, enhances soil structure through aggregation and promotes water infiltration and storage. The benefits support enhanced crop growth through increased nutrient availability, better soil tilth and more available water.

The use of winter cover crops can dramatically affect the hydrology of irrigated fields. Up to 20% of precipitation can run off fields left fallow during the winter. The use of winter cover crops can practically eliminate runoff from agricultural fields. Part of the infiltrated water will recharge groundwater and some is lost from the crop through transpiration. Loss through transpiration can negate groundwater recharge if the cover crop is left too long in the field, therefore timely cover crop removal is required to maximize the potential to infiltrate precipitation. Further study on exactly how much additional water will recharge groundwater is required to better determine the benefits of winter cover crops. The use of winter cover crops should be considered in areas prone to flooding. In addition, winter cover crops “catch” or utilize residual fertilizer nitrogen for growth and reduce the potential for nitrate leaching and the emission of the greenhouse gas nitrous oxide. These complimentary benefits represent significant steps towards cropping system sustainability. However, the adoption of winter cover cropping practices has been very slow for primarily three reasons. First, the establishment of winter cover crops following the harvest of a summer crop depends on timing of the first winter rains. Winter rains may impede entry to fields to establish a cover crop. Conversely, sufficient winter rains are needed to establish and maintain the cover crop until rainfall becomes persistent during the months of January and February. Secondly, managing the winter cover crop requires timely field entry to stop growth and reduce excessive soil water depletion. Delayed field entry from late spring rains can make a winter cover crop difficult to deal with because of the excessive biomass accumulation. The depletion of soil water reserves from excessive winter cover crop growth may require additional summer irrigation. Third, winter cover crops enhance soil infiltration overall by building soil structure through the input of organic matter. This results in increased infiltration of irrigation water at the head end of the field and may require additional water to irrigate the tail end of the fields in furrow irrigation systems. For these reasons, the use of winter cover crops has not been widely adopted. It may be possible to compensate farmers through conservation payments or some sort of crop insurance model to make utilizing cover crops more of a standard practice.

Overall, the use of the sustainable practices of reduced tillage, soil amendments and winter cover cropping likely have benefits that outweigh their challenges. Reduced tillage will decrease fuel consumption and benefit both the grower economically and reduce fossil fuel greenhouse gas emissions. The use of winter cover crops will increase soil water storage from winter rains and may lead to decreased summer irrigation inputs. Cover crops also build soil organic matter and reduce nitrous oxide emission. These practices could place California agriculture in the forefront of sustainable agriculture management and provide a unique competitive market position. The competitiveness of California agriculture will be bolstered compared to other regions adopting these practices because of the high productivity and diversity of crops.

Photo by Lan Ngo

Increasing soil organic matter improves fertility by adding nutrients and cation exchange capacity, enhances soil structure through aggregation and promotes water infiltration and storage.

While people typically think of old vine zinfandel or century old walnut trees when they think of dry farming, it’s important to recognize that farmers successfully dry farm in a variety of current conditions.

In the Mediterranean region, olives and grapes have been dry farmed for thousands of years. Even today, vast swaths of Spain, Greece, France, and Italy dry farm these crops. In some regions of Europe, it is illegal to irrigate wine grapes during the growing season.

California has dry-farmed vineyards from Mendocino to Santa Barbara counties. Inland, there are dry-farmed vineyards in Lodi and the Sierra foothills. In addition to wine grapes and olives, a wide range of crops including tomatoes, pumpkins, watermelons, cantaloupes, winter squash, garbanzos, apricots, apples, grains, and potatoes are dry farmed in California.

While dry farming is not for every grower or every region, it is a promising system of crop management that offers greater crop security in times of uncertain water supply and can offer a higher-quality product.

Water Savings

Until now, supportive water policies have allowed farmers access to plentiful water, but increasing urban and environmental demands are putting greater pressure on limited supplies. In addition, other policies have essentially ignored agriculture’s potential to address climate change through soil carbon sequestration and reduction of nitrous oxide emissions from more efficient fertilizer and water use practices. The confluence of these two shifts in farming considerations provides a perfect economic and environmental opportunity for farmers to adopt more efficient water management practices. In addition, building soil organic matter through carbon sequestration enhances soil quality and has a long-term positive effect on crop production.

Mulching, cover cropping and reduced tillage can all serve to minimize the need for applied water. Building soil organic matter and improving soil structure are key drought-management strategies. No-till, for example, can increase organic matter in the soil’s surface by 1,000 lbs/ac/yr. According to the Encyclopedia of Soil Science, each kilogram of soil organic matter can absorb 20 times its weight in water. In effect, the soil can be viewed as a sponge or a water reservoir in its own right. Furthermore, improving soil structure, particularly in the surface layer, can improve infiltration and percolation. Rodale Institute studies report that the open soil surface structure resulting from building soil organic matter increases percolation 25 to 50% over unimproved soils.

One of the challenges in implementing reduced tillage practices is finding compatible irrigation practices. Both subsurface drip and overhead sprinkler would be compatible with reduced tillage since crop residues would not interfere with water application as with furrow irrigation. Comparative studies of drip systems versus furrow irrigation have shown that drip systems can be more efficient in terms of N use (10-20% less) and water use (20-50% less) with comparable yields. Similar water savings can be realized with overhead sprinkler technology.

Applications

The implementation of reduced tillage, winter cover crops and more efficient irrigation practices would address sustainability issues related to water resources, climate change and the economic viability of farm operations. The reduction in tillage can best be managed with subsurface drip and overhead sprinkler irrigation. Subsurface drip irrigation can also address nitrous oxide emission through efficient fertigation compared to the standard approach of one-time application of fertilizers. A recent study at UC Davis has shown that subsurface drip irrigation can significantly reduce nitrous oxide emission especially following a winter cover crop compared to furrow irrigation. Currently, fewer than 5% of growers in California implement reduced tillage and winter cover crops; there is significant potential for expansion. Studies at UC Davis have shown that row crops in any region could benefit from these approaches. The scale of the farming operation may impact the potential to implement these practices. Subsurface drip systems are expensive and are often used on higher value crops such as tomatoes, grapes and tree crops. Overhead sprinkler technology is also expensive but combined with reduced tillage and water savings could make them more affordable. The reduction of water, fertilizer nitrogen and fuel inputs will increase the competitiveness of California agriculture and provide growers an opportunity to participate in emerging carbon and greenhouse gas markets as an additional income source.

Photo by Lan Ngo

The use of winter cover crops can dramatically affect the hydrology of irrigated fields.

Additional Benefits

The environmental and social benefits of more efficient on-farm water and energy use are a necessity to addressing the increasing demands for water and reduction in energy inputs. The approaches outlined in the above sections provide the means to sustain California agriculture through addressing emerging environmental concerns and maintaining or enhancing the livelihoods of farmers and ranchers. The main benefits to adopting reduced tillage and winter cover cropping – aside from reducing water and energy use – is the building of soil quality. Increasing soil quality will reduce loss of nutrients to surface and groundwater supplies. Maintaining the quality of groundwater will become increasingly important as we rely on groundwater reserves to address gaps in water supply due to urban and environmental demands and the effects of climate change. Soil erosion is also significantly reduced by improving soil structure and reducing the flow of water over the soil surface. Reduced tillage also reduces fuel consumption and soil compaction. The combined benefits of addressing water limitations, energy use and increasing soil quality will ensure the viability of California agriculture.

Photo by Lan Ngo

Page Credit

Content for this page was originally developed by William Horwath, UC Davis Department of Land, Air, and Water Resources. Various others have since contributed content.