The following text was written by Jim Leap (with editorial support from Martha Brown) for inclusion in the University of California Santa Cruz Center For Agroecology and Sustainable Food Systems (CASFS) curriculum manual with support from Beginning Farmer/Rancher Development Program (BFRDP) funding through USDA.
“Dry Farming” is a term commonly used by growers and consumers here on the Central Coast of California to describe summer harvested orchard, vineyard and vegetable crops grown to maturation without the application of supplemental irritation water from sources other than winter rainfall. This method of production for annual crops is fairly unique to our region and requires a very specific set of criteria for it to be successfully implemented. Though many crops can be grown utilizing dry farming techniques, the most notable in our region is tomatoes. Perhaps one of the most unique aspects of dry farming is the intensity of the flavors of the dry farmed crops compared to the same crops produced with supplemental irrigation. It is these intense flavors that make dry farmed tomatoes highly sought after by savvy consumers and chefs in our region. The production and sale of dry farmed tomatoes has become a very important and economically viable niche market for small-scale organic specialty crop growers in our region.
Another term commonly used in agricultural production vernacular is “dry land farming”. For the sake of clarification, dry land farming typically refers to winter grain production from non-irrigated crop-land. Dry land grain is typically planted in fall and harvested in spring/early summer utilizing winter rainfall for growth and development. A dry land grain crop typically requires between 10 and 15 inches of annual precipitation for economic yields. In areas where rainfall is less than 10 inches, with careful soil management, grain can be produced every other year. The important distinction here is that the grain crop is “rain irrigated” during most of its growth cycle compared to dry farmed crops where there is little or no rainfall during the growth cycle of the crop. It is important to understand the distinctions and differences between dry farming and dry land farming. In this article we are specifically referring to “dry farming”.
Criteria necessary for successful dry farming:
An important aspect of dry farming that makes it regionally unique is that this production method is definitely specific to our Mediterranean climate here in Central California. In normal years our rainfall comes from storms that initiate in the Gulf of Alaska and sweep south and then east moving from the Pacific Ocean across our region from November through February and into March. Our region is typically dominated by high pressure from April through September and often into October. This high pressure keeps the potential for precipitation primarily to our north during this time period. Our region rarely receives significant rainfall from May through September. Rainfall amounts vary considerably across our region based mainly on the location and orientation of our numerous mountain ranges. Depending on their orientation and height these ranges can have significant impacts on rainfall amounts as the storms move across Central California. Steeper ranges parallel to the coast can cause significant orographic lifting which can result in very high rates of precipitation on the west sides of these slopes. These ranges can also create rain shadows on the east (inland) sides, which results in reduced rainfall in these areas. In agricultural producing areas, from San Luis Obispo County in the South to San Mateo County in the north, rainfall amounts vary from about 8 inches up to about 35 inches per year depending on the effects of the mountain ranges and specific storm dynamics.
Adequate winter rainfall
Most dry farmed crops require about 20 inches of rainfall during the rainy season. The challenge of the dry farm producer is to capture and hold as much of this precipitation in the soil as possible so that the dry farmed crops can access this “stored” moisture for growth and maturation during the dry summer months. If a grower is considering dry farming it is advisable to bore numerous holes up to 4 feet in depth throughout the production area to definitively determine suitability based on clay content and moisture holding capabilities of the lower soil horizon. An ideal tool for boring these holes is a 2 inch slide hammer soil probe.
The best soils for dry farming tend to have relatively high clay content. Sandy loam soils or loam soils that overlay deeper clay soils also work well for dry farming. Soils higher in sand content do not hold soil moisture as well as clay and clay loam soils and therefore are typically not utilized for dry farming.
In any dry farming system variety selection is absolutely critical for optimal success. Varieties that do well in dry farming typically have a very deep and aggressive root system capable of reaching deep into the soil horizon to tap the deeply stored residual rain moisture. It is interesting to note that growers in our region have trialed literally hundreds of varieties of heirloom, open pollenated and hybrid tomatoes and, to date, none have compared to “Early Girl” in their ability to aggressively set roots deep and consistently produce a high yield of flavorful and marketable fruits of high quality with no irrigation. There is a new variety available on the market that is very closely related to Early Girl. The new variety is called “New Girl” and appears to have many of the favorable characteristics of Early Girl.
The valleys along the coast in Central California that receive significant summer time marine influence in the form of early morning fog and mild afternoon high temperatures (highs in the mid 80’s) and evapotranspiration (Et) rates in the range of .15 inches per day seem to be ideal for dry farm production. Higher afternoon temperatures and Et rates in the range of .33 inches per day, typically encountered in the more inland valleys with less marine influence, are much less ideal for dry farming, especially of tomatoes, since it can be difficult for the plant to access deeper moisture quickly enough to maintain turgidity during periods of high evapotranspiration. Though not within the scope of this article, it is common practice in California, on small acreages, to dry farm wine grapes, olives and apricots in areas with little or no maritime influence.
Proper soil preparation is critical for success in dry farming. Prior to planting out dry farm crops, residual rain moisture within a root zone is typically lost through both deep percolation beyond the root zone and surface evaporation. Water moves down through the soil horizon with the help of gravity. High clay content in the soil, and to a lesser extent soil organic matter (humus), greatly facilitates the soils ability to hold water against the pull of gravity. Soil water has the ability to move upward through the soil through very small channels between soil particles. These small channels are thought of as capillaries within the soil horizon. Upward movement of water in the soil is greatly facilitated by polar bonds between water molecules and the forces of cohesion. As water near the soil surface evaporates water lower in the soil is pulled nearer the surface. For this reason it is critical to break up the capillaries near the surface to halt unnecessary evaporative loss of residual rain moisture during late spring and summer in soils destined for dry farming. This breaking of capillaries is most typically accomplished with relatively shallow mechanical soil tillage. The depth of tillage is usually in the range of from 8 to 10 inches. Tillage tools most commonly used for this operation are rototillers and disc harrows often followed by secondary tillage implements such as spring tooth harrows. The resultant tilled zone is called a “dust mulch”. This dust mulch provides an effective barrier from the potential evaporative loss of residual rain moisture held in the soil within the root zone of the soon to be planted dry farmed crop. The timing of the creation of this dust mulch is critical. There is usually a very small window for the initial dust mulch formation since it is necessary to trap as much rain moisture in the soil as possible while at the same time avoiding working the soil too wet. Working the soil when too wet, especially on heavier soils, might lead to the formation of clods and a compacted zone from the weight of the tractor and tillage implement due to the fact that wet soils tend to be highly susceptible to compaction. It is important to minimize the depth of tillage when preparing soil for planting out annual dry farm crops since deeper tillage could disrupt the lower soil capillaries which are critical for soil water movement below the tilled zone. The dust mulch needs to be maintained with fairly frequent and light tillage operations (every two or three weeks) from the time of initial tilling until the crops are too large to cultivate effectively. During dry springs it is sometimes necessary to add additional water to the soil prior to planting using either overhead irrigation or drip lines. This type of “pre-irrigation” is sometimes necessary for optimal stand establishment when soil conditions warrant it. On a garden scale it is sometimes necessary to hand water the newly planted plants to assist in rooting and uniform establishment. When a mechanical spader is used to incorporate a high residue cover crop prior to dry farming it is often necessary, in the absence of post tillage rain events, to pre-irrigate with overhead sprinklers. Our typical springtime dry farm tillage and crop culture sequence at the UCSC Farm is as follows:
- Flail mow cover crop
- Incorporate cover crop residue with mechanical spader
- Form beds with rolling cultivator
- In the absence of rain – pre-irrigate beds with over head irrigation at a rate of 1.5 inches per acre (when spring rains are adequate this step is unnecessary)
- Wait for weed flush and create dust mulch with rolling cultivator
- Maintain dust mulch with rolling cultivator as needed until planting time
- At time of planting break open bed middles with Alabama shovels and plant tomato transplants deeply into moisture using hand trowels
- Cultivate with sweeps and side knives when first weeds appear in furrow bottoms or as necessary to maintain dust mulch
- Once plants reach adequate height reform beds by throwing dirt into bed middles with rolling cultivator – when timed well this last cultivation pass will also effectively smother weeds starting to establish within the plant line
Dry farmed crops with extensive root systems can effectively extract deep residual rain moisture from a fairly large area within their roots grasp. Competition from other nearby crop plants or weeds can result in water stressed plants that produce very little fruit and remain stunted. For this reason it is critical to plant out dry farmed crops in a much wider spacing than is typically used for irrigated crops of the same type. Good weed management in a dry farm system is critical as well since most weeds have very aggressive root systems capable of out competing most crop plants for both water and nutrients. As an example of plant spacing, irrigated tomatoes are commonly spaced 2 feet apart within the row with rows spaced 4 feet apart thus giving roughly 5,400 plants per acre. A typical spacing for dry farmed tomatoes (depending on soil type and rainfall amounts) would be 6 feet between rows and 6 feet between plants giving a total plant population of 1210 plants per acre. As you can see from this example a significant yield reduction can be expected from most dry farmed crops simply based on per acre plant populations. A higher price premium for dry farmed tomatoes will often make up for the yield loss related to wider spacing.
Crops suitable for dry farming
Certainly the most notable dry farmed crop produced in our region is tomatoes. In our area dry farmed tomatoes are typically transplanted into the field from May through June. It is advantageous to plant the tomatoes as deep as possible into the residual rain moisture after the dust mulch has been created and when soil temperatures are adequate for strong growth (>55 degrees f). Growers typically plant several successions spaced 2 to 3 weeks apart to provide an extended fall harvest period. Some growers stake and tie the tomatoes for ease of harvest and to enhance fruit quality and some growers let the plants vine out on the ground without support. Early Girl and/or New Girl seem to be the varieties of choice. The fruits are easy to handle, they don’t crack and the flavor is remarkable. One issue with these varieties, when grown without irrigation, is that they are prone to a physiological condition known as blossom end rot. Blossom end rot is related to the plants inability to move calcium to the blossom end of the fruit and the symptom is a black sunken spot on the blossom end of the fruit that is prone to rot depending on the severity of the symptom. Fruit showing symptoms of blossom end rot are not marketable. Other annual vegetable crops that have been successfully dry farmed in our region include dry corn, dry beans and winter squash, all of which are direct seeded into residual rain moisture after the creation of the dust mulch. In a trial conducted at the UCSC farm in the mid 90’s we showed no significant difference in yield between irrigated and dry farmed Red Curry, Butternut and Spaghetti squash.
Advantages of dry farming
As a rotation within a diverse irrigated cropping system, dry farming has many advantages. The lack of irrigation in a dry farmed production block can lead to improved soil tilth since dry surface soil is not prone to compaction or clod formation from both foot traffic associated with harvest and tractor compaction from cultivation operations. Problem weeds are much easier to deal with when irrigation is eliminated for a season and weed seed development is easily minimized in a dry farmed block. If water is a limited resource on a farm then dry farming makes perfect sense as a means of maintaining production while eliminating the need for irrigation. Forcing deep rooting of dry farmed crops can also facilitate the extraction of nutrients that have leached below the root zone of most irrigated crops through excessive rainfall or irrigation. A good understanding of the basic principles of dry farming will most surely lead to a greater understanding of the complexities of water/soil dynamics, tillage, weed management, fertility management and ultimately lead to a greater understanding of your particular production system. Conservation of water in our farming systems is critical and the application of some of the dry farming principles to irrigated systems will only lead to improved water use efficiencies, better weed management and improved soil tilth and productivity.