Growing Magazine - June, 2012
A Dirty Balancing Act
Making the most of your soil management
"Soil is a perfect harbinger of plant development," says John Boelts of Desert Premium Farms in Yuma, Ariz.
A number of management tools can improve soils, from adding organic material to changing tillage practices to rotating crops. Often one technique can mitigate more than one problem. On the other hand, a technique that solves one problem can make another one worse.
Part of the balancing act for growers is to choose between sometimes conflicting soil management techniques, and to compensate when one that could benefit their soil isn't possible in their area.
Potatoes being harvested in the San Luis Valley of south-central Colorado. Rotating potatoes with cover crops provides many benefits, including nitrogen management, improved soil and water quality, bigger potatoes and higher yields.
PHOTO COURTESY OF THE USDA ARS.
Boelts is partners with Kent Inglett in the 1,500-acre operation. Both are members of the Arizona Farm Bureau. Most of their acreage is planted twice a year. In the winter they grow iceberg, romaine and leaf lettuce, about 300 acres of alfalfa and vegetables for seed production. In the spring they grow cotton, vegetable seed crops, grass and durum wheat. Most of the produce is sold to food services across the U.S. and in Canada.
"Every place in the country has unique challenges with its soils," Boelts says.
Microbes in the soil need oxygen to become active and break down organic matter, and plant roots need pores between the soil particles in order to expand. Although sandy and silty soils have large air-filled pores and don't compact easily, plant nutrients leach out with irrigation or rainfall. Clay soils compact easily and have less aeration, but they can contain plant nutrients.
In the Yuma area, quality soil is crucial, because crops are grown intensively, he says. On the whole, the soil is generally good, although it can be saline and poorly drained, with little organic matter. The area receives little rain, even in winter.
Erosion is a major problem in many parts of the country. When wind and rain erode the topsoil, which typically has more nutrients than the soil below, growers lose their best soil. Also, much of the soil is compacted, especially by heavy equipment.
With one of his sons in tow, John Boelts inspects his fields.
PHOTO COURTESY OF JULIE MURPHREE, ARIZONA FARM BUREAU.
"We harvest all year," Boelts says. "When there's a long-term crop like alfalfa, we'll get 10 to 11 cuttings annually. You run into a situation where you're going into the fields a lot more times." In addition, vegetable fields are watered as close to harvest as possible. By necessity, the soil is wet when the harvesting equipment comes through.
The Yuma area also has low organic matter in the soil and a high water table, and to top that off, the water in the aquifer is extremely salty.
"Salinity is a huge issue here," he says. "The Yuma area is one of the saltiest places on Earth."
The Colorado River's water is somewhat saline, but less so than the local groundwater. While the goal of growers is to move their salty water down and replace it with less-salty irrigation water, compaction can be a limiting factor.
There's also inconsistency in the soil, even within a single field.
"Our soils were laid down by rivers, and it was either feast or famine," Boelts says. One corner of a field may be a sandy wash and the rest something entirely different.
Boelts does soil and petiole sampling if he's unsure about the soil conditions and also to establish what nutrients new plants will need. Although sampling can be expensive, the cost of fertilizer is rising, he notes, and an increasing number of growers are having their soil tested to avoid applying too much fertilizer.
He's seen growers progress toward other, more efficient growing practices as well, such as making sure to apply only as much irrigation water as the crops need and combining operations.
"Less is more," he says. "Just make sure you're doing the right things."
Boelts adds that it's important to begin early on; the earlier you do, the greater results you'll have.
"We're trying to achieve and maintain soil with the best tilth we can," Boelts says. "Compost is one of the best things you can add to it."
Although, he notes, any time you add to the soil, you're changing the soil chemistry. On the other hand, if you do nothing, the soil makes its own changes.
Compost made completely from plant residues is essentially salt-free, but when it includes manure, it may be high in salt. Compost can be used at almost any rate without burning plants, but it isn't a good source of readily available nutrients because it releases nutrients slowly.
Manure is an excellent soil amendment once it's composted, and it provides both organic matter and nutrients. Most manure comes from confined animal feeding operations and dairies.
Decomposition of organic matter is accelerated by warm, wet weather and by organisms such as earthworms, arthropods, protozoa, nematodes, bacteria and fungi, which improve the soil in many ways. Mycorrhizae extend the reach of root hairs into the soil and increase the uptake of water and nutrients, especially in less fertile soils.
Growers in the Yuma area have little access to manure, and because heat breaks down organic matter, as each summer comes and goes, the organic matter in the soil continues to break down. To compensate, they grow high-residue crops and plow them in at the end of the growing season, Boelts explains.
Deep-rooted crops add organic matter to the soil. Shallow tillage incorporates residues and speeds decomposition by adding oxygen that microbes need to break down the organic matter.
Organic fertilizers supply the major plant nutrients and many essential micronutrients. They are commonly made from waste products such as manure, peat, seaweed and worm castings, as well as compost, blood meal, bonemeal, fish meal and feather meal. These fertilizers release nutrients slowly.
Commercial fertilizers also have their place in sustainable agriculture. The advantages are that they release nutrients immediately and are less bulky to apply.
In Yuma, irrigation water holds the soil down, so wind is more likely to deposit desert soil onto a farm than blow topsoil away, Boelts explains. Since growers level their fields by either GPS or laser, every drop of irrigation water stays in the field.
"In most of our fields, the perimeter is higher than the field," he says. "Water flows evenly across the field and sinks into the soil profile."
Other practices, such as growing cover crops, prevent erosion by protecting the topsoil from wind and rain. Cover crops also suppress weeds, help break pest cycles and provide food sources for beneficial insects. Examples of cover crops include wheat, rye, buckwheat, cowpeas and millet, as well as crimson, subterranean, red and sweet clover.
John Boelts inspects a stalk of celery.
PHOTO COURTESY OF JULIE MURPHREE, ARIZONA FARM BUREAU.
Some cover crops perform double duty. When the stalks of high-residue crops are plowed into the soil, they add to the organic matter. Legume cover crops add nitrogen. Nitrogen-fixing bacteria form nodules on their roots and convert atmospheric nitrogen into nitrogen that is available to future crops.
Another practice is the use of mulch as a soil cover. Often a wood product, the mulch adds organic matter to the soil when it decomposes.
Crop rotation reduces erosion because the ground is covered for a longer period of time than it would be with only one crop. It also improves soil quality and reduces the buildup of insect, disease and weed pests, and thus the reliance on agricultural chemicals. Succeeding crops should be of a different genus, species, subspecies or variety than the previous crop. For example, row crops after small grains or grains after legumes. The rotation can be for two years or longer.
Conservation tillage leaves crop residues on the soil, which reduces the velocity of water and its ability to transport soil. It also increases water infiltration and retention and helps build organic matter in the soil. These methods include no-till, ridge-till, zone-till and variations of chisel plowing and disking. Unfortunately, conservation tillage can favor insects and disease, as it gives them a place to hide and grow.
"We have a bevy of practices that promote good drainage," Boelts says. "We do a lot of heavy tillage, but not always between every crop. Oftentimes it precedes the planting of a vegetable crop, and there may be two or three crops planted until more heavy tillage is done."
Subsoilers are used to break up the soil more than 3 feet down, but subsoilers also aerate the soil, and aeration speeds up the decomposition of what little organic matter they have.
Strategically applied irrigation can improve drainage. Growers in the Yuma area use furrow irrigation with either gravity-fed water or water that needs little pumping. While the water is in the furrows, they "spike" the furrows to break the hardpan that forms on the surface and prevent it from developing again.
"We're standing on the shoulders of giants," he says. "The people who surveyed and laid these irrigation and drainage systems were very forward-thinking. It was very expensive, but this gravity-fed water doesn't cost us much to convey the water."
In addition to subsoiling, Boelts mitigates compaction by planting deep-rooted crops.
"The roots of deep-rooted crops like cotton and Sudan grass expand and break up the soil," he says. This also allows roots to reach a larger supply of water and nutrients.
"Various tillage practices can improve salinity," Boelts states. "They maintain drainage and keep the hydrology moving the right way. In the summer we do heavy tillage, subsoiling and tandem disking.
When growers in the area irrigate, whether with furrows or sprinklers, they apply just the right amount of water, when it's needed and uniformly. The irrigation water continues to move down through the soil, forcing the salty water deeper down.
Groundwater is pumped into drainage systems to lower the water table, which also moves the saline water down and allows them to place the higher quality irrigation water on top. "It's expensive, but necessary," he says.
Pest and disease control
Rotating crops increases biological diversity. Because pests and diseases are usually attracted to certain crops, rotating them gives disease and insect pests only one year to breed. Growing in more than two rotations is the most effective.
Rotating crops can also suppress weeds. Cultural practices that enhance the crop may enhance the weed species as well. When crops are rotated, the environment changes and the weeds no longer thrive.
The author is a freelance writer based in Altadena, Calif.