Most farmers don’t like to hear that they’re “mining” their soils. They will quickly tell you they’re producers, not miners.

But growers have to be careful they don’t take out more nutrients than they put back in when they’re trying to maximize yields, a farm adviser with the University of California Extension Service says.

“A grower might ask ‘here’s my potassium level. How much should I apply?’” says Dr. Scott Stoddard, who is located in Merced County in the central part of the state. “Based on our research, you need to be in the 250-pound range.”

While soil-test K may not always serve as a good indicator of potassium availability because of the different types of soils used to grow crops, Stoddard says growers should consider putting back at least as much nutrient as a crop is removing from the field.

“If you’re averaging a yield of 40 bins, well multiply that by four and you’re removing 160 pounds of potash per acre. So, you’d better be putting out at least that much potash – 250 pounds, or more, would probably be better.”

While California isn’t often thought of as a hotbed of sweetpotatoes, Stoddard said some of the soils in Merced County are well adapted to the crop, which was planted on 15,000 to 17,000 acres in 2009.

“There’s a big area of very sandy soils and sweetpotatoes do well in them,” says Stoddard. “You tend to get better shape and higher quality roots in sandier soils.”

Production differences

Asked how the crop is grown in Merced County compared to other regions of the United States, Stoddard says “wedo things a little different here.”

Merced County sweetpotato operations are at least 95 percent drip irrigated, one of the differences from other sweetpotato production areas.

“One thing that’s interesting is we have almost no pest management to worry with. There’s very little spraying for insects. We’ll sometimes treat for caterpillars or beet army worm – but not consistently, not every field.

“We do fumigate soils, though, and that’s essentially our pest control. Telone is the common product, which is also available to growers in the South. It’s used on tomatoes, strawberries and things like that.”

As for varieties, Beauregard (a Louisiana sweetpotato) and Covington (a North Carolina variety) represent about half the Merced County market.

The other half of market is made up of varieties not grown elsewhere. “In California, we have four distinct markets: the traditional yam like Beauregard; a red yam market for potatoes with dark red, or maroon-colored skin with orange flesh; a market for ‘sweets’ which consists of potatoes like the old Jersey sweet with a pale, light-tan color; and a market for the Japanese, or Oriental, yam which is almost purple-skinned with white, dry flesh.”

Typical growing season

Sweetpotato, says Stoddard, “is a 120-day crop that requires nine months to grow. That always puzzles some people.”

California growers start in early February with a hot-bed operation and nurseries where transplants are raised.

“We usually begin transplanting in early April with the bulk in May through mid-June. However, people will continue to plant ‘seed fields’ – the crop saved for the next year’s seed – all the way through mid-July.”

At the beginning of transplanting “we give them a big shot of water, probably 3,000 gallons of water per acre. Then, the drip irrigation system is fired up. We plant on raised beds – two rows to a bed with one drip line between them.”

In the spring, growers often provide a fumigation. Two to four weeks later, “they’ll come in to work the field: disk it, fluff it up, and pull beds. Many times when they pull beds they’ll also add in a pre-plant fertilizer. Then, the beds are ready for transplanting.”

Once the transplanting is done and the drip line is in place, growers begin feeding nitrogen – sometimes potassium – through the line. “A lot of nitrogen” is provided through fertigation.

The potatoes are cultivated a few times, perhaps hand-hoed to control weeds. “We don’t have a lot of herbicides available although we have some for good grass weed control.”

Depending on the variety, it may be mid-July when harvest begins. The bulk of harvest is in October and is usually finished by early November. A lot of sweetpotatoes go into storage sheds for the Thanksgiving and Christmas holidays.

Typical Yields

“A really good Beauregard yield is about 80,000 to 100,000 pounds per acre. Growers here like to think about yield as ‘bins per acre.’ At the end of the day, they can see how many bins are stacked up at the end of the field. In those terms, a really good yield would 60 to 80 bins.”

A more average yield is around 40 bins.

Fertilizer programs are interesting and vary greatly, says Stoddard. “Pesticides are more regulated and when you have a problem the toolbox is limited as is the way to apply them. Fertilizer isn’t like that and, as a result, you hear of all kind of secret combinations or recipes. They’ll throw in a bit of this, a little of this and some of the other.

“In our case, we put down the majority of the potassium, all the phosphorus along with a few pounds per acre of zinc up front. These sandy soils tend to be deficient in zinc. Those could be broadcast and incorporated although we like to use a lot of liquids in this state. So, when farmers are pulling beds they can also be injecting 800 pounds of an 8-8-8 or 1,000 pounds of 10-10-10. Zinc and sulfur might be mixed in. That’s common, although it varies. Instead of 10-10-10, you might have 18-5-12. “

Growers might decide to put out potassium – sulfate of potash, for example – and “because the drip-line is on the surface, not buried, they’ll sometimes run a band of potash right down the bed’s center. They’ll pull a small ‘V’ and then pour the band of potassium sulfate.”

The band is often covered with an inch of dirt and the drip tape placed atop it. “So, when they irrigate, the water runs right through the band of potassium. The nice thing about that is you’re providing potassium with the water where the roots can access it. You don’t have a bunch of potassium on the shoulder of the bed where there are no roots.”

Potassium trials

In 2007 and 2008, Stoddard conducted several trials using sulfate of potash. The 2007 trial “was pretty simple, mostly a nitrogen trial although there was a potassium fertilizer component. That looked at 200 units of K20 per acre from KCl (potassium chloride) versus 200 units of K20 per acre SOP (sulfate of potash).”

For the 2008 trial, Stoddard decided to do a more intensive potassium evaluation. That one looked at different rates of only SOP ranging from zero to 300 pounds. “I had zero, 100, 200 and 300 pounds pre-plant. On top of that, I put another zero, 50 or 100 pounds of potassium from potassium nitrate, which was injected.

“Besides studying crop response, another thing we wanted to see was whether it made any difference in the stored potatoes. So the 2008 trial didn’t really end until May of 2009.”

In a nutshell, the 2007 trial results suggested a trend for improved yields for SOP versus KCl. Yield increases amounted to a few bins per acre. “In 2008, I evaluated that again and didn’t see the same trend,” he said.

Another nice thing about the trial was Stoddard was able to look into potassium removal. “I did a lot of root sampling. As you increase the fertilizer K, you see a pretty good correlation with increased root potassium. That makes sense. For example, at zero pounds of applied potassium I got averaged about 1.25 percent root potassium in the May-stored roots. Where we applied 400 pounds of potassium, we were at about 2 percent. That’s a linear relationship and was expected.”