Wednesday, June 16, 2021

Potatoes and nutritional density

I found out that if you were able to choke down 3 pounds per person per day you would get a bit over 1000 calories/day.   From reader Uninformed

It is worse than that.


I will be using "specific gravity" as a proxy for caloric density in much of this essay because it is relatively easy to measure specific gravity and that is what most of the literature reports. 

As with many things, details are important. In fact, from the lens of the subsistence gardener, details are very, very important.

The chart shown above is from MSU data. The bright red data point is the average of "red" boiler potatoes like Dark Red Norland and Red Pontiac and you would have to eat six pounds of them to get 1000 Calories. The bright yellow dot is the average for "round, white tablestock" which includes Yukon Gold. The elongated brown data point is the average for "russet tablestock" and the boomerang shaped data point is the average for "chipping stock" (three pounds to get 1000 Calories).

Potato chips are BIG business in Michigan. Cultural parameters were tweaked to hit a specific gravity of 1.085 for the chipping stock and the others fell wherever.

The big chip processors require a specific gravity of 1.080-to-1.085. Variation in moisture results in chips coming out of the fry-bath soggy or burned.

The second level of detail
Suppose you are a commercial producer of potatoes intended for the consumer's table and you are making variety and growing decisions.

The effect of irrigation on yield and specific gravity
Looking at the extremes, you have the choice of shipping 30,000 kg per ha (approximately 27,000 pounds per acre) with a specific gravity of 1.074 or you can ship 65% more product per unit area of land with a specific gravity of 1.066.

You get paid by the amount delivered and cosmetic appearance. You would be daft to not irrigate to maximize shippable product.

If you are choosing varieties, do you choose Russet Burbanks which might produce 30,000 pounds of US#1 per acre (S.G. of 1.075) or Norkoda Russet at 45,000 pounds of US#1 per acre (S.G. 1.065)?

Most people who buy potatoes at the grocery store are not informed enough to prefer Burbanks over Norkoda nor do they prefer a Kennebec with a specific gravity of 1.074 over a bag with a specific gravity of 1.066.

A specific gravity of 1.085 is not a "hard-stop"

Michigan chipping processors specify the 1.080-1.085 range of specific gravities because they know they will receive some shipments from growers who struggle to get over 1.080 due to soil, sun exposure, late rains and choice of variety. They would LOVE to raise the window to 1.085-to-1.090 or something higher because that would lower their energy costs and improve the throughput at the fryer vat. 

They could raise the window but then the rate of rejects...after frying...would be higher and that is expensive scrap because it is lost salable product.

As a subsistence gardener, it is not rocket science to increase the caloric density of your potatoes

Pre-planting decisions:

  • Choose a site that gets great sun exposure. Just like sun ripens grapes that make good wine, sun makes potatoes. Sites with great sun exposure are rare in cities and suburbs.
  • Ensure the site has adequate potassium.
  • Ensure the site has enough nitrogen to grow a full canopy quickly. Sun that does not fall on leaves does not make nutrition. BUT, don't use any more nitrogen than that amount. This is an experience thing.
  • Choose varieties that are noted for high specific gravity. Those are usually later varieties. Some catalogs (Example) list potatoes as "waxy" or "mealy". You want dry/mealy/floury potatoes to maximize caloric density. Specific gravities for potato varieties can be found on the internet but you have to be careful to not compare values from different sites to avoid apples/oranges comparisons.

Planting decisions:

  • Plant early so you have full canopy during the longest days of summer.
  • Plant closely enough together that they canopy quickly but far enough apart so that potato plants don't compete with each other. This will vary by site fertility, availability of irrigation and vigor of the variety you choose.

Growing decisions:

  • Control weeds. Sunlight that weeds intercept cannot make potatoes.
  • Control insects. The leaves they eat cannot intercept sunlight.
  • Irrigate through early and mid-summer, if necessary, to ensure plants don't shut-down for lack of moisture.
  • Back off irrigation in late-summer (mid-August for me) to ensure natural die-down.
  • Harvest AFTER natural die-down. Your vines will squeeze every bit of goodness out of the vines and pack it into the tubers...if you let them. Some growers like to leave the tubers in the ground for a couple of weeks so they can harden up.

It is difficult to over-emphasize the importance of good sunlight. "Out west" blows the doors off eastern potatoes for specific gravity because they have clear skies and intense sunlight many more days of the year than us "back east". If a grower in Idaho or Washington is striving for high specific gravity rather than maximum tonnage, specific gravities of 1.120 are well within reach.


The nutritional density of potatoes can vary by a factor of three.

It is lunacy to grow, dig and store, cook and eat three times the mass of potatoes when the same gross amount of nutrition can be captured in half or a third of that amount.

Nutritional density of potatoes can be controlled, within limits, by the grower. The economic incentives for a subsistence gardener are significantly different than they are for a commercial farmer.

Attempting to survive solely on potatoes...without additional a huge amount of potatoes, even with the most nutritionally dense potatoes available.

Let me repeat, if potatoes are a cornerstone of your subsistence gardening plans, you need to also have a plan to add fats to your diet.


  1. Does the PD shape of the chipping potato have any relation to eating chips? ---ken

  2. WOW! You gots skilz! THANK YOU for the shared wisdom!
    Types of potatoes better for caloric density...thanks for the link above!

    Potassium for gardening...

    Anybody know of any reference sites I can go poke around in to find sources for this kind of this type of information?

  3. Would you be interested in a swap of ammo for services? I'm in the area and want to plant a few fruit trees. Would like to do a ~2 apple trees and ~2 pear trees, but am a novice at this and figured an expert would be the right way to go.

    1. What town are you in? Maybe we have a mutual acquaintance.

      Regarding ammo: Sadly, I lost most of my firearms in a boating accident and then my wife sold the rest to a middle-aged man at our yard sale. Unfortunately, she sold them for what I had told her I paid for them.

    2. Tragic that. I live in Bath. Any recommendation/names of companies/people to contact would be appreciated. Seen too many people get random trees started that die after a few years or never take off to spend money on this without a more experienced eye to guide the start. I can also be emailed at this name @ Thanks!

  4. Thanks for your excellent scholarship. Again. Glad I planted my walnut tree 35 years ago. Perhaps I'll try growing some maize / corn after all. And will definitely start watching for a nice white potato variety for next year's crop. On the bright side, if I'm trying to lose weight, eating reds is better than whites.

    Right now my current question (and no pressure here) is why my same variety of spud from one seed potato supplier is growing so much better than the second supplier? Vaguely I recall it's the number of generations since the seed was started from vegetative growth only, but I can't yet rediscover that information.

    1. From the Michigan Seed Potato Association:

      Seed is one of the most important ingredients to the total potato production management program. There is a variation among states as to how seed plots are maintained, however, there has been a trend in the adoption of the limited generation programs. Michigan was one of the leaders in the United States to adopt this scheme when it was initiated in 1964.

      The limited generation program in Michigan utilizes the tissue culture system. Within the tissue culture system plants are produced under aseptic conditions, tested for disease, and transplanted in the greenhouse to produce the nuclear class of seed or what is commonly known as mini-tubers. These mini-tubers, under normal field conditions, produce the field year 1 class. The following year, the field year 2 class is produced from the field year 1 class seed. This process continues through the field year 6 class, after which the seed lot is no longer eligible for recertification.

      ERJ notes: Potato lines pick up virus from leaf-hoppers, nematodes and perhaps can be spread by cuts from implements and "seed cutters". The tissue used in tissue culture is nipped from the extreme growing tip and, in some cases, the donating plant is dosed with an antivirals or grown under extreme heat to suppress viral loading.

    2. Thanks Joe, that's got me headed in the right direction. Now I'll follow the many links on the British Columbia Certified Potato Seed Growers.

  5. Thanks for the info. Lots I didn't know... sigh

  6. Given that the current caloric problem in the US is one of excess, Growers should be encouraged to grow lower SG spuds, no?

  7. bile... I think you may have missed the gist as it were

    1. No. My theme for this age is "Satire is dead"

  8. Completely off topic, it occurs to me that this is a book for you, Mr Joe.

    Winchester is a treasure,