Earlier in the season, I was taking lunch in the car (I think it was raining) and happened to catch Ed Lawrence on CBC. He was speaking about the wet, cloudy start of the season, and noted that the weather conditions were delaying a lot of crops such as tomatoes.
This pleased me, as I felt that having a better known expert than me pointing this out would carry a little more weight than just myself (when explaining to customers why their tomatoes were so late).
However, his next statement caused me a little dismay. He said that the rain would bring about BIG potatoes. I was in the midst of packing the spring harvest of potatoes, and they were not big.
What determines the size of potatoes?
There are several factors that determine the size of potato tubers - distance between plants, the classification of the seed potato, and soil fertility are three.
Like many other crops, the size of the fruits is directly proportional to the distance between plants. Plants that are close together have to compete with one another for sunlight, water, and soil nutrients.
Typically, potatoes are planted between eight and twelve inches apart. In most cases, I plant the maximum twelve inches, though a couple of years ago, I planted a late season bed at ten inches due to having too many seed potatoes. This resulted in a record breaking yield of baby sized potatoes.
As I have written elsewhere on this blog, seed potatoes are produced over several years. First, a potato seed is sown, which grows into a plant that produces pea-sized tubers. The best of these are selected and re-sown. The best and biggest of the subsequent yield are classified as "foundation" potatoes.
These are then either sold to growers such as myself, or replanted to create larger "Elite 2" tubers. These can be sold to growers, or resown again to create "Elite 3" tubers. In theory, one could carry on indefinitely, but in most cases, "Elite 4" is the maximum.
Elite 3 or 4 seed potatoes yield what most folks find in the supermarket. I assume that the potatoes called "Chef size" by wholesalers are grown from Elite 4.
Each generation is a clone of the the parent seed potato. With each generation, the number of genetic mutations build up, eventually resulting in a weaker plant. Knowing this, my seed potato supplier has started to focus on selling elite 2 tubers, and in most cases, that is what I purchase from him.
I don't have enough experience growing potatoes yet to have been able to observe a difference, but I think I can accept the opinion of a third generation organic seed potato producer.
Soil fertility is the biggest determinant of any crops growth, be it size, overall plant health, or resistance to insects and disease.
Our field's fertility is a work in progress. Older, more established farms that have been maintaining and building soil fertility for years have a more advanced and complex soil than I do, but I have the advantage of being on a field that supported organic cattle many years ago. One clue to the latent fertility within my field's soil is the abundance of stinging nettle.
The biggest determinant for soil fertility (according to Eliot Coleman and others), is determined by the crop rotation pattern.
One advantage to placing potatoes before brassica and roots is that brassica and root crops often do not tolerate competition from weeds very well. The hilling and thick foliage of potatoes tends to suppress weed growth, they are often known as a "cleaning crop. As well, when harvesting the tubers, a lot of soil is dug up and persistent roots and grass rhizomes can be extracted from the soil.
All fruit beds receive a soil amendment of rock phosphorus the season before they are planted. This on it's own is not enough, so I often plant buckwheat - which is very good at taking up phosphorus - at some point the following season. Usually, this happens in the fall, so the potatoes don't get this added benefit - the following season's brassica's and roots do.
The other soil amendment that I apply the season before I sow potatoes is compost. Fruit, root, and brassica plants tend to do better when their compost is applied the season before. Roots grown in soil recently amended with compost tend to be covered in lots of tiny rootlets, and fruits tend to grow lots of foliage and less fruit.
So far, so good. I have the potatoes in a reasonable spot in the rotation pattern, and their soil is amended with phosphorus and compost the season before.
So this is not explaining why the tubers are relatively small - even elite 2 seed potatoes at 12 inches (with lots of rain) should be producing tubers 25 per cent larger than I am getting.
In this instance I know why.
As mentioned above, potatoes are an excellent cleaning crop.
As mentioned in other posts on this blog, two parts of my field are relatively new - they were developed within the past three years.
Potatoes have to another characteristic that has made them such an important crop. They are good at scrounging nutrients, even when grown in poor soils.
After developing the two new parts of my filed, I took these three factors into account and decided to concentrate my potatoes in the new beds. The new beds had a lot of weed seed and quack grass rhizome, and they did not have the advantage of several years of accumulating compost applications.
The alternative would have been to plant fruit crops that require high levels of nutrients, such as tomatoes, in these beds.
So this is he reason why our potatoes are smaller. For the time being. In theory, I had concluded that with each passing year, the tubers would gradually reach their natural potential.
This seems to bear out. Customers who received russet potatoes last year might have noticed that they are larger this season than last. I will be growing the russets in the next set of beds in the same "new"field, to gauge the increase in fertility.
As I consider what my production plans are for next year, I look at the beds that are going to have potatoes planted in them. The early leaf crops were cleaned out in time to have a cover crop of buckwheat sown, so I am anticipating another increase in size.
As I have all too often heard since starting to farm...."This is next year's country."
Bob