Looks like another La Niña spring is upon us. while the farm tries to figure out where to put more water we are beginning to work in the greenhouse. We don't really believe in pushing the season too hard on our farm. We shoot for the first week of March for onions and leeks, peppers and eggplants. About the 15th for tomtoes (though I sowed several varieties today). We've found that those dates get the plants big enough to hit the soil running when its time for the field, but not so big that they are rootbound, leggy, and nutrient stressed when its transplant time.
I thought it would be appropriate to share a recipe for potting mix, as much of our energy is in growing starts for transplant right now. The vast majority of premixed bagged potting soil is incredibly deficient when it comes to nourishing the initial growth of our plants. Unless you are fortifying them, they just don't cut it, in our experience.
We really had our mix dialed in a couple years ago, but it was written on a scrap of paper and ... well... you can imagine where the story goes. So we are honing it back in and this is our current incarnation. I've explained some of our choices of materials afterwards. The recipe fills about 4 standard garbage cans so is obviously more related to farm production, but can be scaled down with the same proportions. Its not as good as our old one yet but getting close.
2.5 bricks coco coir (50 gall.) (Soaking for a several hours makes it way easier to finely break up)
4 bags worm castings (30 gall.) - we use Down to Earth's "Wonder Worm"
1 bag (25 gall.) perlite
10 gall. vermiculite
¾ # lime
2 # powdered cal/phos
5# 10-6-1 Bat guano (we also get this from Down to Earth but this specific N-P-K is not always available. You'll need to do some math to reformulate for varying N-P-K versions of the guano.)
4# Alfalfa meal
Add water to create a moist but not soggy consistency and mix very thoroughly. Store in a dry place. Add moisture as needed to keep from getting too dry.
We are very sensitive about the raw materials we use on our farm. For the most part we don't eat animal products in our family, so for a couple years we experimented with plant based fertility mixes. The problem is, plant based nitrogen, while great (we use lots of OG alfalfa meal in the field), is generally not that available on the time scale that a growing start needs it. They tend to be mid-slow releasing sources. While we wouldn't rule it out, we really struggled to produce vigorous robust starts in exclusively plant based mixes. Bone meal, blood meal, and feather meal are typical sources of nitrogen for potting mixes but, regardless of how you feel about animal based agriculture, these products are the dregs of industrial, horribly abusive, animal systems. It doesn't seem to make sense to us to be working to create something so fundamentally opposed to those industrial ag systems while being dependent on their waste products.
What we settled on for quick release nitrogen is bat guano. Its mined out of caves, is probably somewhat invasive, but is in some cases a domestic product, and seemed to us the least bad of several unsavory options. We don't love it but have learned to live with it.
We use Coir in place of peat as peat mining is pretty unsustainable and we have been pretty happy with it as an alternative. Coir comes from the pith between the inner and outer shells of coconuts. The one we use comes from Sri Lanka and while that's a long way away, its pretty compact and Id guess the overall footprint is smaller than that of peat in the long run.
So, A note about aging potting mix. There seems to be a sweet spot with using your home mixed potting mix and here's how we think about it. We feel as though there is a window soon after mixing when the materials are just getting to know each other, a little soil mixer, but nobody has hooked up yet and the nitrogen is still available to the growing seedling. If you don't plant in it in the first week or two though, the nitrogen gets all kissy kissy with the carbon of the base material (in our case coir), breaking it down, and is less available to the growing needs of the plant. After about a month or month and a half of aging everybody realizes there is enough love to go 'round and the seedlings can get in on the action and have access to the nutrients again. In a perfect world we'd plan ahead and mix well in advance of the greenhouse season. Our reality more often looks like "Oh S$%@! I thought that trashcan was full! Quick, mix some up today, we have hundreds of tomatoes to pot up to 4"s. So, you do the best you can.
Thanks for reading and I hope you find this useful.