Fertiliser - you only get back what you put in

We all know that plants need nutrients to grow. If you're like me pre-gardening course, you just stick them in the soil and assume that that has everything they need in it. After all, that's all they get in the wild. But actually it turns out that there's more to it than that. We don't grow plants – either on farms or in gardens – like they grow in the wild (not even organic farms). In the wild, no one goes round clearing up all the dead plant life that ends up on the soil - or for that matter the animal excrement or dead animals. Sure, some things eat some of these things, but actually they themselves tend to produce excretions that end up back in the soil.

Modern human life avoids this. We clear up dead plants and, if the garden's lucky, put them in a compost heap and return them to the garden. If it's not so lucky we send them off to the council's green waste collection or just put them in the bin, and after we've up eaten plants we don't return the waste products we create from them to the soil, we transport them away via the sewer system.

Now, don't get me wrong, I'm not suggesting turning you garden into your toilet. For health and disease reasons, that would be a truly bad idea. You don't want the excrement of anything that's not a herbivore on your garden (so horse manure = good, human "manure" or dog or fox mess = health risk). But this does mean that we're taking a lot of nutrients away from the soil in our gardens and farmland that never leave wild areas (because we take away the plants that have grown there). This leaves the soil depleted and less able to support plants. We need to add it back in the form of compost or fertiliser.

The main nutrients plants need are nitrogen (good for leaves and shoots), phosphorous (good for roots) and potassium (good for flowers and fruits) N, P and K on the periodic table of elements). So non-organic farms make sure they add these to the soil. They do it by deriving these from petrochemicals. The problem with this is that although these are the main nutrients plants need, they're not the only ones that they need, and when you use NPK fertilisers made of petrochemicals, these are the only chemicals the soil gets and therefore the plants and ultimately, if you're eating the plant, you get. This gets worse year after year, as the levels of other elements the soil originally contained get further depleted. Also, plants need a wide range of nutrients to help them stay healthy. Here's a page with a really useful picture on it that shows you how to diagnose missing nutrients based on what's wrong with you plant - some things that look like disease actually turn out to be a lack of nutrients.

Organic fertilisers come from things that were originally living organisms and contain more than pure nitrogen, phosphorous or potassium (although not all of them are high in all of those). Here are some organic fertiliser options:

Bone meal

The nitrogen-phosphorus-potassium ratio of bone meal is generally 4-12-0, though some steamed bone meals have a ratio of 1-13-0. Bone meal is also an excellent source of calcium, but does not provide enough nitrogen to be beneficial to plants. Research suggests plants can only get phosphorus from bone meal if the soil pH is below 7.0 (acidic soil).*

Seaweed

Helps to bind soil crumbs together, and it contains all soil nutrients (0.3% N, 0.1% P, 1.0% K, plus a full range of trace elements)*.

Organic chicken manure

According to the RHS website, pelleted poultry manure is a good source of ntirogen and generally contains 4% nitrogen (as ammoniacal nitrogen), 2% phosphorous (as phosphorus pentxide) and 1% potassium (as potassium oxide).Most poultry manure is in the range of pH 6.5-8.0, being neutral to moderately alkaline, so unsuitable for lime-hating plants. According to this website, it also contains a range of trace elements, with amounts varying depending on the diet of the poultry and how they're kept.

Manure

I assume this is horse manure. According to this website, horse manure is only about half as rich as chicken manure, but richer in nitrogen than cow manure. Horse manure is considered "hot", which means it should be composted before it's put on the soil so it doesn't burn the plants it comes into contact with. Also, horse manure often contains a lot of weed seeds, which means it's a good idea to compost it using a hot composting method or to buy it already hot composted.

Green manure - mustard

Green manure - buckwheat

Green manure - red clover

Green manure - alfalfa

Green manures are plants you grow between other crops (often over winter) and then dig into the soil before planting your next crop. They suppress weeds while they're growing and make nutrients in the soil more available to other plants. Many are leguminous and fix nitrogen in the soil. Some also acidify soil. Which nutrients or effects you get depends on which green manure you pick.

Comfrey

Because comfrey has deep roots, it sucks nutrients from deep out of the soil. Comfrey fertiliser makes these available to plants with roots that don't go so deep. My next post will tell you how to make your own comfrey fertiliser.

* Information from Wikipedia on 12 October 2014.