SOIL BUILDING AND MAINTENANCE
Once your patch has been created, most of the hard work is done. Now your two main tasks will be to maintain the soil fertility, and manage weeds.
On this page:
MAINTAINING SOIL FERTILITY
There are several different factors of your soil layer that you will want to pay attention to maintaining. These include nutrient levels, pH (acidity/alkalinity), soil structure and drainage.
The short version of the story to maintaining a healthy soil layer, is to add good compost regularly (about once a year or more often, depending on intensity of cultivation), add lime once every 1-2 years at about a handful per square meter, fertilise your plants during growth (especially heavy feeders) with liquid fertilisers such as urine, liquid seaweed, or comfrey tea, or using mulches such as pea straw or comfrey leaves. After harvest turn the soil over to mix in the partially broken down mulch layer to increase the friability of the soil.
NUTRIENT LEVELS can be supplemented in a couple of ways. This is by either the addition of organic matter and minerals from outside sources (e.g. compost, comfrey leaves, seaweed, manure, straw, leaves, grass clippings etc), or by 'green cropping' with legumes, which 'harvest' nitrogen from the air and fix it into the soil using nodules on their roots.
A good soil layer requires the presence of the main nutrients required for plant growth - Nitrogen, Phosphorous and Potassium, as well as a selection of other 'minor' nutrients and trace elements. Different plants enjoy different ratios of nutrients, with plants that grow a lot of green bulk like leaves requiring lots of nitrogen, while flowering / fruiting vegetables require higher levels of Phosphorous and Potassium.
Details on each plant's preferences can be found in the species specific section of this website. The easiest way to ensure that you're getting a good range of nutrients added to your soil is to add a variety of things to it, rather than just one thing such as manure. Seaweed is an excellent source of a wide range of nutrients and minerals. If you've added a lot of raw (not pre-composted) ingredients to a garden bed, you may need to let it sit for a few weeks to allow the materials to break down a bit before planting.
ORGANIC SOURCES FOR NUTRIENTS
If your plants can't get the nutrients they need from the soil, they may show a wide range of deficiency symptoms, from yellowing leaves to stunted growth.
Like the human diet, it's a good idea to add a mixture of ingredients to your soil, to better ensure that you're covering all the bases of required nutrients. Diversity is a huge asset that reduces the need for specific tests and corrections.
At a very basic level, a soil amended with a rotating selection of inputs like compost, seaweed, grass clippings, mulch, pea straw, and a dusting of lime now and then, will be a healthy soil. For specific ingredients for certain nutrients include:
NITROGEN Animal manures, grass clippings, compost, urine, blood and bone, fish meal
PHOSPHOROUS Seaweed, rock phosphate, bonemeal
POTASSIUM Wood ashes, crusher dust (from quarries), comfrey mulch or tea, seaweed, urine
MAGNESIUM Epsom salts, dolomite lime
CALCIUM Gypsum, garden lime, crushed sea shells
SULPHUR Flowers of sulphur, gypsum
OTHER MICRONUTRIENTS Kelp or seaweed, compost, crusher dust, comfrey mulch or tea, worm farm tea, urine
pH (acidity / alkalinity)
Different plants prefer different pH ranges. On the pH scale of 1 to 14, 7 is neutral pH, with below 7 being acidic and above 7 being alkaline. Different pH levels affect the bio-avaliability of the soil nutrients to your plants roots, as illustrated by the picture below.
There are several ways of keeping an eye on the pH of your soil, including inexpensive pH test kits (avaliable at garden centres), litmus paper, or observations of the growth habits of the plants and any pH related diseases / problems that you may see arising.
The decomposition process of organic matter within the soil usually results in a gradual shift toward acidic soil. For 'all purpose' use garden beds it is best to keep the pH near neutral or ever so slightly acidic. pH can be adjusted back toward neutral from acidic using lime. It's best to use lime in as raw of a form as possible, such as crushed limestone or dolomite lime.
The more processed forms of lime, such as hydrated lime, are very potent and you can easily overdo it or burn your plants with them. The raw forms are slow release and only need to be applied once every 1-4 years depending on your application rate and preferences.
SOIL STRUCTURE AND DRAINAGE
Again, different types of vegetables prefer different types of soil structure, with some preferring denser, boggier soils, some liking light, humusy soils, and so on.
In general, it's good to aim for a middle ground, where the soil is free draining but also holds a decent amount of moisture. If the soil becomes really squeltchy / muddy when it's wet, or if puddles form, you probably need to take action to improve the drainage.
One method is to build the border of your patch higher up and fill that with light, friable soil materials like straw, hay, dried grass clippings, dried leaf rakings, etc. This can then be mixed in with the bottom soil layer and should improve the overall drainage.
If the problem is really serious you may need to dig out your soil layer and create some drains underneath by digging trenches and filling them with stones, ceramic piping, etc. before covering over again with dirt.
COVER CROPS / GREEN MANURES
A green crop, also known as cover crop or green manure, is a crop of quick growing plants sown densely on vacant ground or within a vegetable rotation and then dug into the top soil. Within the soil it breaks down to humus and releases nutrients. This is a primary soil building technique within sustainable agricultural systems at both the backyard and the farm scale.
In the garden the green crop is usually planted in late autumn, grows through the winter and is dug-in or mulched on top of the soil in the spring. After digging-in it is best to have a delay of up to three weeks before planting the next crop. If space or time is at a premium the green crop may be better harvested them composted or left as a mulch to allow immediate planting of the crop to follow.
The green crop has several functions:
As a protective cover for the soil (stops the winter weather washing and blowing the soil away.
To gather nutrients for use by crops to follow
Maintenance of humus levels
Roots preserve soil structure
Shelter and a food source for predators.
Some Useful Green Crops
My favourite green crop regime is a vetch/oat mix sown in the early autumn. Other winter hardy legumes can be substituted for the vetch. This provides the best of both worlds of green cropping - nitrogen from the vetch (legume) and carbon/biomass from the oats. Use 1 part vetch to 2 parts oat seeds. Broadcast the seeds so that they are about 5-8cm apart from each other. Dig the crops in to the ground in the spring before seed heads/pods develop.
If you have some weeds in your patch that you want to reduce, I have found that a thickly sown crop of phacelia can be very effective at smothering out competing crops. If sown in the early spring, the decent sized plants should die off in the depths of winter and form a natural mulch layer just waiting for you to plant into in spring.
Other green crops include:
Grow legumes as part of a crop rotation to provide nutrients for nitrogen hungry plants (corn, tomatoes, pumpkins), for best effect sow them as a green manure and dig them in just prior to flowering, at least two weeks befor you want to plant your next crop, or if you want to eat the produce, just snip them off at ground level when they are finished producing and let all that yummy nitrogen soak into the soil...
The main leguminous green crops are:
Lucerne (called alfalfa in some countries)
Broad beans (called fava beans in some countries)
Include oats, rye and buckwheat. These can produce a large bulk of growth through the winter but if left in too long quickly become woody. If you are wanting to add physical bulk to your soil, woodyness can be good but you'll need to add a bit of extra nitrogen (or grow mixed with a legume) to assist in the decomposition process. The strong root system of cereal crops improves soil structure. Their main function is to build up carbon materials to add to the soil.
Other Green Crops:
Almost any crop can be used as a green manure. The trick is to make sure to turn them in to the soil before they develop seed heads, otherwise you may end up with a lot more green crops than you wanted! Other commonly used crops include mustard, phacelia, buckwheat and peas.
This is useful as permanent stands harvested for green manure or composting. Comfrey have roots that penetrate 3m or more into the subsoil. Thus nutrients gathered from the subsoil will replenish the top soil. While it does not spread too vigorously, an established patch is hard to get rid of. As such, plant comfrey somewhere in your garden where you don't mind it being for quite some time!
Many weeds have strong, deep root systems which improve soil structure and gather nutrients from the subsoil. It is beneficial to let a bed lie fallow for a season as every weed brings different minerals to the garden. However, the seeds left in the soil after digging-in are likely to cause problems for the following crop. As such, its best to pull up or chop weeds before they reach seeding stage. Non-seeding weeds can be 'chop and drop' simply left on the surface of the soil to naturally compost, or can be added to the compost heap or worm farm.
Crop rotation is the practice of alternating the types of crops grown in each bed, to make the most of the effects of each crop on the soil, and to avoid pest and disease problems which can become established if the same crop is grown in the same place for too long.
There are many different potential crop rotation regimes. Some gardeners adhere to a strict 8 year crop rotation schedule which means that each crop is grown in each bed only once every eight years. However, this requires having eight or more garden beds, and goes on the assumption that you want to grow eight different crop types in equal ratios.
In practice, a simpler crop rotation regime will suffice for the home gardener. Never grow the same crop type in the same spot twice in a row. This puts your garden patch at risk of developing a resident infection such as potato blight or club root. A rough example crop rotation schedule that one could follow is:
Green crop (cereal) > Legumes > add lime > Brassicas > Leafy veg > Root veg > Add compost > Pumpkin/zuch/corn > potatoes
But essentially the idea is to keep changing the crop type in the bed. If you notice that a particular crop suffers a disease problem, take note of this and don't grow that type of crop in that bed for a few years if possible. This will provide time for the spores of the disease to die off.
If you have club root in your garden, a disease which can affect brassicas and make them grow very poorly, you can try to hasten the process of club root clearing from your soils. You do this by getting a bunch of brassica leaves (maybe from a friend, as you should be holding off growing brassicas yourself if you have club root). Crush up the leaves and soak them in a large bucket or barrel of water. Then, thouroughly water your garden with the brassica juice. The spores of the club root will become activated, thinking that there are brassicas present, but since there are no actual brassica plants to attach to, the germinated spores will then die off. If you preform this treatment a few times over the course of a year or two you should have a good chance of eliminating club root disease.