Just as you and I need the right kinds of food, plants need the right amounts of different nutrients. You couldn’t live on chocolate all the time (you might think that’s a pity; I know I do!), and if you drank a gallon of orange juice, you’d probably make yourself feel quite sick, even though orange juice in moderate amounts is good for you.
The three primary nutrients we need to think about are NPK—nitrogen, phosphorus, and potassium. These are the most important for plants. Nitrogen helps plants grow; phosphorus stimulates root growth, and potassium increases plants’ strength and disease resistance as well as helping them grow good fruit.
Next to NPK come a scattering of other nutrients, such as calcium, which is important for both root health and leaf growth; magnesium, a key component in chlorophyll and necessary so the plant can get energy from sunlight; and sulfur, which is necessary for chlorophyll formation. Other trace minerals in the soil have specific characteristics which can also help or hinder plant development.
Let’s look at NPK first. N is for nitrogen, which is a key element in plant growth. Soils high in organic matter usually have higher nitrogen content. That’s particularly the case if the soil has previously hosted plants like beans and peas, which fix nitrogen in their roots. Artificial fertilizers are made by using nitrogen to make nitrates (converting the gas to mineral form) that can then be applied to the soil.
A big problem with nitrates is that they’re easily leached out of the soil. Heavy rainfall could wash all the nitrates away, taking all the goodness out. So you need to make sure you reduce runoff, for instance by mulching. When you mulch, you’re putting a layer of some other substance above the topsoil, to prevent falling rainwater from carrying the soil away with it, as well as to keep moisture in. You might mulch with cardboard, wood chips, or straw, for instance (there’s a whole chapter on mulching later on.).
You’ll easily see if you don’t have enough nitrogen. Plants won’t thrive. The leaves might turn yellow and start to fall, even though the plant is still growing. Fruit will be small and of poor quality. So, you need to make sure your plants get enough nitrogen. Also check for potassium deficiency, as fruit can also be small and low quality where the plant is not getting enough potassium.
But you also need to ensure that the nitrogen in the soil is balanced with carbon; gardeners sometimes call these two components ‘green and brown.’ For instance, when you’re making compost, you need to balance ‘green’ inputs (like grass clippings or vegetable peelings) with ‘brown’ (like shredded twigs or cardboard) to achieve a satisfactory mix. Carbon is both an energy source and structural building block for plants, while nitrogen helps plants build tissue. We also need to think, of course, about what all the little micro-organisms in the soil need because they’re responsible for keeping the soil healthy. It seems that both those microbes and fungi, and most plants, have evolved to do best with a carbon-to-nitrogen ratio (CNR) of between 24:1 and 30:1. Even better when you have a CNR within this range, the microbes can help release other nutrients such as phosphorus and zinc.
Coal, charcoal, and diamonds are all forms of pure carbon. However, plants find carbon more useful in certain compound forms such as calcium carbonate (the active component of agricultural lime). You might have carbon in your kitchen, in the form of bicarbonate of soda. Sawdust is also nearly pure carbon: it has a CNR of 400:1. If you tried growing plants in pure sawdust, there wouldn’t be enough nitrogen for the bacteria and fungi. If you put lots of sawdust in the soil, the bacteria and fungi would have to get their nitrogen from the soil instead of the sawdust, and that would lead to soil depletion. This is one reason you should be careful about putting too much sawdust in the garden, though it’s great to add in small handfuls to a mainly ‘green’ compost heap. Composted manure, by contrast, has a CNR of 20:1, so you can add as much as you like to your soil, as it is so close to the desirable CNR for the soil itself.
When organic matter rots, its CNR changes. The nitrogen content will usually go up while the carbon content stays pretty much the same. In the process of decomposing the organic matter, the soil micro-organisms need to use a lot of nitrogen, but when they die, they’ll give it back to the soil. It’s just a matter of time and that’s why you can’t expect soil improvements to work immediately. Nitrogen can be added to the soil in a number of ways. You could buy a nitrogen-rich fertilizer, though that’s not a very sustainable way of doing things. You could use fish emulsion, which is a more natural way of adding nitrogen.
Or you could depend on natural processes to do the work for you. You could add composted manure—chicken, rabbit, or goat droppings are all good, as well as horse manure—but they all need to be composted first. Coffee grounds are also excellent for digging into the soil or composting. You could use green manure, planting fast-growing seeds like mustard, fenugreek, winter grazing rye, or alfalfa, then cutting them down and chopping them into the soil. If you have a lawn, use your grass clippings as mulch to put nitrogen back into the soil; the grass clippings will decompose gradually, liberating large amounts of nitrogen. Finally, you could grow nitrogen-fixing plants like beans and peas. These will take nitrogen out of the atmosphere and ‘fix’ it in their roots. But remember that you’ll need to rotate the crop, and not grow these plants in the same place two years in a row, otherwise the balance of the soil can get out of kilter.
You’ll probably find that a mix-and-match of different techniques works best for your garden.
Next up is phosphorus. This helps plants develop strong root systems and helps them resist pests and diseases. It improves flowers and seed creation. Flowers aren’t just nice to have; if you’re growing fruit, including tomatoes or peppers, you’ll want plenty of flowers to give you plenty of fruit. Also, when you’re growing your own vegetables, phosphorus really brings out the flavor in fruit and veg.
A good way to add phosphorus organically is to use a liquid kelp fertilizer. It works very well in the short term if you’ve spotted that your plants are in trouble, but the nutrients won’t last long term. Both bone meal and fish meal also deliver phosphorus; they release into the soil very quickly and give a big phosphorus boost and a small addition of nitrogen.
Over the longer term, if your soil is low in phosphorus, you’ll benefit by using rich composted manure rather than other types of compost. Manure is higher in phosphorus, and of course, it will help condition your soil too.
The last of the NPK elements is potassium. This is particularly important for disease resistance, and also for fruiting. It also helps plants process water and nitrogen. But if plants get too much potassium, they may be inhibited from absorbing other minerals that they also need and it can lead to nitrogen deficiency. So again, you need to get the right balance.
To add potassium to the soil, you simply need to add a little wood ash. Don’t add too much—you only need 25 lb per 1,000 square feet a year or about a third of an ounce per foot.
Diagnosing mineral deficiencies
|Mineral||Not enough||Too much|
|Nitrogen||Yellowed leaves – tends to affect older leaves first, sometimes with pink tint Anaemic appearance Stunted growth.||Spindly with weak stems Leaves browning from the edge inwards Shiny and abnormally dark green leaves|
|Phosphorus||Leaves going purple or red Flowers drop ‘Burnt’ leaf tips Slow growth Dull yellow foliage||Stunted growth Leaves turn yellow or dark|
|Potassium||Leaves go yellow or purple with browning at the leaf edge or look scorched Poor flowering and fruiting Wilting old growth||They will have a nitrogen deficiency —yellowing leaves, anemic appearance, stunted growth Veins of leaves will have a red tint Newer leaves will be smaller|
|Calcium||Inhibited growth Brown spots on leaves New leaves beginning to yellow with brown spots Tips of leaves turning black Distorted, crumpled leaves Blossom end rot Plants have bushy appearance||Yellowing between leaf veins which can then die|
|Magnesium||Yellowing between the leaf veins, sometimes with reddish-brown tints Early leaf fall||Inhibits growth Darkening leaves|
|Sulfur||Looks like nitrogen deficiency, but affects younger leaves first Pale green, yellowish-green, yellowish Small plants with narrow leaves||Uncommon: can stunt growth Dark in color|
Calcium is another mineral that plants need. Without calcium, they can’t manage to process nitrogen, even if there’s a lot of nitrogen in the soil. Calcium also helps the plant grow strong cell walls, helping it resist disease.
Adding calcium is really simple. You can add lime to the soil, usually in the fall so it is well absorbed by the time you start planting in spring. In the days before artificial fertilizers became so widespread, farmers used to dig into chalky cliffs to extract marl (calcium-rich, crumbly stone) for the fields. Some dredged local ponds and streams for lime.
For your home garden, you can also add crushed eggshells, which are full of calcium. Gypsum, too, adds a calcium boost and can help to break up heavy and sticky soils.
Magnesium contributes to plants’ green coloring. It’s the mineral that makes photosynthesis work properly. Very light soil, and soil without much organic matter, can easily lead to magnesium deficiency. You’ll spot this by the yellowing of leaves; later the leaves may begin to turn purple or brown. The best way to make sure your plants have enough magnesium is simply to make sure they have rich, organic compost added at least once a year. But you can also add Epsom salts, one tablespoon to every gallon of water, as a foliar spray or in watering (spraying gets it into the plant’s systems a little more quickly).
Finally, sulfur is an important nutrient. Plants don’t need a lot of it, but it does have to be there. It helps plants form enzymes and other essential proteins. It can also help reduce the sodium content of soils (useful if you live near the sea or brackish water), and its compounds are partly responsible for the flavor of mustard, onions, garlic and other alliums.
You can use fertilizers containing sulfur, but if you use animal or chicken manure for your plants, that will contain sulfur too, so you won’t need to buy additional fertilizer. If you have soil that’s very alkaline (high pH) adding sulfur will help to make it a little more acidic (sulfur is a manufactured fertilizer but is accepted as part of an organic regime, since the basic materials are natural).
I want to stress that if you use plenty of organic matter for your soil and add mulch, top dressings, and compost on a continuous basis, you probably will end up with soil that’s healthy and in balance. But occasionally, soils are more extreme or for whatever reason (e.g., leaching, or nitrogen-hungry plants growing on one patch for a long time) can become depleted. So you should really look at adding minerals in two situations: firstly, when you start tackling a piece of land that hasn’t been cultivated recently or hasn’t been looked after; and secondly, when any of your plants show symptoms of a mineral deficiency.
Generally, you should try to achieve a good balance with your soil. However, that balance might be altered a bit for different specific needs. For instance, compost with higher PK levels will promote root growth, so use that for taking cuttings or transplanting seedlings. To get really good flowers, add a little more phosphorus than you’d normally use.
Different types of soil
Part of achieving the best balance of nutrients in the soil is knowing what kind of soil you’ve got to start with. We usually divide soil into six types: clay, sandy, silt, peat, chalky, and loam.
- Clay soils are rich in nutrients, but you need to add organic matter to break up the soil and stop it from compacting or waterlogging. Both oxygen and water move slowly in clay because there’s less room for them to move, so microorganisms and roots have a tough time getting what they need. You may also need to improve your drainage by, for example, digging drainage ditches.
- Sandy soils have very few nutrients and are free draining. You’ll need to add organic matter to bind the soil and improve its ability to hold water. You may also need to add nutrients. But sandy soil is well-ventilated, so once you’ve added organic matter (see the next chapter), your plants will have no problem establishing their root systems.
- Silt can easily be compacted by treading on it or using heavy machinery, but it’s also prone to erosion by wind and water. Adding organic matter can help improve the soil, but you may not need to add nutrients.
- Peat soil is acidic (good for blueberries) and retains moisture, but it generally doesn’t have much in the way of nutrients. You’ll need to add lime if you want to reduce the acidity, plus plenty of organic matter.
- Chalky soils contain lime, so they’re alkaline and won’t support acid-loving plants like blueberries and rhododendrons. On the other hand, grape vines love chalk. You can add sulfur to help make the soil less alkaline (if you have hard water and get scale in the pipes, you probably have chalky soil, too).
- Loam is the ideal soil, containing a balance of all the different elements. However, you’ll still need to add organic matter regularly.