Identify Nutrition Deficiencies in Your Vegetable Plants
When you grow vegetables organically it is sometimes a struggle to obtain and incorporate a sufficient amount of compost and other nutrient-rich organic matter into your soil to maintain its health and fertility. You must be diligent to "feed your soil" after every harvest to sustain the soil micro-organisms that drive the life cycle of the soil and your vegetable crops. Even then there will be circumstances when your soil or your crops may suffer from a nutrient deficiency. Fortunately, if you are alert to the symptoms of nutrient deficiency you can act quickly to correct the problem without significant harm to your harvest.
Nitrogen, phosphorus, and potassium are the three all-important "macro" elements for plant growth and each present distinctive, common symptoms of nutritional deficiencies or excesses.
Nitrogen is one of the three primary nutrients required by plants in relatively large quantities and is necessary for healthy leaf growth. If it is deficient in the soil, or simply unobtainable by the plants, then your vegetables will suffer visible symptoms of nitrogen deficiency.
Nitrogen deficiencies are fairly easy to identify because they affect the entire plant:
- Growth is poor and stunted
- Small leaves
- Yellowing foliage
- Limited shoot growth
- Reduced flowering
- Small, poor fruit
In the Brassicas (cabbage family), older leaves commonly develop a red, orange, or purple tint.
Signs of Excess
- Rampant leaf growth
- Dark green color
- Succulent leaves, and stems
- Little or no fruit
The lack of fruit is due to the plant taking up the excess nitrogen and storing it in its leaves for future use. While this may be prudent of the plant, it does little to advance the main interest of the gardener, which is a nice harvest of vegetables.
Since plants take most of their nutrients from the soil, the most common causes are a deficiency of nitrogen in the soil, or soil conditions that make the nitrogen unavailable to the plants. If you add sufficient supplies of nitrogen sources at the beginning of the season, remember that nitrogen is removed from the soil by intensive cropping more than any other nutrient. Consequently, if you succession plant or inter-crop it is important to replace nitrogen between crops with organic matter or soil amendments.
Big nitrogen feeders (such as tomatoes, spinach, cabbage, and lettuce) also appropriate most of the nitrogen around them, requiring the gardener to replenish it mid-season. I amend my soil with compost and amendments in November before I put in my lettuce crop. When I test the soil in March I usually can't get even a trace of nitrogen. So I put in a crop of peas, which actually have the ability to fix nitrogen in the soil with the assistance of a certain bacteria from the genus Rhizobium, and then amend the soil with more compost and steer manure just before I put in the tomato crop. Then I "side dress" with fertilizer throughout the season to ensure the tomatoes receive a continuous supply of the nutrients they need.
It is possible, however, that your soil contains nitrogen but conditions simply will not allow the plants to obtain it. Cold or excessively wet soil makes nitrogen unavailable because the soil microorganisms that break down organic matter to nutrient forms plants can use are made sluggish or inactive in such conditions. An alkaline (high pH) soil also limits the availability of nitrogen, and acidic soils limit the activities of the micro herd. Lastly, plants need adequate amounts of other macro-nutrients, such as calcium and phosphorus, to uptake and process nitrogen so a nutrient imbalance in the soil may be the culprit. Consider your soil conditions and maybe have the soil tested before you just start adding more nitrogen.
Sources and Remedies
The best source of nitrogen is organic matter or compost. This, however, is somewhat slow acting because the micro herd must break it down before your plants can use it. Other good sources of nitrogen are green manures, blood meal, alfalfa meal, and fish emulsion.
Long term, apply compost, manure, alfalfa meal, or cottonseed meal to your beds in the spring a week or two before planting. Do not add nitrogen to the soil in the fall if you intend to let the soil lie fallow. Nitrogen is easily leached out of the soil by rainfall so you will waste your efforts and your resources.
Fish Emulsion and Foliar
If your plants are in immediate need of a nitrogen fix, the best thing you can do is mix up a batch of fish emulsion and foliar to feed them. Don’t go overboard on this because, as explained, a nitrogen excess is almost as bad as a deficiency. For non-leafy crops, a weekly nitrogen feed should be enough when combined with a “top dressing” of blood meal or alfalfa meal placed around the plant’s root line. The foliar feed will give the plants an immediate boost and the slow release of the top dressing will provide slow and steady growth.
Phosphorous is one of the three primary nutrients required by plants in relatively large quantities and is essential for cell division, strong stems, and good root development. Phosphorous is the most important nutrient for seedlings when they are growing quickly.
Initial symptoms of phosphorus deficiency are not very distinctive or dramatic: Typically, it shows up as a red or purple tint on leaves. The first time I grew tomato seedlings I was delighted with the beautiful, delicate purple hue of their young leaves. I thought that was simply the way they looked. Only later did I learn they were suffering from a classic case of phosphorous deficiency.
Later on, symptoms may manifest themselves as:
- leaf bronzing and mottling
- thin stems
- Stunted or slow growth
- Little or no fruit
Phosphorous readily combines with iron and aluminum to form insoluble phosphates, so even if it is present in the soil it is mostly unavailable to plants, whose roots can only absorb a soluble mix of phosphorous ions and water. So the problem is not a soil deficiency but rather getting the soil to release the phosphorous. Soil pH is also important because phosphorous only becomes available when the soil is close to neutral.
Sources and Remedies
It is difficult to deal with a phosphorous deficiency because it is so slow to release and become available to plants. It is frequently recommended that you ensure seedlings get as much phosphorous as you can provide to fuel their rapid cell division and growth, and to get a good jump on root system development. Plants with vigorous root systems can forage farther and deeper once planted out, so they have a better opportunity to locate available phosphorous.
The most common organic fertilizer that supplies phosphorous is rock phosphate. The finest ground variety is known as colloidal phosphate, which becomes available to plants the fastest. Unlike other amendments, you can’t use too much rock phosphate so don’t skimp.
There are two quick fixes for deficiencies. One is to foliar plants weekly with fish emulsion until symptoms abate. You can also try making bat guano or worm casting “tea” and applying it as a soil drench. Use this in conjunction with a top dressing of bonemeal, which is phosphorus-rich. Be sure to thoroughly “water in” the top dressing.
For a long-term fix, mix rock phosphate or aged poultry manure into the soil and your compost pile in fall. This gives the micro herd time to work its magic and also leaches some of the nitrogen out of the poultry manure, which is often too nitrogen “hot” to use in the spring.
Potassium is one of the three primary nutrients required by plants in relatively large quantities and is essential for photosynthesis, protein formation, and good root growth.
Symptoms of potassium deficiencies are among the most dramatic of the plant nutritional disorders. Yellowing starts on older leaves and progresses to the younger ones
- Leaf edges are brown and scorched
- Leaves roll inward
- Interveinal yellowing
- Little or no fruit and weak stems
The first time I saw symptoms of potassium deficiency in my green beans I thought they were suffering from sunburn and covered them up! Obviously, this did no good at all!
Symptoms of excess are similar to those of nitrogen excess: Succulent growth that emphasizes leaves instead of fruit. Succulent growth makes the plants more attractive to insect pests so they suffer from attacks, become weakened, and they are vulnerable to disease. Not a pretty picture.
Potassium is a naturally-occurring nutrient in most soils and is easily available to plants because it is so soluble. Unfortunately, that is also its downfall; potassium is so soluble it is easily leached from the soil by rainfall or irrigation water. Potassium also prefers a neutral soil and may not be available in acidic soils with a pH of 5 or less.
Sources and Remedies
The best-known sources for potassium are greensand and granite dust. Greensand looks just like you might think. It is naturally occurring and mined from locations once covered by oceans. Granite dust is valuable because it can supply both immediate nutrients and slow release nutrients so plants get a meal today and more next week. Greensand takes more time, so it is good to add a season ahead of time and to add it to the compost bin. There are a few other remedies as well:
Another good source of potassium is wood ashes—the kind you can scrape out of your fireplace. Just ensure you don’t burn any plastics or other contaminants with the wood. Blackwood ashes are best, but the grey variety will also do. Unlike the rock powders, wood ashes are highly soluble. Use it sparingly! An overabundance will release more potassium then is required, which will raise the soil pH. Fortunately, it also leaches from the soil just as fast. If you over apply wood ashes, a good watering will correct the situation.
For a quick fix spray potassium-deficient plants weekly with fish emulsion until symptoms abate. For the long term apply granite dust or greensand to the soil in fall. Hardwood ashes may be applied to soil anytime. Just sprinkle it on the surface around the root line and lightly water in.
Unfortunately, there are other deficiencies that will cause your plants grief including calcium, magnesium, and boron. Be sure to keep feeding the soil after every harvest and "in between" if the crops require a boost.
See you in the garden!