Most people are well aware that plants need sunlight and water to grow. However, what is less well understood is that they also require a range of nutrients in order to survive and thrive. Gaining at least a basic understanding of the needs of your plants can help you understand how to care for them better. While pests and disease are usually more of a threat to your plants, certain deficiencies in key nutrients or certain minerals can also be responsible for certain problems. Learning about plant deficiencies and how to combat them can help you when maintaining your polytunnel garden.
Keeping a close eye on your plants and knowing them well is the first stage and most important factor in maintaining a healthy garden. A full-size polytunnel or walk-in fruit cage can make it easier for you to get up close to all your crops and see quickly when anything is awry. Having easy access can also make it easier for you to take care of the soil and your plants and maintain your garden.
What Key Nutrients and Minerals Do Plants Need?
In addition to the carbon, oxygen and hydrogen plants obtain from the air, all plants need three key nutrients from the soil for their survival. These are:
When plants are lacking in one of these three key nutrients, they can fail to thrive and may not even survive. In addition to these three nutrients, plants also require:
- and calcium
in large quantities.
In addition to these essential nutrients, plants also require smaller quantities of:
- and zinc.
In all, then, there are 17 nutrients that plants need to acquire from the air and from their growing medium (usually the soil). Signs that plants are deficient in one or more of these nutrients can range from a slight failure to thrive, right through to plant death. How badly plants are affected will, of course, depend on the degree to which they are deprived of the nutrient or nutrients that they need.
Plant deficiencies are strongly linked with soil quality. Where the soil texture is poor, where soil lacks sufficient water retention, or is prone to waterlogging, nutrient uptake can be reduced and the soil’s nutrient profile can be compromised. Soil pH also plays an important role in how easily and effectively plants are able to take up available nutrients from the soil around them. Finally, other environmental factors such as excessive rainfall (or watering), and temperature can also be involved in plant deficiencies.
Before adding any additional minerals, therefore, it is first important to analyse the soil and surrounding environment to work out what may be contributing to, or indeed be the root cause, of the problems.
Why Is Identifying Plant Deficiencies Important?
Identifying plant deficiencies can play an important role in informing the decisions you make with regards to your planting plan. Knowing why plants are not thriving can also help you to form practices to improve soil quality and feed plants. When using a polytunnel to intensively produce food all year round, without perfect practice, it can be easy for nutrient deficiencies to creep in over time. The good news is that these deficiencies are generally easy to remedy with organic gardening practices.
Plant Deficiencies You May Encounter in Your Plants:
Below is a list of some plant deficiencies that you may encounter in your plants. Some of the plant deficiencies below are more common than others, but all are potentially possible in gardens in the UK, and could be considered in a ‘differential diagnosis’ if you encounter a problem.
Nitrogen Deficiency in Plants
Nitrogen promotes healthy green, leafy growth and while it is important for all plants, leafy vegetables are particularly hungry for this key nutrient. A nitrogen deficiency often results in yellowing of the leaves. Sometimes leaves will also have a pinkish tint and plants can look weak and spindly. If leaves have yellowed after heavy watering, nitrogen deficiency can often be the cause, since nitrogen is very soluble and is easily washed out of the soil by over watering.
Potassium Deficiency in Plants
Potassium is required for controlling water uptake and for photosynthesis. It promotes good flowering, fruiting and the general hardiness of your polytunnel plants. Plants suffering from potassium deficiency will tend to have yellow or purple leaf tints, browning at the leaf edges and will not flower or fruit well. Potassium shortage is more likely to occur on free-draining chalky or sandy soils, where the water soluble potassium is easily washed away, while clay soils are better at holding potassium within their structure. Potash, comfrey-based organic tomato feeds and other organic sources of potassium such as banana skins can be added to remedy the problem.
Phosphorus Deficiency in Plants
Phosphorus is the third main nutrient required by plants. It is essential for healthy growth of shoots and roots. Phosphorus deficiencies are rare in the UK but may occur occasionally where there is heavy rain and clay soil. Symptoms include yellow, dull foliage and slow growth. Adding bone meal can remedy the deficiency.
Calcium Deficiency in Plants
Calcium is one of the most important nutrients for plant growth. Acidic and sandy soils often contain less calcium, though more commonly, calcium deficiency is caused not by a lack of calcium in the soil but by water shortages or high nitrogen levels in the soil which lead to poor uptake. High phosphorus soils can be prone to creating insoluble forms of calcium. Calcium deficiency can cause die back of certain parts of a plant, such as blossom end rot in tomatoes. Generally improving soil with organic matter, ensuring good watering practice and liming acid soils can help with this problem.
Magnesium Deficiency in Plants
Magnesium is needed by plants to create healthy leaves and for photosynthesis. It is not uncommon to see magnesium deficiency in tomatoes grown in polytunnels with light and sandy soils. The over-use of tomato fertilisers that are high in potassium can cause magnesium deficiency, as plants take up potassium in preference to magnesium. On plants with a magnesium deficiency you will see yellowing between the veins of plant leaves, and sometimes reddish brown tints and early leaf fall. Epsom salts can be used as a foliar feed in summer to remedy this deficiency.
Sulphur Deficiency in Plants
Sulfur is one of the molecular building blocks for a number of different proteins, vitamins and hormones in plants. It also plays an important role in water equilibrium in plants (and in soil). Sulfur deficiency is uncommon with in-ground growing, though can occasionally develop with plants grown in containers. With sulfur deficiency, you may see de-colouration of the leaves and a strong purple colouration in the leaf stems. Lowering the pH of the growing medium and adding sulphur are the main ways to remedy this deficiency.
Manganese & Iron Deficiency in Plants
Manganese and iron deficiencies are uncommon in UK soils but can be seen where the soil is particularly alkaline. These nutrients are required in order for photosynthesis to take place. With these plant deficiencies, you will usually see yellowing between leaf veins and browning of leaf edges on ericaceous (acid-loving) plants. Chelated iron and manganese treatments are available, though redressing the pH problem for particular plants will usually help.
Molybdenum Deficiency in Plants
Molybdenum is needed for a range of different plant processes, though only in tiny quantities. Soil shortages of molybdenum in UK soil is also rare, yet can sometimes be seen in cauliflowers and other brassicas when these are grown in insufficiently alkaline soil. Brassica with this problem will have elongated, twisted leaves. Liming the soil to make it more alkaline can help to solve this problem in the long term.
Boron Deficiency in Plants
Boron is required for healthy cell formation in plants. It is rare for soil shortages in the UK, though this nutrient can become less available in alkaline soil conditions. If you have a boron deficiency in your polytunnel you may see stunted growth and tip dieback on lettuce, brown cracks in celery, or rotting swedes, turnips or celeriac. A boron foliar spray is available.
It can often be difficult to determine which deficiency plants are suffering from, as many have similar visual symptoms. That said, an understanding of the surrounding conditions can often help to narrow down root causes and get to the bottom of the problem.
If you believe you have a plant deficiency and wonder if anyone can help to identify it, or have tips to share regarding dealing with a plant deficiency you have encountered, please feel free to comment below.
Elizabeth Waddington is a writer and green living consultant living in Scotland. Permaculture and sustainability are at the heart of everything she does, from designing gardens and farms around the world, to inspiring and facilitating positive change for small companies and individuals.
She also works on her own property, where she grows fruit and vegetables, keeps chickens and is working on the eco-renovation of an old stone barn.