Lupin Leaves Turning Purple: What You Need To Know

Lupin leaves that are turning purple indicate a nutrient deficiency in the plant. Healthy lupin leaves should be green, with short silver hairs on them. Any deviation in color or appearance could indicate a nutrient deficiency, fungal or viral infection, and therefore must be diagnosed and treated.

Lupin leaves turn purple as a result of a phosphorous deficiency. Smaller leaves and shorter roots also characterize deficient plants. This phosphorus deficiency impedes the plants’ ability to grow healthy roots, stems, and leaves, and in severe cases, it will lead to the death of lupin plants.

The rest of this article will explore the importance of phosphorus in growing lupins. We will also look into the effects of phosphorus deficiency in lupins and the methods you can use to correct this deficiency and grow healthy lupins.

Table of Contents

Why Do Lupin Leaves Turn Purple?

Lupin leaves turn purple when they store excess carbohydrates, resulting from imbalances caused by a phosphorus deficiency. Additionally, phosphorus deficiency also causes overproduction of anthocyanin, which causes the leaves to attain a purple or reddish hue.

Phosphorus is an essential nutrient for plant growth, and it is a crucial component in the following plant processes:

Stimulates shoot, root, and stem growth
Energy reactions within the plant (including phosphorylation)
Transport of nutrients within the plant
Formation of flowers and seeds
Nitrogen fixation in root nodules
Genetic transfer and new cell formation in the plant
Resistance to diseases
Timely maturity of the plant

Lupin leaves synthesize carbohydrates through the process of photosynthesis. These carbohydrates provide the energy needed for the plant to grow and conduct other essential functions such as nutrient uptake from soil.

ATP is a crucial phosphorus compound that converts carbohydrates to energy so nutrients from the soil can move through the plant’s cell walls.

When the lupin plant lacks sufficient phosphorus, they do not use the synthesized carbohydrates entirely, causing them to accumulate in the leaves. Carbohydrate accumulation results in darkened leaves.

Phosphorus Deficiency and Anthocyanin Production in Lupins

Overproduction of anthocyanin in lupin leaves causes them to attain a purple or reddish pigmentation. This overproduction can point to phosphorus deficiency in the plant.

Separate studies conducted by scientists on soybean (which, like the lupin, belongs in the legume family of plants) and maize plants indicated that phosphorus deficiency in plants causes increased anthocyanin production by promoting the genetic expression of genes involved in synthesizing anthocyanin.

Anthocyanin is useful in lupins for seed dispersal and growing plant tolerance to environmental stressors. However, too much anthocyanin impedes the uptake of much-needed nitrogen by the lupins. Nitrogen is essential for plant growth, and deficiencies lead to stunted growth in lupins.

How Else Can Phosphorus Deficiency Manifest in Lupins?

A combination of factors can diagnose phosphorus deficiency in plants. Usually, the signs appear in older leaves before moving up to newer shoots.

In addition to purple-colored leaves, you may also notice the following signs of phosphorus deficiency in the lupins:

Smaller plants
Underdeveloped leaves
Leaves may start to curl
Stems and petioles are narrow
Older leaves turn brown, droop, and die-off
Slow plant growth
Brown spots develop when the plant is left untreated.
Lupins may also fail to bloom during the blooming season or produce small flowers.

Untreated lupins may eventually succumb and die off, especially in cases where all the leaves have dried up.

Treating Phosphorus Deficiency in Purple-Leafed Lupins

As stated before, phosphorus is an essential nutrient for lupin growth and development. Lupins are known to adapt well to phosphorus-deficient soil, but when the plant exhibits signs of deficiency, you need to increase the phosphoric content of the earth.

Here are some valuable suggestions for how you can achieve this:

Apply Phosphorus fertilizer as per soil test recommendations. A soil test conducted by an agronomist will guide you on how much Phosphorus your soil needs. This information is necessary when applying fertilizer so that you neither under nor over-apply.
Add composted manure to the soil. Compost manure is a great organic source of phosphorus. Additionally, it optimizes the soil pH level to allow effective phosphorus uptake by the plant.
Keep the roots at an optimum temperature above 15 degrees Celsius (59 degrees Fahrenheit). Temperatures lower than this will affect the absorption of phosphorus by lupin roots. If you live in a colder state such as North Dakota or Minnesota, consider using vegetation heating pipes to maintain this.
Use soils with Proper Drainage. Water-logged lupin roots do not receive sufficient oxygen, interfering with their ability to absorb nutrients—plant lupins in well-drained soil. You can enhance drainage by combining the soil with organic matter.
Incorporate bone meal. Yet another organic solution, bone meal enhances the phosphorus content of soil by gradually releasing the nutrient into the ground. It is also safer for the environment as compared to fertilizers.
Apply a mix of Kelp and Fish Meal. Primarily applied as a foliar fertilizer, kelp (seaweed) enriches the soil with phosphorus and nitrogen. When mixed with fish meal, the NPK ratio is increased, making it more beneficial to the lupin plants.

How Long It Takes To Reverse Purple Leaves in Lupins

Lupins take about one week to recover from phosphorus deficiency after treatment. Older leaves are unlikely to recover since they are usually the worst hit and may have died off. However, after a week, you will begin to see signs of recovery, such as healthier leaves, stems, and shoots.

The Problem of Overtreating Purple Lupin Leaves

Too much phosphorus can cause phosphorus toxicity in lupins, so as you treat purple-leafed lupins, this is a potential risk that you should consider. In severe cases, the lupins may die from excess phosphorus’s effects.

Phosphorus toxicity can cause the following effects:

Yellowing or darkening of leaves.
Calcium deficiency in the plants (since excess phosphorus impairs calcium uptake)
Zinc and Iron Deficiency in plants.
Higher susceptibility to plant diseases
Death of lupin plants in severe cases
Water pollution due to run-off during rains.

To avoid overtreating the lupins with phosphorus, conduct a soil test before treatment. Doing this will give you a clear picture of the state of your soil and how much phosphorus you should incorporate. Conduct regular soil tests even after treatment to track the progress of your soil and adjust future therapies as needed.

Spraying foliar zinc and iron on lupins growing in high-phosphorus soils will also mitigate the effects of excessive phosphorus by providing the plants with these much-needed nutrients. However, this method should be combined with other control methods to protect the plant effectively.

How To Reduce Excess Phosphorus in Soil

You can correct excess phosphorus in soil and protect your lupins by following these practical suggestions:

Avoid adding phosphorus in any form. Doing this includes avoiding fertilizers containing phosphorus, organic compost, and manure for as many seasons as it takes for the soil to return to safe phosphorus levels.
Loosen the soil and water often. Phosphorus is water-soluble, so loosening the ground with a shovel and watering the soil frequently will gradually flush out excess phosphorus. Remember to maintain proper drainage in the soil, so the roots do not get waterlogged and start to rot.
Remove rocks where possible. Stones hold minerals slowly released into the soil, so removing these rocks will prevent additional phosphorus from getting into the ground.
Chemical treatment. Incorporating chemicals such as alum and ferric chloride has also been suggested as a suitable technique to reduce phosphorus levels in the soil.

How To Maintain Healthy Lupins

Proper care for lupins is necessary to keep them healthy and promote plant growth. It also ensures they bloom beautifully during the blooming season and yield good yields. Here are a few pointers to help you maintain healthy lupins:

Maintain slightly acidic soil pH for optimum nutrient uptake.
Keep the soil well-watered and well-drained to prevent dehydration or root rot.
Apply a light mulch to the base of the plant to protect it from adverse heat or cold, which can negatively affect the plant.
Light is essential to healthy lupin growth, so if you keep potted lupins, ensure they receive at least six hours of direct sunlight daily.
Inspect the plants regularly for appearance, size, or color changes that may indicate a nutrient deficiency or plant disease.
Regularly spray the lupins with pesticides and fungicides to control the spread of pests and fungi, inhibiting plant growth.
Prune regularly to encourage root and shoot growth since removing overgrown stems will direct the plant’s energy toward new shoots and roots.

Final Thoughts

Purpling of lupin leaves indicates a nutritional deficiency in the plant, which results in excess carbohydrate storage and overproduction of anthocyanin. Phosphorus deficiency in plants can cause slow growth, deformed leaves, and thinner and smaller plants.

Treating phosphorus deficiency in lupins is a delicate process because, when overdone, it can lead to phosphorus toxicity and, in severe cases, plant death. The most effective way to navigate this process is by conducting regular soil tests to understand the needs of your soil and treat it appropriately to encourage lupin growth.