Ruth Vaughan, Technical Director Crop Nutrition Laboratory Services Ltd.
Boron is the first element in Group 13, the aluminium family, of the Periodic Table. It is quite different from the other members in that it is not a metal and is classified as a metalloid. Boron has been used for centuries to make glasses and glazes and more recently is used in agriculture, fire retardants, soaps and detergents, alloy manufacture, medicines, nuclear power stations and insecticides.
It is naturally found in the earth’s crust at an average of 10 ppm, and in surface, ground and seawater. High boron in water tends to come from human activity and pollution.
Boron is an essential micro-nutrient for certain organisms, notably vascular plants and diatoms. In addition, boron is required by nitrogen fixing bacteria and is beneficial in animals. Boron is required in very small amounts and deficiencies can cause serious growing and yield problems.
What boron does it do in plants?
Boron is essential for many plant functions, including but not limited to the following roles: –
1) Boron is used with calcium in cell walls and is essential for proper cell wall formation, functioning and strength.
2) Boron requirements are much higher for reproductive (generative) growth as it is important in pollination and seed production. Low boron often manifests itself at flowering as bud abortion (flower drop), low seed set and deformed fruits.
3) Boron plays an important role in the proper functioning of cell membranes. This controls the transport of potassium to the guard cells in the stomata for proper control of the internal water balance in plants. Boron regulates the movement of sugars through plant membranes.
4) Boron is important in the translocation of sugars and carbohydrate, and in maintaining the balance between sugars and starched.
5) It is essential for normal cell division, nitrogen metabolism and protein formation
6) Boron is used in RNA, DNA and some plant auxins.
Unlike other micronutrients (e.g. iron, manganese), boron deficiency does not cause plant chlorosis. It is used for cell division and a deficiency results in cell wall abnormalities. Deficiency symptoms first appear at growing points of the shoot and roots and in the flowering and fruiting structures. Often terminal buds die and shoot internodes shorten, leading to stubby, distorted new growth emerging from side shoots causing a bushy rosetted look. Stems may be brittle, fragile, hollow or cracked. Leaves may be smaller, thicker, distorted & cupped upwards, with slight yellowing/mottling and the tips turning brown or black and dying off. Roots are often short and stubby with few root hairs, with reduced nutrient uptake.
Most of a plant’s boron is bound in its cell walls, so it is not very mobile in plants. Within minutes a temporary boron shortage in plant sap can cause distortion of the growing points of the pollen tube tips resulting in flower drop / bud abortion.
Boron helps transports sugars around the plant and a deficiency causes a reduction in exudes and sugars from plant roots – this can affect attraction and colonization of mycorrhizal fungi and other beneficial microbes in the root zone.
A boron deficiency will not kill plants and generally allow plants to grow to a reasonable size, but just in a weaker state with reduced plant development and result in non-productive plants.
Boron is a very sensitive micronutrient, there is a very fine line between too much and too little. Apply too little and you have a deficiency. Apply too much and you have a toxicity. A toxicity manifests as strong leaf deformation, leaf yellowing or mottling, gum spots underneath leaves and premature leaf drop. The leaves of many types of plants can be damaged by foliar sprays of boron and it better to apply it to the soil / media solution.
What causes boron deficiency?
Low boron in the root zone and fertilizer program. Do a complete soil and irrigation water analysis to assess the boron status in your area and apply if required.
Soil pH. Boron can easily get locked up at higher pH, at pH>7.5 in soils and pH>6.5 in soil-less media. At low pH boron is more soluble and leaches more easily.
Too much water. Boron is very mobile and leaches easily, especially in low CEC sandy soils, coarse soils and low organic matter soils. Boron deficiencies are most common in acidic sandy soils, over-irrigated crops and during high rainfall periods.
Too little water. Boron is taken up with water through the roots in the transpiration stream. Drought conditions / low moisture in the root zone reduces boron uptake.
Low transpiration. Cool cloudy weather reduces boron uptake due to lower transpiration rates. A boron deficiency causes stunted roots further aggravating boron uptake.
Nutrient in-balances in the root zone: – High calcium levels in the soil or nutrient solution with reduce boron uptake. In cases of boron toxicity, applications of soluble calcium will reduce toxic effects. Applying high rates of potassium in situations where boron is limiting or marginal can further depress boron uptake and yields. High zinc applications can reduce boron accumulation, whereas phosphorous applications can increase boron accumulation.
Nitrogen stress – low nitrogen availability decreases the vigour of the plants to the extent that they cannot take up adequate amounts of other nutrients.
What to do?
Check your soil pH, texture, CEC, organic matter, and boron levels. Check the boron status in the plant with a leaf analysis. Apply measured, targeted amounts of the correct boron fertilizer at the correct crop stage for your plant species. Pay special attention to boron during flowering, fruiting and the rainy season.
While boron is an essential nutrient for all plants, the following crops have been found to be very responsive to boron applications: – lucerne, clover, sunflower, maize, fruit (citrus, mango, pawpaw, apples, peaches, pears), conifers, carrots and celery, broccoli, brussel sprouts, cabbages, cauliflower, peanuts, beets, tomatoes, sunflower, canola and sweet potatoes.
To find out more about boron and check your boron status, please contact us on email@example.com. For boron products and stockists look at the following link:- http://shambaza.com/boron-fertilizers-ws