Written by Ed Trotter
Understanding Calcium Beyond Soil pH
Calcium is often overlooked as a nutrient, especially when talking about it as nutrition for plants. Instead, it is far more common to discuss soil pH and liming. Soil with a low pH has essentially a higher concentration of hydrogen ions which means calcium is much less prevalent. When calcium is at correct levels, pH is adjusted to the sweet spot where plants like to grow and nutrients are more available. At pH 6.7 soil microorganisms thrive, and soil structure performs well. A soil at pH 6.0 is ten times more acidic than a Ph 7.0 soil, and pH 5.0 is one hundred times more acidic due to the logarithmic scale.
Soils are constantly losing Ca and Mg ions through leaching and acidification of the soil. Heavy rain can cause losses of these nutrients over time and allow hydrogen ions to take their place. Annual application of ammonium fertilisers also pumps soils full of hydrogen ions.
Soil Structure and the Process of Flocculation
You might have heard of flocculation, it is the process where fine particles are induced to aggregate and form larger, looser clusters known as flocs. Calcium is a primary promoter of flocculation, acting as a cationic bridge that brings negatively charged particles together. This is how gypsum helps open a soil which is dominated by ‘sticky’ magnesium, by draining it out and replacing it with calcium. You can get over-flocculation from too much calcium, particularly in lighter soils which then slump leading to a dense, compact soil.
The Physiological Importance of Calcium in Plants
Calcium equally has very important roles in the plant. It is largely involved in the structural integrity of cell walls and membranes, signalling pathways and enzymatic activities.
Strategies for Liming and Calcium Application
Liming increases soil pH by adding substances like calcium carbonate, which neutralise acidic hydrogen ions (H+) in the soil. Soils must be tested every 5 years as a minimum for compliance rules, but it is useful, particularly on lighter soils and those which have a history of requiring lime to test more regularly.
Synthetic and Compound Fertiliser Options
Some growers have shifted to applying calcium through CAN (calcium ammonium nitrate), to supply some in the spring as well as applying nitrogen. This does come at a premium and the rate of calcium is 8% from a standard 27% N product, so one split of 60kgN will only provide just under 18kg/ha of CaO. This is a healthy top up but not sufficient to adjust soil pH. This product could be very useful placed near the seed at planting with an applicator or used as part of a whole programme. Brassicas and higher pH loving crops like barley would be most suited to this type of product.
Polysulphate is also a useful supply of CaO, as growers are already using it for the S and K. It contains 17% CaO and used at 100kg/ha provides 17kg/ha CaO, as well as 48% SO3 and 14% K2O. Calcifert lime has a very high CaO content at 55% and Calcifert S 39%, which makes them some of the highest concentration calcium products you can apply through a fertiliser spreader. To raise soil pH with calcifert lime would be very costly, but as a top up on specific sites and soils it could be quite beneficial, especially applied in the row at drilling.
Bulk Liming Materials
Agricultural limestone is the cheapest product for reducing pH significantly at approximately 50% neutralising value (NV). It can only be spread by large, rear rotary spreaders. Limex, a similar type of product comes as 45 or 70 grade which is 25% and 29% NV value respectively. Both these products have a significant amount of P, Mg and S within them as an added benefit of application but come at a premium due to much higher application rates for liming.
Comparison of Calcium Sources for Soil Liming
| Ag Lime | Calcium Ammonium Nitrate (CAN) | Polysulphate | Calcifert Lime / S | Limex | |
|---|---|---|---|---|---|
| Calcium content | Approximately 50% NV | 8% CaO | 17% CaO | 55% CaO (lime), 39% CaO (S) | 25 – 29% NV |
| Cost | Cheapest CaO | Premium | Moderate, but low CaO | Premium | Premium |
| Application | Large rear rotary spreaders | Tractor spreader | Tractor spreader | Tractor spreader | Large, rear rotary spreaders |
| Other Nutrients | None | Nitrogen | S, K, Mg | Sulphur (Calcifert S) | Phosphorous, Magnesium, Sulphur |
| Use Case | Raising soil pH | Top-up calcium with nitrogen | Supplying S and K | Top-up on specific sites | Raise Ph, P, Mg, S |
Crop-Specific Responses to Soil pH
Crops react differently to soil pH, the crops which suffer the most from acidic soil are brassicas, beans (legumes), beets and barley – all the b’s. Crops like potatoes are quite tolerant, as well as oats and grass, which can still thrive at pH 5.5 to 6 but any lower will cause significant problems. However, testing for calcium in these crops will still be important and applying it possibly still beneficial. Wheat will struggle once pH drops to around 6 and below.
Nutrient Antagonism and Deficiency Risks
Low soil pH means less available calcium for plants due to displacement. Equally plants will struggle to access other nutrients particularly phosphorus, magnesium, molybdenum, and very importantly nitrogen, due to chemical binding and reduced microbial activity. Conversely, toxicity increases for aluminium and manganese, meaning that applications of manganese can be very detrimental to the crop in acidic soils. So, it is imperative that diagnosis is correct when crops appear yellow. Check plant tissues or sap to ensure accuracy.
High calcium is antagonistic in soil to phosphorous, potassium, magnesium, manganese, boron, zinc and iron. Phosphate is well known for lock up (chemically binding to calcium) whilst positively charged cations like potassium and magnesium are displaced and compete for room in the soil. Manganese, boron and zinc are all less available as pH rises, which makes it important to apply them through the leaf to correct deficiency.
The Benefits of Foliar Calcium
Foliar calcium is becoming increasingly viewed as a solution for providing strength to cell walls and therefore bringing wider benefits than just yield, such as standing ability, disease and pest resistance. A prime example is clubroot in brassicas such as OSR, where applying calcium can bring excellent results.
Conclusion: A Balanced Approach to Agronomy
Testing the soil, plant and grain gives a season long view of where the plant is, in terms of nutrition. Getting the balance right will provide wider benefits to soil health and in turn plant health to ultimately give better crop performance and return. Growing healthy crops can be made cheaper by reducing extra sprays and improving input efficiency. Understanding the knock-on effects of low nutrient levels for other factors like soil and plant health, shows how your system can function with less artificial intervention.
