You can add the right fertiliser at the right time, and still get a poor crop, if the soil pH is wrong.
Soil pH measures how acid or alkaline the soil solution is. It is often called the master variable of soil chemistry, because it controls which nutrients a root can reach, which elements become toxic, and which microbes are active.
Acid soils cover roughly a third to a half of the world's potentially arable land, especially in the humid tropics.
And liming them is one of the oldest and most profitable soil improvements there is. In this video: what pH means and how it is measured, why soils turn acid, how pH controls nutrients and aluminium toxicity, which crops tolerate what, why a pH test alone cannot give a lime dose,
liming materials, a worked lime sum, good practice and safety, and lowering pH.
The advice is general and global.
Your soil test and your local extension service come first.
The pH scale is logarithmic, so each whole unit is a tenfold change. A soil at pH five has ten times the acidity of one at pH six,
and a hundred times that of one at pH seven.
Below about four and a half is extremely acid, as in acid sulphate soils and peats.
Four and a half to five and a half is strongly acid, common in humid tropical uplands and leached sands. Five and a half to six and a half covers most productive humid-region farmland. Six and a half to seven and a half is near neutral. Above that, soils are alkaline, like the calcareous soils of dry regions,
and above eight and a half they are often sodic, with too much sodium.
Laboratories measure pH with a glass electrode,
usually in one part soil to two and a half parts water. Many also measure it in a weak calcium chloride solution,
which gives a steadier reading, usually about half a unit lower.
So always check which method was used before you compare two numbers.
Acidity is a natural result of rain and plant growth, and farming speeds it up. Where rainfall is higher than evaporation, water washes out calcium, magnesium, potassium and sodium, and hydrogen and aluminium take their place.
Fertiliser plays a part too. When soil bacteria turn one ammonium ion into nitrate, they release two hydrogen ions. So ammonium-based fertilisers and urea acidify the soil, steadily, year after year.
Harvested crops also remove calcium and magnesium,
especially forages and legumes.
Decomposing residues add organic acids.
And when coastal marshes containing pyrite are drained, the sulphides oxidise into sulphuric acid, and the soil can fall below pH three and a half. These are acid sulphate soils.
Soils of dry regions stay alkaline, because there is too little water to wash the bases out, and calcium carbonate builds up.
Here is why pH matters so much. Most nutrients are most available between pH six and seven, which is why that range suits most crops.
Phosphorus is most available around six to seven. In acid soils it is locked up by iron and aluminium,
and in calcareous soils it forms calcium phosphates. Iron, manganese, zinc and copper become less soluble as pH rises,
so iron chlorosis, yellow young leaves with green veins, is common on calcareous soils.
Molybdenum is the exception: it becomes more available as pH rises, so molybdenum deficiency in legumes and brassicas is a problem of acid soils.
Calcium and magnesium are scarce in strongly acid soils, simply because they have been washed out.
The biggest danger is aluminium.
Below about pH five and a half, aluminium dissolves,
and it is toxic to roots.
Root tips stop growing, and become stubby and brown, so the plant cannot explore the soil for water and phosphorus.
Manganese toxicity can appear too.
And bacteria, including the rhizobia that fix nitrogen for legumes, work poorly in strongly acid soil.
Crops differ in what they tolerate.
Tea, blueberry, cranberry and pineapple prefer acid soils.
Potatoes are often grown slightly acid, to reduce common scab. But lucerne, also called alfalfa, barley and many legumes are sensitive to acidity and aluminium.
So a low pH is not always bad. The harm comes mainly from aluminium and manganese toxicity,
and from too little calcium, magnesium, phosphorus and molybdenum, and it hits sensitive crops hardest.
Know your crop before you decide how far to lime.
The hydrogen ions a pH meter measures are only a small part of a soil's acidity. That part, in the soil solution, is called active acidity, and it is tiny.
Behind it is a reserve of aluminium and hydrogen held on clay and humus, which moves into solution as soon as the active acidity is neutralised.
The reserve is often a thousand times larger than the active pool. This is buffering: the soil resists a change in pH.
It is why a clay or organic soil needs much more lime than a sand to raise its pH by the same amount.
And it is why a pH test alone cannot give a lime dose.
Laboratories use a buffer pH test, or the soil's cation exchange capacity, to calculate it.
Liming materials are compounds of calcium or magnesium that neutralise acid. When calcium carbonate reacts, the calcium replaces acidity on the clay,
and the toxic aluminium is locked away as harmless aluminium hydroxide.
Materials are compared by their calcium carbonate equivalent, how much acid they neutralise compared with pure calcium carbonate at one hundred per cent. Ground limestone is the reference, slow but safe.
Dolomitic limestone is about one hundred and nine per cent, and also supplies magnesium.
Slaked lime is about one hundred and thirty-five per cent, and fast.
Quicklime is about one hundred and seventy-nine per cent, the fastest.
And gypsum supplies calcium, but it does not raise pH at all.
Fineness matters as much as chemistry.
Lime reacts only at the surface of its particles, so finely ground lime acts within months, while coarse grit may take years.
Wood ash works as lime too, often with thirty to seventy per cent of the value of calcium carbonate, but its strength varies, so use it with a soil test.
Here is a worked lime sum.
A soil test reports a cation exchange capacity of twelve,
and a base saturation of fifty per cent. The farmer wants eighty per cent base saturation in the top twenty centimetres.
The soil needs thirty per cent of twelve, which is three point six units of calcium.
Calcium carbonate supplies one unit per half a gram, so that is one point eight grams of calcium carbonate per kilogram of soil. One hectare to twenty centimetres, at a bulk density of one point three, weighs about two point six million kilograms. So the soil needs about four point seven tonnes of pure calcium carbonate per hectare.
Farm lime is not pure. With a lime of ninety per cent value and eighty per cent fineness efficiency, the effective value is seventy-two per cent. So apply about six and a half tonnes per hectare, usually split over two seasons, and mixed into the soil. Your own numbers will differ: use your laboratory's recommendation.
Test first, and lime to the crop's need, not to neutral.
Most crops do well at pH six to six and a half. On many weathered tropical soils, the yield benefit stops once the aluminium is neutralised, often around five and a half to six, so they are limed only that far.
Apply lime well before sowing, and mix it in, because lime moves very slowly down the soil.
Avoid overliming: pushing a weathered soil above pH seven can cause deficiencies of zinc, manganese, iron and boron, and lower phosphorus availability.
Do not mix lime with ammonium fertilisers or fresh manure at application, because ammonia is lost to the air.
And where acid subsoil limits the roots, gypsum can carry calcium down and reduce subsoil aluminium, even though it does not change the pH.
A word on safety.
Quicklime and slaked lime are caustic, and fresh wood ash is too.
Protect your skin and eyes when you handle them, and spread on a calm day.
Alkaline soils are harder to change, especially when they contain free calcium carbonate, which buffers them strongly.
Elemental sulphur, which soil bacteria turn into sulphuric acid, is used to acidify soil for acid-loving crops,
and acid-forming fertilisers help slowly.
Sodic soils are a different problem.
There the trouble is sodium, and the remedy is gypsum and leaching, not acidification.
To recap.
One pH unit is a tenfold change, and the method matters: water or calcium chloride.
Soils turn acid through leaching, ammonium fertilisers,
crop removal and drained sulphide soils.
Most nutrients are most available at pH six to seven,
and aluminium turns toxic below about five and a half. Buffering is why the pH test is not the dose: use a buffer test or the exchange capacity. Compare limes by calcium carbonate equivalent and fineness,
and remember that gypsum does not raise pH.
Lime to the crop's need, before sowing, and do not overlime.
For grow guides, the pest and disease encyclopedia,
and more soil explainers, visit laisha.ai. Thank you for watching.
