Is Volcanic Soil Good for Farming? Limits, Crops and Fixes

Is volcanic soil good for farming? Often yes, but not automatically. FAO soil notes describe Andosols, the main volcanic ash soils, as fertile on the whole and easy to till, with phosphorus lock-up as their main limit. Wet, heavily leached ash soils can be acidic and short of nutrients.

So the useful question is which volcanic soil you have and what it needs. This guide explains how these soils behave, what limits them, which crops have measured targets on them, and what ashfall does to a working farm. It also has a calculator that tests whether a phosphorus application could pay on your field.

Is volcanic soil good for farming? The short answer by soil type

“Volcanic soil” covers several different soils. Age, rainfall and parent rock matter as much as the volcano. The table compares types described by soil scientists in Hawaii, New Zealand and at FAO.

Two trusted sources can seem to disagree here. FAO calls Andosols fertile by and large. New Zealand’s soil portal says natural fertility is low in its Allophanic Soils, which form mainly in North Island volcanic ash and other materials. Both can be right, because they describe different groups of soils under different conditions. Physical quality is often excellent, while chemical fertility depends on the site.

What volcanic soil is and where farmers find it

The soil order is called Andisols in USDA Soil Taxonomy and Andosols in the world reference system. The name comes from Japanese words for black soil. The University of Idaho explains that these soils form in volcanic ash or other volcanic ejecta. Much of their behaviour comes from glass and weathering products such as allophane, imogolite and ferrihydrite.

They are rare. Andisols cover about 1% of the world’s ice-free land and about 1.7% of US land, according to the same source. FAO puts the Andosol area at about 110 million hectares, with more than half in the tropics. Its map notes show major areas around the Pacific rim, including Alaska, the Rocky Mountains, Central America, the west coast of South America, Japan and New Zealand. In Europe, FAO lists Italy, France, Germany and Iceland.

In the US, ash from the Mount Mazama eruption about 7,700 years ago shaped soils in the Pacific Northwest. Idaho soil scientists note that US Andisols include very productive forests in that region. Hawaii’s Maui and Big Island also have important farm soils in this order, as the University of Hawaii soil classification chapter explains.

Farming in these places is what people often call volcano agribusiness: growing, processing and selling crops from volcanic landscapes. The term is broad, and it is not a soil class. It tells you little about a field until you have a soil test and a soil map.

Properties that make volcanic soil good for crops

The main advantages are physical. FAO’s description of Andosols gives numbers that a grower can picture.

• Low weight. Bulk density is typically below 0.9 Mg/m³ and can be as low as 0.3 Mg/m³ in highly hydrated soils. Roots meet little resistance.
• Many pores. The pore share rises from about 50% to more than 75% of soil volume as the material weathers.
• Organic matter. The surface layer averages about 8% organic matter, and the darkest profiles can reach 30%.
• Drainage. FAO says most Andosols drain very well inside the profile, helped by high porosity and high ground.
• Water supply. Available water is generally greater than in other mineral soils.
• Stable crumbs. Good aggregate stability and permeability give some resistance to water erosion.

University of Hawaii material adds a simple reason for the water and nutrient storage. Amorphous volcanic minerals have a very high surface area, so they can hold large amounts of water and stored nutrients, depending on the degree of weathering.

New Zealand’s soil portal describes similar field behaviour. Its Allophanic Soils are easy to dig, have little resistance to root growth, and have topsoils that stand up to machinery and grazing animals in wet weather. They also carry large populations of soil organisms. Erosion rates are generally low there, except on steep slopes or exposed sites.

Why the idea of rich minerals needs care

Volcanic rock does contain nutrients, and weathering does release them. That does not mean plants can reach them or that they stay in the root zone. FAO says base saturation in Andosols is generally low because of strong leaching, apart from some very young soils and soils in dry regions.

Older blog claims that volcanic soils hold a fixed percentage more phosphorus or potassium than other soils are too broad. Total phosphorus and plant-available phosphorus are different things, and on these soils the gap can be large. Your own test result is the number to use.

▶ Why soil matters to farming: a living system that holds water and nutrients. · 9:58

Where US soils stand: Andisols are a small share

Few farmers work this soil order. The chart shows how small it is beside the larger US orders, using University of Idaho figures. That is one reason general fertilizer advice may not fit it well, so local guidance matters.

Chart

Share of US land by soil order

Percent of US land area, eight selected soil orders of twelve

The phosphorus problem on volcanic soil

Phosphorus is the issue that catches new growers out. Idaho soil scientists say these soils can fix large quantities of phosphorus and make it unavailable to plants. The University of Hawaii says volcanic soils tend to have the greatest phosphorus sorption of all soils, because they hold so much amorphous material.

In practice, the same fertilizer dose leaves less available phosphorus on a volcanic soil than on a moderately weathered grassland soil. The range inside one island is wide too. A University of Hawaii chapter says organic soils among young ‘a’ā lava sorb almost no phosphorus, while weathered ash soils on the Hamakua Coast are among the strongest phosphorus-sorbing soils in the world. On those soils, phosphorus acts more as a soil amendment than as a routine fertilizer.

FAO lists lime, silica, organic material and phosphate fertilizer as ways to reduce the effect. A test tied to your soil type decides how much is worth applying. Follow local rules on phosphorus, since losses in eroded soil can reach streams and groundwater.

Crop phosphorus targets

Crops differ a lot in how much phosphorus they need in the soil solution. The University of Hawaii phosphorus chapter gives levels linked with 80-95% of maximum yield. These were set on particular Hawaiian soil series, some volcanic and some not, so treat them as a guide. The chapter’s point is simple: a soil test tells you little until you match it to the crop.

Could a phosphorus application pay?

The chapter also gives a general equation for a soil whose series is not known: 2.17 × (target Modified Truog test level − current level) gives milligrams of phosphorus per kilogram of soil. It assumes about 2 million lb of soil in the top 6 inches of an acre. The calculator below uses that method. Volcanic soils are often lighter than that and may fix more than the general curve, so ask your lab or adviser for a curve closer to your soil.

Chart

Soil-solution phosphorus linked with 80-95% of maximum yield

mg P per liter of soil solution, crops grown in Hawaii on named soil series

Try the numbers for your own farm below.

Soil acidity and lime on volcanic soil

Many volcanic soils in wet areas are acidic. In Hawaii, the University of Hawaii liming chapter lists common unlimed pH ranges of 4.8-5.8 for the Hilo and Akaka Andisols, 5.2-6.0 for Kula and 5.5-6.5 for Waikaloa. Those ranges alone show why one lime rate cannot fit every volcanic field.

The same chapter gives lime requirement curves for 22 acid soils. We solved the published equations for the six Andisols in that table, for a target of pH 6.0. The answers run from about 1.2 tons of calcium carbonate per acre on Hilo to about 4.3 on Piihonua. The curves start from different pH values, so read the chart as a sign of how much soils differ, and not as a rate for your field.

Other points from that chapter help when planning the job:

• Lime works poorly unless it is mixed well into the soil. Recommendations are often based on a 6-inch plow depth.
• Change the rate in proportion if you work the soil to a different depth.
• Finer material costs more and reacts faster. Check the neutralizing value on the product you buy.
• Soil moisture is needed for the reaction to occur.

A pH reading can also mislead. The University of Hawaii notes that the Hilo soil, heavily depleted of bases, can show pH values approaching 6. That soil may need calcium as a nutrient and not as a pH amendment. Its soil chapter says amendment rates may be 10 times the rates used to correct a calcium or magnesium deficiency, so the wrong diagnosis costs real money. Sulphate forms of calcium and magnesium can work better there because they are more soluble and mobile than lime. Check product labels and local rules, and get professional advice on rates.

Chart

Lime to reach pH 6.0 on Hawaiian Andisols

Tons of CaCO3 per acre, solved from published curves; starting pH differs by soil. General curve combines all 22 acid soils, not only Andisols

Other challenges of farming volcanic soil

Beyond phosphorus and pH, the common limits come from the landscape and from the soil’s unusual water content.

• Steep ground. FAO names steep topography as a serious constraint in places and says Andosols on steep slopes are perhaps best kept under forest.
• Weak wet soils. Strongly hydrated Andosols can have low bearing capacity and be sticky, which limits machinery. Some Hawaiian Hydrudands are 60-70% water by volume below the top 10 inches.
• Drying damage. FAO warns that excessive air-drying can permanently harm water holding, ion exchange and soil structure. In extreme cases the soil falls apart into dust that blows away.
• Erosion after clearing. Highly hydrated types that dried out strongly, for example after forest clearing, can crumble into hard granules that runoff carries off.
• Hardpans under paddy. FAO notes dense hardpans can form in places under long-term paddy rice.

Ground cover, contour work and terraces are common options on slopes. Their cost and benefit depend on the site, so a local conservation adviser is a good first call. In the US that is often your local NRCS office.

Crops on volcanic soil: what the sources show

FAO says Andosols in the tropics grow a wide variety of crops, including sugarcane, tobacco, sweet potato, tea, vegetables, wheat and orchard crops. It notes that sweet potato tolerates low phosphate levels. Paddy rice is a major use in lowlands with shallow groundwater. In Hawaii, one very wet group in this order, the Hydrudands, has supported a sugar industry for more than a century, even though these soils rate as infertile by almost any test. The University of Hawaii says rain and cloud limit them more than nutrients, because fertilizer can correct the low fertility.

The Hawaiian trials give a useful way to sort crops by phosphorus demand:

Coffee, cacao and specialty fruit are often linked with volcanic regions, and many fine growing areas are volcanic. Elevation, rainfall, variety and processing all shape quality and price as well. Treat “volcanic” as a selling story to test with your buyers, not as a promise of a yield gain or a price premium.

How to choose a crop for your field

1. Get the soil series from a soil map and the current pH and phosphorus levels from a lab.
2. List crops with local buyers and a suitable climate at your elevation.
3. Compare each crop’s phosphorus demand and pH range with your test result.
4. Cost the soil correction, and spread lasting amendments over the years they should help.
5. Prefer perennial cover or tree crops on steep ground where tillage would raise erosion risk.

Irrigation and water on volcanic ground

These soils often hold plenty of water and also drain freely, which sounds odd but fits their structure. The pores store water while excess moves through. Even so, slopes, shallow soils over lava, and dry seasons can still leave crops short.

Some simple rules follow from that behaviour:

• Measure water at root depth in your own field, since water content varies widely between volcanic soils.
• Use smaller, more frequent applications on slopes, where runoff and erosion are the risk.
• Keep soil covered, because FAO warns about permanent damage from excessive air-drying in these soils.
• Watch nutrient losses. The University of Hawaii notes nitrate moves with excess water, and potassium can be lost to leaching.
• Check local water rights and any permits before building storage or drawing from a well.

Drip and micro-irrigation suit many high-value crops on slopes because they place water near the roots. Savings vary with crop, slope and the system being replaced, so compare quotes and metered use and do not rely on a general percentage.

What volcanic ash does to a working farm

Many searchers ask if volcano ash is good for plants. In the long term, ash is the parent material of these soils. In the short term, a fresh fall is a hazard. FAO notes thin ash layers may rejuvenate surface soil while thicker layers bury it, and a new profile then develops in the fresh ash.

The US Geological Survey’s guidance on ash and agriculture says damage depends on ash thickness, crop type and condition, soluble fluoride, later rainfall, pasture and animal condition, and access to clean feed and water. USGS also says crop-loss forecasts are hard to make and usually exaggerated.

The classes come from the USGS rehabilitation guidance. It draws on the Mount St. Helens eruption of 1980 and Ruapehu in 1995-96, so USGS says it may only apply in temperate places. Where ash is deep, the land may no longer pay as pasture and can sometimes move to a use such as forestry.

USGS says stock grazing ash-covered grass can suffer fluorine poisoning or fatal gut blockages, and shortages of clean feed and water can also kill. Move animals to clean feed and water first and call a veterinarian. Ash also wears down machinery, though USGS says extra maintenance and a few precautions can greatly cut that cost.

Sustainable management of volcanic farmland

Good practice here protects what makes the soil valuable: its structure, organic matter and cover. Volcanic landscapes often include forests and steep watersheds, so what happens on one farm can affect people downstream.

• Keep the surface covered with cover crops, mulch or pasture, which limits drying and runoff.
• Keep trees on the steepest land. FAO’s view on steep Andosols is the place to start. Shade trees in coffee or cacao can also keep some cover in place on sloping fields.
• Add organic material. It is one of the measures FAO lists against phosphate fixation.
• Test on a schedule and apply phosphorus to a target, because overapplication can still lead to losses by runoff or leaching, as the University of Hawaii points out.
• Keep records of inputs and field boundaries if you sell to buyers who ask for origin or carbon data.

Satellite maps can help you see where a field is lagging behind the rest. They show vegetation change, not the cause, so a low patch still needs a soil test or a field check. Farmonaut Satellite + AI Farm Monitoring System provides this kind of field map on web, Android and iOS, which can be handy on steep or scattered blocks that take a long time to walk.

Related pages on our site: admin tools for larger farms, web app access, carbon footprint tracking, satellite and weather API documentation, fleet management, product traceability and API access.

▶ How satellite NDVI maps can show crop stress across a large farm. · 4:09

Agribusiness jobs in volcanic regions

Farming these soils draws on the same value chain as other specialty agriculture. Soil chemistry and sloping land are the local skills to look for. Common roles include:

• Soil scientists and agronomists, who read soil tests and set phosphorus, lime and crop plans.
• Irrigation technicians and engineers, who design and maintain systems on steep ground.
• Processing and packing staff, especially for coffee, cacao, tea and fruit.
• Farm and sustainability managers, who handle crews, certification and buyer records.

Pay depends on country, role and employer. We did not find reliable pay data for volcanic regions as a group, so use your national labor statistics for the job title and area. The video in this section covers typical US pay across agriculture careers.

▶ Twenty careers in agriculture, with typical US pay. · 7:04

A checklist before you buy or plant volcanic land

1. Find the soil series on a national soil map, such as USDA Web Soil Survey in the US.
2. Test pH, phosphorus, calcium, magnesium, potassium and organic matter, and ask the lab which methods suit ash soils.
3. Ask whether a phosphorus curve or lime curve exists for your soil or a close relative.
4. Walk the slopes after rain and look for runoff paths and soft ground that could limit machinery.
5. Check the hazard maps from your national volcano agency, and plan clean feed and water for stock.
6. Cost the first-year soil correction before you agree a land price. Get professional agronomic and legal advice before you commit.

FAQ

Why is volcanic soil good for farming?

Mainly because of its structure. Andosols are porous, light and easy to till, and they generally store more available water than other mineral soils (FAO). Many also hold high levels of organic matter. Fertility varies by site.

Is volcanic soil good for farming all crops?

No. Crops differ in phosphorus and pH needs, and in the Hawaiian trials head lettuce needed about 10 times the soil-solution phosphorus of cabbage at the low end. Sweet potato tolerates low phosphate, according to FAO. Compare your soil test with each crop’s needs first.

Is volcano ash good for plants?

Not straight away. Thin layers can renew surface soil over time, but USGS warns that crop and pasture survival is often severely limited above 100-150 mm of ash. Fluoride on ash can also harm grazing animals.

How is volcanicity important to farmers?

Volcanic activity creates the parent material of these soils, shapes slopes and water movement, and brings an ongoing hazard. Farmers in active areas gain useful soils but need an ash plan for stock, machinery and feed.

Why do volcanic soils need so much phosphorus?

Amorphous minerals and active aluminium bind phosphorus so plants cannot easily take it up. The University of Hawaii says volcanic soils tend to have the greatest phosphorus sorption of all soils, so rates need to match the soil and crop.

Do volcanic soils need lime?

Many acid ones do, but test first. Published curves for six Hawaiian Andisols imply about 1.2 to 4.3 tons of calcium carbonate per acre to reach pH 6.0, and some soils with a pH near 6 need calcium as a nutrient, at much lower rates.

The bottom line

Volcanic soil offers good structure, easy rooting and water storage, and it grows a wide range of crops. It is not a shortcut to high yields. Phosphorus fixation, acidity on leached sites, steep land and ash hazard are the costs that come with it. Identify the soil, test it, match the crop, and budget the correction before you plant.