The three correct answers are soil nutrients, pH, and organic matter. These are widely recognized in agricultural science as the three foundational pillars of soil fertility, and they work together to determine how well plants can grow, access food, and resist stress.
Why These Three Factors Matter
Soil fertility isn’t just about dumping fertilizer on the ground. It depends on a balance of chemical, biological, and physical conditions. Nutrients feed the plant directly. pH controls whether those nutrients are actually accessible. Organic matter ties everything together by improving soil structure, feeding microorganisms, and releasing nutrients slowly over time. Remove any one of these three from the equation and plant growth suffers, even if the other two are in good shape.
Soil Nutrients
Plants need 18 essential elements to survive. Three of them (carbon, hydrogen, and oxygen) come from air and water. The rest must come from the soil. The most important soil-supplied nutrients fall into three groups:
- Primary nutrients: nitrogen, phosphorus, and potassium. These are needed in the largest amounts. Nitrogen is a building block of proteins and chlorophyll. Phosphorus drives energy transfer and is a core component of DNA. Potassium supports photosynthesis, drought tolerance, and protein production.
- Secondary nutrients: calcium, magnesium, and sulfur. Required in moderate amounts, these support cell structure, enzyme function, and chlorophyll production.
- Micronutrients: iron, boron, copper, manganese, zinc, and others. Plants need only traces, but deficiencies can still stunt growth or reduce yields significantly.
A soil can be rich in total mineral content yet still leave plants starving if the nutrients are locked in chemical forms that roots can’t absorb. That’s where pH comes in.
Soil pH
Soil pH measures how acidic or alkaline the soil is, on a scale from 0 to 14. For most crops, the optimal range is 6.0 to 7.5. Within that window, the majority of essential nutrients dissolve into forms that plant roots can actually take up. Move too far in either direction and specific nutrients become chemically unavailable, even if they’re physically present in the soil.
In highly acidic soils (below about 5.5), aluminum and manganese can reach toxic levels, damaging roots and blocking nutrient absorption. At the same time, phosphorus becomes less available. In overly alkaline soils, iron, zinc, and manganese tend to lock up. Correcting pH is one of the fastest ways to boost fertility without adding a single extra nutrient. Applying agricultural lime to acidic soils raises pH, reduces aluminum toxicity, and improves both nutrient uptake and crop yields. Research on wheat and maize grown in acidic soils found that lime application significantly increased grain yields and how efficiently plants used nitrogen, outperforming even crop residue incorporation alone.
Organic Matter
Organic matter is the decayed remains of plants, animals, and microorganisms mixed into the soil. It typically makes up a small fraction of total soil weight, but its influence is enormous. In loam soils, 2% to 3% organic matter is common. Sandy soils often sit below 1%, while clay soils can reach 4% to 5% or higher. There’s no single “ideal” number because what counts as good depends heavily on soil texture. Two percent organic matter in a sandy soil is excellent and hard to achieve, while that same percentage in a clay soil signals serious depletion.
Organic matter improves fertility through several overlapping mechanisms. It acts as a slow-release nutrient source: as microorganisms break down plant residues and compost, they gradually release nitrogen, phosphorus, and sulfur in forms plants can use. This slow release reduces the feast-or-famine cycle that comes with synthetic fertilizers alone.
It also transforms the physical structure of soil. Organic matter helps soil particles clump into aggregates, creating pore spaces that let air and water move through more easily. Better aeration matters because roots need oxygen to function, and compacted soil restricts how deeply oxygen can penetrate. Improved structure also increases water-holding capacity, meaning the soil retains moisture longer between rains or irrigation events. In compacted layers deeper than about 30 centimeters, oxygen diffusion slows dramatically, which limits root growth and nutrient uptake. Organic matter helps keep upper soil layers loose and well-aerated.
Perhaps most importantly, organic matter fuels the soil’s living community. Bacteria, fungi, and other microorganisms depend on it as a food source. In return, these organisms perform critical work: nitrogen-fixing bacteria convert atmospheric nitrogen gas into biologically available forms, which is the largest natural source of new nitrogen entering most land ecosystems. Mycorrhizal fungi extend the reach of plant roots, helping them access phosphorus and water they couldn’t reach alone. Without organic matter to sustain these organisms, the biological engine of fertility stalls.
How the Three Factors Work Together
These three factors are deeply interconnected. Adding nutrients to soil with the wrong pH is like putting premium fuel in a car with a clogged engine. The fuel is there, but the system can’t use it. Similarly, raising pH without addressing depleted organic matter may unlock nutrients temporarily, but the soil loses its ability to hold and cycle them over time.
Long-term field trials illustrate this clearly. Research spanning eight years on acidic soils found that combining balanced fertilizer application with crop residue incorporation (to build organic matter) and lime (to correct pH) produced the best results for both crop yields and nutrient efficiency. Each factor alone helped, but the combination outperformed any single intervention.
Practices like crop rotation and cover cropping support all three factors at once. Rotating legumes into a planting schedule adds nitrogen through biological fixation. Cover crops prevent erosion, suppress weeds, and add organic matter when they decompose. Over years, these practices build the kind of self-sustaining fertility that reduces dependence on external inputs.