How Soil Testing is Revolutionizing Farm Nutrition for Rice-Blackgram Cropping Systems
For centuries, farmers fertilized their fields based on guesswork. Today, agricultural scientists are prescribing nutrients with the precision of medical doctors.
Imagine visiting a doctor who prescribed medication without checking your vital signs, past medical history, or current symptoms. This scenario would rightfully alarm most people, yet for generations, farmers have often applied fertilizers to their crops with similarly limited information about their soil's actual nutrient needs 2 . The consequences of this approach—diminishing returns on fertilizer investments, environmental damage from nutrient runoff, and gradual deterioration of soil health—have prompted agricultural scientists to develop a more precise alternative.
The Soil Test Crop Response (STCR) approach represents a paradigm shift in agricultural nutrition. By integrating soil testing with tailored fertilizer prescriptions, this method allows farmers to apply exactly what their crops need to reach specific yield targets—no more, no less 1 . When applied to complementary cropping systems like rice and blackgram, this precision nutrition strategy offers a path to enhanced productivity, improved sustainability, and greater profitability for farming communities.
Traditional fertilizer recommendations typically offer blanket suggestions for entire regions, ignoring the substantial variation in nutrient levels between different fields—and sometimes even within the same field 1 .
Conventional fertilizer practices have nutrient use efficiencies of just 30-50% for nitrogen, 15-20% for phosphorus, and 60-70% for potassium 1 . The remainder represents wasted expense and environmental contamination.
The Integrated Plant Nutrition System (IPNS) framework within STCR offers a solution by combining organic manures with chemical fertilizers. This integration significantly improves nutrient availability and use efficiency through multiple mechanisms: enhancing nitrogen availability via mineralization, increasing potassium availability by reducing fixation, and improving overall soil biological activity 1 .
The fundamental principle behind STCR is elegantly simple: different soils possess different natural capacities to supply nutrients, and crops require specific amounts of nutrients to produce each unit of yield. By quantifying both these factors, scientists can create precise fertilizer prescriptions tailored to individual fields and yield goals 5 .
At the heart of the STCR approach lies a mathematical relationship known as the fertilizer prescription equation. This formula integrates four critical parameters for each essential nutrient (N, P, and K):
Fertilizer Nutrient Required = [(NR × T) - (CS × STV)] / CF
Where T represents the yield target and STV signifies the soil test value 1 .
| Parameter | Nitrogen (N) | Phosphorus (P) | Potassium (K) |
|---|---|---|---|
| Nutrient requirement per Mg of grain (kg) | 2.01 | 0.44 | 3.06 |
| Contribution from soil (%) | 22.05 | 37.34 | 41.48 |
| Contribution from fertilizer (%) | 38.08 | 49.93 | 252.98 |
| Contribution from FYM (%) | 23.25 | 28.34 | 16.80 |
| Contribution from fertilizer + FYM (%) | 44.83 | 60.57 | 278.70 |
To understand how STCR works in practice, let's examine a comprehensive field study conducted specifically on the rice-blackgram cropping sequence—a system where two crops are grown in succession on the same field.
Researchers conducted a multi-year experiment at the Agricultural College Farm in Bapatla, India, during the 2017-18 and 2018-19 growing seasons. The study employed a Randomized Block Design with ten different nutrient management treatments, each replicated three times to ensure statistical reliability .
Integrating soil test-based chemical fertilizers with organic manure
Using only chemical fertilizers based on soil tests
The conventional blanket recommendation
Representing traditional methods 1
Rice grain yield increased significantly with STCR treatments incorporating FYM. These results were statistically comparable to STCR without FYM and superior to all other approaches .
The content and uptake of nitrogen, phosphorus, and potassium in both grain and straw were significantly higher in treatments receiving soil test-based fertilizer recommendations with FYM .
The fertilizer use efficiency was notably highest with the application of 5.5 t ha⁻¹ target yield recommendation with FYM compared to all other treatments .
| Treatment | Grain Yield (t ha⁻¹) | Fertilizer Use Efficiency | Nutrient Uptake |
|---|---|---|---|
| STCR with FYM | Highest | High | Highest |
| STCR without FYM | Statistically similar to STCR with FYM | Moderate | High |
| General Recommended Dose | Moderate | Lower | Moderate |
| Farmers' Practice | Lower | Lowest | Lower |
| Absolute Control | Lowest | Not applicable | Lowest |
Implementing the STCR approach requires specific tools and materials that enable researchers—and increasingly, agricultural extension services—to develop tailored fertilizer prescriptions.
| Tool/Material | Function | Application in STCR |
|---|---|---|
| Soil sampling equipment | Collecting representative soil samples | Obtaining accurate baseline soil nutrient data |
| Farmyard Manure (FYM) | Organic nutrient source | Improving soil structure and providing slow-release nutrients |
| Vermicompost | Enhanced organic amendment | Boosting microbial activity and nutrient availability |
| Chemical fertilizers | Precise nutrient sources | Urea (N), Single Super Phosphate (P), Muriate of Potash (K) |
| Soil testing laboratory | Analyzing soil parameters | Measuring pH, organic carbon, available N, P, K |
| Fertilizer prescription equations | Calculating nutrient requirements | Determining exact fertilizer doses for target yields |
The process begins with proper soil sampling—the foundation of any STCR approach. As Purdue Extension specialists advise, "The first step of gathering a good soil sample is to draw a diagram of your property and indicate where you will take soil samples from. It's important for you to take samples from different parts of your property that have different characteristics" 2 . This careful spatial approach ensures that the soil analysis truly represents the field's variability.
The organic components like farmyard manure and vermicompost play dual roles: they supply nutrients while simultaneously improving soil health parameters. Research on sesame has demonstrated that the combination of organic and inorganic nutrient sources creates a more balanced, sustainable nutrition system 3 .
The implications of STCR research extend far beyond experimental plots. With global food demand projected to rise significantly in coming decades—and with increasing concerns about agriculture's environmental footprint—precision nutrient management offers a pathway to sustainable intensification.
For the rice-blackgram cropping system specifically, STCR enables farmers to optimize the complementary relationships between these crops. Blackgram, as a legume, can fix atmospheric nitrogen, potentially reducing the nitrogen fertilizer requirement for subsequent crops in the rotation . The STCR approach can quantify this contribution and adjust fertilizer prescriptions accordingly.
The transition to prescription-based fertilization does face implementation challenges, including the need for accessible soil testing services and technical support for farmers. However, the compelling research findings—yield increases of 20-32% for direct-seeded rice using STCR compared to general recommendations, similar dramatic improvements for green gram, and enhanced nutrient use efficiency across multiple crop systems—present a strong case for wider adoption 1 5 .
As we look to agriculture's future, the paradigm of fertilizing crops based on precise prescriptions rather than generalized recommendations offers hope for addressing one of humanity's most fundamental challenges: meeting our nutritional needs while stewarding the natural resources that make food production possible. In the words of one research team, "The Inductive cum Targeted Yield Model used to develop fertilizer prescription equations provides a strong basis for soil fertility maintenance consistent with high productivity and efficient nutrient management in farming for sustainable and enduring agriculture" 3 .
The prescription for healthier crops, more profitable farms, and more sustainable agricultural systems begins with understanding what's happening beneath our feet.