Abstract

This project aimed to identify the quantities of nitrogen, phosphorus, potassium, and sulfur needed that should be included in the fertilizer addition for Uruguayan forage plant species. This was performed via the extraction of soil samples from three paddocks and laboratory analysis. After the results were handed back it can be concluded that nitrogen should be applied in the vegetative stage of crops at different times of the year. Additionally, 100 Kg/Ha of phosphorus should be added to the 0-40+5S fertilizer to increase the phosphorus levels in the soil by 2.5 or 3 ppm. 100 K/Ha had to be applied to a 0-0-60 fertilizer to increase the potassium soil levels to 0.1 meq/100 gr. And sulfur applications should be divided at different times of the year with the combination of nitrogen and phosphorus. 

Key Words: macronutrients, micronutrients, soil sampling, nitrogen, phosphorus, potassium, sulfur. 

Introduction

The objective of this project was to determine the quantity (in mEq per 100 gr.) of a diverse range of macronutrients and micronutrients in various soil samples from three different paddocks in the Uruguayan lands. With this information, I would later regulate the fertilizers needed for each field, which are growing specific plant species. 

All plant species, for their proper and maximum productivity, require different amounts of nutrients. Based on the number of nutrients required, they can be classified into macronutrients and micronutrients. On the one hand, macronutrients are nitrogen, phosphorus, potassium, and sulfur. On the other hand, the micronutrients are zinc, iron, boron, copper, magnesium, manganese, calcium, and molybdenum. 

The plants that mostly make up artificial pastures in Uruguay are the forage species. Usually, the nutrients these plants need can come from two sources: either from the soil (organic matter and soil clays), or from aggregates through fertilizers. To give you a general idea; the optimal moments when fertilizer addition should be made to pastures are at the planting stages, during the development of annual crops, and mostly in autumn annually.  The main nutrients to monitor from the fertilizers applied to these plants are nitrogen, phosphorus, potassium, and sulfur. 

Methods and Analysis

Now that the purpose of this project has been introduced, I will now proceed to explain how it was conducted. The soil samples were taken on April 8th of 2022. To begin, I had to establish a plan for sampling, preparing a map to include the boundaries for each field, different slopes, elevations, crop growth, and degrees of erosion. The samples gathered were taken from all over the area, since taking soil from a single place would not adequately represent the entire population of ground from the field section. To do this, many samples were taken from different points of the paddock. The apparatuses used were a soil auger to remove the samples with a minimum depth of six inches, a spade, and soil sampling tubes. After taking ten or fifteen samples from each field, I placed the soil in a clean container, mixed it thoroughly, and took approximately one pint for analysis. This process was repeated for the three paddocks of analysis. 

Afterward, on April 11th, the three samples were sent to the “Lavsa Laboratory” to determine the quantity (mEq/100 gr) of nitrogen, phosphorus, sodium, potassium, sulfur, zinc, boron, calcium, manganese, magnesium. Other data collected were the pH, and percentages of carbon monoxide and molybdenum. 

The analytical methods used to determine these values were: 

For nitrogen: Kjeldahl
For phosphorus Bray I: spectrophotometric method
For sodium and potassium: flame photometry
For sulfur, zinc, boron, calcium, manganese, and magnesium: atomic absorption spectroscopy

Results and Discussions

Table 1 displays the results obtained by the “Lavsa Laboratory.” 

After analyzing the results can gather information into how the different nutrients will be applied to crops through the use of fertilizers. 

Nitrogen

To begin with, nitrogen should be applied at different times of the year (since it is a very mobile nutrient in the soil), mainly in the vegetative stage of crops. Nitrogen is a compound used in high quantities by plants, especially by legumes; such as lotus, alfalfa, white clover, and red clover. These plants have the particular ability to fix or transform nitrogen from the air into a “profitable” type of nitrogen for the plants. They do this by a symbiosis interaction at the root level with a bacteria of the Rhizobium genus. Therefore, via this transformation, legumes do not need to apply artificial nitrogen fertilizers. However, forage grasses; like fescue, Dactylis, sabadilla, oats, or ryegrass, do not have the ability to “fix” the nitrogen from the air to their own advantage. Hence, these Uruguayan pastures need nitrogen added via fertilizers or nitrogen transferred from legumes in mixed grasslands. 

Phosphorus

This nutrient should be added at the planting stage, because it is a poorly mobile nutrient in the soil, and in annual re-fertilizations in the fall. From what has been discovered until now, the only way to maintain or increase the phosphorus levels in the soil is via the addition of fertilizers. These levels are usually measured in parts per million, through the Bray 1 methodology. Each plant species has an optimum level of phosphorus in the soil needed to grow effectively. For example, in the case of lotus corniculatus, this minimum level is 12 ppm, for red clover and fescue the optimum level is 14 ppm, in white clover, it is 16 ppm and in the case of alfalfa is around 18 or 20 ppm. Nevertheless, if the soil levels are below those values we must find what is missing through fertilizers to reach the desired levels. To make these calculations we have to take into account different points. Firstly, the soil levels from the field or paddock we are analyzing, which is measured through soil analysis. Secondly, the requirements of the plant species that will be farmed in that sector. Thirdly, the type of soil, meaning its texture- portions of sand, silt, and clay, have to be taken into account. And lastly, the type of chemical available, including the proportion of soluble phosphorus over the total concentration of each fertilizer. Finally, we could conclude that 100 Kg/Ha had to be applied of a 0-40 + 5S fertilizer, to provide an increase of 2.5 or 3 ppm of phosphorus levels in the soil. 

Potassium

Like phosphorus, the only way to maintain or increase the soil level of potassium is through the use of fertilizers. Although Uruguayan soils originally had high levels of potassium, prolonged stages of agricultural crops without the replenishment of this particular nutrient led to the fact that today, most agricultural fields have deficiencies in the addition of this chemical. Therefore, a level of 0.45 meq/100 grams of soil is critically low; so the addition of fertilizers was needed for fields “Perdido” and “5b”. Consequently, 100 Kg/Ha had to be applied with a 0-0-60 fertilizer to increase 0.1 meq/100 gr. level of soil in the field. 

Sulfur

Finally, since sulfur is a very mobile nutrient in the soil, it is very convenient to divide its applications at different times. In addition to this, its absorption and use within plant species are closely associated with phosphorus and nitrogen. Hence, all applications of phosphorus at planting should be done together with sulfur applications. Nitrogen additions (in the re-fertilizations) should be done together with sulfur applications too. Usually, carbamide or urea (the chief nitrogenous end product of the metabolic breakdown of proteins) of 46-0-0 is the maximum used and almost all include sulfur. 

Now that we have gathered this information, we can conclude that the quantities of each nutrient needed for the two different fertilizers used are shown in Table 2.