A garden centre sells nitrogen fertiliser as small round granules, shown in the photograph. On the bag the composition is printed as an NPK grade, which giv...
Question 1 Report
A garden centre sells nitrogen fertiliser as small round granules, shown in the photograph. On the bag the composition is printed as an NPK grade, which gives the percentages of nitrogen, phosphorus and potassium (as their oxides) in the fertiliser. The table gives the grades of three products.
| Fertiliser | Main compound(s) | Grade (%N : %P2O5 : %K2O) |
|---|---|---|
| fertiliser A | ammonium nitrate | 34 : 0 : 0 |
| fertiliser B | ammonium phosphate and potassium chloride | 15 : 15 : 15 |
| fertiliser C | potassium chloride | 0 : 0 : 60 |
Use these relative atomic masses: H = 1, N = 14, O = 16, K = 39, Cl = 35.5.
(a) State which fertiliser, A, B or C, is a straight nitrogen-only fertiliser, and give a reason from the table. [2]
(b) Explain why fertiliser B is described as a balanced (complete) NPK fertiliser but fertiliser C is not. [2]
(c) Fertiliser C is potassium chloride, KCl. Calculate the percentage of potassium by mass in potassium chloride. [3]
(d) Explain why fertilisers are manufactured as small granules rather than as a fine powder. [2]
(e) The nitrogen in fertiliser A is present as ammonium nitrate. Explain why ammonium nitrate provides nitrogen to plants in two different ways. [2]
(f) A farmer needs to add 6.8 kg of nitrogen to a field. Using the grade in the table, calculate the mass of fertiliser A that must be spread. [3]
(g) Describe and explain two problems that can be caused by spreading too much of fertiliser A on farmland. [4]
Answer Details
Labelled answer diagram:
This question reads an NPK grade table and then uses percentage by mass in two calculations.
(a) Fertiliser A [1]. Its grade is 34 : 0 : 0, meaning 34 % nitrogen but 0 % phosphorus and 0 % potassium, so it supplies nitrogen only - a "straight" nitrogen fertiliser [1]. [2]
(b) A balanced (complete) NPK fertiliser must supply all three major nutrients. Fertiliser B has grade 15 : 15 : 15, so it provides nitrogen, phosphorus and potassium together [1]. Fertiliser C is 0 : 0 : 60, supplying only potassium, so it is not complete [1]. [2]
(c) For potassium chloride, \(\text{KCl}\):
\[ M_r = 39 + 35.5 = 74.5 \quad[1] \]
\[ \%\,\text{K} = \frac{39}{74.5}\times100 \quad[1] = 52.3\,\% \quad[1] \]
The accepted answer is 52.3 %. [3]
(d) Granules have a much smaller surface area to volume ratio than a fine powder, so they dissolve more slowly [1]. This releases the nutrients gradually and means they are less easily washed away by rain, and they are also easier to spread evenly without blowing about [1]. [2]
(e) Ammonium nitrate, \(\text{NH}_4\text{NO}_3\), contains two nitrogen-bearing ions: the ammonium ion \(\text{NH}_4^{+}\), which supplies nitrogen [1], and the nitrate ion \(\text{NO}_3^{-}\), which also supplies nitrogen [1]. So a single compound feeds nitrogen to the plant in two forms. [2]
(f) Fertiliser A is 34 % nitrogen by mass, so every 100 kg of fertiliser carries 34 kg of nitrogen. To supply 6.8 kg of nitrogen:
\[ \text{mass of fertiliser} = \frac{100}{34}\times6.8 \quad[1][1] = 20\ \text{kg} \quad[1] \]
The accepted answer is 20 kg. [3]
(g) Problem 1: excess fertiliser is washed into rivers and lakes and causes eutrophication [1]; the resulting algal bloom dies and decays, bacteria use up the dissolved oxygen, so fish and other aquatic life die [1]. Problem 2: nitrate leaches into groundwater and drinking-water supplies [1], where high nitrate concentrations are harmful to human health [1]. [4]
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