Fertility and Nutrients
Fertilization to improve crop quality and health
By Presented by Jeff Schoenau, College of Agriculture and Bioresources, University of Saskatchewan at the Top Crop Manager Plant Health Summit, Feb 25-26, 2020, Saskatoon.
A well-fed crop is a strong crop.
We think a lot about yield, but another important part of crop production is the influence that plant nutrition has on crop quality and health. And that transcends to human health as well. Soil health affects plant health, and that ultimately affects human health.
A basic premise of plant health is that a well-nourished crop with the balance of essential macro and micro nutrients really provides the best yield, and also the best ability to fend off foes, whether it’s diseases or maybe even insects. We can probably sum it all up into a single short statement: “a well-fed crop is a strong crop.”
The first key principle of plant health from a nutritional standpoint is concentration of nutrients in plant material. Concentration may be linked to nutrient density. I like to think of nutrient-dense foods defined as a food that is rich in minerals, vitamins, and other components that are desired for human health.
To increase the nutritional value of foods requires an understanding of those factors that affect the concentrations that are found in the plant. The most important is genetics. For example, canola has a high requirement for sulphur because of the sulphur containing compounds that are involved in the physiology of those Brassicae crops. So, genetics is important.
Anybody that does crop scouting and takes tissue samples is certainly aware of the importance of the effect of plant part and age on nutrient concentrations in a plant. For example, annual crops take up most of the nutrients they need early on in their life cycle, but as they grow and photosynthesize and fix carbon, that concentration decreases.
Environment is very important. For example, nutrient concentration might be higher if something shuts down plant growth, so that nutrient isn’t diluted by photosynthetic fixation of carbon and higher yield.
Another consideration is the availability of a nutrient in the soil. Nutrient concentration in plant tissue can be influenced through fertilization. That concentration typically follows an S-shape relationship in soils with very low levels of nutrients. For example, nitrogen concentration in plant tissue may actually decrease initially with fertilization in a very nitrogen-deficient soil. That is called growth dilution. A little bit of nutrient added under a condition of extreme deficiency results in an explosion of plant growth and a production of dry matter such that the concentration may actually decrease initially.
Beyond that, as we increase the nutrient availability, the concentration in plant tissue increases. Finally, depending on the nutrient, as we increase nutrient availability further, there may be no further increases in concentration as the plant reduces or ceases uptake. But for some nutrients – for example, nitrogen – as nutrient availability increases further in that soil, the plant continues to take it up beyond the point of maximum yield. There’s no further increase in yield, but the concentration increases, and this is referred to as “luxury uptake.”
In cereals there is a critical level where further increases in nitrogen fertility are not associated with any yield increase but instead the nitrogen is going towards increasing protein content. In spring wheat, that is around 13.5 or 14 per cent.
A study by Amy Mangin and Don Flaten at the University of Manitoba showed that split applications of nitrogen – for example, 80 pounds (lbs.) of nitrogen per acre at planting, plus 30 to 60 lbs. at stem elongation to flag leaf – bumped protein content up by about half a protein unit. Late supply of nitrogen in a cereal crop will go more towards protein and less towards yield. High yielding spring wheat varieties need high rates of N to optimize both yield and protein.
Sulphur is another important nutrient, and is a building block of two amino acids, cysteine and methionine. Research by Rigas Karamanos showed that wheat grown on soil that is highly deficient in available sulphur may respond to sulphur fertilization and increase protein concentration.
There is also some research that shows that having adequate amounts of sulphur-containing amino acids can also contribute to increased protein quality and improved bread-making quality.
Forages benefit from fertilization. For example, a study conducted by one of my graduate students, Bayartulga Lkhagvasuren, looked at the response of brome grass to nitrogen fertilization. At one of our sites close to Colonsay, Sask., 50 to 60 lbs. of nitrogen per acre added as dribble banded urea ammonium nitrate solution maximized yield and also produced significant increases in protein content of brome grass. Rates of nitrogen fertilizer added above that continued to increase protein, but there was no further increase in the brome grass yield.
In pulse crops, protein content is generally not highly responsive to nitrogen fertilization. One of my graduate students, Harshini Dona, conducted a field study in south-central Saskatchewan with soybean and lentil and different rates of starter fertilizer. A starter blend of urea and monoammonium phosphate – a 50-50 blend of 11-52-0 and 46-0-0 – was applied at increasing rates of zero, 10, 20 and 30 lbs. N per acre. There was not any increase in protein content with increasing N rates. The reason is that with a legume, the biological nitrogen fixation process will generally compensate for low nitrogen availability when conditions for fixation are good.
Some studies have reported that under phosphorus deficient conditions, starter phosphorus can give a protein boost to pulses, and this is related to the importance of phosphorus in the nitrogen fixation process.
Micronutrients are also important for crop quality. The contents of bioavailable zinc and iron in grains are of special interest. For example, zinc deficiencies in humans are estimated to affect over 30 per cent of the world’s population. There are areas that have quite widespread crop zinc deficiency. One particular area is the Middle East, where human deficiencies of zinc are quite widespread in the population. A high content of bioavailable zinc is desirable in pulses that are exported to countries like Pakistan, India and Bangladesh.
On the Prairies, we’re quite fortunate because our soils, in general, are relatively high in available zinc. As a result, our pulse crops tend to be quite high in zinc content compared to some other parts of the world where the soils are low in available zinc. That gives our pulse crops a bit of an edge when it comes to marketing and selling.
There are several ways to help ensure high zinc content in pulse crops. Work at the Crop Development Centre by Dr. Vandenberg and Dr. Warkentin is looking at ways to increase the content of these important micro-elements in grain through plant breeding efforts.
Fertilization is another potential strategy for improving plant health. Sarah Anderson, as part of her MSc work at the University of Saskatchewan, looked at the effects of zinc fertilization on the phytate:zinc molar ratio and the estimated bioavailable zinc in lentil grain.
Phytate is a major storage form of phosphorus in seeds. It has plant and human health benefits but is also anti-nutritional because it inhibits absorption of zinc ions within the human intestine if it is too high in concentration. The phytate:zinc molar is the ratio of phytate to zinc. If the ratio is low, that indicates generally higher bioavailability of the zinc content in the grain.
In Sarah’s research, compared to the unfertilized control, fertilization with zinc decreased the phytate:zinc molar ratio, which is a good thing for human bioavailability. It appeared that the chelated form was slightly more effective than the sulphate form in enhancing bioavailability.
In terms of genetic effects, Sarah looked at three different classes of lentil – Maxim red lentil; Invincible, small green; and Impower, large green lentil at two field sites. At both of the field sites under the same zinc availability status, there was significantly higher concentrations and higher bioavailable zinc in the large green lentil compared to the small green or the red lentil.
Some other work conducted by Noabur Rahman, one of my PhD students, looked at zinc concentration in pea grown on a Brown Chernozem soil that had quite low concentration and supply rate of available zinc. Compared to the unfertilized control, zinc fertilization increased the concentration of zinc in pea seed. The chelated form also produced some of the highest zinc concentrations.
Another question that we addressed in some of our research is how does the addition of phosphorus influence the phytate and the zinc concentrations in grain? As part of the work that Steven Froese did in his thesis work, we looked at the effects of 20 kg of P2O5 per hectare added in different combinations of seed-placed monoammonium phosphate and a mid-season foliar monopotassium phosphate spray to field pea. The treatments include all the phosphorus applied at seeding, three-quarters at seeding and remaining at foliar, a 50:50 split between seeding and foliar, and the entire 20 kg at foliar timing. Each treatment had the same total rate of 20 kg/ha.
There was a trend towards lower phytate concentrations in the seed as the proportion of applied phosphorus at the foliar timing was increased. But as the proportions of foliar P were increased, yield was lower than with seed-placed P. Overall, there was no significant effect on total phytate content between the treatments.
Looking at zinc, there was occasionally slightly higher concentration of zinc in the grain when the phosphorus was applied in the foliar form. This suggests that foliar application may reduce that phytate:zinc molar ratio, but we really didn’t see any large effects, and no effects on the iron concentrations.
Fertilization impact on plant disease
Nutrient management can affect the incidence of a plant disease. It can stimulate root and shoot growth, but very high rates of nitrogen can produce a heavy crop canopy with high humidity within that canopy that may favour the spread of certain pathogens. Work done by Randy Kutcher at Melfort showed that very high rates of nitrogen in canola, for example, were associated with increased incidences of blackleg and sclerotinia. With high fertilization rates, attention must be paid to address potential disease issues from a heavy canopy through fungicide application.
The second way that nutrient manipulation can influence plant disease is by changing the physical or biological micro-environment in the soil. A good example is how different fertilizers may affect the pH of the soil. For example, ammonium produces acidity when it’s oxidized to nitrate in the nitrification process, versus nitrate, which is not associated with acidity. Some work conducted on winter wheat in northwestern United States showed some interesting interactions of how ammonium versus nitrate influenced the population of pseudomonas bacteria in the rhizosphere, which is actually an antagonist to root rot disease that was affecting the winter wheat.
Another way that nutrient manipulation can impact plant health and disease is by increasing plant vigour and strength. Some recent research found that the optimum rate of phosphorus fertilizer for field pea is higher when the root system is compromised by Aphanomyces. The crop may be especially challenged in accessing an immobile nutrient, like phosphorus, in the soil under conditions of reduced vigour and root growth. However, it is desirable to try to get rid of the source of the problem rather than using a Band-Aid to try to address it.
Copper and ergot
Based on work by Ieuan Evans in Alberta in the 1990s, lower copper fertility was identified to aggravate ergot infections in wheat. A copper deficiency causes the self-pollinating florets to remain open longer, which increases the likelihood of the infection entering into that floret and the ergot body developing. On copper-deficient soil, copper fertilization may help to reduce the incidence of ergot in cereals, although this may not be a 100 per cent effective management strategy. And I think you’re only going to see this effect on soils that are truly copper-deficient.
Rotation can also impact micronutrient nutrition and response. My colleague, Dr. Ryan Hangs, looked at copper and zinc in a wheat-pea rotation – copper added to wheat, followed by zinc added to pea. Soil was collected from 47 different locations across Western Canada. Some of these soils had available concentrations and supply rates of a micronutrient that indicated potential responses to fertilization.
When copper was applied to wheat on 12 mineral soils that were suspected to be responsive to copper, there was a significant yield response. Foliar and banded application of copper sulfate significantly increased yield. However there was a lower yield from banded chelated copper compared to the control, which I think was because our rate was too high, and we saw some toxicity show up. We always need to be aware, with micronutrients in particular, there can be a fine line between sufficiency and toxicity.
The next year peas were grown on the wheat stubble, and were fertilized with zinc. There was a bit of response to zinc, but in many cases, it wasn’t statistically significant. But the interesting thing was that pea yield was significantly higher in the treatments where we had applied copper to the wheat the year before. It had us puzzled. We really couldn’t find evidence that it was a nutritional response, but pondered that perhaps it had something to do with the effect of the copper in reducing disease pressure in the pea. I think this is something that deserves some further attention down the road.
Chloride fertilization can also play a role in plant health. Potash (KCl 0-0-60) is the most common source used to meet chloride recommendations on the Prairies. We’ve known for a long time – for example reported in literature in the 1990s south of the border in Montana – of the role that chloride fertilization can play on low-chloride soils in reducing the incidence of leaf diseases and root diseases in cereals. Brian Fowler at the University of Saskatchewan, in his program with winter wheat, also showed some responses of the winter cereals to chloride fertilization in the reduction of leaf spot in winter wheat.
I was involved in a study in the 1990s looking at the application of 40 lbs. of KCl at foot slope positions versus upper slope positions in the landscape in a farm field southeast of Saskatoon. We were interested in this topographic effect because the foot slope positions in this landscape had very low chloride levels in the top two feet. In the two years of the study, one year we had a significant wheat yield response to the application of potash in the foot slope regions, which we were able to attribute to the effect of the chloride. Was it a disease thing, or was it a nutritional factor? Unfortunately, we did not do any disease ratings. I think it had to do with a lot of snow the winter before that leached the chloride out of the upper profile in the depressions where the snowmelt water accumulated. The next year was drier and there wasn’t any response to the potash at either the upper or lower slopes.
There is less talk about chloride as a limitation in recent years, and I think one of the reasons is that there is more widespread use of potash in fertilizer blends, particularly for cereals. With chloride, the important thing to remember is that almost all of the chloride remains in the straw, which means that that all of it gets recycled back into the soil if the straw isn’t baled off the field.
A question was raised in the past about whether boron addition can reduce the incidence of clubroot in canola. Some early work indicated that boron addition reduced the infection of Brassicae by clubroot, but it was also associated with phytotoxicity. Follow-up work that was done in 2014 in a study conducted in Alberta, Ontario, and Quebec showed no reduction in clubrooted incidence with boron fertilization and no yield response except on one organic soil in Ontario.
Calcium is another nutrient that some researchers are looking at to help control clubroot. It seems that soil pH of 7.2 or 7.4 would be an optimum pH to help reduce the incidence of clubroot. We also need to be thinking about how fertilizers may be influencing soil pH and how that might perhaps have an indirect effect on the incidence of this disease.